Question
In many plant species from fire-prone ecosystems germination is promoted by smoke. Mediterranean Basin (MB) flora is no exception. However, most information regarding germination response to smoke in the MB comes from a few experiments performed in laboratory conditions. This approach does not consider factors that occur in the field, such as species interactions, density-dependent processes or the fact that seeds spent time in the soil seed bank. In addition, species selection has been biased (e.g. there is a lack of information about annual species). Hence the importance of smoke relative to other fire cues is not clear, and we have a biased knowledge of post-fire community assembly in the MB. In this framework, we tested the following hypotheses: (1) smoke enhances seedling emergence and establishment from the soil seed bank of MB species, and (2) annual species are an important component of this smoke-stimulated flora.
Location
Mediterranean fire-prone shrublands in eastern Spain. Western Mediterranean Basin.
Methods
We performed a field experiment in which we applied a liquid smoke treatment and tracked seedling emergence and seedling establishment during 1 yr. Differences between smoke and control subplots with respect to seedling emergence and seedling establishment were analysed at different scales: community, growth form (annual or perennial), family and species level.
Results
At the community level, smoke played a clear role in seedling recruitment, increasing seedling emergence and seedling establishment. In addition, for most plots, families and species, establishment was higher in smoke subplots compared to the control. Annual species establishment was clearly stimulated by smoke but no effect was detected for perennials.
Conclusions
Smoke derived from wildfires has a key effect on plant recruitment and hence on community assembly in the MB vegetation.
+We applied liquid smoke in 21 plots and tracked seedling emergence and establishment during one year. Smoke increased seedling emergence and establishment globally and for most plots, families and species, and was most evident for annuals. Our results suggest that smoke derived from wildfires has a key effect on plant recruitment and community assembly in the Mediterranean Basin vegetation. +
Question
Revealing how plant traits vary over disparate spatial scales and how ecological processes mediate such variation is important for understanding plant community assembly. However, to what extent does the distribution of trait variation among ecological scales differ between leaf and wood tissues and between physical and chemical traits? What are the consequences of resource competition and/or habitat filtering on the community assembly with respect to differences between leaf and wood traits, and between physical and chemical traits?
Location
Subtropical evergreen broad-leaved forests in five sites in the Ningbo area (29°41–50′ N, 121°36–52′ E) in eastern China.
Methods
Traits of 96 woody plant species were sampled and variation of ten physical- and chemical-based leaf and wood traits were partitioned across six ecological scales (site, plot, species, individual plant, twigs and leaf age) using a linear mixed model.
Results
From individual plant to site scales, variance partitions were distinct between leaf and wood traits. In leaf tissues, physical and chemical traits showed a consistent pattern, with the majority of variation found among species and individual plants, with little among plots. For wood tissues, the largest variation in physical traits was at the species and individual plant scales, with the largest variation in chemical traits observed at the plot scale. Variance partition was markedly similar within and across species.
Conclusion
Leaf and wood traits vary differently in relation to ecological scale, suggesting that trait variability is tissue-specific. The large variability of wood traits at the plot scale suggests a strong habitat filtering process. The large variation in leaf traits within plots may reflect niche differentiation across species and the importance of intra-specific variation that affects species co-existence. Our study demonstrated that physical and chemical traits may be independent. These decoupled trait axes may increase the dimensionality of niche space and facilitate species co-existence in forest communities.
+In subtropical forests in southeast China, 10 physical- and chemical-based traits vary differently between leaf and wood tissues across six ecological scales. The large variability for wood traits at the plot scale suggests a habitat filtering process, but for leaf traits within plots reflects niche differentiation process. These decoupled trait axes may increase the dimensionality of niche space and facilitate species coexistence. +
Question
Poikilohydric organisms can survive desiccation without damage and recover soon after re-wetting. In this work we explore the realized niches of epilithic, poikilohydric organisms, i.e. lichenized and lichenicolous fungi and bryophytes, grouped into 15 functional guilds based on growth form, reproductive strategy and photosynthetic traits. We hypothesize that in Mediterranean ecosystems, the distribution of these guilds is controlled by water availability and solar radiation, together with characteristics of the substrate.
Location
Western Sardinia (Italy). The area hosts Mediterranean Quercus ilex forests on basaltic substrata within an agroforestry landscape.
Methods
The presence or absence of species from each guild was recorded in 1025 small quadrats (10 × 10 cm). Four environmental variables were measured: solar radiation, microtopography (potential water run-off), substrate roughness and number of cracks. The occurrence of each guild was related to environmental variables using non-parametric multiplicative regression, and ecological niches of the guilds were derived from those models.
Results
The occurrence of all functional guilds varied significantly according to the selected environmental variables. In most cases, solar radiation was the most important variable; TI was included first in the model of two sorediate guilds, whereas four sexually reproducing functional guilds had surface relief as first variable. The growth forms were separated along a gradient of increasing solar radiation, whereas water run-off and surface roughness mainly discriminated reproductive strategies and photobionts. Vegetatively reproducing guilds with other photobionts than Euphyta-like pigments were confined to the border of the ecological space, defined by gradients of environmental factors, which was largely occupied by more competitive, sexually reproducing organisms.
Conclusions
Functional traits of epilithic poikilohydric organisms were associated with ecological adaptations to the stressful environment of Mediterranean outcrops. We showed that their response in terms of probability of occurrence is coherent with quantitative gradients of solar radiation and water availability at the micro-scale.
+Functional traits of epilithic poikilohydric organisms were associated to ecological adaptations to the stressing environment of Mediterranean outcrops. Their response in terms of probability of occurrence is coherent with quantitative gradients of solar radiation and water availability at the micro-scale. +
Questions
What are the general tree communities found in tidal freshwater swamps along four large coastal rivers in the southeastern United States (US)? How do these communities compare to other tidal freshwater swamps in the US and South America?
Locations
Tidal floodplains of major rivers along the Atlantic and Gulf coasts of the southeastern US: Savannah, Altamaha, Suwannee and Apalachicola Rivers.
Methods
An extensive survey of trees and shrubs was conducted to describe the communities from a range of tidal freshwater swamps. River basins studied include micro-tidal (Gulf coast) and meso-tidal (Atlantic coast) regimes, and study areas were located both near and distant to primary channels. A total of 128 plots (100 m2 each) were inventoried, distributed evenly over the Savannah and Altamaha Rivers along the Atlantic coast, and the Suwannee and Apalachicola Rivers along the Gulf coast. Multivariate statistics helped discern communities and the significant indicator species in each.
Results
Four general communities were characterized and named according to the strongest individual indicator species in each: Water Tupelo (Nyssa aquatica) Community, Swamp Tupelo (Nyssa biflora) Community, Dwarf Palmetto (Sabal minor) Community and Cabbage Palm (Sabal palmetto) Community.
Conclusions
Descriptions of most tidal freshwater swamps in the southeastern US fit within the communities described in this study. Because studies that make inferences between environmental drivers (e.g. salinity, hydroperiod, hurricanes) and specific community types are best applied to the same communities (but perhaps different river systems), this work provides a framework by which tidal freshwater forested wetlands can be accurately compared based on their tree communities. We suggest that, within the broad range of our inventories, the four communities described identify the primary associations that should be tracked within most tidal freshwater swamps of the US. However, we identify some river basins in the US that do not fit this construct. Diversity of major tree communities in tidal freshwater swamps outside the US is generally much lower (with the notable exception of Amazonian hardwood tidal várzea), as are basal area values.
+Tidal freshwater forested wetlands are a poorly studied ecosystem that continues to be heavily impacted by global climate change, specifically via sea-level rise. We provide this four-community framework as a means to more accurately compare and predict ecosystem responses between different river systems. We describe how most published accounts fit into this framework and identify those that do not fit. +
Question
The ephemeral layer is an important component of the plant community in cold deserts. Conventional wisdom has it that the ephemeral layer primarily depends on early spring snowmelt water for germination and development. However, this hypothesis has never been subjected to rigorous testing.
Location
Southern edge of the Gurbantunggut Desert, a cold desert and a natural habitat of ephemeral plants (the inter-sand dune lowland), in the far northwest of China and around the geographical centre of the Eurasian continent.
Methods
At the end of the snow cover season between 2009 and 2011, four different snow treatments (0, 50, 100 and 200% snow cover, with 100% representing the natural amount of snow) were applied, using five replicates. Seedling establishment, species diversity and plant growth, population, cover and productivity were monitored throughout or sampled at the end of the growing season.
Results
The ephemeral plant germination and seedling density were significantly influenced by changes in snow cover. Germination, density of mature ephemeral plants and plant coverage rose as snow cover depth increased. However, the height, rooting depth, numbers of lateral roots and above-ground biomass of individual plants decreased as snow cover increased, and plant size was negatively correlated with plant density. At the ephemeral layer level, above-ground biomass showed no significant variation between snow treatments. There was also no significant difference in species richness among the snow treatments.
Conclusions
Increased snow resulted in higher plant density of ephemerals. However, this high density induced strong competition, reducing the body size of individual plants. By the end of the growing season, snow treatments did not induce significant changes in primary productivity of the ephemeral layer. Primary productivity or growth after seedling establishment depended mainly on rainfall that occurred after the snow had melted, not on snowmelt water itself. Thus conventional wisdom was shown to be only partially true. Snow was important to these ephemeral plants, but its role was limited to seedling establishment.
+Conventional wisdom has it that ephemeral plants in Gurbantonggut Desert mainly depend on snow-melted water to grow. Our results show that snow water is important for ephemeral plants, but not that important. Its role is limited to seedling establishment. Plant growth after then mainly depends on the rainfall of the growing season, not on snow-melted water. +
Questions
Does the importance of biotic interactions between tree species increase during secondary forest succession? Do functional trait values become increasingly divergent from early towards late successional stages and how is functional diversity affected by trait identity, species identity and species richness effects?
Location
Gutianshan National Nature Reserve, Zhejiang Province, southeast China.
Methods
Based on 26 leaf and wood traits for 120 woody species, we calculated functional diversity, using Rao's formula for quadratic entropy, trait dissimilarity, defined as half the mean trait-based distance of all species in the community, and functional evenness, defined as the degree to which functional diversity is maximized. We employed randomization techniques to disentangle the effects of trait identity, species identity and species richness on these three components of functional diversity.
Results
Against expectations, functional diversity did not show any successional trend because the communities compensated for a loss in trait dissimilarity by distributing the trait values more evenly among the resident species, thus increasing functional evenness. Randomization tests showed that functional diversity was not affected by trait identity, by species identity or by species richness, which indicates that functional diversity was neither determined by particular single traits or by single species with outstanding trait values.
Conclusions
The constant functional diversity suggests constant functionality in this subtropical forest, which might temporally maintain stable immigration conditions during the course of succession, and thus provides an explanation why these subtropical forests become more species-rich with time.
+Analysing functional diversity along a subtropical forest chronosequence we encountered a constant functional diversity with increasing successional age because the communities compensated for a loss in trait dissimilarity by distributing the trait values more evenly among the resident species, thus increasing functional evenness. This constant functionality might provide an explanation for the temporally stable immigration conditions in this succession series. +
Question
How do canopy disturbance and soil properties structure vascular plant community species composition and resilience to encroachment by exotic species in a tropical montane wet forest?
Location
Hawai'i Experimental Tropical Forest (HETF), a tropical montane wet forest, on Mauna Kea, Hawai'i Island, Hawai'i, USA.
Methods
Previous studies employing airborne LiDAR were used to define three zones across an elevation gradient from 900 to 1500 m. Within each zone, a ~1000-m block transect was selected to cross two different volcanic substrates: one derived from surface lava and one derived from thick ash deposits. Non-metric multidimensional scaling (NMS) scores of vegetation data were related to independently-derived environmental NMS scores and spatial location with generalized additive models (GAM).
Results
Vascular plant species composition in all elevation zones consists of three NMS axes, which are best modelled by one of three possible environmental NMS axes or by location. The first NMS axis of species composition in the lowest elevation zone (40% variance explained (VE)) is a function of location on volcanic substrates (61% deviance explained (DE)). The second lowest elevation axis (27% VE) is a function of unexplained spatial heterogeneity (31% DE). The third lowest elevation NMS axis (24% VE) is a function of the spatial mosaic of canopy disturbance (16% DE). In the middle elevation zone, species composition most strongly relates to the interaction between volcanic substrate and the condition of the soil surface for all three NMS axes (41%, 27%, 24% VE; 70%, 16%, 24% DE). The primary axis of species composition in the highest elevation zone (41% VE) corresponds with substrate and soil condition (55% DE) but the second and third axes of species composition (27% and 25% VE) relate to canopy dieback disturbance (36%, 14% DE). Counts of exotic species and 0–2 m height class native tree species respond to the type of volcanic substrate and soil surface condition in all three elevation zones. Lava-derived substrates have a higher incidence of exotic species and less native tree regeneration; whereas ash-derived substrates have higher numbers of native tree species regenerating and many fewer exotic species.
Discussion
The tropical montane forests on Mauna Kea reflect a native-dominated plant community response to disturbance on both lava- and ash-derived volcanic substrates, and a higher propensity for exotic species to occur on the lava-derived substrate. Native plant communities on ash-derived soils may have higher resilience to exotic invasion than communities on lava-derived substrates. Our results indicate resource managers should explicitly account for variation in soils and substrate type when prioritizing, implementing and monitoring management interventions to foster native plant assemblages and control the spread of exotic and invasive species.
+In the tropical montane forests on Mauna Kea, Hawai'i, native plant communities on ash-derived soils may have higher resilience to exotic invasion than communities on lava-derived substrates. Resource managers should explicitly account for variation in soils and substrate type when fostering native plant assemblages and controlling the spread of exotic and invasive species. + +
Question
Climate change is driving shifts in the composition of vegetation but the lack of controls and confounding spatial factors pose challenges for detecting the climate signal in observed changes through time and space. We tested whether climate can be isolated as a driver of spatial vegetation composition at the landscape scale in mediterranean southern Australia by considering landscape factors (e.g. soil gradients) and spatial structure (relative geographic isolation). The aim was to develop principles for selecting spatial analogues for climate change and provide a spatially predictive baseline for monitoring.
Methods
A landscape-scale monitoring transect spanning 550 km was established. Whole community presence/absence of vascular plant species in plots was modelled as a multivariate response to environmental and spatial variables. Species and phylogenetic composition–environment relationships were also explored using indirect gradient approaches, partial correlations and distance decay models.
Results
A total of >2900 occurrences of >400 plant species were recorded. Relative vegetation composition was predicted by mean temperature and soil properties, such as electrical conductivity and texture. Spatial structure was critical, as decay in compositional similarity with geographic distance and spatial autocorrelation of nested plots were involved in turnover patterns. The rate of change in species composition with changes in temperature equated to complete species turnover within the habitats sampled.
Conclusions
The influence of climate on spatial variation in vegetation composition can be quantified, accounting for distance decay. Landscape gradients (particularly soil properties) tended to be orthogonal to climate and explained some turnover. Spatial analogues for climate change would need to be similar in soil properties and not too geographically distant. Composition resulting from more extreme climate change scenarios may have no spatial analogue due to the importance of neutral distance decay at larger spatial scales in determining compositional differences. We illustrate these principles with a sequence of warming, drying analogues. The spatial transect provides a framework for monitoring composition by directly incorporating temporal data and using spatial analysis to inform the expected direction of compositional shifts with climate change.
+Linking changes in vegetation to climate change is challenging because there are no static-climate controls for comparison. Guerin et al. use a monitoring transect in southern Australia to isolate the influence of climate on vegetation composition, considering landscape factors. They select spatial climate change analogues and establish a baseline for monitoring at multiple scales that predicts climate influences.
Questions
To assess the relationship between modern pollen rain and Andean montane forest vegetation for diversity, and provide a basis for interpretations of palaeoecological data in the northern Andes, we asked: (1) can the reduction of plant and pollen data to family level preserve information about diversity in both data sets; (2) how precisely do tree pollen and spore types represent richness patterns along an altitudinal gradient on tropical mountains; and (3) how similar are tree pollen and spore family richness in relationship to tree family richness?
Location
Tropical montane rain forests, Podocarpus National Park in the Andes of South Ecuador (3°S, 79°W, 1000–3000 m a.s.l.).
Methods
We analysed tree diversity and species composition in three different rain forest types: Premontane (PMF), lower montane (LMF) and upper montane (UMF). We investigated modern pollen rain using pollen traps. After testing the reliability of a taxonomic surrogacy on the plant data, we compared abundance and representation, as well as diversity of the two data sets at family level. This was done using rarefaction and Sørensen indices.
Results
The correlation between tree species and families was high (r = 0.81, P < 0.001). Sample rarefaction on tree pollen and plant family data revealed highest pollen diversity on sites of the UMF, but highest tree diversity on LMF and PMF sites. The Sørensen indices indicate down-drift of pollen from higher altitudes in PMF and LMF and up-drift in UMF. Between 1% and 50% of pollen taxa of each sample originate from outside the plot.
Conclusions
Taxonomic surrogacy at family level is a good tool for comparing presence–absence patterns of plant and pollen data in tropical regions with high tree diversity. On a family basis, pollen presence–absence data represent the corresponding tree vegetation data, but uncertainties increase with decreasing altitude. The higher diversity in pollen data of the UMF, but slightly lower diversity in the LMF and PMF, can at least partly be explained by wind patterns, local abundance of shrubs and herbs and differences in evenness.
+We assessed the relationship between modern pollen rain and Andean forest vegetation regarding diversity. We compared presence-absence patterns of tree and pollen data using taxonomic surrogacy on a family level. Pollen presence-absence data represent tree vegetation data well, but uncertainties increase with decreasing altitude. Differences in diversity patterns can mainly be explained by wind-patterns and differences in evenness. +
Questions
How have the historical frequency and severity of natural disturbances in primary Picea abies forests varied at the forest stand and landscape level during recent centuries? Is there a relationship between physiographic attributes and historical patterns of disturbance severity in this system?
Location
Primary P. abies forests of the Eastern Carpathian Mountains, Romania; a region thought to hold the largest concentration of primary P. abies forests in Europe's temperate zone.
Methods
We used dendrochronological methods applied to many plots over a large area (132 plots representing six stands in two landscapes), thereby providing information at both stand and landscape levels. Evidence of past canopy disturbance was derived from two patterns of radial growth: (1) abrupt, sustained increases in growth (releases) and (2) rapid early growth rates (gap recruitment). These methods were augmented with non-metric multidimensional scaling to facilitate the interpretation of factors influencing past disturbance.
Results
Of the two growth pattern criteria used to assess past disturbance, gap recruitment was the most common, representing 80% of disturbance evidence overall. Disturbance severities varied over the landscape, including stand-replacing events, as well as low- and intermediate-severity disturbances. More than half of the study plots experienced extreme-severity disturbances at the plot level, although they were not always synchronized across stands and landscapes. Plots indicating high-severity disturbances were often spatially clustered (indicating disturbances up to 20 ha), while this tendency was less clear for low- and moderate-severity disturbances. Physiographic attributes such as altitude and land form were only weakly correlated with disturbance severity. Historical documents suggest windstorms as the primary disturbance agent, while the role of bark beetles (Ips typographus) remains unclear.
Conclusions
The historical disturbance regime revealed in this multi-scale study is characterized by considerable spatial and temporal heterogeneity, which could be seen among plots within stands, among stands within landscapes and between the two landscapes. When the disturbance regime was evaluated at these larger scales, the entire range of disturbance severity was revealed within this landscape.
+How have the frequency and severity of natural disturbances in primary Picea abies forests varied during recent centuries? The disturbance regime is characterized by considerable spatial and temporal heterogeneity, which could be seen among plots within stands, among stands within landscapes. When the disturbance regime was evaluated at larger scales, the entire range of disturbance severity was revealed. +
Questions
Recent research has revealed that genotypic diversity may have a similar role to species diversity in understanding competitive relations and ecological functioning of a community. Despite theoretical advancement in how competition and co-existence influence the pattern of genetic diversity between interacting species/genotypes, this topic has received little empirical attention. The key question is how the distribution of a given species/genotype is influenced by inter-specific interactions.
Location
A sclerophyll shrubland (kwongan) at Beekeepers Nature Reserve, 300 km north of Perth, Western Australia.
Methods
We examined whether there are any genetic or ecological consequences arising from the interspersion of three Banksia species. We investigated patterns of local abundance, mortality after environmental stress (viz. severe drought) and microsatellite allelic richness for populations of the three co-occurring banksias distributed on the crests and slopes of a mosaic of sand dunes.
Results
Local abundance of the fire-killed Banksia hookeriana was positively correlated with that of the lignotuberous resprouter B. attenuata, which has similar shrub morphology, but negatively with that of the less similar clonal resprouter, B. candolleana. In contrast, population allelic richness of B. attenuata and B. hookeriana were negatively correlated, while allelic richness of B. candolleana and the other two banksias were uncorrelated. Positive correlation between local abundance of B. attenuata and B. hookeriana was likely a consequence of a similar response to the same environmental resources, while B. candolleana has greater environmental tolerances than the other banksias but a slower dispersal rate as it rarely produces seedlings. Negative correlation in allelic richness between the ecologically similar B. attenuata and B. hookeriana was likely the result of competition for niche space at the genotype level. While local exclusion of inferior genotypes might occur by competition, exclusion of entire species is avoided and stable co-existence is achieved.
Conclusions
Our results indicate that mechanisms of co-existence and isolation among plant species may operate at several genetic scales, and that subtle genotypic variation is of potential importance in maintaining co-existence among ecologically matched species.
+Through investigating patterns of local abundance, mortality after environmental stress, and microsatellite allelic richness, we revealed genetic or ecological consequences arising from the interspersion of three co-occurring banksias species. Mechanisms of coexistence and competition among plant species may operate at several genetic scales, and that subtle genotypic variation is of potential importance in maintaining coexistence among ecologically matched species. +
Question
Both genetic diversity within a dominant species and species diversity have been shown to affect productivity. However, these two levels of diversity have the potential to affect productivity in similar or opposing ways. Our study assessed direct and indirect relationships between genetic diversity of a dominant grass species and plant community diversity on productivity within a mesic grassland with naturally co-occurring genotypes and species.
Location
Konza Prairie Biological Station, Kansas, USA (39°05′35″ N, 96°33′31″ W).
Methods
We conducted a study at two sites (~65 plots per site) in intact tallgrass prairie that have been subjected to frequent (burned 14 times) and infrequent (burned nine times) fire treatments for ~30 yrs. With frequent burning, the dominant C4 grass, Andropogon gerardii, typically has higher abundance; in contrast, infrequently burned sites typically have higher species diversity but lower abundance of A. gerardii. Therefore, we hypothesized that genetic diversity within A. gerardii would have a significant direct relationship with productivity for the frequently burned site due to higher population abundances of A. gerardii overall, whereas species diversity would have a significant direct relationship with productivity for the infrequently burned site.
Results
Contrary to our predictions, genetic diversity of A. gerardii was not related to above-ground productivity either directly or indirectly via traits, while diversity at the community level had a negative indirect relationship with productivity via a negative effect of A. gerardii abundance on community diversity for both frequently and infrequently burned sites.
Conclusions
While much of the focus of diversity–productivity research has been on direct relationships between diversity at the community or population level separately, future studies should examine the indirect and potentially interactive effects of both levels of diversity in natural communities.
+Genetic diversity of a dominant species and species diversity can separately or interactively effect productivity in similar or opposing ways. We examined the relative importance of these two levels of diversity on productivity in frequent and infrequently burned, intact tallgrass prairie, and found that genetic diversity of the dominant species did not have a direct or indirect effect on productivity. +
Questions
How do rates of endozoochory change across gradients of environmental variation, grazing intensity and herbivore body size? Is there evidence that plants experience a trade-off in successful germination in dung vs soil, and if so, are opposing strategies associated with traits indicative of evolutionary grazing history?
Location
Serengeti National Park, Tanzania and a greenhouse in Birmensdorf, Switzerland.
Methods
We compared rates of seedling emergence from the dung of six grazing herbivore species and soil collected across an environmental gradient in the Serengeti; we experimentally exposed both dung and soil to smoke to test for the effects of breaking seed dormancy. We analysed overlap in plant composition of seedlings germinated from dung and soil and asked whether species found in dung (but not soil) or soil (but not dung) differed in traits that are associated with evolutionary grazing history, such as leaf height and seed head height.
Results
Spatially, dung samples collected in the heavily grazed Serengeti plains contained more seeds than more northern sites. Median seedling emergence from dung varied among herbivores: hartebeest and wildebeest had the most, topi and zebra had intermediate numbers and Thomson's and Grant's gazelle the fewest, with Grant's gazelle dung being composed almost entirely of herbaceous dicots. Smoke failed to induce a germination response when applied to dung, but the composition of plant species germinating in ‘dung only’ or ‘soil only’ was surprisingly distinct. Moreover, seed head heights and leaf heights were shorter for plant species that germinated from dung compared to those that germinated from soil. Finally, the abundance of seeds germinating in dung and soil showed opposite patterns across the transition from short-grass, historically grazed sites in the south to tall-grass, fire-prone sites in to the north.
Conclusions
Together, our results suggest that some Serengeti plant species may be specialized to disperse via the dung of large herbivores. Consequently, endozoochory may be an important, and thus far overlooked, ecological process in Serengeti and in tropical savannas dominated by large herbivores more generally.
+In Serengeti National Park rates of seed germination from dung (endozoochory) and seed germination from soil showed opposite patterns across a gradient from short-grass, grazed to tall-grass, fire-prone sites. The height and composition of plants germinating from dung and soils were also surprisingly distinct, suggesting endozoochory is an important ecological and evolutionary mechanism in savannas dominated by large mammalian herbivores. +
Questions
What factors explain the variation in plant survival parameters across species and ecosystems?
Location
Western North America.
Methods
We compiled six long-term data sets from western North America to test for ecosystem-dependent demographic responses for forbs and grasses. Based on these data, we characterized 123 survivorship curves for 109 species. Three demographic parameters were extracted from these survivorship curves: survival rate at age 1, life expectancy at age 1, and a parameter describing the shape of the survivorship curve. We used a mixed effects model to compare the differences in demographic parameters between life forms (forbs or grasses) and among ecosystems, incorporating ‘ecosystem’ as a random factor, with life form treated as a categorical factor, and mean annual precipitation and mean annual temperature treated as continuous variables.
Results
Grasses had higher survival and longer life expectancy than forbs at 1 yr of age. Both forbs and grasses followed Type III survivorship curves, although forbs were closer to Type II compared to the grasses. Averaging across species, hazard ratios for whole survivorship curves differed among most ecosystems. While mean annual precipitation had no effect on any demographic parameter, mean annual temperature had a significantly negative effect on both first year survival rates and life expectancy for forbs.
Conclusions
Our results demonstrate that life form exerts a strong influence on demographic parameters and their response to temperature variation among ecosystems. This unprecedented information on the age-specific demography of herbaceous plants has implications for population modelling and research on life-history evolution and senescence.
+We compiled six long-term datasets from western North America to test for ecosystem-dependent survival patterns of forbs and grasses. We found that life form exerts a strong influence on survival parameters and their response to temperature variation among ecosystems. The age-specific demography of herbaceous plants has implications for population modeling and research on life history evolution and senescence. +
Questions
What is the relative importance of topographic (elevation), edaphic (soil salinity, nitrogen and particle size) and hydrologic (estuarine river flow) gradients for variation in tidal wetland plant composition and diversity?
Location
Four Oregon estuaries: a marine-dominated lagoon, two tidal-driven bays, and a river-dominated site.
Methods
We surveyed species presence, cover and richness; and environmental factors (soil salinity, grain size, soil nitrogen and elevation) in plots in marsh and swamp. We assessed patterns of community structure and the relative importance of environmental gradients with hierarchical partitioning, ordination, species accumulation curves and path analysis.
Results
The relative importance of measured environmental gradients on plant occurrence differed by species. Soil salinity or elevation explained the most variation in the majority of common species. Estuarine hydrology, soil nitrogen and soil clay content were usually of secondary or minor importance. Assemblage composition and species richness varied most strongly with tidal elevation. Local soil salinity also affected composition, but differences in estuarine hydrology had comparatively less effect on composition and richness. Higher-elevation wetlands supported larger species pools and higher plot-level richness; fresher wetlands had larger species pools than salt marsh but plot-level richness was relatively invariant to differences in soil salinity.
Conclusions
Elevation and salinity tended to exert more influence on the vegetation structure of tidal wetlands than estuarine hydrology or other edaphic variables. With relative sea-level rise expected to increase both flooding intensity and salinity exposure in future wetlands, global climate change may lead to changes in species distributions, altered floristic composition and reduced plant species richness.
+Environmental gradients in tidal wetlands can overlap in complex ways. Using ordination and path analysis we found that tidal elevation was the principle correlate of plant composition and diversity. Soil salinity also affected composition, but major hydrologic differences among estuaries played a more minor role. Future climate change may alter the spatial distribution of gradients, thereby affecting vegetation structure. +
Question
Is the spatial distribution and density of the maple spindle gall mite Vasates aceriscrumena (MSGM) positively correlated with the distribution and density of its host? Is the distribution of MSGM influenced by non-host species and abiotic factors?
Location
Temperate mixed hardwood forest stand, Haliburton Forest and Wildlife Reserve, Ontario, Canada.
Methods
We used the mapped locations of host and non-host trees to investigate the cause of spatial variation in the density of MSGM within an 8.8-ha forest plot in central Ontario, Canada. Gall densities were determined from fallen leaves, collected at 20-m intervals. We used Mantel and partial Mantel tests to compute the correlation between gall density and several spatially variable biotic and abiotic factors: (1) host density and basal area, (2) density of non-host stems, (3) overall stem density, (4) stem species diversity and (5) topography.
Results
The density of leaf galls was weakly correlated with host density and basal area. Although the correlation with host density and basal area was statistically significant, leaf gall density was more strongly correlated with overall tree species richness and overall stem density. Gall densities were highest at the boundaries of neighbourhoods containing high and moderate sugar maple (Acer saccharum) densities. Partial Mantel tests indicated that the observed spatial correlations held when controlling for the potential influence of topography.
Conclusions
Based on the spatial relationships documented here, we speculate that the mechanism responsible for the correlation between the MSGM and non-host stems is parasite-induced host stress. Separate studies have established a strong negative impact of the MSGM parasite on sugar maple stem growth. We suggest that by weakening the competitive ability of its host, the parasite indirectly promotes local species diversity through competitive release. Given the high diversity and prevalence of leaf gall parasites in mixed hardwood stands, depression of host dominance by leaf parasites may represent an unexplored mechanism for the maintenance of species diversity in northern temperate forests.
+Here we find that the distribution and density of a host-specific canopy galling arthropod in a temperate forest is linked to local tree species diversity. Based on this and prior work examining galling impacts we suggest that parasite-induced host stress here might promote local species diversity through competitive release by weakening the competitive ability of the tree host species. +
Questions
How do semi-deciduous and evergreen shrubs exploit environmental resources during summer drought? What is the contribution of the understorey shrubby layer to ecosystem carbon assimilation? To what extent are carbon balance and transpiration impacted by a rain pulse?
Location
Cork oak open woodland in the Mediterranean region.
Methods
We used closed dynamic light and dark chambers to measure gas exchange (CO2 and H2O) in the dominant shrub understorey species Cistus salviifolius, Cistus crispus (semi-deciduous) and Ulex airensis (evergreen), together with plant physiological and morphological measurements during summer drought and autumn recovery. A hyperbolic light response model constrained by vapour pressure deficits was fitted for up-scaling shrub photosynthesis to the ecosystem level. The data were compared, on a daily and daytime basis, with gross primary productivity estimates from ecosystem eddy-covariance flux measurements.
Results
The onset of summer drought led to a significant leaf area reduction in semi-deciduous species. A general decrease in photosynthesis in all species was observed, while evapotranspiration and above-ground respiration fluxes contrasted among species during summer progression and autumn recovery. The shallow-rooted C. salviifolius was able to use light more efficiently than the other two species, although with poor stomatal control over water loss and consistently higher above-ground respiration rates, leading to lower water and carbon use efficiencies when compared with C. crispus. The deep-rooted shrub U. airensis maintained higher leaf water potentials and very low photosynthetic rates while decreasing transpiration rates throughout the summer drought. A summer rain pulse showed that shallow-rooted shrubs use water in an opportunistic way, with immediate leaf rehydration and concomitant photosynthesis increments. Conversely, deep-rooted shrubs (U. airensis) were unresponsive, only recovering photosynthesis with high soil water content. An opportunistic growth response may be disadvantageous to shallow-rooted shrubs in a future climate with extended dry summers and higher probability of rain pulse events. The prominent increase in transpiration rates and plant respiration costs observed during the dry conditions that followed the rain pulse, led to a reduced plant ability to recover after autumn rains.
Conclusions
The shrubs that naturaly colonized this montado understorey showed contrasting strategies to overcome summer drought, suggesting an efficient mosaic exploitation of seasonal environmental resources. The contribution of these shrubs to total ecosystem CO2 uptake during summer and autumn recovery was 17%. This high contribution implies that shrub density management decisions should consider a carbon balance perspective.
+Carbon and water fluxes in evergreen and semi-deciduous shrubs were studied in a Mediterranean cork oak woodland from early summer to autumn recovery. Rooting depth and leaf habits explained the contrasting strategies to exploiting light, water and carbon among species. Summer rain pulses mislead water use opportunistic shrubs. Average contribution of this shrub understory to ecosystem carbon assimilation was 17%.
Questions
Could mountain pine (Pinus mugo Turra subsp. mugo) located above the timberline and at the southern limit of its potential geographic distribution be a promising species to monitor climatic change in specific environments such as the Mediterranean mountains? Does mountain pine krummholz upward expansion result from current climate change? Has the relationship between climate and tree ring growth of mountain pine changed? To what extent does climatic variability affect tree ring growth? Could mountain pine upward expansion contribute to loss of biodiversity in the undisturbed tree line ecotone on the Majella massif?
Location
Majella National Park, Apennines, Italy.
Methods
Dendroclimatological analysis was performed to examine the relationship between tree rings of mountain pine and climatic parameters at the tree line. Climate–growth relationships were analysed using correlation function (CF) analysis and moving CF (MCF).
Results
In the Majella massif, climate variables affected mountain pine tree growth at the tree line ecotone. Significant correlations were found with spring temperatures and summer precipitation. Essentially, three months played a key role in mountain pine growth: April and May of the current year and October of the previous year. MCF showed that mountain pine is a sensitive species, in terms of tree ring growth.
Conclusions
Growing above the limit of tree vegetation, mountain pine proved to be a promising species for monitoring climate- and land use-driven influences over tree ring growth at high elevations. Our results show that mountain pine is particularly sensitive to climate at the beginning and at the end of the growing season.
+This study contributed to better understand the vegetation dynamics at the treeline on Mediterranean mountains. The response of the krummholz species Pinus mugo Turra subsp. mugo to climate in Apennines (Central Italy) and the temporal variation of tree ring sensitivity to climate were used for monitoring climate-driven influences over ring width growth at high elevation. +
Question
What factors best characterize tree competitive environments in this structurally diverse old-growth forest, and do these factors vary spatially within and among stands?
Location
Old-growth Picea abies forest of boreal Sweden.
Methods
Using long-term, mapped permanent plot data augmented with dendrochronological analyses, we evaluated the effect of neighbourhood competition on focal tree growth by means of standard competition indices, each modified to include various metrics of trees size, neighbour mortality weighting (for neighbours that died during the inventory period), and within-neighbourhood tree clustering. Candidate models were evaluated using mixed-model linear regression analyses, with mean basal area increment as the response variable. We then analysed stand-level spatial patterns of competition indices and growth rates (via kriging) to determine if the relationship between these patterns could further elucidate factors influencing tree growth.
Results
Inter-tree competition clearly affected growth rates, with crown volume being the size metric most strongly influencing the neighbourhood competitive environment. Including neighbour tree mortality weightings in models only slightly improved descriptions of competitive interactions. Although the within-neighbourhood clustering index did not improve model predictions, competition intensity was influenced by the underlying stand-level tree spatial arrangement: stand-level clustering locally intensified competition and reduced tree growth, whereas in the absence of such clustering, inter-tree competition played a lesser role in constraining tree growth.
Conclusions
Our findings demonstrate that competition continues to influence forest processes and structures in an old-growth system that has not experienced major disturbances for at least two centuries. The finding that the underlying tree spatial pattern influenced the competitive environment suggests caution in interpreting traditional tree competition studies, in which tree spatial patterning is typically not taken into account. Our findings highlight the importance of forest structure – particularly the spatial arrangement of trees – in regulating inter-tree competition and growth in structurally diverse forests, and they provide insight into the causes and consequences of heterogeneity in this old-growth system.
+Our findings demonstrate that inter-tree competition continues to influence tree growth in this old-growth Picea abies system, with crown volume being the size metric best explaining neighborhood competition. Our findings also demonstrate that stand-level tree spatial patterning can influence competitive environments, which suggests caution in interpreting traditional competition studies where spatial patterning is not taken into account.
Questions
The coastal heathlands of northwest Europe are classified as highly endangered and a habitat of high conservation importance throughout their geographic range. Previous research into heathland vegetation dynamics has typically been carried out within single sites or regions, and hence little is known about the variability of land-use effects and successional dynamics along biogeographic gradients. We test the hypothesis that the bioclimatic gradient is a key factor shaping post-fire regeneration dynamics in Calluna heathlands, with progressively slower regrowth and lower post-fire pioneer species richness towards colder climates.
Location
Wet and dry Calluna heath vegetation in five sites spanning a 340-km latitudinal gradient along the west coast of Norway (60.70°–63.79° N).
Methods
A repeated measures design was used, with floristic data recorded from permanent plots in a number of experimental post-fire successions over a 3-yr period (n = 344). The data were analysed using multivariate ordination techniques: detrended correspondence analysis, partial redundancy analysis and principal response curves, and mixed effects models.
Results
Regeneration rates decrease from south to north and the wet heath stands regenerate faster towards the pre-fire community composition than the dry stands. Calluna decreases immediately after fire in all sites, but increases from the second year onwards, with the southernmost site having the fastest return. Regeneration of grasses, herbs, mosses and lichens is also faster in the south. Across all sites, species richness decreased the first year after fire, followed by an increase that exceeds pre-fire levels by the second year. The number of pioneer species entering the heathlands after fire decreases northwards.
Conclusions
We found considerable geographic variation, not only in species composition, but also in post-fire successional trends and dynamics. This is probably linked to higher productivity and larger pools of pioneer species in southern sites, and higher growth rates of shrubs and mosses in the wet heath habitat. This calls for conservation of a geographically diverse set of heathland sites, as well as development of regional- and site-specific management plans.
+The coastal heathlands of north-west Europe have a wide distribution and a high conservation importance throughout their geographic range. We test the hypothesis that the bioclimatic gradient is a key factor shaping the post-fire regeneration dynamics in Calluna heathlands. Our results show considerable geographic variation, not only in species composition, but also in post-fire successional trends and dynamics.
Smith (2013), in this issue, reviews the consequences of extended leaf phenology of invasive plant species in native deciduous forests. How important is early leaf emergence and/or late leaf senescence for the success of non-native species? What are the direct and indirect impacts on invaded communities and ecosystems? We are just at the very early stage in answering such questions.
+Smith (2013), in this issue, reviews the consequences of extended leaf phenology of invasive plant species in native deciduous forests. How important is early leaf emergence and/or late leaf senescence for the success of non-native species? What are the direct and indirect impacts on invaded communities and ecosystems? We are just at the very early stage in answering such questions. +
The role traits play in species distributions is central to developing a mechanistic and predictive plant ecology. Developments in generalized linear mixed models described by Jamil et al. in this issue provide a significant step in this endeavour. Trait-based GLMMs are intuitive parametric models for exploring and predicting the distribution of species along environmental gradients, as affected by species traits.
+The role traits play in species distributions is central to developing a mechanistic and predictive plant ecology. Developments in Generalized Linear Mixed Models described by Jamil et al. in this issue provide a significant step in this endeavour. Trait-based GLMMs are intuitive parametric models for exploring and predicting the distribution of species along environmental gradients as affected by species traits. +
Many high-impact invaders of temperate deciduous forests of Eastern North America exhibit extended leaf phenology compared to native species, with leaf emergence occurring earlier in the spring and abscission occurring later in the autumn. This observation indicates that extended phenology may be an important invasion mechanism for this system. However, most evidence that extended leaf phenology drives species invasion is anecdotal, and most studies that directly address the role of leaf phenology in invasion focus solely on increased invader growth. Beyond increasing invader growth, extended leaf phenology may drive invader impact on natives by (1) facilitating resource competition through nutrient pre-emption and shading, (2) altering production of secondary chemicals by invasive plants, (3) altering apparent competition dynamics mediated by native herbivores, (4) allowing temporal enemy escape, and (5) altering behaviour of native pollinators. I review current evidence from the literature regarding each of these possible consequences of extended invader phenology and emphasize the need for experimental manipulations to measure the extent to which extended leaf phenology mediates impact on the native community. Understanding the importance of leaf phenology in species invasion will facilitate prevention of future invasions while elucidating the role of seasonality in shaping species interactions.
+Many plant invaders of Eastern deciduous forests exhibit extended leaf phenology compared to natives. Beyond providing invaders an advantage in resource competition, extended leaf phenology may have diverse community-level effects by changing interactions between plants and their enemies, mutualists, and competitors. Investigating the role of leaf phenology in invasion will inform our understanding of seasonality in shaping species interactions. +
Question
Quantification of the effect of species traits on the assembly of communities is challenging from a statistical point of view. A key question is how species occurrence and abundance can be explained by the trait values of the species and the environmental values at the sites.
Methods
Using a sites × species abundance table, a site × environment data table and a species × trait data table, we address the above question using a novel generalized linear mixed model (GLMM) approach. The GLMM overcomes problems of pseudo-replication and heteroscedastic variance by including sites and species as random factors. The method is equally applicable to presence–absence data as to count and multinomial data. We present a tiered forward selection approach for obtaining a parsimonious model and compare the results with alternative methods (the fourth corner method and RLQ ordination).
Results
We illustrate the approach on a presence–absence version on two data sets. In the Dune Meadow data, species presence is parsimoniously explained by moisture and manure on the meadows in combination with seed mass and specific leaf area (SLA). In the Grazed Grassland data, species presence is parsimoniously explained by the grazing intensity and soil phosphorus in combination with the C:N ratio and flowering mode.
Conclusions
Our GLMM approach can be used to identify which species traits and environmental variables best explain the species distribution, and which traits are significantly correlated with environmental variables. We argue that the method is better suited for providing an interpretable and predictive model than the fourth corner method and RLQ.
+We propose a Generalized Linear Mixed Model (GLMM) for the functional response of species to environmental change. The model can be used to identify which functional traits and environmental variables are significantly related and which best explain the species distribution. We argue that the method is better suited for providing an interpretable model than the fourth corner method and RLQ.
Questions
Is productivity linked with clonal traits through their indirect effect on competitive interactions? Are clonal traits better predictors of productivity than foliar traits?
Location
Rennes, France.
Methods
We used a wide-scale mesocosm experiment based on several assemblages of species differing in clonal traits, and evaluated if the relationship between biomass production and clonal traits is consistent at different ecological scales.
Results
Results showed that at the individual level, foliar traits were independent from clonal traits in most studied species. Community specific above-ground net primary productivity was significantly correlated to community-aggregated values of clonal and foliar traits. Nevertheless, a stronger relationship with clonal traits was indicated, emphasizing a plant foraging strategy along the horizontal plant plane, which was a determinant of community productivity. An inverse relationship between clonal traits and biomass production was observed at the individual and community levels, which was attributed to modifications in resource acquisition processes resulting from competitive interactions.
Conclusions
We demonstrated that clonal traits are correlated with productivity at the individual and community scales. These traits were indicators of resource acquisition processes mediated through competitive interactions.
+Based on a wide-scale mesocosm experiment comprising several assemblages of species differing in clonal traits, we demonstrated that these traits can be better predictors of the production function than foliar traits at the individual and the community scales. These traits were indicators of resource acquisition processes mediated by competitive interactions.
Question
Many calcareous grasslands in Europe have declined in species richness in recent decades. This loss of species could be partly due to habitat loss and fragmentation, leading to increased distances and reduced connectivity through seed flow between calcareous grassland patches resulting in increased local extinction risk related to small population size, and partly due to abandonment of traditional management practices that fostered dispersal within and between patches. Here, we quantify short- and intermediate-distance dispersal ability of dry calcareous grassland species and relate these to dispersal traits.
Location
Schaffhauser Randen, Switzerland.
Methods
We studied wind dispersal of diaspores under natural conditions within and beyond two replicate calcareous grassland patches. Funnel traps (n = 230) were set up at heights of 0.2 m and 0.7 m along ten transects traversing the calcareous grassland and extending 40 m into the surrounding landscape. We developed a new method for quantifying short - (0–1 m) and intermediate-distance (1–40 m) dispersal ability, related these to species traits, and tested whether they were able to explain dispersal rates into the adjacent landscape.
Results
While grasses could be categorized as good dispersers over short or intermediate distances, or both, forbs were generally poor dispersers over both distance classes. Only small numbers of diaspores were found in the adjacent landscape, and these were predominantly grasses. Diaspore traits, such as terminal velocity or diaspore mass, contributed little to explaining dispersal ability, whereas release height was an important predictor, especially for intermediate-distance dispersal.
Conclusions
Under natural field conditions, dispersal into the adjacent landscape depends on release height rather than terminal velocity, and is heavily biased towards grasses, so that seed rain does not reflect the species composition of the calcareous grassland community. Thus natural regeneration of species richness of degenerated calcareous grassland communities even over short distances should not rely on wind dispersal alone.
+Habitat loss, fragmentation and abandonment of traditional management practices have reduced connectivity by seed flow between Europe‘s calcareous grassland patches. Here we quantify the wind dispersal ability of calcareous grassland species using funnel traps. We show that dispersal into the adjacent landscape is species-specific and conclude that natural regeneration even over short distances should not rely on wind dispersal alone.
Question
We analyse the contribution of plant–plant interactions, particularly the outcome of plant competition and plant facilitation, on vegetation dynamics as a result of extreme drought episodes. These events will likely become more frequent under climate change, can induce vegetation die-off and alter community dynamics. We study succession in a shrubland that tends to be replaced by juniper (Juniperus phoenicea) woodland. Due to drought, shrubland die-off may result in competition release favouring J. phoenicea juveniles, and accelerating shrubland replacement. Alternatively, deleterious abiotic stress may increase after loss of vegetation cover protection.
Location
Mediterranean coastal shrublands, South Spain (Doñana National Park).
Methods
Field estimates of plant growth, production of needle-like leaves, water-use efficiency (WUE; leaf δ13C) and N leaf content of J. phoenicea juveniles in relation to plant size, drought-induced damage, cover and habit characteristics of surrounding vegetation, and drought-induced defoliation of the surrounding vegetation.
Results
Juniperus phoenicea juveniles growing beneath a dense vegetation canopy, particularly trees and large shrubs, were less damaged during the extreme drought episode. Plant size correlated negatively with damage. Post-drought growth was higher in juveniles partially released from the vegetation canopy, supporting the existence of a balance between competition and facilitation. Cover of pines, large shrubs and spiny shrubs favoured growth of juveniles. Needle-like juvenile leaves were more abundant in plants covered by the surrounding vegetation or in moderately damaged plants, but less abundant in plants without damage. Higher leaf δ13C values – indicating water stress – were measured in plants more damaged by drought and in those without canopy protection, or under vegetation strongly affected by drought. Leaf N content was lower in undamaged plants and individuals covered by surrounding vegetation.
Conclusion
We did not find evidence that gaps opened by drought promoted growth of the potential replacing J. phoenicea. Thus, drought-induced enhancement of successional replacement of shrublands with woodlands was not supported; instead, our findings foresee shrubland prevalence under future climate change conditions. Plant facilitation will play a relevant role in this process. Thus, we herein extend the relevance of plant–plant interactions to extreme drought episodes related to climate change, highlighting their role as drivers of community dynamics.
+The study illustrates the outcome of competence and facilitation to community dynamics under a climate change scenario where extreme climatic episodes are becoming more frequent. Empirical data from a Mediterranean shrubland subjected to successional replacement, shows that drought events disrupt this dynamics diminishing the facilitative effect of shrubland cover on the establishment of juveniles of the short tree Juniperus phoenicea. +
Questions
The high competitiveness of exotic invasive species has often been demonstrated, but usually with respect to native species known to have low competitive ability. Considering five exotic and five native riparian species with close characteristics regarding competitive ability, habitat and growth form, we addressed the following questions: (i) do the selected invasive plants produce more biomass than the selected native dominants under competitive pressure; and (ii) are the selected invasive species better competitors than the selected native dominants?
Location
Common garden experiment at the Henri Gaussen Botanical Garden, Toulouse, France.
Methods
We selected five native dominant species and five exotic invasive species co-occurring along a riparian successional gradient of the middle Garonne River (SW France). Young plants of each species were planted in pots in ten intra- and 17 inter-specific combinations in conditions of high water and nutrient availability. To simulate the effects of hydrological disturbance during earlier growth stages, a partial cutting of plants was applied 6 weeks after planting. We measured above-ground and below-ground biomass of individuals of each species after 6 mo of growth.
Results
There were large disparities among species performances, regardless of whether the species were exotic or native. The exotic species produced more above-ground and below-ground biomass than the natives species for 73% of the selected species pairs. The exotic species had higher competitive ability than the native species, mainly related to the high competitive effect of I. glandulifera. The two species with the highest biomass production and competitive ability were invasive exotics, whereas the two species with the lowest were dominant natives.
Conclusions
Our results predict that competition among young individuals could play a major role for the invasion success of the studied exotic species in European riparian areas.
+Mean competition effect (± 95 %-bootstrap confidence interval) on the individual biomass according the origin of the species. Considering five exotic invasive and five native dominant plant species, we found that the exotics produced more biomass than the natives in 73 % of the selected species pairs. The exotic species had higher competitiveness than the natives, mainly related to the high competitiveness of I. glandulifera. The two species which displayed the lowest production and competitive abilities were natives.
Questions
How is natural regeneration of a patchy landscape affected by within-patch species interactions and among-patch dispersal after an extreme disturbance? Do landscape dispersal processes facilitate the invasion of native-dominated patches by exotic species in adjacent patches?
Location
Irvine Ranch Natural Landmark, Irvine, California, USA.
Methods
We monitored plant community composition in paired grassland patches that were initially dominated by native or exotic grasses at eight sites. We followed recovery of native and exotic grassland species over time, starting in a record drought year prior to an intense fire, and then for 3 yr with more typical rainfall patterns after the fire. Additionally, we compared seed rain of native and exotic species across native and exotic patches, quantifying how seed rain influenced species abundance in the following year. Multivariate and regression analyses were used to assess the potential homogenization of the landscape.
Results
Following the extreme drought/fire disturbance, the exotic annual grasses quickly recovered in abundance in patches that they dominated prior to the disturbance. However, the native grass, Stipa pulchra, was not able to recover in the patches it once dominated. As the exotic grasses gradually increased in the native patches, the paired patch types became more similar in composition over time. Exotic grasses produced up to 28 times more seed than the native dominant grass, Stipa; even in the patches initially dominated by Stipa, exotic seed rain was equivalent or greater than the native. Seed rain was positively correlated with the following year's abundance for both exotic and native species.
Conclusions
After an extreme disturbance, recovery of native patches can be stalled by an influx of propagules from neighbouring exotic patches. This exotic seed rain can allow the invasion of areas once dominated by natives, thus inhibiting regeneration. The matrix surrounding remnant native stands can be a critical factor in determining whether an extreme disturbance enhances native diversity vs. increasing its susceptibility to invasion.
+A severe drought paired with an extensive wildlife created an extreme disturbance in Southern California grasslands. Our study shows that the recovery after such an extreme disturbance is driven by landscape dispersal processes, where the propagule pressure from neighboring exotic annual grasses can hinder the recovery of slower growing native perennial dominated grasslands. +
Aims
Plant facilitation is recognized as an important process, contributing not only to the ecology of individual species, but the diversity, organization and stability of communities as a whole. While decades of studies have enhanced our knowledge of the mechanisms contributing to plant facilitation, greater attention is now being given to the conditionality of the mechanisms, whereby multiple biotic and abiotic factors may act independently and jointly with one another in shaping plant–plant facilitation. Our objective was to evaluate the conditionality and hierarchy of mechanisms contributing to cactus germination and early seedling recruitment arising from plant–plant facilitation, including the effects of seed banks, rainfall, granivory and nurse plants.
Location
Bahia de Kina, Sonora, Mexico; Sonoran Desert of North America.
Methods
We employed descriptive analyses and a factorial experiment to examine the consequences of rainfall, nurse plants, granivory and seed banks for senita cactus recruitment (Pachycereus schottii) in two plant communities of the Sonoran Desert: one degraded of its key nurse plants and the other with its nurse plants intact.
Results
Seed banks did not occur in either of the plant communities, indicating that seed banks and seed dormancy do not affect senita recruitment and thereby do not modulate other mechanisms that contribute to nurse plant facilitation and cactus recruitment. Juvenile senita cacti were consistently associated with nurse plants in both plant communities. Yet, the density and species composition of nurse plants differed between the communities, resulting in reduced senita recruitment and shifts to alternative nurse plants in the community degraded of its key nurse plants compared to the intact community. In a factorial experiment manipulating rainfall, granivory and nurse plants, we found that rainfall and granivory had significant main factor effects, leading to increases and decreases in seedling recruitment, respectively. There was no main factor effect of nurse plants, but there was a significant nurse plant by granivory interaction, indicating that nurse plants increase seedling recruitment through escape from granivory conditional upon rainfall.
Conclusions
Taken together, these results reveal a conditional hierarchy of mechanisms of plant–plant facilitation shaping senita cactus recruitment, namely the predominant effects of rainfall increasing recruitment and of granivory decreasing recruitment, for which nurse plants played a key role in increasing seedling recruitment with rainfall and by reducing granivory.
+Our results show the hierarchical effects that shape seedling recruitment of a desert cactus, namely the predominant effects of rainfall in increasing recruitment and of granivory in decreasing recruitment. It is primarily under adequate rainfall conditions that nurse plants play a key role in increasing seedling recruitment and survival.
Questions
How does litter accumulation through succession affect secondary seed dispersal and buried seed viability and consequently control seed bank formation for small- and large-seeded species?
Location
A post-mined peatland in northern Japan (45°06′ N, 141°42′ E) where the chronological sequence of plant community succession is known.
Methods
The movements of seeds after reaching the ground surface and the availability of viable seeds potentially contributing to seed bank formation at various depths were experimentally investigated for 1 yr in four species that produce different-sized seeds: Drosera rotundifolia (seed mass 0.01 mg), Lobelia sessilifolia (0.25 mg), Rhynchospora alba (0.87 mg) and Moliniopsis japonica (1.82 mg). The experiments were conducted in three successional stages with 0-, 4- and 9-cm thick litter layers.
Results
Seed emigration decreased and seed retention increased with an increase in litter thickness. Large seeds were retained within the litter throughout the experimental period, and fewer seeds were buried in peat compared to small seeds trapped by thick litter, which had shifted downward by the following early spring. Litter contributed to increasing the number of viable and ungerminated seeds. The number of viable seeds for all species was nearly zero on the bare peat surface. The numbers of viable seeds on and beneath the peat surface increased with increases in litter cover thickness.
Conclusions
The patterns of secondary seed dispersal and the availability of viable seeds were altered by litter accumulation through the progress of succession. Moreover, the effects of litter on seeds varied among species for which seed size differed. Overall, our results suggest that temporal changes in litter thickness through the progress of succession can play an important role in seed bank formation, which has potential impacts on the long-term dynamics of plant populations and the whole community.
+We found that the patterns of secondary seed dispersal and buried seed viability varied with litter accumulation through succession in a peatland. Small seeds passed through the litter and formed a persistent seedbank at the peat surface, while large seeds developed a transient seedbank within the litter. The temporal changes in litter thickness can control the spatial distribution of seedbank.
Question
Aluminium (Al) toxicity is one of the most important factors restricting plant establishment on acidic soils, but its ecological significance for the occurrence of species along natural pH gradients is still under investigation. Are species occurring on acidic sandy soils less susceptible to Al toxic effects on germination and seedling root growth rate than species from calcareous sandy soils? How strong is the explanatory power of species' Al susceptibility for their occurrence along a pH gradient, as represented by their Ellenberg indicator value (EIV) for soil reaction (R)? Can Al tolerance of species be used as an independent trait to support Ellenberg's empirically-derived reaction indicator values?
Location
Dry sandy grasslands in Southern Germany, with soil reactions ranging from acidic to calcareous.
Methods
We tested early seedling responses to different Al concentrations in 15 species from dry sandy grasslands. A filter paper-based system was used to germinate seeds under Al concentrations of up to 10 mM. Germination, absolute root growth and the length of the root hair zone were recorded 7 and 14 d after first germination. Al concentrations that reduced root growth by 50 or 95% (ED50 and ED95, respectively) were correlated to Ellenberg indicator values (EIV) for soil reaction.
Results
EIV explained 66% of the variance in species' Al sensitivity. Tolerated Al concentrations resemble those concentrations the individual species are exposed to in their natural habitats.
Conclusions
Among all soil factors varying with soil pH, Al is one of the strongest restrictions to species' occurrence in acidic soils. Al acts as an environmental filter by allowing only Al-tolerant seedlings to grow roots and establish. Al sensitivity is a measurable objective trait that could form a crucial physiological component in defining R indicator values.
+Seedling roots of species from Central European sandy grasslands were exposed to different concentrations of aluminium (Al). Al-tolerance was found to strongly correlate with Ellenberg-indicator-values for soil reaction. Hence, Al acts as an environmental filter in acidic soils by allowing only Al-tolerant seedlings to grow roots. Al-tolerance is a measurable objective trait with the potential to underpin subjectively derived Ellenberg-indicator-values. +
Questions
Species–habitat associations may contribute to the maintenance of species richness in tropical forests, but previous research has been conducted almost exclusively in lowland forests and has emphasized the importance of topography and edaphic conditions. Is the distribution of woody plant species in a Peruvian cloud forest determined by microhabitat conditions? What is the role of environmental characteristics and forest structure in habitat partitioning in a tropical cloud forest?
Location
Cloud Forest, north Peruvian Andes.
Methods
We examined species–habitat associations in three 1-ha plots using the torus-translation method. We used three different criteria to define habitats for habitat partitioning analyses, based on microtopography, forest structure and both sets of factors. The number of species associated either positively or negatively with each habitat was assessed.
Results
Habitats defined on the basis of environmental conditions and forest structure discriminated a greater number of positive and negative associations at the scale of our analyses in a tropical cloud forest.
Conclusions
Both topographic conditions and forest structure contribute to small-scale microhabitat partitioning of woody plant species in a Peruvian tropical cloud forest. Nevertheless, canopy species were most correlated with the distribution of environmental variables, while understorey species displayed associations with forest structure.
+We studied tree species-habitat associations within three 1 ha plots in a Peruvian cloud forest. The micro-scale distributions of canopy species were sensitive to topographic conditions, while low stature species were more sensitive to forest structure. We demonstrate that the inclusion of both topographic and forest structure variables in habitat association analyses provides a greater evidence of micro-habitat partitioning than analyses incorporating either set of variables in isolation. We conclude that micro-scale habitat associations contribute to the maintenance of species richness in this neotropical montane forest. +
Questions
(1) Does climate amelioration in semi-arid ecosystems increase seed bank richness and seed density in both total and persistent seed banks? (2) Does herbivory modulate climate effects on soil seed banks? (3) Is this effect mediated by changes in the above-ground vegetation? (4) To what extent do environmental conditions affect similarity between the total and persistent seed banks, and between above-ground (vegetation) vs below-ground (seed bank) community compartments?
Location
Ecuadorian Dry Mountain Scrub Ecosystem.
Methods
Our study was conducted along a climatic gradient (altitude) under two different management conditions: undisturbed and grazed. The effect of grazing, altitude and their interaction on seed abundance and richness of the total and persistent soil seed banks were evaluated with non-linear mixed models.
Results
Altitude, grazing and their interaction exerted a significant effect on richness and abundance of the total seed bank, but not on the persistent seed bank. At highly stressful climate conditions, differences in species richness between the total and persistent seed banks were small or absent, while at milder environmental conditions the total seed bank showed greater seed species richness and abundance than the persistent one. However, under grazing conditions, species richness in both seed banks was rather similar along the climatic gradient. Relationships in terms of species richness between above-ground vs below-ground compartments, shifted from being dominated by the above-ground component at low altitudes (more stress) to being dominated by the seed bank component at higher elevations (milder conditions). In grazed locations, vegetation and seed bank showed similar species richness throughout the altitudinal gradient.
Conclusions
This study demonstrates that not only climate acted as a key environmental filter on soil seed bank properties, but also that grazing greatly modulated the primary effect of climate. Nevertheless, the effect of climate and grazing was restricted to the total seed bank (the fertility compartment) and no effect was observed in the persistent seed bank (longevity compartment). In this tropical dry scrub, richness and abundance of seeds increased upwards along the altitudinal gradient, most likely because at higher altitudes environmental conditions improve and become more benign for plant development.
+We wish to improve our understanding on the effect of plant patch on soil seed bank properties and, more specifically, how these seed reservoirs (seed bank patches) are affected by biotic (i.e. herbivory) and abiotic (i.e. climate) stressors and how they interact with the standing vegetation. Our results showed that although climate is the primary factor determining soil seed bank properties, its effect was profoundly modulated by grazing, as suggested by the existence of an important interaction between both stressors. Not only seed abundance and richness but also the relative dominance of total and persistent seed banks was affected by both environmental stressors.
Photo Credit: M. Smith
Questions
In species response modelling, can a hierarchical logistic regression framework compete against GAM in terms of statistical inference? Are bimodal shapes useful to model species responses along ecological gradients?
Location
Germany.
Methods
In hierarchical logistic regression modelling [also known as Huisman, Olff, Fresco (HOF) models], the best model is chosen from a set of predetermined models using statistical information criteria, i.e. a balance between model fit to the data and simplicity of the model. We extended the classical five model types with two bimodal shapes. We improved the model optimization process to inhibit unrealistically steep slopes and abrupt changes. The stability of model choices is safeguarded through bootstrapping. The framework was tested on a data set of 547 vegetation plots of arable land with measured soil pHKCL. The ability to reproduce known shapes was tested with artificial data sets. Shape parameters, e.g. niche width and range, slope (turnover) and species optima, can be calculated from the models and used for further analyses. The model framework together with advanced plot functions is included in the package eHOF for the statistical software environment R.
Results
Based on the AIC, 66 out of 131 species are modelled with a better compromise between model fit and model complexity by one of the logistic regression models as compared to GAM with automatic smoother selection. Within the model framework, 17 species (13%) are best modelled with one of the new bimodal types. The test with artificial data of known shape reveals good reliability of eHOF models for unimodal responses in areas with homogeneous information, but increasing uncertainty if the sampling is uneven or if only a part of the response is covered within the observed gradient range.
Conclusions
Hierarchical logistic regression models offer a flexible way to efficiently fit species response data. They propose a sound theoretical background for ecological interpretation. Extended HOF models as presented here are judged as an effective tool for univariate species response modelling.
+The shape of species responses along ecological gradients is of great theoretical and practical interest. However, ecological species responses are influenced by many factors and cannot easily be generalized. We compare a hierarchical set of 7 logistic regression models (eHOF) with General Additive Models. eHOF models perform well and can be used to derive several species characteristics for further analyses. +
Questions
How do two distant mountain ranges differ in their vegetation–environment relationships, overall species composition and its variability and species richness of vascular plants and bryophytes in alpine fens and bogs? Is the floristic difference consistent along the acidity–alkalinity gradient?
Location
West Carpathians (Poland, Slovakia) and Swiss Alps.
Methods
Vascular plant and bryophyte species compositions and environmental characteristics were sampled along the large acidity–alkalinity gradient within the alpine belts. PERMANOVA was used for testing floristic differences between the regions, DCA for the patterns description, CCA for testing the vegetation–environment relationships and linear models for testing the local species richness determinants. Compositional β-diversity (rate of compositional turnover) and regional species pool were compared using PERMDISP and sample-based rarefaction curves.
Results
Most floristic variation was explained by water pH in both regions. Macroclimate was the second most important gradient in the Alps, correlating with the second DCA axis delimiting the moderately poor brown-moss fens from the Sphagnum bogs. The species composition differed significantly between the regions in all three pH classes, expect for bryophytes in the middle pH class. Bryophytes exhibited higher similarities between the regions, except in acidic mires. Vascular plant composition differed most in the middle pH class. The local species richness was predominantly determined by pH, except for bryophytes in the Alps. As compared to bryophytes, vascular plant local species richness was affected by more factors and showed a clear linear response to pH. Within the particular pH classes, there were few differences in local species richness, compositional β-diversity and regional species pool between the two mountain ranges.
Conclusions
The pattern in inter-regional floristic dissimilarities differs between bryophytes and vascular plants, probably because of better dispersal ability of bryophytes, and different causes of these dissimilarities: lower mass effect at extreme ends of the acidity–alkalinity gradient in the case of vascular plants and higher compositional β-diversity of acidic mires in the case of bryophytes. Contrary to previously explored alpine springs of the same regions, local species richness and composition were determined rather by pH than mineral richness, and all patterns were more consistent between the regions and the taxonomic groups.
+The manuscript deals with comparison of overall species composition and its variability and species richness in mires between two distant mountain ranges. The paper is based on original recently collected data, thus sampled using the same protocol and the same effort in bryophyte determinations, what allowed direct comparisons between vascular plants and bryophytes. +
Questions
Two alternative mechanisms of abrupt vegetation change across ecological boundaries have been proposed: (1) concomitantly abrupt gradients in physical environmental variables and vegetation across the boundary, and (2) gradual environmental gradients that vegetation responds to in a non-linear or threshold manner. Here, we evaluate spatial patterns of climate and vegetation across a grassland–forest biome boundary to examine evidence in favour of either of these alternatives.
Location
Minnesota, USA.
Methods
Vegetation data represented the presence of prairie vs. forest vegetation in Minnesota from 1847 to 1908, generally prior to European settlement of the region, while the climatic variables represented an index of long-term average moisture availability (precipitation minus potential evapotranspiration (P – PET). Using linear and sigmoidal regression models, we evaluated spatial patterns of change in vegetation, climate and vegetation–climate relationships across 22 transects (170–400 km) oriented perpendicular to the biome boundary. We also evaluated boundary characteristics in light of dominant topographical controls and position along the boundary.
Results
Vegetation followed a sigmoidal pattern of change across the boundary, with mean boundary width of ca. 100 km. The P – PET increased by ca. 100 mm across the boundary following a comparatively smooth pattern of change. Climate–vegetation relationships were clearly non-linear across the boundary, indicating these variables did not change in a common spatial pattern. Regional topographical controls modified relationships between vegetation and climate along the length of the boundary.
Conclusions
Our results document strong non-linear relationships between the presence of forest vegetation and its dominant climate control across a grassland–forest biome boundary. An average change of ca. 100 mm in P – PET moving across the boundary is about 40% of the long-term mean annual range of this variable, suggesting that modest changes to P – PET may potentially cause substantial shifts in the location of the prairie–forest boundary.
+We use data from a historical grassland-forest biome boundary to show that forest vegetation responds in a non-linear or threshold manner to comparatively gradual spatial changes in climate. Thus, modest climatic changes may have potential to cause substantial locational shifts in this biome boundary.
Questions
What are the differences between the tree spatial patterns (TSP) of various recruit and mortality waves in alluvial hardwood forests and mountain fir–beech forests? Are there any statistically significant differences between the mean TSP of these forest types? Are these differences stable over time?
Location
Alluvial floodplain forests at the confluence of the Morava and Dyje rivers, and mountain fir–beech forests in the Outer Western Carpathians, Czech Republic.
Methods
In both forest types, seven 2-ha rectangular plots were analysed. The pair correlation function g(r) was used to describe tree density variability of trees with DBH ≥ 10 cm. The analyses were carried out for data sets from the 1970s, 1990s and 2000s. A bootstrap method was used to test for significant differences between the mean values of g(r) from alluvial forests and from fir–beech forests.
Results
Recruits in mountain fir–beech forests revealed consistent clustering to at least 5 m. In alluvial hardwood forests, recruits also showed random distribution as well as occasional regular distribution at distances over 20 m. Bootstrap significance tests revealed significant differences between the mean values of g(r) for alluvial forests and fir–beech forests. Alluvial floodplain forests showed statistically significant stronger clustering up to a distance of 4 m in all study periods. At distances over 20 m, mountain fir–beech forests demonstrated stronger clustering. In the 1970s, this was statistically significant only at a distance of 32 m, but in the 2000s, it was at intervals of 22–30 and 34–38 m.
Conclusions
The methods of data analysis in this study enabled us to find significant features of TSP at finer resolution than the common resulting trichotomy of univariate analysis: clustering, randomness and regularity. We believe that, on the basis of detailed spatial analyses, it is possible to create a TSP model that reflects the typical features of particular forest types.
+As alluvial hardwood forest and mountain fir-beech forest differ completely in a majority of characteristics, we expected significant differences between their tree spatial patterns too. Our study really demonstrated actual differences in tree spatial patterns of these forest types, although at surprising distances, and even in character. The main carrier of differences was the pattern of recruits and small-sized trees.
Questions
El Niño Southern Oscillation (ENSO) is a strong driver of climatic and ecosystem variability in coastal NW Peru. La Niña amplifies the already dry local conditions, and led to depleted ecosystems in 2011. However, the 2012 La Niña event triggered rainfall far above the average. (1) Did plant species diversity, primary productivity and vegetation assemblages change along a climatic gradient between two climatologically different La Niña years; (2) Is there a difference in explanatory power of environmental predictors between the 2 yr; and (iii) is it possible to predict the observed vegetation patterns spatially?
Location
Transect along a climatic gradient in the Sechura Desert of Piura, NW Peru (corresponds to the terrestrial part of the El Niño region 1 + 2) – a region of extremely high climatic variability.
Methods
We visited 50 30 m × 30 m randomly sampled plots in 2011 and 2012. A Procrustes analysis of two non-metric multidimensional scaling (NMDS) ordinations provided information on the temporal change of species assemblages. Variation partitioning revealed the differences in explanatory power of the predictors. We employed a generalized additive model (GAM) to fit the scores of the first ordination axis with a floristic gradient map as a result.
Results
Generally, higher rainfall resulted in a positive feedback when considering biodiversity, productivity and vegetation assemblages. The floristic gradient map resulting from the GAM displayed the spatial distribution of the three main assemblages along the climatic gradient. Edaphic variables added no independent portion to the explanation of the vegetation assemblages, but explained in conjunction with topography and NDVI a considerable amount of the variance.
Conclusions
Strong Atlantic easterly winds crossing the Andes can boost plant growth even during a La Niña situation. This underscores the need for a deeper understanding of ENSO-related climate variability of ENSO. Combining vegetation maps with accurate predictions of such climatic anomalies would aid the effective execution of conservation and recovery strategies. Additionally, coupling an unconstrained ordination with a GAM appears to be a promising tool for vegetation mapping, especially in the presence of a non-linear gradient.
+We analyze the vegetation composition of an endangered tropical dry forest formation along a climatic gradient. The vegetation composition is related to climatic and edaphic factors for two meteorologically different La Niña years. Furthermore, we modeled the scores of an ordination with a map of the floristic gradient as a result. This novel approach allows to spatially depict vegetation assemblages. +
Question
We connected tree-census and dendrochronological research data (74.2 ha) in order to answer the following questions: How do we apply an individual-based approach, which allows us to abandon the traditional patch model, in the research of disturbance history through spatial scales? What is the disturbance history of the natural forest? How do we understand the influence of the Kyrill storm, which affected the reserve on 18 January 2007? How does the disturbance history in dendrochronological records correspond to real disturbances?
Location
Žofínský Prales (48 °40′ N, 14 °42′ E, 735–830 m a.s.l., granite, Czech Republic)
Methods
Repeated tree-censuses (>20 000 trees) were carried out in 1975, 1997 and 2008. These surveys served as the basis of dendrochronological research, during which 3 020 trees were sampled. A boundary line (BL) approach and geostatistical techniques were used to detect disturbance intensity and its spatial pattern.
Results
The mean disturbance rate was 11.0% canopy loss per decade (maximum 33.7%) in the core zone. Bordering forests were directly affected by humans after 1800. A maximum of nine disturbance events were recorded per tree, but there were also 3% of non-suppressed trees without records of disturbance events. The Kyrill storm represented a singular event in the forest history from the point of view of its exceptional spatial pattern (ranges up to 320.0 m), but not from the perspective of disturbance intensity. The relationship between disturbance intensity (y) and the range of spatial autocorrelation (x) can be explained by the equation y = 10.6863 + 0.0783*x (R2 = 0.546, P = 0.009). The spatial variability of releases ≥50% of BL corresponds to real disturbance events, while the insertion of weak releases led to the overestimation of range. The frequency of releases was affected by a decrease in air pollution at the beginning of the 1990s.
Conclusions
Effects of small-scale disturbances predominate in forest dynamics. Disturbance intensity was often not directly in accordance with the range of spatial autocorrelation. The ecological role of disturbance is therefore biased if disturbance intensity is used as the sole criterion. The results suggest the importance of historical and geographical contingency in ecosystem development.
+Effects of small-scale disturbances predominate in forest dynamics. The Kyrill storm (18th Jan 2007) represented a singular event in the forest history from the point of view of its exceptional spatial pattern, but not from the perspective of disturbance intensity. The ecological role of disturbance is biased if disturbance intensity is used as the sole criterion. +
Questions
It is hypothesized that species and genetic diversity are correlated because niche differentiation among species and genotypes is either affected by the same processes (positive) or each level restricts the amount of diversity in the other (negative). Although many studies have observed both positive and negative relationships, others have found no correlation between the two diversity measures. Are measures of species (richness, diversity and evenness) and genetic diversity correlated, and how does resource (soil moisture, light, nitrogen and phosphorus) quantities and heterogeneity affect both levels of diversity?
Location
Intact tallgrass prairie at Konza Prairie Biological Station, northeast Kansas, US.
Methods
We investigate the correlation between plant species and genetic diversity in a long-term precipitation manipulation experiment – the Rainfall Manipulation Plots (RaMPs) – located in intact tallgrass prairie as well as adjacent non-manipulated prairie. The RaMPs experiment has been imposing ambient and more variable precipitation regimes (a 50% increase in timing between rainfall events without changing total rainfall amount) during the growing season since 1998, resulting in reduced mean soil moisture and increased soil moisture variability. Thus, the RaMPs and non-manipulated prairie plots capture a range of soil moisture amounts and variability. Genetic diversity (measured as genotype richness and genomic dissimilarity among individuals) was quantified for the dominant grass species, Andropogon gerardii, which has large impacts on plant community structure and ecosystem function.
Results
We found species and genetic diversity were not significantly correlated. Genotypic richness was negatively related to soil moisture variability, but measures of species diversity were not. In the non-manipulated plots only, we found generally negative relationships between resource quantity (light and nitrogen) and community diversity, and positive relationships between resource heterogeneity (CV of light) and community diversity.
Conclusions
Our results suggest that a lack of a positive or negative relationship between species and genetic diversity could be due to these two levels of diversity responding differently to the identity, quantity and heterogeneity of resources.
+In tallgrass prairie, the genetic diversity of the dominant species Andropogon gerardii is not related to plant community diversity. Here, we investigate how environmental resources, their quantity and heterogeneity, might affect correlations between these two levels of plant diversity. We found that species and genetic diversity are related to different environmental resources potentially explaining the lack of relationship.
Photo Credit: M. Smith
Question
Explaining the biomass–species richness relationship is key to understanding vegetation dynamics. Several possible mechanisms have been suggested, but complex analysis of plant strategies, major biomass and species richness components along a long productivity gradient is still lacking. We provide a detailed analysis of the relationship between major biomass components (total above-ground biomass, green biomass and litter), plant strategies and species richness along a long gradient of alkali and loess grasslands in a steppe landscape in Central Europe.
Location
Hortobágy, Great Hungarian Plain, East Hungary.
Methods
Above-ground biomass of characteristic alkali and loess grassland stands was sampled along a gradient of increasing productivity. In each grassland stand, a 25-m2 sample site was randomly selected. Within each site, ten above-ground biomass samples (20 × 20 cm) were collected randomly in June 2009, at the peak of biomass production. We classified all species into mixed C-S-R strategy types. To obtain correlations between various biomass and species richness data, Spearman rank correlation was used. The relationship between plant strategies and species composition were displayed with a DCA ordination.
Results
The frequently detected humped-back relationship was valid for the relation of total biomass and species richness. With increasing amount of total biomass, we detected an increasing proportion of competitors, and a decreasing proportion of stress tolerators in green biomass. A low proportion of ruderals was detected at both low and high biomass levels. Species richness was affected positively by litter at low litter scores, but there was a negative litter effect from much lower scores than detected previously (from 400 g·m−2). There was a positive relationship between green biomass production and species richness.
Conclusions
The study revealed that at the initial part of a productivity gradient, stress is likely responsible for low species richness. Our results show that litter can shape changes in species richness along the whole biomass gradient, thus the litter effect is one of the major mechanisms structuring grassland diversity.
+We found a humped-back relationship between total biomass and species richness. A negative litter effect on species richness was detected at much lower scores (from 400 g/m2) than reported in former studies. Our study revealed that litter can shape species richness along the whole biomass gradient, thus it is one of the major factors in structuring grassland biodiversity.
The biomass and dynamics of disturbed and degraded tropical forests have mostly been ignored in the recent scientific literature, partly because of a spotlight on old-growth forests but also due to a lack of long-term data from degraded forests. There is a pressing need to understand the rates and patterns of growth, mortality and recruitment in degraded forests, not only because they are increasing in area relative to old-growth forests, but also due to their potential capacity to sequester large amounts of carbon dioxide from the atmosphere. This paper introduces a permanent forest plot network and database, FORMNET-B (GIVD ID# NA-BZ-001), designed to study the long-term dynamics of disturbed and degraded tropical forests in Belize, Central America.
+Disturbed and degraded tropical forests have received little attention in long-term studies of forest dynamics. We present a database designed to monitor tropical forests in permanent plots as they transition from mature to degraded to recovering forests after various degrees of natural and anthropogenic disturbance. We describe the plot methodology, the plot disturbance history and the database structure and contents. +
Ibanez et al. (Journal of Vegetation Science, 2013, 24: 177) used fire modelling to examine the differential fire tolerance of secondary forest species in New Caledonia savanna. Misinterpretation of the concept of flame-front residence time and inadequate choices and linkages of fire behaviour and fire effects models affected the estimation of both stem and canopy injury. The most relevant concern is that the depth of fire-induced necrosis in the stem was overemphasized by a factor of two to three, underestimating tree tolerance to fire and overestimating the tree size thresholds for escaping fire.
+Fire simulation modelling is a useful and insightful tool in fire ecology research, provided that modelling choices are thoughtful. Deceptive fire-vegetation relationships will emerge when model concepts and assumptions are overlooked or poorly understood and when model inputs are not representative.