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Plant-plant interactions may critically modify the impact of climate change on plant communities. However, the magnitude and even direction of potential future interactions remains highly debated, especially for water-limited ecosystems. Predictions range from increasing facilitation to increasing competition with future aridification., The different methodologies used for assessing plant-plant interactions under changing environmental conditions may affect the outcome but they are not equally represented in the literature. Mechanistic experimental manipulations are rare compared with correlative approaches that infer future patterns from current observations along spatial climatic gradients., Here, we utilize a unique climatic gradient in combination with a large-scale, long-term experiment to test whether predictions about plant-plant interactions yield similar results when using experimental manipulations, spatial gradients or temporal variation. We assessed shrub-annual interactions in three different sites along a natural rainfall gradient (spatial) during 9 years of varying rainfall (temporal) and 8 years of dry and wet manipulations of ambient rainfall (experimental) that closely mimicked regional climate scenarios., The results were fundamentally different among all three approaches. Experimental water manipulations hardly altered shrub effects on annual plant communities for the assessed fitness parameters biomass and survival. Along the spatial gradient, shrub effects shifted from clearly negative to mildly facilitative towards drier sites, whereas temporal variation showed the opposite trend: more negative shrub effects in drier years., Based on our experimental approach, we conclude that shrub-annual interaction will remain similar under climate change. In contrast, the commonly applied space-for-time approach based on spatial gradients would have suggested increasing facilitative effects with climate change. We discuss potential mechanisms governing the differences among the three approaches., Our study highlights the critical importance of long-term experimental manipulations for evaluating climate change impacts. Correlative approaches, for example along spatial or temporal gradients, may be misleading and overestimate the response of plant-plant interactions to climate change. [ABSTRACT FROM AUTHOR]

Abstract: Site productivity and disturbances are among the main factors determining plant community composition. With increasing site productivity, plant diversity is often reduced and the importance of traits associated with light competition increases. Small-scale disturbances created by mole activity are assumed to affect this dynamics by two factors: increasing site heterogeneity and providing survival opportunity for weaker light competitors. These effects are expected to become stronger with increasing site productivity. To test this hypothesis we compared species composition on molehills and in undisturbed plots within grasslands and along a productivity gradient. The differences in species composition were evaluated both by species diversity and by traits representing trade-offs between species’ response to competition and disturbance. Species diversity generally decreased with site productivity, while disturbances had no significant contribution to the variation among samples. When analyzing shifts in species traits, we found a central role of site productivity, but also a significant, though secondary, effect of molehill disturbances. In particular, communities showed increased potential height and specific leaf area and decreased seed weight and Ellenberg’s light indicator values with increasing site productivity, supporting the assumption of increasing light limitation along the gradient. Species observed on molehills were generally less shade tolerant, especially in the more productive grasslands. We conclude that, although overall plant community composition is mainly shaped by large environmental gradients such as site productivity, small-scale disturbances may contribute to local environmental heterogeneity and trait variation by enabling species less adapted to light competition to survive. [ABSTRACT FROM AUTHOR]

Coexistence of woody and herbaceous plants may be governed by a complex set of direct and indirect interactions, whose relative importance have been rarely assessed. We experimentally studied woody species establishment in a mixed plant community by disentangling the potential role of such biotic interactions and the effect of environmental variations on them. Seedling establishment of the common eastern Mediterranean shrub species Sarcopoterium spinosum was investigated under different rainfall and light conditions, combined with the effect of the presence of adult shrubs and annual neighbors. We predicted that seedlings will be directly affected by competition with annuals with increasing water availability, while direct effects of adult shrubs will be positive via amelioration of water stress. Indirect effects were expected beneath shrub canopies due to reduced water stressed and light availability for both annuals and shrub seedlings, which may intensify competition between annuals and shrub seedlings. To test these predictions we performed field and garden experiments in which we combined manipulation of shrub and annual presence with manipulations of water availability and light conditions to simulate the effect of shrub canopy. In contrast to our prediction, shrub seedling establishment was not facilitated but inhibited by adult shrubs because of light limitation. As expected, annuals had direct negative effects on shrub seedlings under wet conditions, which shifted to neutral or positive effects under dry conditions. Thus, interactions among shrubs and annuals, and in particular the release from competition during drought years, leads to a counterintuitive positive effect of drought on shrub seedling establishment. Our findings point to the importance of experimentally studying multidimensional interactions for coexistence of different life forms and to the underestimated role of light for success in water-limited ecosystems. [ABSTRACT FROM AUTHOR]

Defining species by their climatic niche is the simple and intuitive principle underlying Bioclimatic Envelope Model (BEM) predictions for climate change effects. However, these correlative models are often criticised for neglecting many ecological processes. Here, we apply the same niche principle to entire communities within a medium/long-term climate manipulation study, where ecological processes are inherently included. In a nine generation study in Israel, we manipulated rainfall (Drought −30%; Irrigation +30%; Control natural rainfall) at two sites which differ chiefly in rainfall quantity and variability. We analysed community responses to the manipulations by grouping species based on their climatic rainfall niche. These responses were compared to analyses based on single species, total densities, and commonly used taxonomic groupings. Climate Niche Groups yielded clear and consistent results: within communities, those species distributed in drier regions performed relatively better in the drought treatment, and those from wetter climates performed better when irrigated. In contrast, analyses based on other principles revealed little insight into community dynamics. Notably, most relationships were weaker at the drier, more variable site, suggesting that enhanced adaptation to variability may buffer climate change impacts. We provide robust experimental evidence that using climatic niches – commonly applied in BEMs – is a valid approach for eliciting community changes in response to climate change. However, we also argue that additional empirical information needs to be gathered using experiments in situ to correctly assess community vulnerability. Climatic Niche Groups used in this way, may therefore provide a powerful tool and directional testing framework to generalise and compare climate change impacts across habitats. [ABSTRACT FROM AUTHOR]

Abstract: In common garden experiments, a number of genotypes are raised in a common environment in order to quantify the genetic component of phenotypic variation. Common gardens are thus ideally suited for disentangling how genetic and environmental factors contribute to the success of invasive species in their new non-native range. Although common garden experiments are increasingly employed in the study of invasive species, there has been little discussion about how these experiments should be designed for greatest utility. We argue that this has delayed progress in developing a general theory of invasion biology. We suggest a minimum optimal design (MOD) for common garden studies that target the ecological and evolutionary processes leading to phenotypic differentiation between native and invasive ranges. This involves four elements: (A) multiple, strategically sited garden locations, involving at the very least four gardens (2 in the native range and 2 in the invaded range); (B) careful consideration of the genetic design of the experiment; (C) standardization of experimental protocols across all gardens; and (D) care to ensure the biosafety of the experiment. Our understanding of the evolutionary ecology of biological invasions will be greatly enhanced by common garden studies, if and only if they are designed in a more systematic fashion, incorporating at the very least the MOD suggested here. [Copyright &y& Elsevier]

A pertinent debate in plant ecology centers around the generality of the self‐thinning rule. However, studies focused on highly simplified settings such as even‐aged monospecific populations or optimal conditions. This neglects the fact that most natural communities, to which the classical self‐thinning slope is often applied, are age‐structured, composed of multiple species and exposed to various types of abiotic stress. With the help of an individual‐based model, we relax these simplified assumptions and systematically test for changes in the biomass–density relationships of uneven‐aged, functionally diverse plant communities across a complete stress gradient, using excessive to insufficient soil water as a case study. We show that frequent recruitment, which resulted in an uneven‐aged community, and stress intensity caused predictable changes in the entire biomass–density trajectory. Increasing stress resulted in steeper (more negative) slopes and increased the intercept in the classical self‐thinning section irrespective of excessive or insufficient soil water as a stress type. Recruitment steepened the slope, too and enabled a novel section in the biomass–density trajectory. This novel section represented a quasi‐steady state of the density‐dependent dynamics of new generations which occurred locally within patches of recruitment. At the community level, the slope of the biomass–density relationship at quasi‐steady state had a significantly flatter slope of −1.1 under optimal soil water conditions. Functional diversity showed little impact on density‐dependent mortality. Namely, it resulted in an earlier onset of mortality but not in changes in the values of the slope and intercept. We conclude that the classical −3/2 slope is not useful to describe the biomass–density relationship in natural and semi‐natural plant communities. The magnitude and direction of variation in the slope are related to the age‐structure and abiotic stress intensity in the plant community. [ABSTRACT FROM AUTHOR]

Long and short-term climatic variation affect the ability of plants to simultaneously cope with increasing abiotic stress and biotic interactions. Specifically, ecotypes adapted to different climatic conditions (i.e., long-term legacy) may have to adjust their allocation to chemical defenses against enemies under acute drought (i.e., short-term response). Although several studies have addressed drought effects on chemical defense production, little is known about their intraspecific variation along resource gradients. Studying intraspecific variation is important for understanding how different environments select for defense strategies and how these may be affected directly and indirectly by changing climatic conditions. We conducted greenhouse experiments with the annual Biscutella didyma (Brassicaceae) to test the effects of long-term climatic legacy versus short-term drought stress on the concentrations of defense compounds (glucosinolates). To this aim, four ecotypes originating from a steep aridity gradient were exposed to contrasting water treatments. Concentrations of chemical defenses were measured separately in leaves of young (8 weeks) and old (14 weeks) plants, respectively. For young plants, ecotypes from the wettest climate (long-term legacy) as well as plants receiving high water treatments (short-term response) were better defended. A marginally significant interaction suggested that wetter ecotypes experienced a larger shift in defense production across water treatments. Older plants contained much lower glucosinolate concentrations and showed no differences between ecotypes and water treatments. Our results indicate that younger plants invest more resources into chemical defenses, possibly due to higher vulnerability to tissue loss compared to older plants. We propose that the strong response of wet ecotypes to water availability may be explained by a less pronounced adaptation to drought. [ABSTRACT FROM AUTHOR]

Water is one of the major drivers determining distribution and abundance of plant species. Namely, plant species’ presence and location in the landscape can be explained using metrics of soil water because plant species are restricted to a species-specific range of soil water conditions, i.e. their hydrological niche. However, little is known about the specific traits that determine the hydrological niche of a plant species. To investigate the relationship between plant functional traits, community structure and hydrological niche segregation, we developed a new generic individual-based model PLANTHeR which describes plant functional trait abundance as a function solely of soil water potentials and individual behavior. An important innovation is that there are no a priori defined trade-offs so that the model is neither restricted to a certain set of species nor scaled to a specific ecosystem. We show that PLANTHeR is able to reproduce well-known ecological rules such as the self-thinning law. We found that plant functional traits and their combinations (plant functional types − PFTs) were restricted to specific ranges of soil water potentials. Furthermore, the existence of functional trait trade-offs and correlations was determined by environmental conditions. Most interestingly, the correlation intensity between traits representing competitive ability and traits promoting colonization ability changed with water stress intensities in a unimodal fashion. Our results suggest that soil water largely governs the functional composition, diversity and structure of plant communities. This has consequences for predicting plant species’ response to changes in the hydrological cycle due to global change. We suggest that PLANTHeR is a flexible tool that can be easily adapted for further ecological-modelling studies. [ABSTRACT FROM AUTHOR]

Predicting responses of plant communities to environmental changes is a key challenge in ecology. Here we examined climate-related mechanisms regulating seedling dynamics of a common Mediterranean shrub. Our objective was to analyze effects of water availability on seedling survival and to determine whether geographical gradients can serve as proxy for predicting local climate change effects. We conducted a field experiment along a natural rainfall gradient with additional long-term rainfall manipulations at a Mediterranean site, enabling the investigation of the relative importance of biotic and abiotic factors on seedling dynamics. Along the natural and artificial rainfall gradient seedling survival increased with increasing soil water availability. However, seedling survival at the Mediterranean site yielded a clear trend of decreasing seedling survival with artificially increasing aridity whereas at the dry end of the geographical gradient seedling survival was relatively high. We attribute this pattern to biotic interactions, which appeared less negative at the dry end. These findings indicate that ignoring biotic interactions is misleading when predicting shifts in the distribution of species under climate change and that because of the complex interplay between abiotic and biotic factors, environmental gradients can be poor proxies for predicting the response of plant species to climate change. [ABSTRACT FROM AUTHOR]

Abstract: Disturbances are characteristic for many ecosystems. However, we still lack generalizations concerning their role in shaping communities, particularly when disturbances co-occur. To study such effects, we used a novel modeling approach that is unrestricted by a priori tradeoffs among specific plant traits, except for those generated by allocation principles. Thus, trait combinations were emergent properties associated with biotic and abiotic constraints. Specifically, we asked which traits dominate under specific disturbance regimes, whether single and combined disturbance regimes promote similar trait tradeoffs and how complex disturbance regimes affect species richness and functional diversity. Overall, disturbances’ temporal properties governed the outcome of combined disturbances and were a stronger assortative force than spatial disturbance properties: low temporal predictability decreased seed-dispersability and dormancy, but increased competitive ability and disturbance tolerance. Evidence for tradeoffs between different colonization modes and between dormancy and disturbance tolerance were found, while surprisingly, the widely accepted colonization–competition tradeoff was not generated. Diversity was highest at intermediate disturbance intensity, but decreased monotonically with increasing unpredictability. In accordance with our results, future models should avoid restrictive assumptions about tradeoffs to generate robust and more general predictions about the role of disturbances for community dynamics. [Copyright &y& Elsevier]

1. An unresolved question in plant ecology is how the balance between positive and negative interactions changes with environmental conditions. Recently, the debate has been expanded by Brooker & Kikvidze (2008) , reintroducing the importance, rather than the intensity, of interactions as the appropriate concept for empirical studies. Importance is the difference in performance with interactions relative to all affecting factors. 2. Positive interactions among plants (facilitation) are common in nature. However, the advocated importance indices fail to address facilitation and can thus not be applied to studying shifts in the balance between positive and negative interactions. The deficiencies of the current importance indices are both conceptual as well as mathematical. In particular, their maximum performance estimator is based on the assumption that it can only be attained in the absence of neighbours. 3. We suggest two major improvements. First, we use the target’s overall maximum performance as a surrogate for the plant optimum, i.e. we allow the possibility of higher performance with neighbours (i.e. facilitation). Second, we propose an alternative approach to treat the factors affecting measured performance. The new index is limited in its range [−1, 1], is symmetrical for competition and facilitation, and it preserves the intuitive nature of the original index by Brooker & Kikvidze (2008) . 4. Synthesis. The concept of interaction importance is useful for studying interactions along environmental gradients. In contrast to previous indices, our index accommodates facilitative interactions and thus offers a significant conceptual and methodological advancement. We advocate its use in future empirical studies. [ABSTRACT FROM AUTHOR]

1. Once neglected, the role of facilitative interactions in plant communities has received considerable attention in the last two decades, and is now widely recognized. It is timely to consider the progress made by research in this field. 2. We review the development of plant facilitation research, focusing on the history of the field, the relationship between plant–plant interactions and environmental severity gradients, and attempts to integrate facilitation into mainstream ecological theory. We then consider future directions for facilitation research. 3. With respect to our fundamental understanding of plant facilitation, clarification of the relationship between interactions and environmental gradients is central for further progress, and necessitates the design and implementation of experiments that move beyond the clear limitations of previous studies. 4. There is substantial scope for exploring indirect facilitative effects in plant communities, including their impacts on diversity and evolution, and future studies should connect the degree of non-transitivity in plant competitive networks to community diversity and facilitative promotion of species coexistence, and explore how the role of indirect facilitation varies with environmental severity. 5. Certain ecological modelling approaches (e.g. individual-based modelling), although thus far largely neglected, provide highly useful tools for exploring these fundamental processes. 6. Evolutionary responses might result from facilitative interactions, and consideration of facilitation might lead to re-assessment of the evolution of plant growth forms. 7. Improved understanding of facilitation processes has direct relevance for the development of tools for ecosystem restoration, and for improving our understanding of the response of plant species and communities to environmental change drivers. 8. Attempts to apply our developing ecological knowledge would benefit from explicit recognition of the potential role of facilitative plant–plant interactions in the design and interpretation of studies from the fields of restoration and global change ecology. 9. Synthesis: Plant facilitation research provides new insights into classic ecological theory and pressing environmental issues. Awareness and understanding of facilitation should be part of the basic ecological knowledge of all plant ecologists. [ABSTRACT FROM AUTHOR]