Showing papers in "Journal of Ecology in 2011"

An ecological perspective on extreme climatic events: a synthetic definition and framework to guide future research

Abstract: Summary 1. Growing recognition of the importance of climate extremes as drivers of contemporary and future ecological dynamics has led to increasing interest in studying these locally and globally important phenomena. 2. Many ecological studies examining the impacts of what are deemed climate extremes, such as heat waves and severe drought, do not provide a definition of extremity, either from a statistical context based on the long-term climatic record or from the perspective of the response of the system – are the effects extreme (unusual or profound) in comparison to normal variability? 3. A synthetic definition of an extreme climatic event (ECE) is proposed that includes ‘extremeness’ in both the driver and the response: an ECE is as an episode or occurrence in which a statistically rare or unusual climatic period alters ecosystem structure and/or function well outside the bounds of what is considered typical or normal variability. This definition is accompanied by a mechanistic framework based on the concept that extreme response thresholds associated with significant community change and altered ecosystem function must be crossed in order for an ECE to occur. 4. Synthesis. A definition and mechanistic framework for ECEs is used to identify priorities for future research that will enable ecologists to more fully assess the ecological consequences of climate extremes for ecosystem structure and function today and in a future world where their frequency and intensity are expected to increase.

Using plant functional traits to understand the landscape distribution of multiple ecosystem services

Abstract: Summary 1. Spatially explicit understanding of the delivery of multiple ecosystem services (ES) from global to local scales is currently limited. New studies analysing the simultaneous provision of multiple services at landscape scale should aid the understanding of multiple ES delivery and trade-offs to support policy, management and land planning. 2. Here, we propose a new approach for the analysis, mapping and understanding of multiple ES delivery in landscapes. Spatially explicit single ES models based on plant traits and abiotic characteristics are combined to identify ‘hot’ and ‘cold’ spots of multiple ES delivery, and the land use and biotic determinants of such distributions. We demonstrate the value of this trait-based approach as compared to a pure land-use approach for a pastoral landscape from the central French Alps, and highlight how it improves understanding of ecological constraints to, and opportunities for, the delivery of multiple services. 3. Vegetative height and leaf traits such as leaf dry matter content were response traits strongly influenced by land use and abiotic environment, with follow-on effects on several ecosystem properties, and could therefore be used as functional markers of ES. 4. Patterns of association among ES were related to the dominant traits underlying different ecosystem properties. The functional decoupling between height and leaf traits provided alternative pathways for high agronomic value, as well as determining hot and cold spots of ES. Traditional land uses such as organic fertilization and mowing or altitude summer grazing were also linked with ES hot spots, because functional characteristics supporting fodder production and quality are compatible with species and functional diversity. 5. Synthesis. Analyses of ES using plant functional variation across landscapes are a powerful approach to understanding the fundamental ecological mechanisms underlying ES provision, and trade-offs or synergies among services. Sustainable management of species and functionally diverse grassland could simultaneously aim at conserving biodiversity and locally important ES by taking advantage of correlations and trade-offs among different plant functional traits.

Seed dispersal distance is more strongly correlated with plant height than with seed mass

Abstract: Summary 1. It is often assumed that there is a trade-off between maternal provisioning and dispersal capacity, leading small-seeded species to disperse further than large-seeded species. However, this relationship between dispersal distance and seed mass has only been quantified for species from particular sites or with particular dispersal syndromes. 2. We provided the first large-scale, cross-species quantification of the correlations between dispersal distance and both seed mass and plant height. Seed mass was positively related to mean dispersal distance, with a 100-fold increase in seed mass being associated with a 4.5-fold increase in mean dispersal distance (R2 = 0.16; n = 210 species; P < 0.001). However, plant height had substantially stronger explanatory power than did seed mass, and we found a 5-fold increase in height was associated with a 4.6-fold increase in mean dispersal distance (R2 = 0.54; n = 211 species; P < 0.001). 3. Once plant height was accounted for, we found that small-seeded species dispersed further than did large-seeded species (R2 = 0.54; n = 181 species; slope = −0.130; P < 0.001); however, seed mass only added 2% to the R2 of the model. Within dispersal syndromes, tall species dispersed further than did short species, while seed mass had little influence on dispersal distance. 4. Synthesis. These findings enhance our understanding of plant life-history strategies and improve our ability to predict which species are best at colonizing new environments.

The ecological role of climate extremes: current understanding and future prospects

Abstract: Summary 1. Climate extremes, such as severe drought, heat waves and periods of heavy rainfall, can have profound consequences for ecological systems and for human welfare. Global climate change is expected to increase both the frequency and the intensity of climate extremes and there is an urgent need to understand their ecological consequences. 2. Major challenges for advancing our understanding of the ecological consequences of climate extremes include setting a climatic baseline to facilitate the statistical determination of when climate conditions are extreme, having sufficient knowledge of ecological systems so that extreme ecological responses can be identified, and finally, being able to attribute a climate extreme as the driver of an extreme ecological response, defined as an extreme climatic event (ECE). 3. The papers in this issue represent a cross-section of the emerging field of climate extremes research, including an examination of the palaeo-ecological record to assess patterns and drivers of extreme ecological responses in the late Quaternary, experiments in grasslands assessing a range of ecological responses and the role of ecotypic variation in determining responses to climate extremes, and the quantification of the ecological consequences of a recent ECE in the desert Southwest of the USA. 4. Synthesis. The papers in this Special Feature suggest that although the occurrence of ECEs may be common in palaeo-ecological and observational studies, studies in which climate extremes have been experimentally imposed often do not result in ecological responses outside the bounds of normal variability of a system. Thus, ECEs occur much less frequently than their potential drivers and even less frequently than observational studies suggest. Future research is needed to identify the types and time-scales of climate extremes that result in ECEs, the potential for interactions among different types of climate changes and extremes, and the role of genetic, species and trait diversity in determining ecological responses and their evolutionary consequences. These research priorities require the development of alternative research approaches to impose realistic climate extremes on a broad range of organisms and ecosystems.

Climate extremes initiate ecosystem-regulating functions while maintaining productivity

Abstract: Summary 1. Studying the effects of climate or weather extremes such as drought and heat waves on biodiversity and ecosystem functions is one of the most important facets of climate change research. In particular, primary production is amounting to the common currency in field experiments world-wide. Rarely, however, are multiple ecosystem functions measured in a single study in order to address general patterns across different categories of responses and to analyse effects of climate extremes on various ecosystem functions. 2. We set up a long-term field experiment, where we applied recurrent severe drought events annually for five consecutive years to constructed grassland communities in central Europe. The 32 response parameters studied were closely related to ecosystem functions such as primary production, nutrient cycling, carbon fixation, water regulation and community stability. 3. Surprisingly, in the face of severe drought, above- and below-ground primary production of plants remained stable across all years of the drought manipulation. 4. Yet, severe drought significantly reduced below-ground performance of microbes in soil indicated by reduced soil respiration, microbial biomass and cellulose decomposition rates as well as mycorrhization rates. Furthermore, drought reduced leaf water potential, leaf gas exchange and leaf protein content, while increasing maximum uptake capacity, leaf carbon isotope signature and leaf carbohydrate content. With regard to community stability, drought induced complementary plant–plant interactions and shifts in flower phenology, and decreased invasibility of plant communities and primary consumer abundance. 5. Synthesis. Our results provide the first field-based experimental evidence that climate extremes initiate plant physiological processes, which may serve to regulate ecosystem productivity. A potential reason for different dynamics in various ecosystem services facing extreme climatic events may lie in the temporal hierarchy of patterns of fast versus slow response. Such data on multiple response parameters within climate change experiments foster the understanding of mechanisms of resilience, of synergisms or decoupling of biogeochemical processes, and of fundamental response dynamics to drought at the ecosystem level including potential tipping points and thresholds of regime shift. Future work is needed to elucidate the role of biodiversity and of biotic interactions in modulating ecosystem response to climate extremes.

Seed survival in soil: interacting effects of predation, dormancy and the soil microbial community

Abstract: Summary 1. Plant defence theory provides a robust framework for understanding interactions between plants and antagonists, and for interpreting broad patterns in the functional-trait composition of plant communities. However, this framework has been built almost entirely on traits expressed by seedlings and mature plants. 2. No equivalent seed defence theory exists that recognizes the distinct suite of natural enemies that seeds encounter, and the unique constraints to their response. Furthermore, most attention has been paid to insect and vertebrate seed predators active above ground, whereas microbes in soil also have large effects on seed survival, particularly for plants that recruit from soil seed banks. 3. We suggest that concurrent selection on seed dormancy and resistance to microbial antagonists should result in distinct seed defence syndromes. We predict that species with physical seed dormancy will rely on physical defences to exclude predators and pathogens, and rapid seed germination to escape pathogens at the emergence stage. In contrast, species with physiological seed dormancy will deploy a continuum of physical and chemical defences, depending on soil pathogen pressure and duration of seed persistence. Finally, seeds of some species persist in the soil in a non-dormant, imbibed state, and lack obvious chemical and physical defences. These seeds may be especially dependent upon protection from beneficial seed-inhabiting microbes. 4. Framing a general ‘seed defence theory’ may help to account for the distribution of seed dormancy types across ecosystems. We predict that physiological dormancy will be favoured in dry or well-drained environments where pathogen pressure is relatively low, germination cues are most unpredictable, and seedling recruitment success is most variable. In contrast, physical dormancy should be favoured in warm and moist environments where pathogen pressure is high, and where germination cues are a stronger predictor of recruitment success. Persistent, non-dormant seeds are restricted to relatively aseasonal environments where favourable conditions for recruitment can occur over most of the year. 5. Synthesis. Integrating seed defence and dormancy traits can provide new insights into selection on dormancy types, and will help elucidate major trends in seed ecology and evolution. Understanding how seeds are defended also may improve our ability to predict plant regeneration and help develop innovative management strategies for weedy and invasive species.

Climate is a stronger driver of tree and forest growth rates than soil and disturbance

Abstract: Summary 1. Essential resources such as water, nutrients and light vary over space and time and plant growth rates are expected to vary accordingly. We examined the effects of climate, soil and logging disturbances on diameter growth rates at the tree and stand level, using 165 1-ha permanent sample plots distributed across Bolivian tropical lowland forests. 2. We predicted that growth rates would be higher in humid than in dry forests, higher in nutrientrich than nutrient-poor forests and higher in logged than non-logged forests. 3. Across the 165 plots we found positive basal area increases at the stand level, which agree with the generally reported biomass increases in tropical forests. 4. Multiple regression analysis demonstrated that climate variables, in particular water availability, were the strongest drivers of tree growth. More rainfall, a shorter and less intense dry period and higher temperatures led to higher tree growth rates. 5. Tree growth increased modestly with soil fertility and basal area growth was greatest at intermediate soil fertility. Surprisingly, tree growth showed little or no relationship with total soil nitrogen or plant available soil phosphorus. 6. Growth rates increased in logged plots just after logging, but this effect disappeared after 6y ears. 7. Synthesis. Climate is the strongest driver of spatial variation in tree growth, and climate change may therefore have large consequences for forest productivity and carbon sequestration. The negative impact of decreased rainfall and increased rainfall seasonality on tree growth might be partly offset by the positive impact of increased temperature in these forests.

Species- and community-level patterns in fine root traits along a 120 000-year soil chronosequence in temperate rain forest

Abstract: Summary 1. Below-ground plant functional traits regulate plant–soil interactions and may therefore strongly influence ecosystem responses to global change. Despite this, knowledge of how fine-root functional traits vary among plant species and along environmental gradients has lagged far behind our understanding of above-ground traits. 2. We measured species- and community-level root and leaf trait responses for 50 temperate rain forest species from 28 families of ferns, woody and herbaceous angiosperms and conifers, along a soil chronosequence in New Zealand that exhibits a strong gradient in soil nutrient availability. Relationships among species traits (both above- and below-ground) and their distribution along the chronosequence were tested using phylogenetic generalized least-squares regression to account for plant relatedness. 3. Distinctive root trait syndromes were observed; they were closely linked to species’ distribution along the chronosequence. Species growing in the strongly P-limited late stages of the chronosequence had relatively high specific root length (SRL), thin root diameter, high root tissue density, high levels of root branching and low root nutrient concentrations compared to intermediate stages. Species on the youngest site also had high SRL, but had low root tissue density, thick root diameter and high root nutrient concentrations. 4. Species root and leaf nutrient concentrations were positively correlated, reflecting the strong underlying gradient in soil fertility. In contrast, the relationship between SRL and SLA was more complex; there was a weak positive correlation between SRL and SLA, but this conflicted with stronger patterns of increasing SRL and declining SLA with increasing site age. 5. Community-averaged trait values calculated using presence/absence data showed similar trends to the species-level patterns. In contrast, community averages calculated using species abundance-weighted data showed weaker relationships with site age, particularly for morphological traits. This suggests that much of the variation in morphological traits between sites was driven by shifts in the presence of subordinate or ‘rare’ species rather than by changes in the dominant species. 6. Synthesis. Our study demonstrates co-ordinated species- and community-level changes in root traits along a soil chronosequence. These results highlight the influence of soil nutrition on plant functional traits and contribute to our understanding of the drivers of community assembly in a changing environment.

Multitrophic interactions below and above ground: en route to the next level

Abstract: Summary 1. Plants mediate multiple interactions between below-ground (BG) and above-ground (AG) heterotrophic communities that have no direct physical contact. These interactions can be positive or negative from the perspective of each player, can go from the BG to the AG community or vice versa, and comprise representatives of different phyla. Here we highlight emerging general patterns and discuss future research directions. 2. Ecologists initially postulated that root herbivores induce general stress responses, which increase the levels of primary (nutritional) compounds in the undamaged plant compartment and thereby facilitate future attack by AG herbivores. However, damage can also reduce the levels of primary compounds or increase contents of secondary (defensive) metabolites. Both effects may cause resistance phenotypes that play an important role in mediating BG–AG interactions. Systemically induced resistance does not only affect other herbivores but also pathogens in the AG and BG compartment and may inhibit beneficial organisms such as natural enemies of herbivores, microbial root symbionts and pollinators. Conversely, symbiotic mutualists such as mycorrhiza and rhizobia may affect AG and BG defence levels. Finally, BG–AG interactions may be costly if they impede optimal defence strategies in the undamaged compartment. 3. Synthesis. In order to better understand the adaptive value of BG–AG induced responses for the players involved and to identify the driving evolutionary forces, we need a better integration of studies at the community level with experiments on model systems that allow unravelling the genetic and physiological mechanisms of BG–AG interactions. Experiments preferably should be carried out at realistic densities and using the natural temporal sequence at which the various associations are established, because we can expect plants to be adapted only to events that are common over evolutionary time spans. Detailed mechanistic knowledge will help to reproduce relevant interactions in experiments that study multiple species in the field. This step will ultimately allow us to evaluate the importance of plant-mediated interactions between BG and AG communities for the fitness of the species involved and for the structuring of natural communities.

Ploidy influences rarity and invasiveness in plants

Abstract: Summary 1. The factors associated with plant species’ endangerment and (conversely) invasiveness are of broad interest due to their potential value in explaining the causes and consequences of population status. While most past work has focussed on ecological variables, recent work suggests that genetic attributes may be strongly associated with plant species status. 2. We collated data on chromosome numbers for 640 endangered species (worldwide) and their 9005 congeners, and for 81 invasive species and their 2356 congeners. We related ploidy (diploid versus polyploid) to endangerment and invasiveness. We also related chromosome number (absolute number and relative to the minimum recorded for the genus) to endangerment and invasiveness with a randomization test, taking the variation of reported chromosome numbers into account. All analyses considered the relatedness of the taxa. 3. We found that endangered plants are disproportionately likely to be diploid and to have lower ploidy ratios, whilst invasive plant species are generally found to have high chromosome counts and to be polyploid. 4. While considering the effect of relatedness, being endangered is c. 8% less likely as ploidy ratio doubles and 14% less likely for polyploids compared with diploids. Being invasive is 12% more likely as chromosome number doubles and 20% more likely for polyploids compared with diploids. There was no significant effect of raw chromosome number on endangerment or of ploidy ratio on invasiveness. 5. Our findings demonstrate the importance of genomic attributes as risk factors of vulnerability to endangerment or invasiveness in higher plant species, and raise interesting questions as to potential causes of the pattern. 6. Synthesis. Our findings generate new hypotheses on plant rarity and invasiveness influenced by genomic attributes and further our understanding of the role of ploidy in rarity and invasiveness in higher plants. The cause of these relationships are potentially complex, requiring further research; ultimately, understanding the mechanistic basis of population status could aid conservation programs seeking to identify potentially endangered or invasive species.

Niche overlap reveals the effects of competition, disturbance and contrasting assembly processes in experimental grassland communities

Abstract: Summary1. Understanding the processes by which species sort themselves into communities remains acentralpuzzleforattemptstomaintainbiodiversity.Itremainsunclearwhetheranysingleassemblyprocess is generally dominant or whether the influence of contrasting processes varies in a predict-able way relative to biotic and abiotic gradients. Abundance-weighted niche overlap betweenspeciesprovidesapowerfulmeansofcontrastingtwomajorassemblyprocesses–nichecomplemen-tarityandenvironmentalfiltering.2. We examined mean overlap for four vegetative functional traits, relative to that expected whenabundances were randomly allocated to species co-occurring in experimental plots in a wet mea-dow. This provided a test of whether any single assembly process prevailed for the meadow as awhole and across all traits. The effects of mowing, fertilization and dominant species removal, andassociated gradients of Simpson’s dominance and biomass on the niche overlap of plots, were alsoexamined.3. Nicheoverlapwashigherthanexpectedatrandomforthreeofthefourtraitsstudied(height,leafand stem dry matter content, leaf C:N ratio). However, niche overlap was lower than expected forspecificleafarea.4. Mowingwasthetreatmentwiththegreatesteffectonbothnicheoverlapandbiomass,withover-lap significantly lower in the absence of mowing for three of the traits, while biomass was lower inmown plots. For three of the traits there was evidence of a significant decrease in overlap withincreasing biomass, but notincreasing dominance. None of the significant mowing effects on over-lapremainedwhentheeffectofbiomasshadbeenremoved.5. Synthesis: Theseresultssuggestthattheimportanceofnichedifferencesbetweenspeciesinstruc-turing grassland communities should increase with increasing biomass and decrease with distur-bance in grassland communities. They also emphasize that contrasting community assemblyprocessesmayoccurfordifferentnicheaxes,evenwithinasinglecommunity.Key-words: coexistence, complementarity, fertilization, functional trait, meadow, mowing,null model, plant population and community dynamics, productivity, removalIntroduction

Intra- and interspecific plant-soil interactions, soil legacies and priority effects during old-field succession

Abstract: 1. Legacy effects of plant influences on abiotic and biotic soil properties can result in priority effects that influence the structure and composition of plant communities. To better understand the role of these plant–soil interactions, here we expand the concept of plant–soil feedbacks from a within-species approach (intraspecific plant–soil feedback) to a between-species approach (interspecific plant–soil interactions). 2. In a greenhouse experiment, we tested how the early successional Jacobaea vulgaris affects its own performance and the performance of 30 co-occurring plant species via changes in abiotic and biotic soil conditions. In addition, we examined the reciprocal effect of the co-occurring species on J. vulgaris. 3. Our study had three important results. First, J. vulgaris exhibits strong negative plant–soil feedback. Secondly, there were large differences among the co-occurring species in interspecific plant–soil effects on J. vulgaris growth. Approximately, half the species reduced J. vulgaris performance, whereas the other half had no effect. Thirdly, soil conditioned by J. vulgaris had a positive or neutral effect on the growth of the co-occurring species. 4. To test the soil effects of entire plant communities, in 10 old-fields that differed in time since abandonment we recorded the identity of all plants surrounding J. vulgaris individuals. We calculated the weighted soil effect of this community on J. vulgaris and the reciprocal effect of J. vulgaris on the community. There was a positive linear relationship between time since abandonment and the weighted feedback effect of J. vulgaris on the plant community. 5. We suggest three mechanisms how the legacy of plant–soil interactions may enhance the rate of succession through priority effects: early successional plant species exert negative plant–soil feedback; co-occurring plant species cause negative interspecific plant–soil effects to the early successional species; and the early successional species have overall positive interspecific plant–soil effects on the co-occurring plant species. 6.Synthesis. The performance of an early successional species can be reduced directly by the legacy effects of intraspecific plant–soil feedback, as well as indirectly by the legacy effects of both intra- and interspecific plant–soil interactions. These intra- and interspecific plant–soil interactions can prioritize transitions of plant species in plant communities.

Functional diversity indices reveal the impacts of land use intensification on plant community assembly

Abstract: Summary 1. Functional diversity has been suggested as an important descriptor of an assemblage and as an indicator of ecosystem function. However, there has been little testing of recent functional diversity measures on field data and across environmental gradients of disturbance and productivity. This study tested how three recently developed measures of functional diversity behave in practice and what could be deduced about assembly rules operating in the systems studied. 2. Data from 30 plant communities in a small area, comprising arable fields, mown and grazed grasslands, moorland and woodland, were analysed in terms of these diversity indices. Functional Divergence, Evenness and Richness were independent of each other, but Richness correlated to species diversity through the sampling effect. 3. As evidenced by reduced Functional Richness, habitat filtering operated at a significant proportion of the sites sampled. Functional Richness was also reduced against expectations at high levels of both productivity and disturbance. Functional Divergence showed no clear patterns, but Functional Evenness showed a clear promotion by disturbance and a reduction at high standing biomass. 4. Synthesis. Knowledge of how functional diversity is related to the environment provides indications of the processes governing community assembly, and the possibility of developing them as measures of ecosystem function.

Costs of constitutive and herbivore-induced chemical defences in pine trees emerge only under low nutrient availability

Abstract: Summary 1. Production of antiherbivore chemical defences is generally assumed to be costly in terms of fitness, although some studies have failed to detect such costs. A convincing explanation is that the expression of fitness costs depends on environmental conditions such as nutrient availability. 2. We performed a greenhouse experiment with 33 half-sib families in order to study the phenotypic plasticity of constitutive and methyl jasmonate-induced chemical defences to soil phosphorus (P) availability, the existence of genetic trade-offs (costs) between growth and the production of those defences and the extent to which P availability may modulate the expression of those costs. 3. We measured some proxies of vegetative fitness (primary growth, secondary growth and total biomass), plant reserves (soluble sugars and starch) and the concentration of quantitative chemical defences (diterpene content in the stem, total polyphenolics and condensed tannins in the needles). 4. Phosphorus availability had a considerable effect, both on the allocation of resources to constitutive and induced defences and on the expression of vegetative costs associated with those chemical defences. Constitutive investment in chemical defences was greater under P-limited conditions for all studied traits. Inducibility of foliar phenolic compounds was greater under P-limited conditions, and it was strongly constrained under high P availability. Availability of P did not affect the inducibility of stem diterpenes. 5. All defensive traits showed significant genetic variation, with different levels of genetic control in constitutive and induced modes, and genetic variation in their inducibility. We found significant negative genetic correlations (i.e. trade-offs) between growth and defensive investment, but costs of chemical defences emerged only in P-limited conditions. Vegetative costs of constitutive defences were detected for stem diterpenes but not for needle phenolics, while costs of induced defences were found for leaf phenolics but not for stem diterpenes. 6. Synthesis. Our results indicate that P availability controls the production of chemical defences in this pine species, influencing the resource allocation to constitutive defences, the inducibility of those defences and the emergence of related vegetative costs. Phosphorus availability thus appears as a major driver in the evolution of pine resistance to insects and a potential factor in maintaining genetic variation in defences.

Functional traits shape ontogenetic growth trajectories of rain forest tree species

Abstract: Summary 1. Functional traits are posited to explain interspecific differences in performance, but these relationships are difficult to describe for long-lived organisms such as trees, which exhibit strong ontogenetic changes in demographic rates. Here, we use a size-dependent model of tree growth to test the extent to which of 17 functional traits related to leaf and stem economics, adult stature and seed size predict the ontogenetic trajectory of tree growth. 2. We used a Bayesian modelling framework to parameterize and contrast three size-dependent diameter growth models using 16 years of census data from 5524 individuals of 50 rain forest tree species: a size-dependent model, a size-dependent model with species-specific parameters and a size-dependent model based on functional traits. 3. Most species showed clear hump-shaped ontogenetic growth trajectories and, across species, maximum growth rate varied nearly tenfold, from 0.58 to 5.51 mm year )1 . Most species attained their maximum growth at 60% of their maximum size, whereas the magnitude of ontogenetic changes in growth rate varied widely among species. 4. The Trait-Model provided the best compromise between explained variance and model parsimony and needed considerably fewer parameters than the model with species terms. 5. Stem economics and adult stature largely explained interspecific differences in growth strategy. Maximum absolute diameter growth rates increased with increasing adult stature and leaf d 13 C and decreased with increasing wood density. Species with light wood had the greatest potential to modulate their growth, resulting in hump-shaped ontogenetic growth curves. Seed size and leaf economics, generally thought to be of paramount importance for plant performance, had no significant relationships with the growth parameters. 6. Synthesis. Our modelling approach offers a promising way to link demographic parameters to their functional determinants and hence to predict growth trajectories in species-rich communities with little parameter inflation, bridging the gap between functional ecology and population demography.

Linking patterns in phylogeny, traits, abiotic variables and space: a novel approach to linking environmental filtering and plant community assembly

Abstract: Summary 1. We introduce a novel method that analyses environmental filtering of plant species in a geographic and phylogenetic context. By connecting species traits with phylogeny, traits with environment, and environment with geography, this comprehensive approach partitions the ecological and evolutionary processes that influence community assembly. 2. Our analysis extends RLQ ordination, which connects site attributes in matrix R (here environmental variables and spatial positions) with species attributes in matrix Q (here biological traits and phylogenetic positions), through the composition of sites in terms of species presences or abundances (matrix L). This methodology, which explores and identifies environmental filters that organize communities, was developed to answer four questions: which combinations of trait states are filtered by the environment, which lineages are affected by these filters, which environmental variables contribute to the assemblage of local communities and where do these filters act? 3. At La Mafragh in north-eastern Algeria, our approach shows that plant species traits were distributed according to environmental filters associated with a salinity gradient. Traits associated with the salinity gradient were convergent among Juncaceae, Cyperaceae and Amaranthaceae. The observed phylogenetic and trait patterns were related to how species survived the xeric season. Juncaceae and Cyperaceae, being perennials and anemogamous, tolerate the xeric hot season by restricting their range to the humid centre of the study area (where conditions are close to a subtropical climate). Several Amaranthaceae species co-occur with the Juncaceae and Cyperaceae in two areas with the highest salinity. Most dicots were observed at higher elevations (up to 7 m a.s.l.), had hairy structures that can retain water and reflect solar radiation and were mostly annual or biennial, completing their life cycle before the onset of the xeric season. 4. Synthesis. Our methodology describes environmental filters in terms of identified combinations of traits and environmental factors. It allows spatial and phylogenetic signals to be determined by identifying convergent and conserved patterns in the evolution of traits and spatial scales that structured the environment. Our statistical framework is generic and can be readily extended to a wide range of exciting issues, such as host-parasite, plant-pollinator and predator–prey interactions.

Constitutive and induced subterranean plant volatiles attract both entomopathogenic and plant parasitic nematodes

Abstract: Summary 1. Indirect plant defences are well documented for the above-ground constituents of plants. Although less investigated to date, below-ground defences that mediate multitrophic interactions are equally important. Entomopathogenic nematodes (Steinernema diaprepesi) are attracted to herbivore-induced volatiles from Swingle var. (Citrus paradisi×Poncirus trifoliata) when fed upon by root weevil, Diaprepes abbreviatus. 2. We examined the extent to which below-ground volatiles modify behaviour of nematode species representing various foraging strategies (cruisers versus ambushers) and trophic levels (plant parasites versus entomopathogens). We compared attraction to volatiles of weevil-infested and non-infested roots from Swingle citrus rootstock and a parent line of the Swingle hybrid, Poncirus trifoliata (Pt). 3. Swingle weevil-infested roots attracted more nematodes than non-infested roots irrespective of nematode foraging strategy and trophic status. The parental line, Pt, attracted all nematode species irrespective of insect herbivory. 4. Dynamic in situ collection and GC–MS analysis of volatiles from soil revealed that Pt roots release attracting cues constitutively. A different non-hybrid citrus species (sour orange, Citrus aurantium) released nematode attracting cues only in response to larval feeding, similar to responses found in Swingle. Volatile collections from above- and below-ground portions of citrus plants revealed that above-ground feeding by weevils does not induce production of nematode attracting cues analogous to that induced by root damage, nor does damage by larvae below-ground induce a similar volatile above ground. 5. Synthesis. Our results suggest that release of nematode attractants by citrus roots occurs broadly and can be constant or herbivore-induced. The major constituent of this indirect defence is produced by roots and not shoots and in response to below-ground, but not above-ground herbivory. Our findings suggest that this cue acts on nematode species broadly, attracting entomopathogenic nematodes that exhibit various foraging strategies. Unexpectedly, we also found that this cue attracts a plant parasitic nematode species. It appears, thus, that release of nematode attracting cues by citrus plants can cause ecological costs. The plants, however, appear to counteract against these costs, because constitutive release was found only in a cultivar that is resistant to phytopathogenic nematodes, while herbivore-induced release occurred in lines susceptible to pathogenic nematode species.

Validation of biological collections as a source of phenological data for use in climate change studies: a case study with the orchid Ophrys sphegodes

Abstract: Summary 1. The scarcity of reliable long-term phenological data has severely hindered the study of the responses of species to climate change. Biological collections in herbaria and museums are potential sources of long-term data for such study, but their use for this purpose needs independent validation. Here we report a rigorous test of the validity of using herbarium specimens for phenological studies, by comparing relationships between climate and time of peak flowering derived from herbarium records and from direct field-based observations, for the terrestrial orchid Ophrys sphegodes. 2. We examined herbarium specimens of O. sphegodes collected between 1848 and 1958, and recorded peak flowering time directly in one population of O. sphegodes between 1975 and 2006. The response of flowering time to variation in mean spring temperature (March–May) was virtually identical in both sets of data, even though they covered different periods of time which differ in extent of anthropogenic temperature change. In both cases flowering was advanced by c. 6 days per °C rise in average spring temperature. 3. The proportion of variation in flowering time explained by spring temperature was lower in the herbarium record than in direct field observations. It is likely that some of the additional variation was due to geographical variation in collection site, as flowering was significantly earlier at more westerly sites, which have had warmer springs, over their range of 3.44° of longitude. 4. Predictions of peak flowering time based on the herbarium data corresponded closely with observed peak flowering times in the field, indicating that flowering response to temperature had not altered between the two separate periods over which the herbarium and field data were collected. 5. Synthesis. These results provide the first direct validation of the use of herbarium collections to examine the relationships between phenology and climate when field-based observational data are not available.

Spatial variation in disease resistance: from molecules to metapopulations.

Abstract: Variation in disease resistance is a widespread phenomenon in wild plant-pathogen associations. Here, we review current literature on natural plant-pathogen associations to determine how diversity in disease resistance is distributed at different hierarchical levels – within host individuals, within host populations, among host populations at the metapopulation scale and at larger regional scales. We find diversity in resistance across all spatial scales examined. Furthermore, variability seems to be the best counter-defence of plants against their rapidly evolving pathogens. We find that higher diversity of resistance phenotypes also results in higher levels of resistance at the population level. Overall, we find that wild plant populations are more likely to be susceptible than resistant to their pathogens. However, the degree of resistance differs strikingly depending on the origin of the pathogen strains used in experimental inoculation studies. Plant populations are on average 16% more resistant to allopatric pathogen strains than they are to strains that occur within the same population (48 % vs. 32 % respectively). Pathogen dispersal mode affects levels of resistance in natural plant populations with lowest levels detected for hosts of airborne pathogens and highest for waterborne pathogens. Detailed analysis of two model systems, Linum marginale infected by Melampsora lini, and Plantago lanceolata infected by Podosphaera plantaginis, show that the amount of variation in disease resistance declines towards higher spatial scales as we move from individual hosts to metapopulations, but evaluation of multiple spatial scales is needed to fully capture the structure of disease resistance. Synthesis: Variation in disease resistance is ubiquitous in wild plant-pathogen associations. While the debate over whether the resistance structure of plant populations is determined by pathogen-imposed selection versus non-adaptive processes remains unresolved, we do report examples of pathogen-imposed selection on host resistance. Here we highlight the importance of measuring resistance across multiple spatial scales, and of using sympatric strains when looking for signs of coevolution in wild plant-pathogen interactions. Keywords: Coevolution, host-parasite interactions, life-history, metapopulation dynamics, gene-for-gene, major resistance, sympatric, disease resistance, epidemiology Introduction How plant resistance to disease is distributed across space can affect fundamental components of disease biology and epidemiology. Ultimately the resistance structure of plant populations will determine where pathogens occur, as a pathogen can only infect hosts whose defence strategies it can overcome. Furthermore, host resistance may be considered the main driving force of pathogen evolution, with pathogens evolving to overcome host resistance strategies. This process is believed to be reciprocal so hosts and their pathogens are engaged in a co-evolutionary arms-race with hosts evolving new counter defences to escape attack, and pathogens in turn evolving to overcome these new forms of resistance (e.g. Clay & Kover 1996). The interaction between hosts and pathogens is characterized by a great disparity in genome size, generation time and speed of adaptability which, on the surface, would generally appear to favour the pathogen. Indeed, there are numerous examples of pathogens adapting to their hosts, both from studies of local adaptation (Parker 1985; Thrall et al. 2002; Laine 2005; Niemi et al. 2006; Sicard et al. 2007; Springer 2007) (but see Kaltz et al. 1999 for an exception), and from classic boom-and-bust cycles, particularly in agricultural crops, where newly deployed resistance genes increase in frequency but then rapidly lose their effectiveness as pathogens adapt to that new variety (Browning & Frey 1969). However, there is notably little evidence of the opposite – hosts evolving resistance under the pressure of pathogen attack. At the same time, variation in disease resistance is widespread (Table 1; Salvaudon et al. 2008), which implies that natural plant populations have the capacity to undergo significant adaptive evolution in response to pathogen attack. Table 1 Summary information for studies used for assessing whether there is spatial variation in host resistance at different spatial scales. Studies with superscript ‘1’ in the Reference column were used for the analysis of levels of resistance ... Theoretical models of coevolution imply indirect negative frequency-dependent selection (FDS) between hosts and parasites within local populations, in which the selection rate for resistance depends on the frequency of parasite avirulence alleles and vice versa (Jayakar 1970; Leonard 1977; Bergelson et al. 2001). In systems with such FDS, costs of resistance and virulence have traditionally been required to maintain polymorphism in these traits (e.g. Jayakar 1970; Leonard 1977; Frank 1993), but recent theoretical work has shown metapopulation dynamics and spatially heterogenous selection to be as important, or even a sufficient condition, for the maintenance of polymorphism (Gandon et al. 1996; Gomulkiewicz et al. 2000; Nuismer et al. 2000; Gandon & Michalakis 2002; Thrall & Burdon 2002; Nuismer & Gandon 2008). Indeed, host-pathogen coevolutionary processes are intrinsically spatial as well as temporal, and occur at many different scales (Burdon & Thrall 2001). These range from single populations dominated by demographic and genetic stochasticity (Parker 1985; Burdon & Jarosz 1991; Alexander et al. 1993; Bevan et al. 1993a; Espiau et al. 1998), to metapopulations in which extinction/colonization dynamics have a large influence (Thrall et al. 2001; Ericson et al. 2002; Thrall et al. 2002; Smith et al. 2003; Antonovics 2004; Laine & Hanski 2006; Soubeyrand et al. 2009), and larger geographic regions where phylogenetic patterns and historical events become more important (Burdon et al. 2002; de Meaux et al. 2003; de Meaux & Mitchell-Olds 2003). Coevolutionary interactions between hosts and pathogens may occur at a broad range of spatial scales, from those encompassed by a single individual through various intermediate levels to those of the species as a whole. It has already been noted by several authors that variation in disease resistance is widespread (e.g. Parker 1985; Springer 2007; Jorgensen & Emerson 2008) yet, to date, we know little about how this variation is structured across space, and what are the processes driving host resistance structure at different spatial scales. This is despite the general recognition that variation in host resistance (and pathogen infectivity and aggressiveness) is of central importance to understanding patterns of infection (Hill 1998; Lockett et al. 2001). Indeed, study of the genetic components of host-pathogen interactions has lagged far behind work documenting the demographic impacts of disease. Thus, while some work has shown negative relationships between the overall diversity of hosts and parasitism (Coltman et al. 1999; Meagher 1999), in only a very few cases has host genetic variation been implicated in rates of epidemic spread of disease or patterns of disease prevalence (e.g. Thrall & Burdon 2000; Laine 2004). Here we review studies on how resistance is distributed in wild plant-pathogen interactions at different spatial scales: within individual hosts, within host populations, among populations at the metapopulation scale, and at regional scales. Understanding this hierarchical spatial structure has the potential to provide valuable insights into the influence of pathogens on host evolution, and spatial variation in the magnitude of their effect. Specifically, here we ask: 1) What are the kinds of resistance responses we find in wild plant populations (i.e. qualitative or quantitative)? 2) Given that there is polymorphism in resistance, are wild plant populations more likely to be resistant or susceptible to their pathogens? 3) How do host and pathogen life-history and origin of pathogen strain affect resistance of plant populations? 4) How is resistance diversity structured at these different spatial levels? We include here an analysis of two plant-pathogen interactions that have been studied in detail across several different spatial scales. One of these, the interaction between the wild flax, Linum marginale, and its obligate rust fungus Melampsora lini, is a native host-pathogen association which occurs across southern Australia. The other is the long-standing interaction between Plantago lanceolata and its obligate powdery mildew fungus, Podosphaera plantaginis in the southwest archipelago of Finland. Knowledge of these systems allows us to explicitly estimate how variation in disease resistance is hierarchically distributed at the different spatial scales. Finally, we conclude with a consideration of empirical and conceptual future directions for the study of plant-pathogen interactions.

Sequence of arrival determines plant-mediated interactions between herbivores

Abstract: Induced changes in plant quality can mediate indirect interactions between herbivores. Although the sequence of attack by different herbivores has been shown to influence plant responses, little is known about how this affects the herbivores themselves. We therefore investigated how induction by the leaf herbivore Spodoptera frugiperda influences resistance of teosinte (Zea mays mexicana) and cultivated maize (Zea mays mays) against root-feeding larvae of Diabrotica virgifera virgifera. The importance of the sequence of arrival was tested in the field and laboratory. Spodoptera frugiperda infestation had a significant negative effect on colonization by D. virgifera larvae in the field and weight gain in the laboratory, but only when S. frugiperda arrived on the plant before the root herbivore. When S. frugiperda arrived after the root herbivore had established, no negative effects on larval performance were detected. Yet, adult emergence of D. virgifera was reduced even when the root feeder had established first, indicating that the negative effects were not entirely absent in this treatment. The defoliation of the plants was not a decisive factor for the negative effects on root herbivore development, as both minor and major leaf damage resulted in an increase in root resistance and the extent of biomass removal was not correlated with root-herbivore growth. We propose that leaf-herbivore-induced increases in feeding-deterrent and/or toxic secondary metabolites may account for the sequence-specific reduction in root-herbivore performance. Synthesis. Our results demonstrate that the sequence of arrival can be an important determinant of plant-mediated interactions between insect herbivores in both wild and cultivated plants. Arriving early on a plant may be an important strategy of insects to avoid competition with other herbivores. To fully understand plant-mediated interactions between insect herbivores, the sequence of arrival should be taken into account. © 2011 The Authors. Journal of Ecology © 2011 British Ecological Society.

Is rapid evolution common in introduced plant species

Abstract: Summary 1. While previous studies have demonstrated rapid evolution in introduced plants and animals, most focus on single species. They are therefore unable to show whether these are special cases, or if rapid evolution is a common phenomenon in introduced species. 2. We used over 1900 herbarium specimens to determine whether morphological traits [plant height, leaf mass per area (LMA), leaf area or leaf shape] have shown significant change during the last ∼150 years in 23 plant species introduced to New South Wales, Australia. 3. Seventy per cent of our study species showed a change in at least one trait through time. The most common change was in plant height (eight out of 21 species). Six of these showed a decrease in height through time. Decreases in height mainly occurred in western New South Wales where dry, low nutrient conditions may favour shorter plants. We also found changes in leaf traits, including one decrease in LMA, five changes in leaf shape, and three changes in leaf area. The magnitude of these changes was surprisingly large, up to 125% in 100 years. 4. We used specimens of both native Australian species, and of the introduced species taken from their native range to investigate the possibility that the morphological changes were a response to temporal environmental change rather than to the species’ introduction to Australia. These control species showed significantly (P < 0.05) fewer changes than the introduced species. We included in our analyses information on the region in which each specimen was collected, to account for the possibility that changes through time might be caused by populations radiating in to new environments where a different phenotype might be expressed. Overall, rapid evolution seems the most likely explanation for the changes we observed. 5. Synthesis. The majority of our study species showed morphological change through time. While common garden experiments will be required to rule out phenotypic plasticity as an alternative explanation for these patterns, our results suggest that rapid evolution in introduced plant species could be much more common, and of a much greater magnitude than previously thought.

Colonization of a newly developing salt marsh: disentangling independent effects of elevation and redox potential on halophytes

Abstract: Summary 1. Many characteristics of the salt marsh environment covary with elevation. It has therefore proved difficult to determine which environmental characteristics limit the distributions of particular species in the field. Oxygen supply to the rhizosphere may be particularly important, as it is determined by the duration and frequency of flooding. 2. The re-activation of a salt marsh by managed coastal realignment provided an opportunity to investigate the large-scale manipulation of environmental effects on halophyte distribution in a situation where the usual relationships between environmental characteristics, elevation and succession had been partially uncoupled. 3. Most locations sampled lay between mean neap and mean spring tidal levels. As expected, anoxic conditions occurred at lower elevation, redox potential increased generally with elevation and sediments were oxic on the upper parts. However, sediment oxygenation at any given elevation was variable, particularly at intermediate levels in the tidal range. This imperfect correlation between elevation and sediment redox allowed quantification of their independent effects on species distributions using the statistical technique of Hierarchical Partitioning. 4. Effects of elevation and sediment redox potential were distinguishable from each other. Salicornia europaea occurred predominantly at lower elevation but was not influenced by redox potential. Puccinellia maritima favoured low redox potentials independently of elevation. In contrast, Suaeda maritima tolerated a wide range of elevations but was absent from areas with low redox potential. Atriplex portulacoides was apparently more averse to low redox potential than to low elevation. Elytrigia atherica was restricted to both high redox potential and high elevation. Smaller independent effects of sediment depth, salinity, water content, nitrate concentration, shear strength and loss on ignition were apparent for some species. 5. Synthesis. Although much of the elevational zonation of species on salt marshes is mediated by differential tolerance of the consequences of co-linearly varying variables, particularly sediment anoxia and elevation, these variables have independent effects that are quantifiable in the field. Hierarchical Partitioning provides a valuable tool for distinguishing the mechanisms underlying species zonations on environmental gradients, especially where large-scale environmental manipulations have partially decoupled the usual co-linear variation.

Interactions among plants and evolution

Abstract: Summary 1. Interactions among plants and their consumers, pollinators and dispersers are central to evolutionary theory, but interactions among plants themselves have received much less attention. Thus focusing more attention on the evolutionary role of plant–plant interactions may provide greater insight into the processes that organize communities. 2. Here, we integrate divergent themes in the literature in an effort to provide a synthesis of empirical evidence and ideas about how plant interactions may affect evolution and how evolution may affect plant interactions. 3. First, we discuss the idea of niche partitioning evolving through competitive interactions among plants, the idea of niche construction evolving through facilitative interactions, and the connections between these ideas and more recent research on diversity and ecosystem function and trait-based community organization. 4. We then review how a history of coexistence within a region might affect competitive outcomes and explore the mechanisms by which plants exert selective forces on each other. Next, we consider recent research on invasions suggesting that plant interactions can reflect regional evolutionary trajectories. Finally, we place these lines of research into the context of extended phenotypes and the geographic mosaic of co-evolution. 5. Synthesis. Our synthesis of separate lines of inquiry is a step towards understanding the evolutionary importance of interactions among plants, and suggests that the evolutionary consequences of interactions contribute to communities that are more than assemblages of independent populations.

Nitrification is linked to dominant leaf traits rather than functional diversity

Abstract: Summary 1. The internal cycling of nitrogen (N) is a critical process in terrestrial ecosystems. Nitrification occurs when soil microbes convert ammonium to nitrate. Nitrification is known to be regulated by abiotic soil properties, but less is known about how plant communities influence this important ecosystem function. 2. Two contrasting hypotheses propose mechanisms for how communities influence ecosystem processes. The ‘mass ratio hypothesis’ proposes that dominant species control ecosystem processes, e.g. communities dominated by plants with high leaf N content may increase rates of N cycling. The ‘diversity hypothesis’ proposes that community diversity controls ecosystem processes, e.g. diverse communities may increase rates of N cycling by providing a more consistent supply of organic N as variable litter qualities break down at different rates. Each hypothesis was simultaneously evaluated using structural equation modelling in the context of a ponderosa pine forest. 3. The first principle component extracted from a species–trait matrix captured interspecific covariation in leaf traits such as specific leaf area and leaf N content. This ‘leaf economics spectrum’was scaled to the community-level by calculating community-weighted mean leaf trait axis scores. Quadratic entropy was used as the index of understorey functional diversity to capture variation in functional traits within the community. 4. Structural equation modelling results suggest that soil factors, such as pH, temperature, C:N mass ratio and total N were the dominant factors exerting direct control over nitrification potential. Understorey leaf traits were positively associated with nitrification potential, suggesting that high-quality litter inputs increase rates of nitrification. The negative indirect effects of pine abundance on nitrification mediated through soil properties were much stronger than the direct effect of understorey leaf traits. 5. Synthesis. The leaf economics spectrum is an important gradient of functional trait variation that has consequences for internal N cycling. Nitrification potential was more strongly linked to dominant leaf traits than to functional diversity, thereby lending more support to the mass ratio hypothesis. However, functional diversity still explained some of the observed variation in biomass production. Leaf traits can be used to understand how internal N cycling rates will be affected by changes in vegetation structure.

Extrinsic and intrinsic forcing of abrupt ecological change: case studies from the late Quaternary

Abstract: Summary 1. Abrupt changes and regime shifts are common phenomena in terrestrial ecological records spanning centuries to millennia, thus offering a rich opportunity to study the patterns and drivers of abrupt ecological change. 2. Because Quaternary climate changes also often were abrupt, a critical research question is to distinguish between extrinsic versus intrinsic abrupt ecological changes, i.e. those externally driven by abruptly changing climates, versus those resulting from thresholds, tipping points, and other nonlinear responses of ecological systems to progressive climate change. Extrinsic and intrinsic abrupt ecological changes can be distinguished in part by compiling and analysing regional networks of palaeoecological records. 3. Abrupt ecological changes driven by spatially coherent and abrupt climate changes should manifest as approximately synchronous ecological responses, both among different taxa at a site and among sites. However, the magnitude and direction of response may vary among sites and taxa. Ecological responses to the rapid climatic changes accompanying the last deglaciation offer good model systems for studying extrinsic abrupt change. 4. When abrupt ecological changes are intrinsically driven, the timing and rate of ecological response to climate change will be strongly governed by local biotic and abiotic processes and by stochastic processes such as disturbance events or localized climatic extremes. Consequently, at a regional scale, one should observe ‘temporal mosaics’ of abrupt ecological change, in which the timing and rate of ecological change will vary among species within sites and among sites. These temporal mosaics are analogous to the spatial mosaics observed in ecological systems prone to threshold switches between alternate stable states. The early Holocene aridification of the North American mid-continent and the middle-Holocene aridification of North Africa may be good examples of temporal mosaics. 5. Synthesis. Past instances of extrinsic and intrinsic abrupt change are of direct relevance to global-change ecologists. The former allow study of the capacity of ecological systems to quickly adjust to abrupt climate changes, while the latter offer opportunities to understand the ecological processes causing abrupt local responses to regional climate change, to test tools for predicting critical thresholds, and to develop climate-adaptation strategies.

Functional traits and local environment predict vegetation responses to disturbance: a pan‐European multi‐site experiment

Abstract: 1. Disturbance is one of the most important factors structuring the taxonomic and functional composition of vegetation. Vegetation resistance or resilience to disturbance depends on local environmental conditions, further modifying the pool of species and traits. This paper aims to understand how disturbance and local environment combine to affect the resistance and resilience of vegetation. 2. A functional-trait approach was used to detect traits related to vegetation resistance and resilience, and trait attributes of individual species responding to disturbance. Trait approaches enable comparison of vegetation responses across biogeographic regions containing different species pools. 3. At 35 European forest and grassland sites, experimental disturbance (human trampling) was applied at five intensities. Indices for resistance and resilience were calculated, based on total vegetation cover, and related to climate and local site factors. Additional indices were calculated for the most common species to demonstrate traits that confer resistance and resilience to disturbance. 4. Vegetation resistance was related to occurrence of species with traits selected by a history of intensive land use (smaller leaf size, rosette plant form) and local environmental conditions. Vegetation resilience, however, was associated with ecosystem properties that facilitate higher growth rates. Resilient vegetation occurred where irradiation was higher (grasslands, open forests) with sufficient water availability (summer precipitation, humidity) and comprised of species with traits related to enhanced growth rates (increased specific leaf area, decreased leaf dry matter content). 5. Synthesis. This pan-European disturbance experiment demonstrates that different drivers (land use or climate) of vegetation response show different mechanistic responses to physical disturbance. Resistance depends on the functional composition of predominant species in the assemblage, which is strongly affected by land-use history; resilience is directly connected to growth rates affected by climate. We argue for the inclusion of land-use history and climate into the planning process for visitor management, especially in areas of high conservation interest.

Whitefly infestation of pepper plants elicits defence responses against bacterial pathogens in leaves and roots and changes the below‐ground microflora

Abstract: Summary 1. Upon facing biotic stresses, plants orchestrate defence mechanisms via internal and external mechanisms that are mediated by signalling molecules such as salicylic acid, jasmonic acid, ethylene and various other volatile compounds. Although pathogen- and chemical-induced plant resistance has been studied extensively within the same plant compartment, the effects of above-ground (AG) insect-elicited plant defence on the resistance expression in roots and the below-ground (BG) microbial community are not well understood. 2. We assessed the effect of AG whitefly (Bemisia tabaci) attack on the elicitation of induced resistance against a leaf pathogen, Xanthomonas axonopodis pv. vesicatoria, a soil-borne pathogen, Ralstonia solanacearum, and on BG modifications of the rhizosphere microflora in peppers (Capsicum annuum). 3. Symptom development caused by the two bacterial pathogens on leaves and roots was significantly reduced in whitefly-exposed plants as compared to controls. A combined treatment with benzothiadiazole (BTH) and whitefly caused an additive effect on induced resistance, indicating that whitefly-induced plant defence can utilize salicylic acid (SA)-dependent signalling. To obtain further genetic evidence of this phenomenon, we evaluated the gene expression of Capsicum annuum pathogenesis-related protein (CaPR) 1, CaPR4, CaPR10 and Ca protease inhibitor II, and observed increased expression after BTH and/or whitefly treatment indicating that AG whitefly infestation elicited SA and jasmonic acid signalling in AG and BG. Since the expression pattern of PR genes in the roots differed, we assessed microbial diversity in plants treated with BTH and/or whitefly. 4. In addition to eliciting BG defence responses, a whitefly infestation of the leaves augmented the population of root-associated Gram-positive bacteria and fungi, which may have positively affected plant growth and induced systemic resistance. Whitefly feeding reduced plant size, which usually occurs as a consequence of the high costs of direct resistance induction. 5. Synthesis. Our results demonstrate that whitefly-induced resistance against bacterial pathogens can cross the AG–BG border and may cause further indirect benefits on future plant development, because it can positively affect the association or plant roots with putatively beneficial microorganisms.

Emergence of a mid‐season period of low floral resources in a montane meadow ecosystem associated with climate change

Abstract: Summary 1. Shifts in the spatial and temporal patterns of flowering could affect the resources available to pollinators, and such shifts might become more common as climate change progresses. 2. As mid-summer temperatures have warmed, we found that a montane meadow ecosystem in the southern Rocky Mountains of the United States exhibits a trend toward a bimodal distribution of flower abundance, characterized by a mid-season reduction in total flower number, instead of a broad, unimodal flowering peak lasting most of the summer season. 3. We examined the shapes of community-level flowering curves in this system and found that the typical unimodal peak results from a pattern of complementary peaks in flowering among three distinct meadow types (dry, mesic and wet) within the larger ecosystem. However, high mid-summer temperatures were associated with divergent shifts in the flowering curves of these individual meadow types. Specifically, warmer summers appeared to cause increasing bimodality in mesic habitats, and a longer interval between early and late flowering peaks in wet and dry habitats. 4. Together, these habitat-specific shifts produced a longer mid-season valley in floral abundance across the larger ecosystem in warmer years. Because of these warming-induced changes in flowering patterns, and the significant increase in summer temperatures in our study area, there has been a trend toward non-normality of flowering curves over the period 1974–2009. This trend reflects increasing bimodality in total community-wide flowering. 5. The resulting longer periods of low flowering abundance in the middle of the summer season could negatively affect pollinators that are active throughout the season, and shifts in flowering peaks within habitats might create mismatches between floral resources and demand by pollinators with limited foraging ranges. 6. Synthesis. Early-season climate conditions are getting warmer and drier in the high altitudes of the southern Rocky Mountains. We present evidence that this climate change is disrupting flowering phenology within and among different moisture habitats in a sub-alpine meadow ecosystem, causing a mid-season decline in floral resources that might negatively affect mutualists, especially pollinators. Our findings suggest that climate change can have complex effects on phenology at small spatial scales, depending on patch-level habitat differences.

Direct and indirect root defences of milkweed (Asclepias syriaca): trophic cascades, trade‐offs and novel methods for studying subterranean herbivory

Abstract: Summary 1. Entomopathogenic nematodes can function as indirect defence for plants that are attacked by root herbivores. By releasing volatile organic compounds (VOCs), plants signal the presence of host insects and thereby attract nematodes. 2. Nonetheless, how roots deploy indirect defences, how indirect defences relate to direct defences, and the ecological consequences of root defence allocation for herbivores and plant biomass are essentially unknown. 3. We investigate a natural below-ground tritrophic system, involving common milkweed, a specialist root-boring beetle and entomopathogenic nematodes, and asked whether there is a negative genetic correlation between direct defences (root cardenolides) and indirect defences (emission of volatiles in the roots and nematode attraction), and between constitutive and inducible defences. 4. Volatiles of roots were analysed using two distinct sampling methods. First, we collected emissions from living Asclepias syriaca roots by dynamic headspace sampling. This method showed that attacked A. syriaca plants emit five times higher levels of volatiles than control plants. Secondly, we used a solid phase micro-extraction (SPME) method to sample the full pool of volatiles in roots for genetic correlations of volatile biosynthesis. 5. Field experiments showed that entomopathogenic nematodes prevent the loss of biomass to root herbivory. Additionally, suppression of root herbivores was mediated directly by cardenolides and indirectly by the attraction of nematodes. Genetic families of plants with high cardenolides benefited less from nematodes compared to low-cardenolide families, suggesting that direct and indirect defences may be redundant. Although constitutive and induced root defences traded off within each strategy (for both direct and indirect defence, cardenolides and VOCs, respectively), we found no trade-off between the two strategies. 6. Synthesis. Constitutive expression and inducibility of defences may trade off because of resource limitation or because they are redundant. Direct and indirect defences do not trade off, likely because they may not share a limiting resource and because independently they may promote defence across the patchiness of herbivore attack and nematode presence in the field. Indeed, some redundancy in strategies may be necessary to increase effective defence, but for each strategy, an economy of deployment reduces overall costs.

Mechanisms of monodominance in diverse tropical tree-dominated systems

Abstract: 1.The existence of many types of monodominant forests is readily explainable by ecological theory (e.g. early successional forests). Nevertheless, monodominant stands sometimes occur in areas where a much higher diversity typically occurs. Such 'classical monodominance' is not currently readily explained by ecological theory. 2.We briefly review the published mechanisms suggested to cause classical monodominance and then combine them into a new probabilistic conceptual framework to better understand why these systems occur. We build on two theories proposed to explain monodominance: a lack of exogenous disturbance over long periods and species-specific life-history traits. We suggest that certain traits under certain conditions may generate positive feedbacks leading to a greater probability of monodominance being achieved. Such positive feedbacks have the potential to drive a typically diverse system towards a monodominant one. 3.Synthesis. Classical monodominance in tropical forests is hypothesized to be attained when a group of traits occur together under low exogenous disturbance conditions, this giving rise to a series of positive feedbacks. The presented framework links the differing mechanisms proposed in the literature to explain classical monodominance and shows there are potentially alternative routes to monodominance, thus reconciling apparently contradictory observational and experimental results. © 2011 The Authors. Journal of Ecology