Showing papers in "Journal of Ecology in 2010"

The use of chronosequences in studies of ecological succession and soil development

Abstract: Summary 1. Chronosequences and associated space-for-time substitutions are an important and often necessary tool for studying temporal dynamics of plant communities and soil development across multiple time-scales. However, they are often used inappropriately, leading to false conclusions about ecological patterns and processes, which has prompted recent strong criticism of the approach. Here, we evaluate when chronosequences may or may not be appropriate for studying community and ecosystem development. 2. Chronosequences are appropriate to study plant succession at decadal to millennial time-scales when there is evidence that sites of different ages are following the same trajectory. They can also be reliably used to study aspects of soil development that occur between temporally linked sites over time-scales of centuries to millennia, sometimes independently of their application to shorter-term plant and soil biological communities. 3. Some characteristics of changing plant and soil biological communities (e.g. species richness, plant cover, vegetation structure, soil organic matter accumulation) are more likely to be related in a predictable and temporally linear manner than are other characteristics (e.g. species composition and abundance) and are therefore more reliably studied using a chronosequence approach. 4. Chronosequences are most appropriate for studying communities that are following convergent successional trajectories and have low biodiversity, rapid species turnover and low frequency and severity of disturbance. Chronosequences are least suitable for studying successional trajectories that are divergent, species-rich, highly disturbed or arrested in time because then there are often major difficulties in determining temporal linkages between stages. 5. Synthesis. We conclude that, when successional trajectories exceed the life span of investigators and the experimental and observational studies that they perform, temporal change can be successfully explored through the judicious use of chronosequences.

Intraspecific functional variability: extent, structure and sources of variation

Abstract: Summary 1. Functional traits are increasingly used to investigate community structure, ecosystem functioning or to classify species into functional groups. These functional traits are expected to be variable between and within species. Intraspecific functional variability is supposed to influence and modulate species responses to environmental changes and their effects on their environment. However, this hypothesis remains poorly tested and species are mostly described by mean trait values without any consideration of variability in individual trait values. 2. In this study, we quantify the extent of intraspecific plant functional trait variability, its spatial structure and its response to environmental factors. Using a sampling design structured along two direct and orthogonal climatic gradients in an alpine valley, we quantified and analysed the intraspecific variability for three functional traits (height, leaf dry matter content and leaf nitrogen content) measured on sixteen plant species with contrasting life histories. 3. Results showed a large variability of traits within species with large discrepancies between functional traits and species. This variability did not appear to be structured within populations. Between populations, the overall variability was partly explained by the selected gradients. Despite the strong effects of temperature and radiation on trait intraspecific variability, the response curves of traits along gradients were partly idiosyncratic. 4.Synthesis. Giving a comprehensive quantification of intraspecific functional variability through the analysis of an original data set, we report new evidence that using a single trait value to describe a given species can hide large functional variation for this species along environmental gradients. These findings suggest that intraspecific functional variability should be a concern for ecologists and its recognition opens new opportunities to better understand and predict ecological patterns in a changing environment. Further analyses are, however, required to compare inter- and intraspecific variability.

Intraspecific variability and trait‐based community assembly

Abstract: 1. Trait-based approaches applied to community ecology have led to a considerable advance in understanding the effect of environmental filters on species assembly. Although plant traits are known to vary both between and within species, little is known about the role of intraspecific trait variability in the non-random assembly mechanisms controlling the coexistence of species, including habitat filtering and niche differentiation. 2. We investigate the role of intraspecific variability in three key functional traits – specific leaf area (SLA), leaf dry matter content (LDMC) and height – in structuring grassland communities distributed along a flooding gradient. We quantified the contribution of intraspecific variability relative to interspecific differences in the trait–gradient relationship, and we used a null model approach to detect patterns of habitat filtering and niche differentiation, with and without intraspecific variability. 3. Community mean SLA and height varied significantly along the flooding gradient and intraspecific variability accounted for 44% and 32%, respectively, of these trait–gradient relationships. LDMC did not vary along the gradient, with and without accounting for intraspecific variability. Our null model approach revealed significant patterns of habitat filtering and niche differentiation for SLA and height, but not for LDMC. More strikingly, considering intraspecific trait variability greatly increased the detection of habitat filtering and was necessary to detect niche differentiation processes. 4. Synthesis. Our study provides evidence for a strong role of intraspecific trait variability in community assembly. Our findings suggest that intraspecific trait variability promotes species coexistence, by enabling species to pass through both abiotic and biotic filters. We argue that community ecology would benefit frommore attention to intraspecific variability.

Evidence of the ‘plant economics spectrum’ in a subarctic flora

Abstract: Summary 1. A fundamental trade-off among vascular plants between traits inferring rapid resource acquisition and those leading to conservation of resources has now been accepted broadly, but is based on empirical data with a strong bias towards leaf traits. Here, we test whether interspecific variation in traits of different plant organs obeys this same trade-off and whether within-plant trade-offs are consistent between organs. 2. Thereto, we measured suites of the same chemical and structural traits from the main vegetative organs for a species set representing aquatic, riparian and terrestrial environments including the main vascular higher taxa and growth forms of a subarctic flora. The traits were chosen to have consistent relevance for plant defence and growth across organs and environments: carbon, nitrogen, phosphorus, lignin, dry matter content, pH. 3. Our analysis shows several new trait correlations across leaves, stems and roots and a striking pattern of whole-plant integrative resource economy, leading to tight correspondence between the local leaf economics spectrum and the root (r =0 .64), stem ( r = 0.78) and whole-plant (r =0 .93) economics spectra. 4. Synthesis. Our findings strongly suggest that plant resource economics is consistent across species’ organs in a subarctic flora. We provide thus the first evidence for a ‘plant economics spectrum’ closely related to the local subarctic ‘leaf economics spectrum’. Extending that concept to other biomes is, however, necessary before any generalization might be made. In a world facing rapid vegetation change, these results nevertheless bear considerable prospects of predicting belowground plant functions from the above-ground components alone.

Plant–soil feedback: experimental approaches, statistical analyses and ecological interpretations

Abstract: Summary 1. Feedback between plants and soil organisms has become widely recognized as a driving force of community composition and ecosystem functioning. However, there is little uniformity in quantification and analysis of plant–soil feedback effects. Meta-analysis suggested that the various experimental methods tend to result in different feedback values. Yet, a direct comparison of the different experimental approaches and their statistical analyses is lacking. 2. We used currently applied methods to calculate plant–soil feedback value ranges and compared their statistical analyses to those based on actual biomass data. Then, we re-analysed a case study to compare plant–soil feedback values obtained under the same environmental conditions, but using different experimental approaches: soil sterilization, addition of soil inoculum, and soil conditioning by ‘own’ vs. ‘foreign’ plant species. 3. Different measures to calculate plant–soil feedback values were more variable in positive than in negative feedback values. Analysis of calculated feedback values that are based on treatment averages can lead to a serious inflation of type I errors. 4. In our case study, both the strength and the direction of the feedback effects depended on the experimental approach that was chosen, leading to diverging conclusions on whether feedback to a certain soil was positive or negative. Soil sterilization and addition of soil organisms yielded larger feedback than comparison of own and foreign soil. 5. Synthesis. The ecological interpretation of plant–soil feedback effects strongly depends on the experimental procedure. When the research question focuses on the strength and the sign of plant– soil feedback, soil sterilization (presumed that the side effect of increased nutrient availability can be controlled) or addition of soil inoculum is to be preferred. When the research question concerns the specificity of soil feedback effects, plant performance can be better compared between own and foreign soil. We recommend that when using calculated feedback values, the original data need to be presented as well in order to trace the cause of the effect.

Linkages of plant traits to soil properties and the functioning of temperate grassland

Abstract: Summary 1. Global change is likely to alter plant community structure, with consequences for the structure and functioning of the below-ground community and potential feedbacks to climate change. Understanding the mechanisms behind these plant–soil interactions and feedbacks to the Earth-system is therefore crucial. One approach to understanding such mechanisms is to use plant traits as predictors of functioning. 2. We used a field-based monoculture experiment involving nine grassland species that had been growing on the same base soil for 7 years to test whether leaf, litter and root traits associated with different plant growth strategies can be linked to an extensive range of soil properties relevant to carbon, nitrogen and phosphorus cycling. Soil properties included the biomass and structure of the soil microbial community, soil nutrients, soil microclimate and soil process rates. 3. Plant species with a high relative growth rate (RGR) were associated with high leaf and litter quality (e.g. low toughness, high nitrogen concentrations), an elevated biomass of bacteria relative to fungi in soil, high rates of soil nitrogen mineralization and concentrations of extractable inorganic nitrogen, and to some extent higher available phosphorus pools. 4. In contrast to current theory, species with a high RGR and litter quality were associated with soils with a lower rate of soil respiration and slow decomposition rates. This indicates that predicting processes that influence carbon cycling from plant traits may be more complex than predicting processes that influence nitrogen and phosphorus cycling. 5. Root traits did not show strong relationships to RGR, leaf or litter traits, but were strongly correlated with several soil properties, particularly the biomass of bacteria relative to fungi in soil and measures relating to soil carbon cycling. 6. Synthesis. Our results indicate that plant species from a single habitat can result in significant divergence in soil properties and functioning when grown in monoculture, and that many of these changes are strongly and predictably linked to variation in plant traits associated with different growth strategies. Traits therefore have the potential to be a powerful tool for understanding the mechanisms behind plant–soil interactions and ecosystem functioning, and for predicting how changes in plant species composition associated with global change will feedback to the Earth-system.

Strong facilitation in mild environments: the stress gradient hypothesis revisited

Abstract: 1. The idea that the role of facilitative interactions increases as environmental conditions become more stressful has become a ruling paradigm in ecology. Here, we review three reasons why positive interactions may actually be more prominent than generally thought under moderately stressful rather than under extreme conditions. 2. First, there is evidence that in some communities the net effect of amelioration of shortage of a limiting resource, such as water under the canopy of nurse plants, may be beneficial under moderate conditions whereas it can be overruled by increased competition for the same resource in very harsh environments. 3. Secondly, we show that even in situations where the relative role of facilitation increases monotonically with stress, the absolute effect should as a rule be largest at intermediately stressful conditions. This is because under the harshest conditions, facilitative amelioration of conditions is insufficient to allow growth altogether. Therefore, while facilitation will expand the range of conditions where an organism may occur, the largest absolute effects on biomass will always occur under less stressful conditions. 4. A third reason why facilitation may be more important under moderate conditions than previously thought is that in any ecosystem, the suite of organisms is adapted to local conditions. This implies that even under conditions that appear benign, facilitation may play an unexpectedly large role as organisms are simply more sensitive than those found under harsher overall conditions. 5. Synthesis. We argue that while facilitation will extend the range of conditions where an organism can occur, it should also boost performance of the species well into the more moderate range of conditions. Broadening our search image for facilitative effects towards milder environments will reveal wider than expected prevalence of positive interactions and their effects on stability and diversity in nature.

Carbon quality rather than stoichiometry controls litter decomposition in a tropical rain forest

Abstract: 1. Ecological stoichiometry predicts important control of the relative abundance of the key elements carbon (C), nitrogen (N) and phosphorus (P) on trophic interactions. In a nutrient-poor Amazonian lowland rain forest of French Guiana, we tested the hypothesis that decomposers exploit stoichiometrically diverse plant litter more efficiently, resulting in faster litter decomposition compared to litter with a uniform stoichiometry. 2. In a field experiment in the presence or absence of soil macrofauna, we measured litter mass loss, and N and P dynamics from all possible combinations of leaf litter from four common tree species which were distinctly separated along a C:N and along a N:P gradient. 3. Mean litter mass remaining after 204†days of field exposure varied between 25.2% and 71.3% among litter treatments. Fauna increased litter mass loss by 18%, N loss by 21% and P loss by 14%. Litter species richness had no effect on litter mass loss or nutrient dynamics. In contrast, litter mass and nutrient losses increased with increasing stoichiometric dissimilarity of litter mixtures in presence of fauna, suggesting faster decomposition of a stoichiometrically more heterogeneous litter. 4. However, the effect of stoichiometric dissimilarity was smaller than the strong C quality related litter composition effect and disappeared in the absence of fauna. Increasing proportions of litter that is relatively rich in accessible C compounds (non-structural carbohydrates, phenolics) and relatively poor in recalcitrant C (condensed tannins, lignin), correlated best with litter mass loss irrespective of fauna presence. No correlation was found for any of the nutrient related litter quality parameters and decomposition. 5.Synthesis. Our results suggest that Amazonian decomposer communities studied here are primarily limited by energy, and only secondarily by litter stoichiometry. Tropical tree species might thus influence decomposers and detritivores by the production of litter of specific C quality with potentially important feedback effects on ecosystem nutrient dynamics and availability. (Less)

Unveiling below-ground species abundance in a biodiversity experiment: a test of vertical niche differentiation among grassland species

Abstract: 1. Plant diversity has profound effects on primary production. Plant diversity has been shown to correlate with increased primary production in nutrient-limited grassland ecosystems. This overyielding has been attributed to vertical niche differentiation among species below-ground, allowing for complementarity in resource capture. However, a rigorous test of this longstanding hypothesis is lacking because roots of different species could not be distinguished in diverse communities. 2. Here, we present the first application of a DNA-based technique that quantifies species abundances in multispecies root samples. We were thus able to compare root distributions in monocultures of two grasses and two forbs with root distributions in four-species mixtures. In order to investigate if vertical niche differentiation is driven by soil nutrient depletion, the topsoil layer of the communities were either nutrient-rich or -poor. 3. Immediately in the first year, 40% more root biomass was produced in mixtures than expected from the monocultures, together with significant below-ground complementarity effects, probably preceding above-ground overyielding. This below-ground overyielding appeared not to be the result of vertical niche differentiation, as rooting depth of the community tended to decrease, rather than increase in mixtures compared to monocultures. Roots thus tended to clump in the very dense topsoil layer rather than segregate over the whole profile in mixtures. The below-ground overyielding was mainly driven by enhanced root investments of one species, Anthoxanthum odoratum, in the densely rooted topsoil layer without retarding the growth of the other species. 4. Synthesis. Conventional ecological mechanisms, such as competition for nutrients, do not seem to be able to explain the increased root investments of A. odoratum in mixtures compared to monocultures, with apparently little effect on the root growth of the other species. Instead, the observed root responses are consistent with species-specific root recognition responses. From a community perspective, the observed early below-ground overyielding may initiate the recently reported increased soil organic matter, mineralization and N availability and thus may ultimately be responsible for the higher productivity at high plant species diversity.

Competition modulates the adaptation capacity of forests to climatic stress: insights from recent growth decline and death in relict stands of the Mediterranean fir Abies pinsapo.

Abstract: Summary 1. Long-term basal area increment (BAI) in Abies pinsapo was studied to investigate the way density-dependent factors modulate the responses of radial growth to climatic stresses in relict stands of a drought-sensitive Mediterranean fir. 2. First, we verified that spatially explicit competition predicts mean A. pinsapo BAI at our study site; i.e. it modulates the degree to which the average climate-driven potential for growth is expressed. Second, we verified that the long-term pattern of temperature predicts the long-term pattern of BAI, estimated as the main trend over a time period of c. 40 years. Finally, we assessed whether the intensity of tree-to-tree competition restrains the potential improvements achieved by our model of BAI when a short-term, high-frequency stressor such as drought (inter-annual precipitation variability) is introduced. 3. We applied Dynamic Factor Analysis (DFA) to characterize regional climatic trends and to test the hypothesis that trees subjected to contrasting competition intensity may differ in their growth pattern. Significant long-term climate trends obtained by DFA were used as predictors of long-term BAI. 4. The mean BAI was mainly determined by competition, whereas growth trends obtained by DFA did not differ among dominant, suppressed and dying trees. Common trends of growth decline were strongly related to long-term, late-winter to summer temperatures, while the residuals were related to total annual precipitation, although with decreasing significance as competition increased. Our results support the contention that the reported patterns of A. pinsapo growth decline and death occur as a result of the interacting effects of both competition and climate stressors acting at longand short-term time scales. 5. Synthesis. Long-term climatic drought stress was the main driving factor of growth decline in A. pinsapo. Moreover, trees already suffering from competition (a long-term stress) were predisposed to decline given an additional short-term stress, such as a severe drought.

The impact of lianas on 10 years of tree growth and mortality on Barro Colorado Island, Panama

Abstract: Summary 1. Lianas compete intensely with trees, but few studies have examined long-term effects of liana infestation on tree growth and mortality. We quantified the effects of lianas in tree crowns (n = 2907) and rooted within 2 m of trees (n = 1086) on growth and mortality of 30 tree species from 1995 to 2005 on Barro Colorado Island (BCI), Panama, documented liana infestation in tree crowns in 1996 and 2007 to determine the dynamics of liana infestation, and quantified liana infestation in the crowns of 3231 additional canopy trees (d.b.h. ‡20 cm) in 2007 to compare with the same metric determined by previous studies in 1967 and 1980. 2. Severe liana infestation increased tree mortality: 21% of liana-free trees in 1996 had died by 2007, whereas 42% of trees with more than 75% of the crown infested by lianas in 1996 had died by 2007. 3. Liana infestation of tree crowns significantly reduced tree growth, particularly on sun-exposed trees. The proximity of rooted lianas significantly reduced the growth of shaded trees. 4. Liana infestation was dynamic: 10.9% of trees with severe liana infestation in their crowns in 1996 had shed all of their lianas by 2007 and 5.3% of trees with no lianas in their crown in 1996 had severe liana infestation in 2007. 5. Liana infestation was common: lianas were present in 53% of trees of the 30 focal species. Including lianas rooted within 2 m of the tree increased this percentage to 78%. Using both aboveand below-ground measures may provide a better estimate of liana competition than either measure alone. 6. Liana infestation is increasing on BCI. Lianas were present in the crowns of 73.6% of canopy trees (d.b.h. ‡20 cm). Liana canopy infestation was 57% higher than in 1980 and 65% higher than in 1967, which is consistent with reported increases in liana abundance, biomass, and leaf and flower production. 7. Synthesis. We used one of the largest studies ever conducted on lianas to confirm the negative effects of lianas on tree growth and survival over 10 years. Liana infestation of trees was widespread, dynamic and increasing on BCI.

Diversity enhances community recovery, but not resistance, after drought.

Abstract: Summary 1. There is growing concern that the current loss of biodiversity may negatively affect ecosystem functioning and stability. Although it has been shown that species loss may reduce biomass production and increase temporal variability, experimental evidence that species loss affects ecosystem resistance and resilience after perturbation is limited. 2. Here, we use the response of experimental plant communities – which differ in diversity – to a natural drought to disentangle the effects of diversity and biomass on resistance, recovery and resilience. 3. Resistance to drought decreased with diversity, but this pattern was highly dependent upon predrought biomass. When corrected for biomass, no relationship between diversity and resistance was observed: at each level of diversity, biomass production was reduced by approximately 30%. 4. In contrast, recovery (change in biomass production after drought) increased with diversity and was independent of biomass. Resilience (measured as the ratio of post- to pre-drought biomass) was similar at each level of diversity. 5. Synthesis. On the one hand, our results confirm earlier findings that a positive relationship between diversity and resistance is mainly driven by pre-perturbation performance rather than by diversity. However, the results also show that recovery after drought strongly increased with diversity, independent of performance. We conclude that it is this diversity-dependent recovery which allowed diverse, productive communities to reach the same level of resilience as less diverse (and productive) communities. This finding provides strong experimental evidence for the insurance hypothesis.

Effects of patch size and density on flower visitation and seed set of wild plants: a pan-European approach

Abstract: Summary 1. Habitat fragmentation can affect pollinator and plant population structure in terms of species composition, abundance, area covered and density of flowering plants. This, in turn, may affect pollinator visitation frequency, pollen deposition, seed set and plant fitness. 2. A reduction in the quantity of flower visits can be coupled with a reduction in the quality of pollination service and hence the plants’ overall reproductive success and long-term survival. Understanding the relationship between plant population size and ⁄ or isolation and pollination limitation is of fundamental importance for plant conservation. 3. We examined flower visitation and seed set of 10 different plant species from five European countries to investigate the general effects of plant populations size and density, both within (patch level) and between populations (population level), on seed set and pollination limitation. 4. We found evidence that the effects of area and density of flowering plant assemblages were generally more pronounced at the patch level than at the population level. We also found that patch and population level together influenced flower visitation and seed set, and the latter increased with increasing patch area and density, but this effect was only apparent in small populations. 5. Synthesis. By using an extensive pan-European data set on flower visitation and seed set we have identified a general pattern in the interplay between the attractiveness of flowering plant patches for pollinators and density dependence of flower visitation, and also a strong plant species-specific response to habitat fragmentation effects. This can guide efforts to conserve plant–pollinator interactions, ecosystem functioning and plant fitness in fragmented habitats.

Evidence for climatic niche and biome shifts between native and novel ranges in plant species introduced to Australia

Abstract: 1. The potential invasive success of exotic plant species is thought to be associated with similarity in climate and biome between the original and novel range. We tested this assumption by quantifying the match between the realized climatic niches and biomes occupied in the exotic and native range of 26 plant species introduced to Australia. We then explored correlations between the propensity to shift climatic niche with residence time, invasion status, geographic range size, and species traits. 2. Occurrence data from the native and exotic range of 26 species introduced to Australia were obtained, and the overlap between native and exotic climate niches was calculated using between-class analysis. Shifts into novel biomes were assessed using a Geographic Information System (GIS). Correlations between introduction, distribution and species traits and the degree of climate matching were examined using nonparametric statistical tests. 3. Exotic species frequently occurred in climatic conditions outside those occupied in their native range (20 of 26 species). Nineteen species inhabited biomes in Australia not occupied in the native range and in some instances this shift represented the establishment of populations in novel biomes not present in the native range. No single-species trait, introduction or distributional characteristic was significantly associated with the degree of climatic niche shift. 4. Synthesis. Exotic species are able to occupy climate niches in the new range that differ substantially from those of the native range, and generally do not show biome conservatism between their native and introduced ranges. This implies that novel climatic conditions are not a major obstacle for exotic species establishing populations outside their native range. These results have important implications for the use and interpretation of ecological niche models used to predict the distribution of species in novel climates in time or space. The results also highlight the importance of alternate mechanisms, such as enemy release, phenotypic plasticity or rapid evolution, in the establishment of naturalized and invasive populations.

Can optimal defence theory be used to predict the distribution of plant chemical defences

Abstract: Summary 1. The optimal defence theory (ODT) of chemical defence provides a predictive framework for the distribution of anti-herbivore defences in plants. One of its predictions is that chemical defences will be allocated within a plant as a function of tissue value, where value is correlated with the cost of having that tissue removed. While many studies have examined intra-plant variation in defence chemistry, these results have rarely been compiled quantitatively to assess whether defence allocation is consistent with the prediction of ODT that more valuable tissues should be more defended than less valuable tissues. 2. We performed a formal meta-analysis of published and unpublished studies to examine the predictive utility of ODT. Specifically, we examined whether defence chemicals occur at higher concentrations in flowers versus leaves and in younger leaves compared to older leaves, under the assumption that younger leaves are more valuable than older leaves. We also examined whether the expansion status of younger leaves, nodal position of the leaves, growing conditions and chemical class of defence compounds affected the mean effect sizes. 3. We found that tissues with higher assumed value had significantly higher concentrations of defence chemicals than tissues with lower value. In particular, we found that younger leaves had higher concentrations of defence chemicals than older leaves, consistent with the predictions of ODT. The magnitude of this difference was higher in the younger leaf/older leaf comparison than in the flower/leaf comparison, with no evidence that flowers were more defended than leaves. The overall results were not affected by chemical class, young leaf expansion status, growing conditions or leaf position on the plant. 4. Synthesis. We found general agreement between the predictions of ODT and the intraplant distribution of chemical defence and conclude it is a useful model. The effect size varied depending on the tissue compared. Explicit measures of tissue value, in particular as it relates to relative fitness, are required to further validate the predictive utility and general applicability of ODT.

Alternative community states maintained by fire in the Klamath Mountains, USA.

Abstract: Summary 1. The earliest examples of alternative community states in the literature appear to be descriptions of natural vegetation said to both depend on and promote fire. Nonetheless, alternative community states determined by fire have rarely been documented at landscape scales and in natural vegetation. This is because spatial autocorrelation may confound analyses, experimental manipulations are difficult and a long-term perspective is needed to demonstrate that alternative community states can persist for multiple generations. 2. We hypothesized that alternative community states occur in a largely forested landscape in the Klamath Mountains, north-western California, USA, where shrub-dominated sclerophyllous vegetation establishes after fire that is lethal to forests. Forests redevelop if succession is not arrested by fire. Our hypothesis would require that sclerophyll and forest vegetation states each be maintained by different self-reinforcing relationships with fire. 3. To test this hypothesis, we examined pyrogenicity of forest and sclerophyll vegetation as a function of time since the previous fire, accounting for spatial autocorrelation. Fire exclusion served as a de facto experimental treatment. Areas where fire had proceeded to occur served as controls. 4. Our findings are consistent with the occurrence of alternative community states established and maintained by different self-reinforcing feedbacks with fire. Sclerophyll vegetation was more pyrogenic, especially where time-since-fire (TSF) was relatively short, a favourable relationship for this fire-dependent vegetation. Forests were much less pyrogenic, especially where TSF was long, favouring their maintenance. Fire exclusion therefore has led to afforestation and rapid retreat of fire-dependent vegetation. 5. Synthesis: We have documented how different self-reinforcing combustion properties of forest and sclerophyll vegetation can naturally produce alternative states coexisting side-by-side in the same environment. Such fire-mediated alternative states may be underappreciated, in part, because they are difficult to demonstrate definitively. In addition, the dynamics they exhibit contrast with common perceptions that fire hazard increases deterministically with TSF in forests and shrublands. Addressing the impacts of fire exclusion will probably require a management shift to better allow fire to perform its ecological role in shaping landscape diversity and maintaining firedependent biota.

Functional traits of alien plants across contrasting climatic and land-use regimes: do aliens join the locals or try harder than them?

Abstract: Summary 1 Two main views have been put forward to explain whether coexisting alien and resident plant species should show converging or diverging functional attributes According to the ‘try-harder’ hypothesis, successful aliens should differ from resident species with traits that allow them to deal better with the local conditions than resident species On the other hand, the ‘join-the-locals’ hypothesis stresses the importance of filtering by environmental factors and predicts strong functional trait similarities between alien and native species, especially among the dominants 2 On the basis of a functional trait comparison between native and alien species of central-western Argentina across five contrasting ecosystems and four land-use regimes, we tested these hypotheses over a broad range of habitats We built a data set with common measurement methods and biogeographical factors but strongly varying environmental conditions, ranging from mesic to extremely dry, and from nearly pristine to heavily disturbed 3 When considering all species together, the main trend of variation in trait syndromes was between acquisitive (tender, large leaves, with high specific area) and conservative (tough, small leaves, with low specific area and low nutrient content) Although both native and alien species appeared to be well spread across the whole range of trait variation, woody alien species showed a significantly more acquisitive set of attributes (higher specific leaf area, larger and thinner leaves, lower wood density) than native species No significant difference was detected between herbaceous alien and native species These general trends were maintained under contrasting climatic and landuse conditions 4 Synthesis The patterns detected for herbaceous species were in line with the ‘join-the-locals’ hypothesis In contrast, those found for woody species, with woody alien species showing more acquisitive attributes than native species in more resource-rich habitats, provide partial support for the ‘try-harder’ hypothesis Overall, our findings reinforce the idea that a universal suit of attributes is unlikely to explain alien plant distribution They also stress the need for caution when mixing major life-forms in comparative plant trait analysis

Influence of plant species and soil conditions on plant-soil feedback in mixed grassland communities

Abstract: 1. Our aim was to explore plant-soil feedback in mixed grassland communities and its significance for plant productivity and community composition relative to abiotic factors of soil type and fertility. 2. We carried out a 4-year, field-based mesocosm experiment to determine the relative effects of soil type, historic management intensity and soil conditioning by a wide range of plant species of mesotrophic grassland on the productivity and evenness of subsequent mixed communities. 3. The study consisted of an initial soil conditioning phase, whereby soil from two locations each with two levels of management intensity was conditioned with monocultures of nine grassland species, and a subsequent feedback phase, where mixed communities of the nine species were grown in conditioned soil to determine relative effects of experimental factors on the productivity and evenness of mixed communities and individual plant species performance. 4. In the conditioning phase of the experiment, individual plant species differentially influenced soil microbial communities and nutrient availability. However, these biotic effects were much less important as drivers of soil microbial properties and nutrient availability than were abiotic factors of soil type and fertility. 5. Significant feedback effects of conditioning were detected during the second phase of the study in terms of individual plant growth in mixed communities. These feedback effects were generally independent of soil type or fertility, and were consistently negative in nature. In most cases, individual plant species performed less well in mixed communities planted in soil that had previously supported their own species. 6. Synthesis. These findings suggest that despite soil abiotic factors acting as major drivers of soil microbial communities and nutrient availability, biotic interactions in the form of negative feedback play a significant role in regulating individual plant performance in mixed grassland communities across a range of soil conditions.

Higher treefall rates on slopes and waterlogged soils result in lower stand biomass and productivity in a tropical rain forest

Abstract: Summary 1. Relationships between tropical rain forest biomass and environmental factors have been determined at regional scales, e.g. the Amazon Basin, but the reasons for the high variability in forest biomass at local scales are poorly understood. Interactions between topography, soil properties, tree growth and mortality rates, and treefalls are a likely reason for this variability. 2. We used repeated measurements of permanent plots in lowland rain forest in French Guiana to evaluate these relationships. The plots sampled topographic gradients from hilltops to slopes to bottomlands, with accompanying variation in soil waterlogging along these gradients. Biomass was calculated for >175 tree species in the plots, along with biomass productivity and recruitment rates. Mortality was determined as standing dead and treefalls. 3. Treefall rates were twice as high in bottomlands as on hilltops, and tree recruitment rates, radial growth rates and the abundance of light-demanding tree species were also higher. 4. In the bottomlands, the mean wood density was 10% lower than on hilltops, the basal area 29% lower and the height:diameter ratio of trees was lower, collectively resulting in a total woody biomass that was 43% lower in bottomlands than on hilltops. 5. Biomass productivity was 9% lower in bottomlands than on hilltops, even though soil Olsen P concentrations were higher in bottomlands. 6. Synthesis. Along a topographic gradient from hilltops to bottomlands there were higher rates of treefall, which decreased the stand basal area and favoured lower allocation to height growth and recruitment of light-demanding species with low wood density. The resultant large variation in tree biomass along the gradient shows the importance of determining site characteristics and including these characteristics when scaling up biomass estimates from stand to local or regional scales.

Idiosyncrasy and overdominance in the structure of natural communities of arbuscular mycorrhizal fungi: is there a role for stochastic processes?

Abstract: 1. Most studies of species abundance patterns focus on conspicuous macroorganisms while microbial communities remain relatively understudied. This bias is a concern given the functional importance and high diversity of microbes. 2. We determine whether a common species abundance distribution (SAD) is observed in communities of a widespread group of soil microbes, the Glomeromycota or arbuscular mycorrhizal (AM) fungi. Using molecular techniques, we intensively sampled the AM fungal community of a woodland–grassland ecotone in Yorkshire, UK. Observed species abundances were compared to theoretical models describing SADs. We also reanalysed 32 previously published data sets in a similar manner. 3. Species abundance distributions in all the AM fungal communities fitted both lognormal and broken-stick models. However, these models consistently and significantly underpredicted the abundance of the most abundant AM fungal taxon. We found that AM fungal communities are typically dominated by a single taxon; representing on average 40% of total abundance within the community. Phylogenetic analysis of the most abundant taxa across data sets showed that the dominant AM fungal type in each community was different and not a widespread generalist. 4. We conclude that a common community structure is present in AM fungal communities from different habitats. The fit to log-normal and broken-stick models suggests the influence of niche differentiation structuring these communities. However, the consistently observed overdominance indicates that local adaptation and stochastic processes may also play important roles in structuring these communities, and we propose a mechanism to explain overdominance in AM fungal communities. 5. Synthesis. This paper applies ecological models derived from studies on larger organisms to microbial communities. Results from this study suggest that a common log-normal SAD is likely to be observed across both microbial and macro taxa. However, due to the distinctive features of microbial biology, some noticeable differences, such as heavy overdominance, may lead to unique structures in microbial communities. This research not only highlights that, to a first approximation, microbial communities follow similar processes and have similar patterns to those of macroorganisms, but also the need for large-scale microbial data sets, if we are to understand the patterns and processes regulating global biodiversity.

Native and exotic invasive plants have fundamentally similar carbon capture strategies

Abstract: 1. Leaf trait relationships of native and exotic invasive species from a range of habitats were compared to assess consistency across habitats and the role of disturbance. 2. One hundred and twenty-two native and exotic species were sampled in five habitats in eastern Australia. Specific leaf area, foliar nitrogen (Nmass), assimilation rate (Amass) and dark respiration (Rmass) were measured for each species. Plants were classified into four types: native undisturbed, native disturbed, exotic invasive undisturbed and exotic invasive disturbed. 3. All traits were positively correlated and slopes were homogeneous within habitats. Significant differences between plant types in slope elevation were found in only two of 18 cases. There were significant shifts in group means along a common slope between plant types within habitats. These shifts were associated with disturbed vs. undisturbed areas, with plant types from disturbed areas having higher trait values. 4. Synthesis. Exotic invasive and native species do not have fundamentally different carbon capture strategies. The carbon capture strategy of a species is strongly associated with disturbance, with species from disturbed sites having traits that confer capacity for fast growth. Thus, differences between exotic invasives and natives may reflect differences in the environmental conditions of the sites where they occur rather than differences between exotic invasives and natives per se.

A link between plant traits and abundance: evidence from coastal California woody plants.

Abstract: Summary 1. A number of recent studies have demonstrated that plant traits play a crucial role in determining the success or failure of species in a given environment. However, whether traits play a role in determining species’ abundance and rarity among the co-occurring species within a community remains an unresolved question. 2. To address this, we analysed the abundance of California coastal woody plant species at landscape and local scales in relation to 11 leaf, wood and seed traits. 3. At the landscape scale, we found no significant relationship between traits and abundance. In contrast, at the local scale we found significant relationships between abundance and four traits: specific leaf area (SLA), height, lumen fraction and wood density. For SLA and height, the relationship was linear; for lumen fraction, it was quadratic. For wood density, the direction of the trait– abundance relationship was dependant on the abiotic context, that is, it shifted across a gradient in soil water content. 4. Synthesis. Understanding the connections between traits and abundance is important for two reasons. First, there is an ongoing debate about the degree to which commonness and rarity are the result of drift among ecologically equivalent species or niche processes. These results suggest that there are non-random, trait-based processes affecting abundance and rarity. Secondly, species’ traits have been shown to have a strong effect on photosynthesis and decomposition rates, mediated by the abundance of the species. The connections between traits and abundance presented here are crucial for scaling from measurements of species’ traits to ecosystem-level processes.

High Arctic plant community resists 15 years of experimental warming

Abstract: Summary 1 Identifying plant communities that are resistant to climate change will be critical for developing accurate, wide-scale vegetation change predictions Most northern plant communities, especially tundra, have shown strong responses to experimental and observed warming 2 Experimental warming is a key tool for understanding vegetation responses to climate change We used open-top chambers to passively warm an evergreen-shrub heath by 10–13 °C for 15 years at Alexandra Fiord, Nunavut, Canada (79 °N) In 1996, 2000 and 2007, we measured height, plant composition and abundance with a point-intercept method 3 Experimental warming did not strongly affect vascular plant cover, canopy height or species diversity, but it did increase bryophyte cover by 63% and decrease lichen cover by 35% Temporal changes in plant cover were more frequent and of greater magnitude than changes due to experimental warming 4 Synthesis This evergreen-shrub heath continues to exhibit community-level resistance to long-term experimental warming, in contrast to most Arctic plant communities Our findings support the view that only substantial climatic changes will alter unproductive ecosystems

Complex responses to climate drivers in onset of spring flowering across a semi-arid elevation gradient

Abstract: Summary 1. Many studies have documented advancement in spring plant phenology; however, studies in dry climates, where water, rather than temperature, is the limiting factor, are rare. To better understand how plants of a water-limited environment may respond to predicted changes in climate, we used a species-rich 20-year data set collected in a semi-arid ecosystem to determine species’ relationships with precipitation and temperature for seasons coincident with and previous to flowering. Our data were collected across a 1200-m elevation gradient, allowing us to explore the consistency in relationships with climatic variables from desert scrub to pine forest. A second objective was to document evidence of changes in the onset of spring flowering over this 20-year period. 2. Onset of spring flowering for species at the lowest elevations was most commonly driven by temperature and precipitation conditions of the previous autumn. In contrast, onset of spring flowering for species in high-elevation communities was more often associated with spring temperatures, a pattern consistent with communities of higher latitudes. Despite these coarse patterns, species’ relationships to climate variables were highly variable and individualistic. 3. Approximately 10% of species showed a significant trend in changes in first flowering date over the period 1984–2003; most trends were in the direction of later onset. The decrease in autumn precipitation observed over the study period appears to explain the delay in onset observed for many of the species across the elevation gradient. Other species’ delays in spring flowering appear to be related to the slight decrease in spring temperature observed over the study period. 4. Synthesis. The south-western USA is expected to become warmer and drier. Climate relationships documented in this study suggest divergent, individualistic changes in the onset of spring flowering. Low-elevation plants may exhibit delayed spring flowering due to changes in the timing or amount of precipitation or insufficient chilling. High-elevation species may show advancement in spring flowering due to warming temperatures. The highly individualistic responses to climate change may result in significant changes in the diversity, composition and abundance of plants in flower. Variable changes in phenology such as these have major implications for species population dynamics and ecosystem functioning.

Plant survival in relation to seed size along environmental gradients: a long-term study from semi-arid and Mediterranean annual plant communities

Abstract: Summary 1. A positive relationship between seed size and subsequent offspring survival is a key assumption in ecological theory concerning life-history strategies. Yet, this relationship is uncertain in respect to lifetime survival because sound evidence only exists for early seedling stages. Furthermore, the effect of environmental variation in space and time, and of contrasting plant functional groups, on this relationship has been scarcely examined. 2. Here, we investigated survival and between-year variation therein. We tested for both whether their relationship to seed size (i) is maintained up until reproduction, (ii) changes along environmental gradients and (iii) differs between functional groups (grasses, legumes, forbs). 3. Survival was monitored from established seedlings to reproductive plants in 49 annual species under natural conditions during 7 years in three sites along a steep rainfall gradient. We then related average survival per species and between-year variation in survival to seed size, site along the gradient and functional group. 4. Larger seed size was associated with higher survival and lower between-year variation. Across the rainfall gradient, we detected no difference in the seed size–survival relationships; however, variation between years was lowest in the most mesic site where no relationship for between-year variation with seed size was observed. Legumes showed lower survival and higher between-year variation than grasses. 5. Our findings indicate that larger seed size provides survival advantages beyond seedling establishment up until reproduction among annual species. Larger seed size also provides a bet-hedging strategy in temporally unpredictable environments. Increased abiotic favourability along environmental gradients may have little effect on survival rates but reduces survival variation between years and thus reduces the bet-hedging benefit of larger seed size. We suggest that the contrasting response of legumes and grasses may partly result from their disparity in seed dormancy. 6. Synthesis. Current plant life-history theory can be refined by accounting for both benefits of larger seed size, higher survival rates and bet-hedging. Studies along environmental gradients are needed to generalize findings across ecosystems and to predict patterns of plant traits and plant performance under changing environmental conditions.

Changes in pollinator fauna cause spatial variation in pollen limitation

Abstract: Summary 1. Pollen limitation may be a consequence of changes in pollinator abundance, diversity and identity. However, no empirical evidence exists concerning the consequences that the spatial variation in pollinator fauna has on pollen limitation intensity and plant reproduction. In this study, we test the effect that changes in flower-visitor abundance, diversity and identity exert on the occurrence and strength of pollen limitation by experimentally quantifying pollen limitation in eight populations of Erysimum mediohispanicum, a pollination-generalist plant native to the Iberian Peninsula. 2. Pollen limitation was accounted for by using a comprehensive estimator, the net reproductive rate (R0). Nevertheless, we also determined which components of plant reproduction, from ovule fertilization to seedling survival, were more intensely pollen-limited. Finally, we explored whether the spatial variation in pollen limitation intensity was related to among-population changes in flower-visitor abundance, diversity and identity. 3. The whole reproductive cycle of E. mediohispanicum was pollen-limited, although pollen limitation occurred more strongly during the ovule fertilization and seed-production phases than during fruit ripening or seedling emergence and establishment. 4. There was a significant among-population difference in pollen limitation intensity. Pollen limitation strength was associated with variations in flower-visitor diversity, and identity. Populations with lower flower-visitor diversity and with many low-efficiency pollinators (i.e. beetles) showed stronger pollen limitation. 5. Synthesis. Our study shows that the intensity of pollen limitation at the population level may depend on several characteristics of the assemblage of flower-visiting insects, such as their abundance, diversity and identity. Our results suggest that any impoverishment of pollinator diversity or any alteration in the specific composition of the pollinator assemblage may exacerbate pollen limitation.

Does herbivory really suppress mycorrhiza? A meta‐analysis

Abstract: Summary 1. In managed and natural ecosystems, herbivory can have a major impact on plant growth. Defoliation and shoot removal reduce the photosynthetic capacity of plants and are also thought to reduce allocation of carbon to roots and mycorrhizal fungi. However, evidence supporting this assumption remains equivocal. 2. We conducted a meta-analysis of 99 experiments, from 33 publications, measuring arbuscular and ectomycorrhizal colonization of roots after real or simulated herbivory removed leaves or shoots. We also explored how effects were moderated by the type of mycorrhiza, fertilization, experimental setting (laboratory or field), treatment (real herbivory of shoots, real herbivory of leaves, simulated herbivory of shoots, simulated herbivory of leaves), type of host plant, duration of the experiment and year of publication. 3. Overall, herbivory reduced mycorrhizal colonization by about 3 percentage points. Treatment, host plant type and year of publication were the only significant moderators, with real or simulated herbivory of leaves and real herbivory of shoots suppressing colonization, while simulated removal of shoots tended to increase colonization. Herbivory reduced colonization of perennial grasses and deciduous trees by about 4 and 8 percentage points, respectively. Mycorrhizal colonization of annual crops was reduced by about 12 percentage points, although this was not significantly different from zero. Mycorrhizal colonization of perennial forbs and evergreen trees was also unaffected by herbivory, and colonization of mixtures of perennial grasses and forbs increased by about 15 percentage points following herbivory. Effect size increased with year of publication, likely due to shifts in experimental designs towards systems more likely to show positive effects of herbivory on mycorrhiza. 4. Synthesis. Our results challenge the carbon-limitation hypothesis and suggest that herbivory, real or simulated, reduces mycorrhizal colonization by biologically meaningful amounts only in a limited subset of systems.

Tree diversity promotes insect herbivory in subtropical forests of south-east China

Abstract: Summary 1. Insect herbivory can strongly affect ecosystem processes, and its relationship with plant diversity is a central topic in biodiversity–functioning research. However, very little is known about this relationship from complex ecosystems dominated by long-lived individuals, such as forests, especially over gradients of high plant diversity. 2. We analysed insect herbivory on saplings of 10 tree and shrub species across 27 forest stands differing in age and tree species richness in an extraordinarily diverse subtropical forest ecosystem in China. We tested whether plant species richness significantly influences folivory in these highly diverse forests or whether other factors play a more important role at such high levels of phytodiversity. 3. Leaf damage was assessed on 58 297 leaves of 1284 saplings at the end of the rainy season in 2008, together with structural and abiotic stand characteristics. 4. Species-specific mean damage of leaf area ranged from 3% to 16%. Herbivory increased with plant species richness even after accounting for potentially confounding effects of stand characteristics, of which stand age-related aspects most clearly covaried with herbivory. Intraspecific density dependence or other abiotic factors did not significantly influence overall herbivory across forest stands. 5. Synthesis. The positive herbivory–plant diversity relationship indicates that effects related to hypotheses of resource concentration, according to which a reduction in damage by specialized herbivores might be expected as host plant concentration decreases with increasing plant diversity, do not seem to be major determinants for overall herbivory levels in our phytodiverse subtropical forest ecosystem. We discuss the potential role of host specificity of dominant herbivores, which are often expected to show a high degree of specialization in many (sub)tropical forests. In the forest system we studied, a much higher impact of polyphagous species than traditionally assumed might explain the observed patterns, as these species can profit from a broad dietary mix provided by high plant diversity. Further testing is needed to experimentally verify this assumption.

Functional composition controls invasion success in a California serpentine grassland

Abstract: Summary 1. Recent debates about the role of biotic resistance in controlling invasion success have focused on effects of species richness. However, functional composition could be a stronger control: species already in the community with similar functional traits to those of the invaders should have the greatest competitive effect on invaders. Still, experiments assessing effects of functional similarity have found contradictory results. 2. We used experimental communities in a serpentine grassland in California, USA, to assess the extent to which functional composition and functional diversity influenced success of two different types of invading plants: early season annuals (E) and late-season annuals (L) that have been previously shown to differ in patterns of resource acquisition. 3. We seeded known quantities of seed of six different species (three in each functional group) into experimental plots containing established communities differing in functional composition and functional diversity. The experimental communities contained different combinations of E, L, perennial bunchgrass (P) and nitrogen-fixer (N) functional groups, with functional diversity ranging from 0 to 4 groups. Each invading species was seeded into a separate quadrat within each plot to minimize competitive effects of invaders on each other. We measured both seedling and adult success of invaders for two full growing seasons to further understand mechanisms underlying biotic resistance. 4. More functionally diverse communities were less invaded overall, as measured by the average success of individual invaders. However, assessment of invaders by functional groups was more informative: Es in the extant community suppressed E invaders the most, and Ls in the extant community suppressed L invaders the most. 5. We observed a variety of interactions among extant functional groups in reducing invader success, including synergism, complementarity and ‘basement’ effects, where two or more groups negatively affected invaders, but combinations of groups were no more suppressive than single groups. The extant community influenced invaders more strongly through suppression of adult plant growth than through effects on seedling establishment. 6. Synthesis. Contrary to predictions from neutral theory, these results indicate that niche overlap was an important component of biotic resistance in these experimental plant communities and summed up to significant effects of species richness.

Ecophysiological differences between genetic lineages facilitate the invasion of non-native Phragmites australis in North American Atlantic coast wetlands

Abstract: Summary 1. Over the last century, native Phragmites australis lineages have been almost completely replaced along the North American Atlantic coast by an aggressive lineage originating from Eurasia. Understanding the mechanisms that facilitate biological invasions is critical to better understand what makes an invasive species successful. 2. Our objective was to determine what makes the introduced lineage so successful in the study area by specifically investigating if morphological and ecophysiological differences exist between native and introduced genetic lineages of P. australis. We hypothesized a priori that due to phenotypic differences and differences in plant nitrogen (N) content between lineages, the introduced lineage would have a greater photosynthetic potential. 3. In situ ecophysiological and morphological data were collected for 2 years in a mid-Atlantic tidal marsh and in a glasshouse experiment. We measured photosynthetic parameters (Amax, water use efficiency, stomatal conductance) using infrared gas analysis, in conjunction with ecophysiological and morphological parameters [specific leaf area (SLA), leaf area, chlorophyll content, N content]. 4. Introduced P. australis maintained 51% greater rates of photosynthesis and up to 100% greater rates of stomatal conductance which are magnified by its 38–83% greater photosynthetic canopy compared to the native type. The introduced lineage also had a significantly greater SLA and N content. Glasshouse-grown plants and naturally occurring populations demonstrated similar trends in ecophysiological characteristics, verifying the heritability of these differences. These ecophysiological differences, when combined with an extended growing season, provide the mechanism to explain the success of introduced P. australis in North America. 5. Our findings suggest the native type is a low-nutrient specialist, with a more efficient photosynthetic mechanisms and lower N demand, whereas the introduced type requires nearly four times more N than the native type to be an effective competitor. 6. Synthesis. Our study is the first to combine field and laboratory data to explain a biological invasion attributed to ecophysiological differences between genetic lineages. Our data corroborates earlier work suggesting anthropogenic modification of wetland environments has provided the state change necessary for the success of introduced P. australis. Finally, our results suggest that genotypic differences within species merit further investigations, especially when related to biological invasions.