Inclusion of biotic stress (consumer pressure) alters predictions from the stress gradient hypothesis

1.  The stress gradient hypothesis (SGH) predicts a shift from net negative interactions in benign environments towards net positive in harsh environments in ecological communities. While several studies found support for the SGH, others found evidence against it, leading to a debate on how nature and strength of species interactions change along stress gradients, and to calls for new empirical and theoretical work.
2.  In the latest attempt in this journal, it is successfully argued how the SGH should be expanded by considering different life strategies of species (stress tolerance versus competitive ability) and characteristics of abiotic stress (resource versus non-resource based) over wider stress gradients (opposed to low–high contrasts), but the crucial role of biotic stress by consumers is largely ignored in this refinement.
3.  We point out that consumers strongly alter the outcome of species interactions in benign and harsh environments, and show how inclusion of consumer-incurred biotic stress alters the predicted outcome of interactions along resource- and non-resource-based stress gradients for stress-tolerant and competitive benefactors and beneficiaries.
4.   Synthesis. New studies should include stress gradients consisting of both abiotic and biotic components to disentangle their impacts, and to improve our understanding of how species interactions change along environmental gradients.     

Reproductive performance of native and alien colonizing phanerogams on a glacier foreland,Iles Kerguelen

The retreat of the Ampère Glacier, on Kerguelen, has left wide ice-free areas where five native and two alien vascular plant species are involved in primary colonization. The number of seeds produced by individual fertile plants has been determined for each species and germination capacity has been tested under different temperature and light conditions. Colobanthus kerguelensis and Cerastium fontanum produced the highest number of seeds per plant. No dormancy mechanism occurred in either species and they appeared to be the most successful colonizers during the early stages following the deglaciation. No seeds of Azorella selago and Agrostis magellanica had germinated after 2 months. Dormancy was demonstrated only in Poa kerguelensis. Festuca contracta was the only species which exhibited a negative photo-sensitivity. Germination of indigenous seeds required relatively high temperatures, whereas the optimum temperature for germination in the introduced Poa annua was 10°C. These results are compared with the few data available in the subantarctic literature and ecological implications are discussed.     

Primary succession on glacier forelands in the subantarctic Kerguelen Islands

Abstract. Primary succession was studied on recently de-glaciated areas in front of the Ampère Glacier, Kerguelen Islands (49° 30′S, 69° 30′E). Vegetation, colonization processes and soil development were investigated on seven sites on the outwash plain over a distance of 5 km and representing a > 200-yr old chronosequence. Seven species are involved in the succession, showing four patterns of change: (1)‘pioneer’ (Poa kerguelensis, Poa annua, Colobanthus kerguelensis and Cerastium fontanum); (2)‘intermediate’ (Festuca contracta); (3)‘late colonizer’ (Azorella selago); and (4)‘fluctuating’ (Agrostis magellanica). Two of the pioneers are introduced species (Poa annua and Cerastium). The total plant cover increases significantly with the age of the sites but never exceeds 4.2 %. The plant succession pattern observed in this study appears to be related to soil development. Root architecture and fine particle dynamics in the soil may explain the behaviour of the different species along the chronosequence. The pioneer species have a very shallow root system whereas the intermediate and late colonizers are deeper and longer rooted. The dominance of Azorella selago (Apiaceae) in the oldest community may be attributed to its unique root system in relation to the vertical distribution of silt cappings in the soil.     

A field test of the stress‐gradient hypothesis along an aridity gradient

Aims: The stress‐gradient hypothesis (SGH) predicts how plant interactions change along environmental stress gradients. We tested the SGH in an aridity gradient, where support for the hypothesis and the specific shape of its response curve is controversial. Location: Almería, Cáceres and Coimbra, three sites in the Iberian Peninsula that encompass the most arid and wet habitats in the distribution range of a nurse shrub species –Retama sphaerocarpa L. (Boiss) – in Europe. Methods: We analysed the effect of Retama on its understorey plant community and measured the biomass and species richness beneath Retama and in gaps. We estimated the frequency (changes in species richness), importance and intensity of the Retama effects, and derived the severity–interaction relationship pattern, analysing how these interaction indices changed along this aridity gradient. Results and conclusions: The intensity and frequency of facilitation by Retama increased monotonically with increasing environmental severity, and the importance tended to have a similar pattern, overall supporting the SGH. Our data did not support predictions from recent revisions of the SGH, which may not apply to whole plant communities like those studied here or when interactions are highly asymmetrical. Facilitation by Retama influenced community composition and species richness to the point that a significant fraction of species found at the most arid end of the gradient were only able to survive beneath the nurse shrub, whereas some of these species were able to thrive in gaps at more mesic sites, highlighting the ecological relevance of facilitation by nurse species in mediterranean environments, especially in the driest sites.     

Isolation and cross-amplification of the first set of polymorphic microsatellite markers of two high-Andean cushion plants

In the southern Andes mountains (27–\(39{^{\circ }}\hbox {S}\)) Azorella madreporica and Laretia acaulis, two Apiaceae cushion plant species commonly known as yaretas, conform a well-established altitudinal vegetation belt along the lower Andean zone. These species have been considered as fundamental components of several ecological dynamics within their communities; however, high-mountain ecosystems are increasingly threatened worldwide by natural and anthropogenic pressures and the southern Andes are not the exception. Recognizing that genetic information is crucial for the success of any conservation or restoration initiative in wild populations, we developed and cross-amplified 28 specifically designed microsatellite markers (14 in A. madreporica and 14 in L. acaulis), and also tested the cross amplification of 25 markers from the related species Azorella selago. In a region which is particularly vulnerable to global change trends, this new polymorphic microsatellite loci will be useful in the study of the genetic diversity of these high-mountain cushion plants, which are pivotal in the structuring of their native ecosystems.     

Fine nurse variations explain discrepancies in the stress‐interaction relationship in alpine regions

Despite a large consensus on increasing facilitation among plants with increasing stress in alpine regions, a number of different outcomes of interaction have been observed, which impedes the generalisation of the ‘stress‐gradient hypothesis’ (SGH). With the aim to reconcile the different viewpoints on the stress‐interaction relationship in alpine environments we hypothesized that fine nurse variations within a single life form (cushion) may explain this pattern variability. To test this hypothesis, we compared the magnitude of the stress‐interaction relationship in a single study area with that observed in existing studies involving cushions, worldwide. We characterized the nurse effects of cushions on the whole plant community at inter‐specific, intra‐specific and intra‐individual levels along a stress gradient in the dry, alpine tropics of Bolivia (4400 m, 4700 m and 4900 m a.s.l). Using a relative index of interaction (RII) we included our data in a meta‐analysis on the nurse effects of cushions along alpine gradients, worldwide. At inter‐specific level, the loose cushion Pycnophyllum was a better nurse than the compact Azorella compacta. However, at intra‐individual level facilitation was higher at the periphery than at the centre of cushions, exceeding in magnitude the variation observed at inter‐specific level. This pattern was associated with higher minimum temperature and lower mortality at the periphery of cushions. The net effects of cushions on plant communities became more positive at higher elevation, corroborating the SGH. Within our single site in Bolivia, fine morphological nurse variations captured a similar variability in the stress‐interaction relationship as that observed in a subset of studies on cushions on a worldwide scale. This suggests that fine variations in nurse traits, in general those not considered in protocols dealing with facilitation or in restoration/conservation management plans, explain in part the current discrepancies among SGH studies in alpine regions.     

Facilitative interactions do not wane with warming at high elevations in the Andes

Positive interactions between species are known to play an important role in the structure and dynamics of alpine plant communities. The balance between negative and positive interactions is known to shift along spatial and temporal gradients, with positive effects prevailing over negative ones as the environmental stress increases. Thus, this balance is likely to be affected by climate change. We hypothesized that increases in temperature (a global warming scenario) should decrease the importance of positive interactions for the survival and growth of alpine plant species. To test this hypothesis, we selected individuals of the native grass species Hordeum comosum growing within the nurse cushion species Azorella madreporica at 3,600 m.a.s.l. in Los Andes (Chile), and performed nurse removal and seedling survival experiments under natural and warmer conditions. For warmer conditions, we used open-top chambers, which increased the temperature by 4 °C. After two growing seasons, we compared the effect of nurse removal on the survival, biomass, and photochemical efficiency of H. comosum individuals under warmer and natural conditions. Nurse removal significantly decreased the survival, biomass, and photochemical efficiency of H. comosum, demonstrating the facilitative effects of nurse cushions. Seedling survival was also enhanced by cushions, even under warmer conditions. However, warmer conditions only partially mitigated the negative effects of nurse removal, suggesting that facilitative effects of cushions do not wane under warmer conditions. Thus, facilitative interactions are vital to the performance and survival of alpine species, and these positive interactions will continue to be important in the warmer conditions of the future in high-alpine habitats.     

Extreme drought stress shifts net facilitation to neutral interactions between shrubs and sub‐canopy plants in an arid desert

The stress gradient hypothesis (SGH) predicts that the importance or intensity of competition and facilitation will change inversely along abiotic stress gradients. It was originally postulated that increasing environmental stress can induce a monotonic increase in facilitation. However, more recent models predicted that the relationship between severity and interaction exhibits a hump‐shaped pattern, in which positive interactions prevail under moderate stress but decline at the extreme ends of stress gradients. In the present study, we conducted a field experiment along a temporal rainfall gradient for five consecutive years, in order to investigate interactions in a shrub‐herbaceous plant community at the southern edge of the Badain Jaran Desert, and, more specifically, investigated the effects of Calligonum mongolicum, a dominant shrub species, on both abiotic environmental variables and the performance of sub‐canopy plant species. We found that shrubs can improve sub‐canopy water regimes, soil properties, plant biomass, density, cover, and richness and, more importantly, that the positive effect of shrubs on sub‐canopy soil moisture during the summer diminishes as rainfall decreases, a pattern that partly explains the collapse of the positive interaction between shrubs and their understory plants. These results provide empirical evidence that the positive effect of shrubs on understory plant communities in extreme arid environments may decline and become neutral with increasing drought stress.     

植物邻体间的正相互作用

植物间的正负相互作用是构建植被群落的重要因素,也是群落生态学研究的中心内容之一。近20a来,植物间正相互作用的研究得到快速发展。综述了正相互作用的定义,不同植物群落中的直接、间接正相互作用及其发生机制,正相互作用研究的实验和模型方法,正负相互作用随胁迫梯度的变化及正相互作用对群落构建的影响。探讨了正相互作用研究前景:(1)进一步理解正负相互作用的平衡及其对群落构建的影响;(2)加深对全球变暖背景下的正相互作用的认识;(3)需把正相互作用研究同进化联系起来;(4)充分发挥正相互作用在生态系统中的推动力作用,把正相互作用应用到生态恢复中,为恢复退化生态系统服务。     

Plant-soil feedbacks and root responses of two Mediterranean oaks along a precipitation gradient

Aims

Plant-soil feedbacks (PSFs) have been shown to be relevant drivers of forest community dynamics. However, few studies have explored variation of PSFs along environmental gradients. In a framework of climate change, there is a great need to understand how interactions between plants and soil microbes respond along climatic gradients. Therefore, we compared PSFs along a precipitation gradient in Mediterranean oak forests and included trait responses. Following the Stress Gradient Hypothesis (SGH), we expected less negative or even positive PSFs in the physically harsh dry end of our gradient and more negative PSFs in the wettest end.

Methods

We grew Quercus ilex and Quercus suber acorns on soil inoculated with microbes sampled under adults of both species in six sites ranging in annual precipitation. After 4 months, we measured shoot biomass and allocation and morphological traits above and belowground.

Results

We found negative PSFs for Q. ilex independent of precipitation, whereas for Q. suber PSFs ranged from positive in dry sites to negative in wet sites, in agreement with the SGH. The leaf allocation showed patterns similar to shoot biomass, but belowground allocation and morphological traits revealed responses which could not be detected aboveground.

Conclusions

We provide first evidence for context-dependent PSFs along a precipitation gradient. Moreover, we show that measuring root traits can help improve our understanding of climate-dependent PSFs. Such understanding helps to predict plant soil microbe interactions, and their role as drivers of plant community dynamics under ongoing climate change.

     

A temporal dimension to the stress gradient hypothesis for intraspecific interactions

Space and time are the two fundamental drivers of ecological dynamics. Studies exploring the Stress gradient hypothesis (SGH) – which predicts that the patterns of interspecific interactions shift from negative to positive with increasing environmental severity – conceptualize environmental severity predominantly from a spatial perspective. Here, from a temporal perspective and for intraspecific interactions, we asked: do the predictions of the SGH at the intraspecific level apply to seasonal change in environmental severity? We conducted a field experiment, which was complemented by a two‐year field survey of natural populations of the non‐native biennial forb Alliaria petiolata at the Koffler Scientific Reserve, Ontario, Canada. In both experiment and field survey studies we found statistically significant negative density‐dependent survival in the productive summer period and positive density‐dependent survival over the severe winter period. Effects were stronger in the field experiment than in the survey of natural populations. We suggest that the SGH at the intraspecific level may be applicable to seasonal variation in environmental severity, though our ability to detect its effect in natural communities may depend on other factors such as species dominance and environmental heterogeneity.     

Extending the stress‐gradient hypothesis – is competition among animals less common in harsh environments?

The role of positive interactions has become widely accepted as a mechanism shaping community dynamics. Most empirical evidence comes from plant communities and sessile marine organisms. However, evidence for the relative role of positive interactions in organizing terrestrial animal communities is more limited, and a general framework that includes positive interactions among animals is lacking. The ‘stress gradient hypothesis’ (SGH) developed by plant ecologists predicts that the balance between positive and negative interactions will vary along gradients of biotic and abiotic stress, with positive interactions being more important in stressful environments. Paralleling the SGH, stress gradients for terrestrial herbivores could be equated to inverse primary productivity gradients, so we would expect positive interactions to prevail in more stressful, low productivity environments. However, this contradicts the typical view of terrestrial animal ecology that low primary productivity systems will foster intense competition for resources among consumers. Here we use alpine herbivores as a case study to test one of the predictions of the SGH in animal communities, namely the prevalence of positive interactions in low productivity environments. We identify potential mechanisms of facilitation and review the limited number of examples of interspecific interactions among alpine herbivores to assess the role of positive and negative interactions in structuring their communities. A meta‐analysis showed no clear trend in the strength and direction of interactions among alpine herbivores. Although studies were biased towards reporting significant negative inter actions, we found no evidence of competition dominating in harsh environments. Thus, our results only partially support the SGH, but directly challenge the dominant view among animal ecologists. Clearly, a sound theoretical framework is needed to include competition, positive and neutral interactions as potential mechanisms determining the structure of animal communities under differing environmental conditions, and the stress‐gradient hypothesis can provide a solid starting point.     

Shifting Species Interaction in Soil Microbial Community and Its Influence on Ecosystem Functions Modulating

The supportive and negative evidence for the stress gradient hypothesis (SGH) led to an ongoing debate among ecologists and called for new empirical and theoretical work. In this study, we took various biological soil crust (BSCs) samples along a spatial gradient with four environmental stress levels to examine the fitness of SGH in microbial interactions and evaluate its influence on biodiversity–function relationships in BSCs. A new assessment method of species interactions within hard-cultured invisible soil community was employed, directly based on denaturing gradient gel electrophoresis fingerprint images. The results showed that biotic interactions in soil phototroph community dramatically shifted from facilitation to dominant competition with the improvement of microhabitats. It offered new evidence, which presented a different perspective on the hypothesis that the relative importance of facilitation and competition varies inversely along the gradient of abiotic stress. The path analysis indicated that influence of biotic interactions (r?=?0.19, p?<?0.05) on ecosystem functions is lower than other community properties (r?=?0.62, p?<?0.001), including soil moisture, crust coverage, and biodiversity. Furthermore, the correlation between species interactions and community properties was non-significant with low negative influence (r?=??0.27, p?>?0.05). We demonstrate that the inversion of biotic interaction as a response to the gradient of abiotic stresses existed not only in the visible plant community but also in the soil microbial community.     

Spatial variation in plant interactions across a severity gradient in the sub-Antarctic

The stress–gradient hypothesis predicts that the intensity of interspecific positive interactions increases along environmental severity (i.e. stress and disturbance) gradients faster than the intensity of negative interactions. This study is the first to test if the stress–gradient hypothesis is supported for a location in the climatically extreme and species-poor sub-Antarctic. To do so, we investigate the fine-scale spatial distribution of plant species across altitude- and aspect-related abiotic severity gradients on a scoria cone on Marion Island. A clear altitudinal severity gradient was observed across the scoria cone, with lower temperatures, stronger winds and greater soil movement at higher altitudes. The altitudinal severity gradient was matched by stronger interspecific spatial association between the four dominant species at higher altitudes and in areas of lower vegetation cover. This suggests that, relative to the intensity of competition, the intensity of facilitation is greater under more severe conditions, supporting the stress–gradient hypothesis at the community level (i.e. for multiple pairs of species) and corroborating its usefulness for predicting variation in plant interactions at high latitudes and altitudes. Furthermore, the directional intraspecific aggregation and interspecific association plant cover patterns found within the gradient suggest that protection from the prevailing wind and from burial by loose substrate are the dominant facilitative mechanisms. Thus, plants benefit from the presence of neighbours when they provide shelter and substrate stability, and the relative intensity of this positive interaction is greatest at higher altitudes, and varies between species pairs. This study, therefore, not only provides support for the stress–gradient hypothesis in the sub-Antarctic, but also demonstrates fine-scale directional spatial patterns between multiple species nested within the severity gradient. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Resource conversion: a generalizable mechanism for resource-mediated positive species interactions

Positive species interactions are ubiquitous in natural communities, but the mechanisms through which they operate are poorly understood. One proposed mechanism is resource conversion – the conversion by a benefactor species of a resource from a resource state that is inaccessible to a potential beneficiary species into a resource state that is accessible. Such conversion often occurs as a byproduct of resource consumption, and sometimes in exchange for non-resource benefits to the benefactor species. At least five known classes of interactions, including both facilitative and mutualistic ones, may be classified as resource conversion interactions. We formulated a generalizable mathematical model for resource conversion interactions and examined two model variants that represent processing chain and nurse plant interactions. We examined the conditions under which these conformed to the stress-gradient hypothesis (SGH), which predicts increased interaction benefits in more stressful environments. These yielded four key insights: 1) resource conversion interactions can be positive (towards the resource recipient) only when facilitator-mediated resource conversion is more efficient than the baseline, spontaneous, facilitator-independent resource conversion; 2) the sign of resource conversion interaction outcomes never switches (e.g. from net positive to net negative) with changing levels of resource availability, when all other parameters are kept constant; 3) processing chain interactions at equilibrium can never be positive in a manner that conforms to the SGH; 4) nurse plant interactions can be positive and conform to the SGH, although the manner in which they do depends largely on how resource stress is defined, and the environmental supply rate of surface soil moisture. The first two insights are likely to be generalizable across all resource conversion interactions. The general agreement of the model with empirical studies suggest that resource conversion is the mechanism underlying the aforementioned interactions, and an ecologically meaningful way of classifying these previously unassociated positive species interactions.     

What does the stress‐gradient hypothesis predict? Resolving the discrepancies

In recent years the importance of facilitative interactions in ecological communities is increasingly recognized. This phenomenon has been observed repeatedly, particularly in vegetation communities, in a wide range of environmental conditions. The current hypothesis predicts that the role of facilitation becomes increasingly important in conjunction with increasing stress. Several empirical studies, however, failed to detect such patterns, particularly at the extreme ends of the stress gradients. Herein, we present a conceptual model that may resolve discrepancies between expected and observed and provides a more precise framework of the existing hypotheses. By relaxing two common assumptions commonly used by the stress‐gradient hypothesis (SGH) we are able to demonstrate that under some circumstances the importance of facilitation may be less at the extreme ends of these gradients. Namely, we first re‐emphasize the notion that physiological response is not linear with respect to environmental changes along stress gradients. Second, it is argued that the net outcome of facilitative and competitive interactions is reflected in the fitness of individuals as a product of these two processes, in contrast to the commonly applied assumption of additivity. Accordingly, a synthesis of the concepts of population biology (measures of fitness) and plant physiology (nonlinear responses) with the stress gradient hypothesis while retaining the original simplicity of the SGH model contributes to a better specification of the predictions of the stress‐gradient hypothesis and the resolution of observed contradictions.     

Food selection by the guanaco (Lama guanicoe) along an altitudinal gradient in the Southern Andean Precordillera (Argentina)

Wild ungulates like the guanaco are exposed to important changes in climate and plant diversity along altitudinal gradients in the Andes Mountains, such as in the Southern Andean Precordillera where three phytogeographic provinces are present in altitudinal belts. The guanaco’s diet and food availability were seasonally analyzed using microhistological analysis and point-quadrat transects at six sampling sites, representative of the phytogeographic belts along the altitudinal gradient. Plant cover and diversity decreased with growing altitude. Richness of plant species was poorer at the summit than in the lower altitudes, whereas the proportion of species eaten by guanacos increased with altitude. The diet included 77 species. Grasses were preferred and shrubs were avoided all year round. The grass Poa spp. occupied more than 50 % of the diet at all altitudes. Grasses were the main dietary item even at low altitudes, where shrubs constituted the main food available. Decreasing generalism with descending phytogeographic belts agrees with the prediction for altitudinal gradients. The increase of diversity in the diet during the winter decline of plant cover at high and middle altitudes follows that expected from the optimal foraging theory. The winter decline of vegetation and the dietary shift from grazing to browsing proved to be stronger as altitude increases and the climate become more rigorous. Plant species richness, food scarcity, and climate severity are relevant variables to explain altitudinal and seasonal changes in the diet of adaptive ungulates in mountain environments, such as the guanaco in the Southern Andean Precordillera.     

Beyond dual-lattice models: Incorporating plant strategies when modeling the interplay between facilitation and competition along environmental severity gradients

We introduce a spatially explicit model that evaluates how the trade-offs between the life strategies of two interacting plant species affect the outcome of their interaction along environmental severity gradients. In our model, we represent the landscape as a two-dimensional lattice, with environmental severity increasing from left to right. Two species with different strategies, a competitor and a stress-tolerant, interact in the lattice. We find that facilitation expands the realized niche of the competitor into harsh environments by suppressing the stress-tolerant species. Most of their coexisting range is dominated by a positive effect of one species on another, with a reciprocal negative effect from the species receiving the benefits on its benefactor (“+, −”), whereas mutualistic (“+, +”) interactions are only found in the harshest part of the environmental gradient. Contrarily as assumed by models commonly used in facilitation research (e.g. dual-lattice models), our results indicate that “+, +” interactions are not dominant, and that their differences with “+, −” interactions along environmental severity gradients depend on the strategies of the interacting species. By integrating the trade-off between competitive ability and stress tolerance, our model provides a new framework to investigate the interplay of facilitative and competitive interactions along environmental gradients and their impacts on processes such as population dynamics and community organization.     

Temperature variation across Marion Island associated with a keystone plant species (Azorella selago Hook. (Apiaceae))

Microclimate is the most appropriate measure of climate affecting species. Understanding microclimate variation is essential for predicting effects of climate change on species. This study examined (1) variation in microclimate temperatures associated with Azorella selago Hook. (Apiaceae) across Marion Island, (2) differences between microclimate temperature and meteorological station temperatures, and (3) effect of A. selago on microclimate temperatures. Microclimate temperatures were shown to vary significantly with altitude and island side. The microclimate associated with A. selago was also more extreme than meteorological station temperature ranges suggest. A. selago was shown to ameliorate temperature conditions compared to those on the ground. Given the biotic differences that have been documented between the sides of Marion Island, this finding argues strongly for improved understanding of spatial variability in Marion Island’s climate. Such understanding is particularly critical given the rapid rate of climate change currently being experienced by the island.     

Geomorphological disturbance affects ecological driving forces and plant turnover along an altitudinal stress gradient on alpine slopes

The altitudinal gradient is considered as a stress gradient for plant species because the development and fitness of plant communities tend to decrease as a result of the extreme environmental conditions present at high elevations. Abiotic factors are predicted to be the primary filter for species assemblage in high alpine areas, influencing biotic interactions through both competition for resources and positive interactions among species. We hypothesised that the relative importance of the ecological driving forces that affect the biotic interactions within plant communities changes along an elevation gradient on alpine debris slopes. We used multiple gradient analyses of 180 vegetation plots along an altitudinal range from ~1,600 to 2,600 m and single 100 m-bands in the Adamello-Presanella Group (Central Alps) to investigate our hypothesis; we measured multiple environmental variables related to different ecological driving forces. Our results illustrate that resource limitations at higher elevations affect not only the shift from competition to facilitation among species. A geomorphological disturbance regime along alpine slopes favours the resilience of the high-altitude species within topographic/geomorphological traps. An understanding of the ecological driving forces and positive interactions as a function of altitude may clarify the mechanisms underlying plant responses to present and future environmental changes.