Showing papers in "Journal of Ecology in 2009"

Refining the stress-gradient hypothesis for competition and facilitation in plant communities

Abstract: Summary 1. The stress-gradient hypothesis (SGH) predicts that the frequency of facilitative and competitive interactions will vary inversely across abiotic stress gradients, with facilitation being more common in conditions of high abiotic stress relative to more benign abiotic conditions. With notable exceptions, most tests of the SGH have studied the interaction between a single pair or a few pairs of species, and thus have evaluated shifts in the magnitude and direction of pair-wise interactions along stress gradients, rather than shifts in the general frequency of interactions. 2. The SGH has been supported by numerous studies in many ecosystems, has provided a crucial foundation for studying the interplay between facilitation and competition in plant communities, and has a high heuristic value. However, recent empirical research indicates that factors like the variation among species and the nature of the stress gradient studied add complexity not considered in the SGH, creating an opportunity to extend the SGH’s general conceptual framework. 3. We suggest that one approach for extending the SGH framework is to differentiate between the original idea of how ‘common’ interactions might be along stress gradients and the ubiquitous empirical approach of studying shifts in the strength of pair-wise interactions. Furthermore, by explicitly considering the life history of the interacting species (relative tolerance to stress vs. competitive ability) and the characteristics of the stress factor (resource vs. non-resource) we may be able to greatly refine specific predictions relevant to the SGH. 4. We propose that the general pattern predicted by the SGH would hold more frequently for some combinations of life histories and stress factor, particularly when the benefactor and beneficiary species are mostly competitive and stress-tolerant, respectively. However, we also predict that other combinations are likely to yield different results. For example, the effect of neighbours can be negative at both ends of the stress gradient when both interacting species have similar ‘competitive’ or ‘stress-tolerant’ life histories and the abiotic stress gradient is driven by a resource (e.g. water). 5. Synthesis. The extension of the SGH presented here provides specific and testable hypotheses to foster research and helps to reconcile potential discrepancies among previous studies. It represents an important step in incorporating the complexity and species-specificity of potential outcomes into models and theories addressing how plant‐plant interactions change along stress gradients.

Impact of invasive plants on the species richness, diversity and composition of invaded communities

Abstract: Summary 1. Much attention has been paid to negative effects of alien species on resident communities but studies that quantify community-level effects of a number of invasive plants are scarce. We address this issue by assessing the impact of 13 species invasive in the Czech Republic on a wide range of plant communities. 2. Vegetation in invaded and uninvaded plots with similar site conditions was sampled. All species of vascular plants were recorded, their covers were estimated and used as importance values for calculating the Shannon diversity index H\ evenness J and Sorensen index of similarity between invaded and uninvaded vegetation. 3. With the exception of two invasive species, species richness, diversity and evenness were reduced in invaded plots. Species exhibiting the greatest impact reduced species numbers per plot and the total number of species recorded in the communities sampled by almost 90%. A strong reduction of species number at the plot scale resulted in a marked reduction in the total species number at the landscape scale, and in less similarity between invaded and uninvaded vegetation. The decrease in species richness in invaded compared to uninvaded plots is largely driven by the identity of the invading species, whereas the major determinants of the decrease in Shannon diversity and evenness are the cover and height of invading species, and differences between height and cover of invading and dominant native species, independent of species identity. 4. Synthesis. Management decisions based on impact need to distinguish between invasive species, as their effects on diversity and composition of resident vegetation differ largely. Tall invading species capable of forming populations with the cover markedly greater than that of native dominant species exert the most severe effects on species diversity and evenness. Since a strong impact on the community scale is associated with reduction in species diversity at higher scales, invaders with a high impact represent a serious hazard to the landscape.

Global patterns in plant height

Abstract: Summary 1. Plant height is a central part of plant ecological strategy. It is strongly correlated with life span, seed mass and time to maturity, and is a major determinant of a species’ ability to compete for light. Plant height is also related to critical ecosystem variables such as animal diversity and carbon storage capacity. However, remarkably little is known about global patterns in plant height. Here, we use maximum height data for 7084 plant Species · Site combinations to provide the first global, cross-species quantification of the latitudinal gradient in plant height. 2. The mean maximum height of species growing within 15� of the equator (7.8 m) was 29 times greater than the height of species between 60� and 75� N (27 cm), and 31 times greater than the height of species between 45� and 60� S (25 cm). There was no evidence that the latitudinal gradient in plant height was different in the northern hemisphere than in the southern hemisphere (P = 0.29). A 2.4-fold drop in plant height at the edge of the tropics (P = 0.006) supports the idea that there might be a switch in plant strategy between temperate and tropical zones. 3. We investigated 22 environmental variables to determine which factors underlie the latitudinal gradient in plant height. We found that species with a wide range of height strategies were present in cold, dry, low productivity systems, but there was a noticeable lack of very short species in wetter, warmer, more productive sites. Variables that capture information about growing conditions during the harsh times of the year were relatively poor predictors of height. The best model for global patterns in plant height included only one term: precipitation in the wettest month (R 2 =0 .256). 4. Synthesis. We found a remarkably steep relationship between latitude and height, indicating a major difference in plant strategy between high and low latitude systems. We also provide new, surprising information about the correlations between plant height and environmental variables.

Socialism in soil? The importance of mycorrhizal fungal networks for facilitation in natural ecosystems

Abstract: Summary 1. Almost all plants are engaged in symbiotic relationships with mycorrhizal fungi. These soil fungi can promote plant growth by supplying limiting nutrients to plant roots in return for plant assimilates. 2. Many mycorrhizal fungi are not host specific and one fungal individual can colonize and interconnect a considerable number of plants. The existence of these so-called mycorrhizal networks implies that fungi have the potential to facilitate growth of other plants and distribute resources among plants irrespective of their size, status or identity. In this paper, we explore the significance of mycorrhizal fungal networks for individual plants and for plant communities. 3. We address the following questions: (i) are all plant species benefitting from mycorrhizal networks, (ii) is benefit dependent on the size or age of a plant, (iii) is fungal support related to the relative dominance of plants in a community, (iv) are there host dependent barriers and physiological constraints for support and (v) what is the impact of mycorrhizal networks on plant–plant interactions and plant community dynamics? Moreover, using a review of published studies, we test whether mycorrhizal networks facilitate growth of small seedlings that establish between or near larger plants. 4. We found 60 cases where seedling species were grown together with larger plants with or without mycorrhizal fungal networks. Mycorrhizal networks promoted seedling growth in 48% of the cases (for 21 seedling species), while negative effects (25%) and no effects (27%) were also common. Seedlings associating with ectomycorrhizal fungi benefitted in the majority of the cases while effects on seedlings associating with arbuscular mycorrhizal fungi were more variable. Thus, the facilitative effects of mycorrhizal fungal networks depend on seedling species identity, mycorrhizal identity, plant species combinations and study system. We present a number of hypothetical scenarios that can explain the results based on cost–benefit relationship of individual members in a network. 5. Synthesis. Overall, this review shows that mycorrhizal networks play a key role in plant communities by facilitating and influencing seedling establishment, by altering plant–plant interactions and by supplying and recycling nutrients.

Plant invasions and the niche

Abstract: Summary 1. For plant invaders, being different is often equated with being successful, yet the mechanistic connection remains unclear. 2. Classic niche theory predicts that invaders with niches distinct from the native flora should coexist with little interaction with native species, yet such invaders often have substantial impacts. Meanwhile, invaders that overlap in niche space with native species should either be repelled or dominate, yet these invaders often naturalize with little effect. Such discrepancies between theory and observation raise questions about how species differences influence invader establishment and impact. 3. Here, we review these issues in light of recent work on coexistence theory, which shows how niche and fitness differences between natives and invaders interact to determine invasion outcomes. We show how successful invader establishment depends on either a fitness advantage or niche difference from resident species, but that only the former allows invaders to become dominant. 4. By identifying the role of niche and fitness differences in leading invasion hypotheses, we unify their predictions for invasion success while highlighting new approaches for evaluating the importance of species differences for invasion. 5. Synthesis. Situating the invasion process within a recent coexistence framework broadens our understanding of invasion mechanisms and more tightly links problems in invasion ecology with our more general understanding of community dynamics.

Seedling root morphology and biomass allocation of 62 tropical tree species in relation to drought- and shade-tolerance

Abstract: 1. Water availability is the main determinant of species' distribution in lowland tropical forests. Species' occurrence along water availability gradients depends on their ability to tolerate drought. 2. To identify species' traits underlying drought-tolerance we excavated first year seedlings of 62 dry and moist forest tree species at the onset of the dry season. We evaluate how morphological seedling traits differ between forests, and whether functional groups of species can be identified based on trait relations. We also compare seedling traits along independent axes of drought and shade-tolerance to assess a hypothesized trade-off. 3. Seedlings of dry forest species improve water foraging capacity in deep soil layers by an increased below-ground biomass allocation and by having deep roots. They minimize the risk of cavitation by making dense stems, and reduce transpiration by producing less leaf tissue. Moist forest seedlings have large leaf areas and a greater above-ground biomass, to maximize light interception, and long, cheap, branched root systems, to increase water and nutrient capture. 4. Associations among seedling traits reveal three major drought strategies: (i) evergreen drought-tolerant species have high biomass investment in enduring organs, minimize cavitation and minimize transpiration to persist under dry conditions; (ii) drought-avoiding species maximize resource capture during a limited growing season and then avoid stress with a deciduous leaf habit in the dry season; (iii) drought-intolerant species maximize both below- and above-ground resource capture to increase competitiveness for light, but are consequently precluded from dry habitats. 5. We found no direct trade-off between drought- and shade-tolerance, because they depend largely on different morphological adaptations. Drought-tolerance is supported by a high biomass investment to the root system, whereas shade-tolerance is mainly promoted by a low growth rate and low SLA. 6. Synthesis. We conclude that there are three general adaptation strategies of drought-tolerance, which seemingly hold true across biomes and for different life forms. Drought- and shade-tolerance are largely independent from one another, suggesting a high potential for niche differentiation, as species' specialization can occur at different combinations of water and light availability.

Ecological information from spatial patterns of plants: insights from point process theory

Abstract: Summary 1. This article reviews the application of some summary statistics from current theory of spatial point processes for extracting information from spatial patterns of plants. Theoretical measures and issues connected with their estimation are described. Results are illustrated in the context of specific ecological questions about spatial patterns of trees in two forests. 2. The pair correlation function, related to Ripley’s K function, provides a formal measure of the density of neighbouring plants and makes precise the general notion of a ‘plant’s-eye’ view of a community. The pair correlation function can also be used to describe spatial relationships of neighbouring plants with different qualitative properties, such as species identity and size class. 3. The mark correlation function can be used to describe the spatial relationships of quantitative measures (e.g. biomass). We discuss two types of correlation function for quantitative marks. Applying these functions to the distribution of biomass in a temperate forest, it is shown that the spatial pattern of biomass is uncoupled from the spatial pattern of plant locations. 4. The inhomogeneous pair correlation function enables first-order heterogeneity in the environment to be removed from second-order spatial statistics. We illustrate this for a tree species in a forest of high topographic heterogeneity and show that spatial aggregation remains after allowing for spatial variation in density. An alternative method, the master function, takes a weighted average of homogeneous pair correlation functions computed in subareas; when applied to the same data and compared with the former method, the spatial aggregations are smaller in size. 5. Synthesis. These spatial statistics, especially those derived from pair densities, will help ecologists to extract important ecological information from intricate spatially correlated plants in populations and communities.

The role of plant interactions in the restoration of degraded ecosystems: a meta‐analysis across life‐forms and ecosystems

Abstract: Summary 1. Traditionally, techniques of plant manipulation during restoration have focused on the reduction of competition by ‘problematic’ existing vegetation. However, the increasing recognition of facilitation as a main process regulating the composition of communities has brought a change in the practice of restoration towards a better awareness of the benefits inherent to conserving neighbouring vegetation. 2. Here, I provide the results of a meta-analysis of published studies that have manipulated interactions among plants with the objective of restoring degraded terrestrial systems. I created four different data sets corresponding to the variables most commonly used to measure plant performance (i.e. emergence, survival, growth and density), and asked whether the benefits of facilitation as a restoration tool vary depending on the study duration, the life-form of the neighbour and target species, and the ecosystem type. 3. Neighbour effects varied strongly among performance estimators. Positive effects were frequently found for emergence and survival, whereas neutral or negative interactions predominated for growth and density. 4. No clear support existed for a relationship between study duration and neighbour effect. 5. The life-form of the interacting species, particularly of neighbours, largely influenced the interaction outcome. Herbs had strong negative effects, especially on other herb species, whereas shrubs had large facilitative effects, especially on trees. 6. Semiarid and tropical systems showed in general more positive neighbour effects than wetlands and particularly mesic temperate systems, where negative interactions predominated. However, these results were largely influenced by the over-representation of herb species in wetlands and temperate habitats, survival facilitation being found in all systems when only woody species were considered. 7. Synthesis. Pre-existing vegetation can have large impacts on species establishment in degraded habitats. Inhibition predominates in herbaceous communities typical of early-successional stages, whereas facilitation prevailes in communities dominated by shrubs and trees. Even productive systems (e.g. mesic temperate habitats) appear suitable for the application of facilitation as a restoration tool of woody communities. Whereas restoring herbaceous communities seems largely reliable on removal techniques, augmenting populations of nurse shrubs and trees should be considered a promising strategy for restoring woody late-successional communities.

A conceptual framework for predicting the effects of urban environments on floras

Abstract: 1. With the majority of people now living in urban environments, urbanization is arguably the most intensive and irreversible ecosystem change on the planet. 2. Urbanization transforms floras through a series of filters that change: (i) habitat availability; (ii) the spatial arrangement of habitats; (iii) the pool of plant species; and (iv) evolutionary selection pressures on populations persisting in the urban environment. 3. Using a framework based on mechanisms of change leads to specific predictions of floristic change in urban environments. Explicitly linking drivers of floristic change to predicted outcomes in urban areas can facilitate sustainable management of urban vegetation as well as the conservation of biodiversity. 4. Synthesis. We outline how the use of our proposed framework, based on environmental filtering, can be used to predict responses of floras to urbanization. These floristic responses can be assessed using metrics of taxonomic composition, phylogenetic relatedness among species, plant trait distributions or plant community structure. We outline how this framework can be applied to studies that compare floras within cities or among cities to better understand the various floristic responses to urbanization.

Decomposition in tropical forests: a pan‐tropical study of the effects of litter type, litter placement and mesofaunal exclusion across a precipitation gradient

Abstract: Summary 1. Litter decomposition recycles nutrients and causes large fluxes of carbon dioxide into the atmosphere. It is typically assumed that climate, litter quality and decomposer communities determine litter decay rates, yet few comparative studies have examined their relative contributions in tropical forests. 2. We used a short-term litterbag experiment to quantify the effects of litter quality, placement and mesofaunal exclusion on decomposition in 23 tropical forests in 14 countries. Annual precipitation varied among sites (760‐5797 mm). At each site, two standard substrates ( Raphia farinifera and Laurus nobilis ) were decomposed in fine- and coarse-mesh litterbags both above and below ground for approximately 1 year.

The changing global carbon cycle: Linking plant-soil carbon dynamics to global consequences

Abstract: Summary 1. Most current climate–carbon cycle models that include the terrestrial carbon (C) cycle are based on a model developed 40 years ago by Woodwell & Whittaker (1968) and omit advances in biogeochemical understanding since that time. Their model treats net C emissions from ecosystems as the balance between net primary production (NPP) and heterotrophic respiration (HR, i.e. primarily decomposition). 2. Under conditions near steady state, geographic patterns of decomposition closely match those of NPP, and net C emissions are adequately described as a simple balance of NPP and HR (the Woodwell-Whittaker model). This close coupling between NPP and HR occurs largely because of tight coupling between C and N (nitrogen) cycles and because NPP constrains the food available to heterotrophs. 3. Processes in addition to NPP and HR become important to understanding net C emissions from ecosystems under conditions of rapid changes in climate, hydrology, atmospheric CO2, land cover, species composition and/or N deposition. Inclusion of these processes in climate–C cycle models would improve their capacity to simulate recent and future climatic change. 4. Processes that appear critical to soil C dynamics but warrant further research before incorporation into ecosystem models include below-ground C flux and its partitioning among roots, mycorrhizas and exudates; microbial community effects on C sequestration; and the effects of temperature and labile C on decomposition. The controls over and consequences of these processes are still unclear at the ecosystem scale. 5. Carbon fluxes in addition to NPP and HR exert strong influences over the climate system under conditions of rapid change. These fluxes include methane release, wildfire, and lateral transfers of food and fibre among ecosystems. 6. Water and energy exchanges are important complements to C cycle feedbacks to the climate system, particularly under non-steady-state conditions. An integrated understanding of multiple ecosystem–climate feedbacks provides a strong foundation for policies to mitigate climate change. 7. Synthesis. Current climate systems models that include only NPP and HR are inadequate under conditions of rapid change. Many of the recent advances in biogeochemical understanding are sufficiently mature to substantially improve representation of ecosystem C dynamics in these models.

Plant and fungal identity determines pathogen protection of plant roots by arbuscular mycorrhizas

Abstract: This is the peer reviewed version of the following article: BA Sikes, K Cottenie and JN Klironomos. (2009) Plant and fungal identity determines pathogen protection of plant roots by arbuscular mycorrhizas. Journal of Ecology 97: 1274-1280. http://dx.doi.org/10.1111/j.1365-2745.2009.01557.x, which has been published in final form at http://doi.org/10.1111/j.1365-2745.2009.01557.x This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.

The evolution of facilitation and mutualism

Abstract: Summary 1 While the relationship between facilitation and competition has been explored extensively in recent years, there is also a natural link between facilitation and mutualism, as both are interspecific interactions that confer benefits Yet, the relationship between these two interactions has been minimally explored 2 Here, I explore parallels and differences between mutualism and facilitation Five focal areas organize current research on mutualism evolution: trait evolution; the continuum from specialization to generalization; the evolutionary origins and maintenance of the interaction; co-evolution of partners; and the prevalence and implications of cheating These foci are also helpful for investigating how facilitation evolves, a much less explored issue 3 Testable hypotheses regarding the evolution of facilitation include the following: selection should be stronger on traits of facilitated species than on traits of facilitators; facilitative interactions with mutualistic (++) and commensal (+0) outcomes should exhibit greater evolutionary stability than those with antagonistic (+−) outcomes; co-evolution should be possible in mutualistic and antagonistic facilitation only; when co-evolution occurs, it should produce a geographic mosaic of interaction outcomes; and antagonistic facilitation could lead to selection on facilitators to either escape or to tolerate the neighbours that benefit from them 4Synthesis Three gaps in our knowledge currently impede progress on evolutionary questions surrounding facilitation First, reciprocal effects are rarely investigated; facilitation might evolve like mutualism, commensalism or antagonism, depending on effects on the facilitator species Secondly, the genetics of relevant traits are not yet well explored; the traits themselves are better known for facilitator species than for the facilitated, which are more likely to evolve in the context of the interaction Finally, the fitness costs and benefits associated with facilitation have rarely been measured Filling these gaps should permit rapid progress in understanding how facilitation arises, persists and evolves

Winter warming events damage sub-Arctic vegetation: consistent evidence from an experimental manipulation and a natural event

Abstract: Summary 1. The Arctic is experiencing considerable change in climate, particularly in winter, and a greater frequency of extreme climatic events is expected. However, the impacts of winter climate change and extreme events have received far less attention than the impacts of season-long summer warming. Here we report findings from observations following a natural event and from experimental studies to show that short (<10 days) extreme winter warming events can cause major damage to sub-Arctic plant communities at landscape scales. 2. In the landscape observations, impacts were assessed following an extreme winter warming event that occurred in December 2007 in northern Scandinavia. During this event, temperatures rose up to 7 °C resulting in loss of snow cover and exposure of vegetation to firstly warm and then returning cold temperatures. 3. In the following summer, extensive areas of damaged dwarf shrub vegetation could be observed. Ground observations showed damaged areas to have a 16 times greater frequency of dead shoots of the dominant shrub Empetrum hermaphroditum, resulting in 87% less summer growth compared to neighbouring undamaged areas. The landscape scale extent of this damage was confirmed by satellite-derived Normalized Differential Vegetation Index values that showed a considerable 26% reduction (comparing July 2007 with July 2008 values) over an area of 1424 km2. This reduction indicates a significant decline in either leaf area or photosynthetic capacity or efficiency at the landscape scale. 4. Strikingly similar damage was also observed in a field manipulation experiment using heating lamps and soil warming cables to simulate such extreme events in sub-Arctic heathland over two winters. Here, an up to 21 times greater frequency of dead shoots and 47% less shoot growth of E. hermaphroditum was observed in plots exposed to simulated winter warming events compared to unmanipulated controls. 5. Synthesis. These combined landscape observations and experimental findings provide compelling evidence that winter warming events can cause considerable damage to sub-Arctic vegetation. With increasing winter temperatures predicted, any increase in such damage may have major consequences for productivity and diversity of these sub-Arctic ecosystems, in contrast to the greening of parts of the Arctic currently attributed to summer warming.

The allometry of reproduction within plant populations

Abstract: Summary 1 The quantitative relationship between size and reproductive output is a central aspect of a plant’s strategy: the conversion of growth into fitness As plant allocation is allometric in the broad sense, ie it changes with size, we take an allometric perspective and review existing data on the relationship between individual vegetative (V, x-axis) and reproductive (R, y-axis) biomass within plant populations, rather than analysing biomass ratios such as reproductive effort (R ⁄ (R+V)) 2 The allometric relationship between R and V among individuals within a population is most informative when cumulative at senescence (total R–V relationship), as this represents the potential reproductive output of individuals given their biomass Earlier measurements may be misleading if plants are at different developmental stages and therefore have not achieved the full reproductive output their size permits Much of the data that have been considered evidence for plasticity in reproductive allometry are actually evidence for plasticity in the rate of growth and development 3 Although a positive x-intercept implies a minimum size for reproducing, a plant can have a threshold size for reproducing without having a positive x-intercept 4 Most of the available data are for annual and monocarpic species whereas allometric data on long-lived iteroparous plants are scarce We find three common total R–V patterns: short-lived, herbaceous plants and clonal plants usually show a simple, linear relationship, either (i) passing through the origin or (ii) with a positive x-intercept, whereas larger and longer-lived plants often exhibit (iii) classical log–log allometric relationships with slope <1 While the determinants of plant size are numerous and interact with one another, the potential reproductive output of an individual is primarily determined by its size and allometric programme, although this potential is not always achieved 5 Synthesis The total R–V relationship for a genotype appears to be a relatively fixed-boundary condition Below this boundary, a plant can increase its reproductive output by: (i) moving towards the boundary: allocating more of its resources to reproduction, or (ii) growing more to increase its potential reproductive output At the boundary, the plant cannot increase its reproductive output without growing more first Analysing size-dependent reproduction is the first step in understanding plant reproductive allocation, but more integrative models must include time and environmental cues, ie development

Local adaptation occurs along altitudinal gradient despite the existence of gene flow in the alpine plant species Festuca eskia

Abstract: Summary 1 Alpine plant species are particularly vulnerable to climate change. Therefore, estimating the adaptive potential of alpine species is of vital importance for determining their future viability. In alpine plants, adaptive potential depends on (i) altitudinal genetic differentiation among populations, combined with gene flow along an altitudinal gradient; (ii) phenotypic plasticity for the traits under selection and (iii) co-gradient variation between genetic and environmental influences on these traits. 2 The adaptive potential of Festuca eskia Ramond (Poaceae), a perennial alpine grass common in the Pyrenean Mountains, was examined in this study. A reciprocal transplant experiment involving 180 individuals along three altitudinal gradients (from 1500 to 2500 m) was established, and survival, functional and reproductive traits were recorded. In addition, four neutral sequence-tagged site and simple sequence repeat molecular markers were chosen to estimate gene flow among populations. 3 Genetic differentiation attributable to selection was detected in all traits between populations along the altitudinal gradient despite the existence of restricted gene flow. For traits directly related to fitness, local altitudinal adaptation was clearly evident. The patterns of local adaptation suggested that selection patterns differed along an altitudinal gradient. Selection for reproductive output was predominant at low altitudes, whereas differential survivorship was observed at higher altitudes. 4 Genetic differentiation with increasing altitude resulted in reduced plant stature and reproductive output but increased specific leaf area (SLA). This increased SLA at higher altitude is interpreted as a resource acquisition strategy. 5 Phenotypic plasticity was seen in all traits at the population level. Evidence of co-gradient variation between genetic differentiation and plastic response was found for all traits except SLA, suggesting that adaptive phenotypic plasticity operates in F. eskia. 6 Synthesis. Local adaptation occurs in F. eskia. It involves different adaptive traits according to the altitude. Such differentiation occurs at a small scale along altitudinal gradients despite the existence of gene flow and phenotypic plasticity. The coexistence of genetic differentiation, gene flow and phenotypic plasticity along altitudinal gradients provides an adaptive potential for F. eskia to successfully adapt to climate change.

Post-Little Ice Age tree line rise and climate warming in the Swedish Scandes: a landscape ecological perspective

Abstract: Elevational tree line change in the southern Swedish Scandes was quantified for the period 1915-2007 and for two sub-periods 1915-1975 and 1975-2007. The study focused on Betula pubescens ssp. czer ...

Do facilitative interactions increase species richness at the entire community level

Abstract: Summary 1. Although the consequences of facilitation at individual and population levels are well known, the community-level consequences of these processes have received much less attention. In particular, the importance of facilitation in determining richness at the entire community level has seldom been evaluated. 2. In this study, we sampled 11 alpine plant communities along the southern Andes in South America, spanning from tropical (25� S) to sub-antarctic latitudes (55� S). Plant communities were dominated by cushion plants, a particular growth form that acts as a nurse plant for other plant species. Through rarefaction curves, we assessed the effectiveness of community sampling and estimated the number of species present within and outside cushions. Non-metric Multidimensional Scaling ordinations (NMDS) were used to assess differences between the species assemblages growing within and outside cushions. Finally, samples from cushions and open areas were combined in a single matrix accounting for the difference in cover between both microhabitats, and through rarefaction curves we assessed how many more species are added to the community due to the presence of cushions. 3. Samples taken within cushions always contained more species than equivalent samples from open areas. However, the magnitude of this difference varied among communities. NMDS ordination indicated that cushions generate species assemblages structurally different from those found in open areas. Inclusion of samples from cushion and open areas in synthetic analyses – where differences in cover were accounted for – indicated that the presence of cushions consistently increased species richness at the entire community level. The magnitude of these increases in species richness varied with habitat severity, with lower values at both extremes of the environmental severity gradient. 4. Synthesis. Facilitative interactions with cushion nurse plants along the high Andes of southern South America changed plant assemblage structure and increased species richness at the entire community level, indicating that facilitative interactions are pivotal in maintaining the diversity of these harsh environments.

Integrating plant-soil interactions into global carbon cycle models

Abstract: 1. Plant–soil interactions play a central role in the biogeochemical carbon (C), nitrogen (N) and hydrological cycles. In the context of global environmental change, they are important both in modulating the impact of climate change and in regulating the feedback of greenhouse gas emissions (CO2, CH4 and N2O) to the climate system. 2. Dynamic global vegetation models (DGVMs) represent the most advanced tools available to predict the impacts of global change on terrestrial ecosystem functions and to examine their feedbacks to climate change. The accurate representation of plant–soil interactions in these models is crucial to improving predictions of the effects of climate change on a global scale. 3. In this paper, we describe the general structure of DGVMs that use plant functional types (PFTs) classifications as a means to integrate plant–soil interactions and illustrate how models have been developed to improve the simulation of: (a) soil carbon dynamics, (b) nitrogen cycling, (c) drought impacts and (d) vegetation dynamics. For each of these, we discuss some recent advances and identify knowledge gaps. 4. We identify three ongoing challenges, requiring collaboration between the global modelling community and process ecologists. First, the need for a critical evaluation of the representation of plant–soil processes in global models; second, the need to supply and integrate knowledge into global models; third, the testing of global model simulations against large-scale multifactor experiments and data from observatory gradients. 5. Synthesis. This paper reviews how plant–soil interactions are represented in DGVMs that use PFTs and illustrates some model developments. We also identify areas of ecological understanding and experimentation needed to reduce uncertainty in future carbon coupled climate change predictions.

An experimental comparison of chemical traits and litter decomposition rates in a diverse range of subarctic bryophyte, lichen and vascular plant species

Abstract: 1. Climate change in the subarctic is expected to influence vegetation composition, specifically bryophyte and lichen communities, thereby modifying litter decomposition rates and carbon (C) dynami ...

Root exudate is allelopathic in invaded community but not in native community: field evidence for the novel weapons hypothesis

Abstract: Summary 1. Exotic invasive plants can have strong effects on native communities. Centaurea maculosa , a forb that is native to Eurasia, has created near-monocultures in many parts of its invaded range in western North America and produces the root exudate (±)-catechin. Controlled laboratory experiments suggest that the phytotoxic effects of (±)-catechin may be stronger on some North American species than on some European species. 2. We conducted experiments in the field in two different years in the native (Romania) and invaded (MT, USA) ranges of C. maculosa , testing the effects of (±)-catechin on species that co-occur with C. maculosa in both ranges. 3. (±)-Catechin reduced the growth of native plant species in Montana in both years, although there was some variability between species in the effect of (±)-catechin on leaf growth in 2005. There was no effect of (±)-catechin on plants in Romania. 4. This first in situ test of the novel weapons hypothesis supports the notion that novel biochemical constituents of some invasive species may contribute to their success. 5. Synthesis . In addition to providing information useful for understanding invasions, our results indicate that some species in the native range of C. maculosa may be adapted to its particular biochemical traits, raising the possibility that interactions among plant species may be affected by a common evolutionary history.

Grazing triggers soil carbon loss by altering plant roots and their control on soil microbial community

Abstract: Summary 1. Depending on grazing intensity, grasslands tend towards two contrasting systems that differ in terms of species diversity and soil carbon (C) storage. To date, effects of grazing on C cycling have mainly been studied in grasslands subject to constant grazing regimes, whereas little is known for grasslands experiencing a change in grazing intensity. Analysing the transition between C-storing and C-releasing grasslands under low- and high-grazing regimes, respectively, will help to identify key plant-soil interactions for C cycling. 2. The transition was studied in a mesocosm experiment with grassland monoliths submitted to a change in grazing after 14 years of constant high and low grazing. Plant-soil interactions were anal ysed by following the dynamics of plant and microbial communities, roots and soil organic matter fractions over 2 years. After disturbance change, mesocosms were continuously exposed to 13C-labelled C02, which allowed us to trace both the incorporation of new litter C produced by a modified plant community in soil and the fate of old unlabelled litter C. 3. Changing disturbance intensity led to a cascade of events. After shift to high disturbance, photo synthesis decreased followed by a decline in root biomass and a change in plant community struc ture 1.5 months later. Those changes led to a decrease of soil fungi, a proliferation of Gram(+) bacteria and accelerated decomposition of old particulate organic C (< 6 months). At last, acceler ated decomposition released plant available nitrogen and decreased soil C storage. Our results indi cate that intensified grazing triggers proliferation of Gram(+) bacteria and subsequent faster decomposition by reducing roots adapted to low disturbance. 4. Synthesis. Plant communities exert control on microbial communities and decomposition

Soil biota accelerate decomposition in high‐elevation forests by specializing in the breakdown of litter produced by the plant species above them

Abstract: Summary 1 There is mounting evidence that leaf litter typically decomposes more rapidly beneath the plant species it derived from than beneath the different plant species, which has been called home-field advantage (HFA). It has been suggested that this HFA results from the local adaptation of soil communities to decompose the litter that they encounter most often, which probably comes from the plant species above them. 2. To test this hypothesis and to investigate how HFA varies over time and in relation to litter quality, we performed the first detailed assessment of HFA in relation to litter decomposition. We monitored decomposition over time in two reciprocal litter transplant experiments involving three high-elevation tree species that differ in litter quality. The three tree species used were trembling aspen (Populus tremuloides), lodgepole pine (Pinus contorta) and Engelmann spruce (Picea engelmannii). 3. First, we incubated litter from each of these species with soil biota extracted from stands of each tree species in a laboratory experiment and observed greater cumulative respiration, a measure of decomposition, when litter was incubated with its home soil biota. Second, we performed a field experiment, which demonstrated that the decomposition HFA also occurred under field conditions. In addition, this experiment demonstrated that despite increased mass loss at home, litter also immobilized more nitrogen when in its home environment. In both experiments, the HFA was most pronounced for pine litter, which is consistent with the hypothesis that HFA increases with decreasing litter quality. 4.Synthesis. As well as demonstrating conclusively that soil communities specialize in decomposing the litter produced by the plant species above them, our data challenge the widely held view that soil organisms are largely functionally redundant.

Frayed at the edges: selective pressure and adaptive response to abiotic stressors are mismatched in low diversity edge populations

Abstract: Summary 1. Theory predicts that population structure and dynamics affect a population’s capacity for adaptation to environmental change. For isolated, small and fragmented populations at the trailing edge of species distributions, loss of genetic diversity through random genetic drift may reduce adaptive potential and fitness levels for complex traits. This has important consequences for understanding population responses to, for example changing climate, but has rarely been tested in natural populations. 2. We measured the intertidal thermal environment and tidal exposure (emersion) times for natural populations of the intertidal seaweed Fucus serratus at the centre (southwest UK) and southern edge (northern Portugal) of its range in the Eastern Atlantic, and for a congener, F. vesiculosus , whose range extends further south to Morocco. Fitness-related traits of individuals at each location were measured in common garden experiments: physiological resilience to desiccation and heat shock (PSII quantum yield), and the molecular phenotype of the heat shock response (quantitative PCR of heat shock protein gene transcripts). 3. The realized thermal environment experienced by F. serratus was similar at the centre and southern edge of its distribution because the maximum shore height (and emersion period) was reduced in southern populations. For F. vesiculosus , thermal maxima were higher and occurred more frequently in the south, although maximum vertical height (emersion time) remained similar to central populations. 4. Edge populations of F. serratus were less resilient to desiccation and heat shock than central populations, and expression of heat shock genes was higher at the same temperature, suggesting greater cellular stress. In contrast, there was no evidence for physiological divergence in heat shock response in F. vesiculosus , and little variation in gene expression. 5. Synthesis. We provide evidence that compared with range-centre populations upper intertidal limits of F. serratus at the southern edge are ‘pruned back’ by abiotic stressors. Rather than being locally adapted, these small populations are less resilient to abiotic stresses and experience greater cellular stress during heat shock. These results suggest that ongoing climate forcing factors may threaten small, fragmented rear edge populations because of inherently reduced fitness and lower adaptive capacity relative to larger central populations.

Vegetation composition promotes carbon and nitrogen storage in model grassland communities of contrasting soil fertility

Abstract: 1. The benefits of plant functional group and plant species diversity for sustaining primary productivity have been extensively studied. However, few studies have simultaneously explored potential benefits of plant species and functional group richness and composition for the delivery of other ecosystem services and their dependency on resource availability. 2. Here, we investigated in soils of different fertility the effects of plant species and functional group richness and composition on carbon (C) and nitrogen (N) stocks in vegetation, soil and soil microbes and on CO2 exchange and the loss of C and N from soil through leaching. We established plant communities from a pool of six mesotrophic grassland species belonging to one of three functional groups (C3 grasses, forbs and legumes) in two soils of contrasting fertility. We varied species richness using one, two, three or six species and one, two or three functional groups. 3. After 2 years, vegetation C and N and soil microbial biomass were greater in the more fertile soil and increased significantly with greater numbers of plant species and functional group richness. The positive effect of plant diversity on vegetation C and N coincided with reduced loss of water and N through leaching, which was especially governed by forbs, and increased rates of net ecosystem CO2 exchange. 4. Soil C and N pools were not affected by the number of plant species or functional group richness per se after 2 years, but were enhanced by the presence and biomass of the legumes Lotus corniculatus and Trifolium repens. 5. Synthesis. Collectively, our findings indicate that changes in plant species and functional group richness influence the storage and loss of both C and N in model grassland communities but that these responses are related to the presence and biomass of certain plant species, notably N fixers and forbs. Our results therefore suggest that the co-occurrence of species from specific functional groups is crucial for the maintenance of multifunctionality with respect to C and N storage in grasslands

Linkages between plant functional composition, fine root processes and potential soil N mineralization rates

Abstract: Summary 1. Plant functional composition may indirectly affect fine root processes both qualitatively (e.g. by influencing root chemistry) and quantitatively (e.g. by influencing root biomass and thus soil carbon (C) inputs and the soil environment). Despite the potential implications for ecosystem nitrogen (N) cycling, few studies have addressed the linkages between plant functional composition, root decay, root detritus N dynamics and soil N mineralization rates. 2. Here, using data from a large grassland biodiversity experiment, we first show that plant functional composition affected fine root mass loss, root detritus N dynamics and net soil N mineralization rates through its effects on root chemistry rather than on the environment of decomposition. In particular, the presence of legumes and non-leguminous forbs contributed to greater fine root decomposition which in turn enhanced root N release and net soil N mineralization rates compared with C3 and C4 grasses. 3. Second, we show that all fine roots released N immediately during decomposition and showed very little N immobilization regardless of plant composition. As a consequence, there was no evidence of increased root or soil N immobilization rates with increased below-ground plant biomass (i.e. increased soil C inputs) even though root biomass negatively affected root decay. 4. Our results suggest that fine roots represent an active soil N pool that may sustain plant uptake while other soil N forms are being immobilized in microbial biomass and/or sequestered into soil organic matter. However, fine roots may also represent a source of recalcitrant plant detritus that is returned to the soil (i.e. fine roots of C4 and C3 grasses) and that can contribute to an increase in the soil organic matter pool. 5. Synthesis . An important implication of our study is that the simultaneous presence of different plant functional groups (in plant mixtures) with opposite effects on root mass loss, root N release and soil N mineralization rates may be crucial for sustaining multiple ecosystem services such as productivity and soil C and N sequestration in many N-limited grassland systems.

Vegetation response to a short interval between high-severity wildfires in a mixed-evergreen forest

Abstract: Summary 1. Variations in disturbance regime strongly influence ecosystem structure and function. A prominent form of such variation is when multiple high-severity wildfires occur in rapid succession (i.e. short-interval (SI) severe fires, or ‘re-burns’). These events have been proposed as key mechanisms altering successional rates and pathways. 2. We utilized a natural experiment afforded by two overlapping wildfires occurring within a 15year interval in forests of the Klamath‐Siskiyou Mountains, Oregon (USA). We tested for unique effects of a SI fire (15-year interval before 2002 fire) by comparing vegetation communities 2 years post-fire to those following a long-interval (LI) fire (> 100-year interval before 2002 fire) and in mature/old-growth (M/OG) stands (no high-severity fire in > 100-year). 3. Nearly all species found in M/OG stands were present at similar relative abundance in both the LI and SI burns, indicating high community persistence through multiple high-severity fires. However, the SI burn had the highest species richness and total plant cover with additions of disturbance-associated forbs and low shrubs, likely due to a propagule bank of early seral species that developed between fires. Persistence of flora was driven by vegetative sprouting, on-site seed banks, and dispersal from off-site seed sources. Several broadly generalizable plant functional traits (e.g. rapid maturation, long-lived seed banks) were strongly associated with the SI burn. 4. Sprouting capacity of hardwoods and shrubs was unaltered by recurrent fire, but hardwood/ shrub biomass was lower in the SI burn because individuals were smaller before the second fire. Conifer regeneration densities were high in both the SI and LI burns (range = 298‐6086 and 406‐2349 trees ha − 1 , respectively), reflecting similar availability of seed source and germination substrates. 5. Synthesis. SI severe fires are typically expected to be deleterious to forest flora and development; however, these results indicate that in systems characterized by highly variable natural disturbances (e.g. mixed-severity fire regime), native biota possess functional traits lending resilience to recurrent severe fire. Compound disturbance resulted in a distinct early seral assemblage (i.e. intervaldependent fire effects), thus contributing to the landscape heterogeneity inherent to mixed-severity fire regimes. Process-oriented ecosystem management incorporating variable natural disturbances, including ‘extreme’ events such as SI severe fires, would likely perpetuate a diversity of habitats and successional pathways on the landscape.

Positive biodiversity–productivity relationship due to increased plant density

Abstract: 1. Positive effects of biodiversity on plant productivity may result from diversity-induced changes in the size or density of individual plants, yet these two possibilities have never been tested at the same time in a biodiversity experiment with a large species pool. Here, we distinguish between size effects and density effects on plant productivity, using data from 198 experimental grassland communities that contained 1–16 species. Plant modules such as tillers or rosettes were defined as relevant units, being equivalent to plant individuals in the majority of species. 2. In agreement with previous studies, we found positive effects of species richness on above-ground productivity. We show that this positive biodiversity effect resulted from diversity-induced increases in module density rather than from increases in module size. In contrast, variation in productivity within diversity levels was related to module size rather than module density. 3. The size–density relationships varied among plant functional groups and among species but their average response to increasing species richness paralleled the pattern observed at the level of the entire plant communities: species richness had a positive effect on above-ground species biomass and species module density but not on species module size. Twenty-four out of 26 overyielding species had denser populations and 25 out of 28 underyielding species had smaller modules in mixtures than in monocultures. 4. Synthesis: In grasslands, an increase in community productivity must involve an increase in plant size or density. We found that diversity-induced increases in productivity were related to diversity-induced increases in density, whereas diversity-independent increases in productivity were related to increases in plant size. Our results suggest that increased density of overyielding species in mixtures was the main driver of the positive biodiversity–productivity relationship in our experiment. We conclude that the mechanisms leading to enhanced productivity of species-rich as compared with species-poor communities cannot be derived from mechanisms explaining high productivity within communities that contain a particular number of species.

Abiotic and biotic drivers of seedling survival in a hurricane-impacted tropical forest.

Abstract: Summary 1. Many forests experience periodic, large-scale disturbances, such as hurricanes and cyclones, which open the forest canopy, causing dramatic changes in understorey light conditions and seedling densities. Thus, in hurricane-impacted forests, large variations in abiotic and biotic conditions likely shape seedling dynamics, which in turn will contribute to patterns of forest recovery. 2. We monitored 13 836 seedlings of 82 tree and shrub species over 10 years following Hurricane Georges in 1998 in a subtropical, montane forest in Puerto Rico. We quantified changes in the biotic and abiotic environment of the understorey and linked seedling dynamics to changes in canopy openness and seedling density, and to spatial variation in soil type, topography and tree density. 3. Canopy openness was highest when first measured after Hurricane Georges and dropped significantly within c. 3 years, while seedling densities remained high for c. 5 years post-hurricane. When all species and census intervals were analysed together, generalized linear mixed effects models revealed that canopy openness, seedling and adult tree densities were significant drivers of seedling survival. 4. The relative importance of abiotic and biotic factors changed over time. Separate analyses for each census interval revealed that canopy openness was a significant predictor of survival only for the first census interval, with lower survival at the highest levels of canopy openness. The effect of conspecific seedling density was significant in all intervals except the first, and soil type only in the final census interval. 5. When grouping species into life-history guilds based on adult tree susceptibility to hurricane damage, we found clear differences among guilds in the effects of biotic and abiotic factors on seedling survival. Seedlings of hurricane-susceptible and intermediate guilds were more strongly influenced by canopy openness, while seedlings of the hurricane-resistant group were less affected by conspecific seedling density. Individual species-level analyses for 12 common species, however, showed considerable variation among species within guilds. 6. Synthesis. Our results suggest that hurricanes shape species composition by altering understorey conditions that differentially influence the success of seedlings. Thus, predicted increases in the intensity and frequency of hurricanes in the Caribbean will likely alter seedling dynamics and ultimately the species composition in hurricane-impacted forests.

Factors explaining alien plant invasion success in a tropical ecosystem differ at each stage of invasion

Abstract: Summary 1. Understanding why some alien plant species become invasive when others fail is a fundamental goal in invasion ecology. We used detailed historical planting records of alien plant species introduced to Amani Botanical Garden, Tanzania and contemporary surveys of their invasion status to assess the relative ability of phylogeny, propagule pressure, residence time, plant traits and other factors to explain the success of alien plant species at different stages of the invasion process. 2. Species with native ranges centred in the tropics and with larger seeds were more likely to regenerate, whereas naturalization success was explained by longer residence time, faster growth rate, fewer seeds per fruit, smaller seed mass and shade tolerance. 3. Naturalized species spreading greater distances from original plantings tended to have more seeds per fruit, whereas species dispersed by canopy-feeding animals and with native ranges centred on the tropics tended to have spread more widely in the botanical garden. Species dispersed by canopyfeeding animals and with greater seed mass were more likely to be established in closed forest. 4. Phylogeny alone made a relatively minor contribution to the explanatory power of statistical models, but a greater proportion of variation in spread within the botanical garden and in forest establishment was explained by phylogeny alone than for other models. Phylogeny jointly with variables also explained a greater proportion of variation in forest establishment than in other models. Phylogenetic correction weakened the importance of dispersal syndrome in explaining compartmental spread, seed mass in the forest establishment model, and all factors except for growth rate and residence time in the naturalization model. 5. Synthesis . This study demonstrates that it matters considerably how invasive species are defined when trying to understand the relative ability of multiple variables to explain invasion success. By disentangling different invasion stages and using relatively objective criteria to assess species status, this study highlights that relatively simple models can help to explain why some alien plants are able to naturalize, spread and even establish in closed tropical forests.