The interplay of facilitation and competition drives the emergence of multistability in dryland plant communities

Within communities, species are wrapped in a set of feedbacks with each other and with their environment. When such feedbacks are strong enough they can generate alternative stable states. So far, research on alternative stable states has mostly focused on systems with a small number of species and a limited diversity of interaction types. Here, we analyze a spatial model of plant community dynamics in stressed ecosystems such as drylands, where each species is characterized by a strategy, and the different species interact through facilitation and competition for space and resources, such as water. We identify three different types of multistability emerging from the interplay of competition and facilitation. Under low-stress levels, plant communities organize in small groups of coexisting species, maintained by space, competition and facilitation ("cliques"). Under higher stress levels, positive feedback from facilitation lead to the dominance of a single facilitating species ("mutual exclusion states"). At the highest stress levels, the single facilitating species left in the system coexists with the desert state. By linking community ecology and alternative stable states theory using a spatial plant community model for stressed ecosystems, our study contributes to highlight the importance of positive feedback loops for the stability of ecological communities.

Polyploidy and environmental stress response: a comparative study of fern gametophytes

Climate change is rapidly altering natural habitats and generating complex patterns of environmental stress. Ferns are major components of many forest understories and, given their independent gametophyte generation, may experience unique pressures in emerging temperature and drought regimes. Polyploidy is widespread in ferns and may provide a selective advantage in these rapidly changing environments. This work aimed to understand whether the gametophytes of allopolyploid ferns respond differently to climate-related physiological stress than their diploid parents. The experimental approach involved a multifactorial design with 27 treatment combinations including exposure to multiple levels of drought and temperature over three treatment durations, with recovery measured at multiple timepoints. We measured Chl fluorescence from over 2000 gametophytes to evaluate stress avoidance and tolerance in diploid and polyploid species. Polyploids generally showed a greater ability to avoid and/or tolerate a range of stress conditions compared with their diploid counterparts, suggesting that polyploidy may confer enhanced flexibility and resilience under climate stress. Overall, these results suggest that polyploidy may provide some resilience to climate change in mixed ploidy populations. However, all species remain susceptible to the impacts of extreme drought and heat stress.

Microbial functional heterogeneity induced in a petroleum-polluted soil profile

Microbial taxonomic diversity declines with increasing stress caused by petroleum pollution. However, few studies have tested whether functional diversities vary similarly to taxonomic diversity along the stress gradient. Here, we investigated soil microbial communities in a petrochemically polluted site in China. Total petroleum hydrocarbon (TPH) concentrations were higher in the middle (2-3 m) and deep soil layer (3-5 m) than in the surface soil layer (0-2 m). Accordingly, microbial taxonomic α-diversity was decreased by 44% (p < 0.001) in the middle and deep soil layers, compared to the surface soil layer. In contrast, functional α-diversity decreased by 3% (p < 0.001), showing a much better buffering capacity to environmental stress. Differences in microbial taxonomic and functional β-diversities were enlarged in the middle and deep soil layers, extending the Anna Karenina Principle (AKP) that a community adapts to stressful environments in its own way. Consistent with the stress gradient hypothesis, we revealed a higher degree of network connectivity among microbial species and genes in the middle and deep soil layers compared to the surface soil layer. Together, we demonstrate that microbial functionality is more tolerant to stress than taxonomy, both of which were amenable to AKP and the stress gradient hypothesis.

Facilitation and competition deconstructed: a mechanistic modelling approach to the stress gradient hypothesis applied to drylands

Facilitative interactions among species are key in plant communities. While experimental tests support the Stress Gradient Hypothesis (SGH) as an association between facilitation and stress, whether the shape of net effects along stress gradients can be predicted is controversial, with no available mathematical modelling approaches. We proposed a novel test, using a modification of the R* model to study how negative and positive partial effects of plant interactions in drylands combine along two common stress gradients. We modelled different interactions: competition for water and light, amelioration of soil infiltration and/or grazing protection, obtaining that intensity and importance of facilitation did not generally increase along stress gradients, being dependent on the interaction type. While along the water stress gradient net interactions became more positive, reaching a maximum and then waning again, various outcomes were observed along the grazing gradient. Shape variety was mainly driven by the various shapes of the partial positive effects. Under resource stress, additive interaction effects can be expected, whereas when including grazing, the effects were non-additive. In the context of the SGH, deconstructing the effect of positive and negative interaction in a pairwise mechanistic models of drylands does not show a unique shape along stress gradients.

Relative contribution of canopy and soil effects between plants with different metal tolerance along a metal pollution gradient

Multiple effects, operating either on the long-term (soil-engineering effects) or on the short-term during plant life (microclimate modification or resources pre-emption), can act simultaneously and determine the outcome of plant-plant interactions. These diverse effects have not been disentangled along a gradient of metal/metalloid pollution, although this is crucial for understanding the dominant species turnover along the gradient, and thus the driving processes of facilitation recurrently found in metalliferous ecosystems, which could help improving ecological restoration of these degraded ecosystems. Here, we experimentally assessed different short-term effects of two dominant forbs of highly polluted habitats (Hutchinsia alpina and Arenaria multicaulis, tolerant to metal stress) and two grasses of less polluted habitats (Agrostis capillaris and Festuca rubra, less tolerant to metal stress) on target plant species (the same as the dominant species mentioned above) transplanted along a large metal pollution gradient. Additionally, in highly polluted environments, we differentiated short- from long-term effects of the two metallicolous forbs, which had different abilities to concentrate metals in their leaves. In line with other studies along metal gradients, variation of short-term interactions appeared to follow the Stress Gradient Hypothesis for plants less adapted to metal pollution (p = 0.030), with positive interactions dominating in most severe areas. Regarding long-term effects, the species with highest leaf metal-accumulation showed no negative effect contrary to the Elemental allelopathy Hypothesis. Long-term effects of the species with lower leaf-metal accumulation could not be determined because of the occurrence of an unexpected difference in micro-habitat conditions (soil depth and humidity) for this species along the metal pollution gradient. Increasing short-term facilitation along metal pollution gradients, which confirmed previous studies, is promising for improving conditions and restoring the most polluted environments. However, long-term results stressed the difficulty to quantify these effects given that these areas are highly fragmented and heterogeneous.

Two common, often coexisting grassland plant species differ in their evolutionary potential in response to experimental drought

For terrestrial plant communities, the increase in frequency and intensity of drought events is considered as one of the most severe consequences of climate change. While single-species studies demonstrate that drought can lead to relatively rapid adaptive genetic changes, the evolutionary potential and constraints to selection need to be assessed in comparative approaches to draw more general conclusions. In a greenhouse experiment, we compare the phenotypic response and evolutionary potential of two co-occurring grassland plant species, Bromus erectus and Trifolium pratense, in two environments differing in water availability. We quantified variation in functional traits and reproductive fitness in response to drought and compared multivariate genetic variance-covariance matrices and predicted evolutionary responses between species. Species showed different drought adaptation strategies, reflected in both their species-specific phenotypic plasticity and predicted responses to selection indicating contrasting evolutionary potential under drought. In T. pratense we found evidence for stronger genetic constraints under drought compared to more favourable conditions, and for some traits plastic and predicted evolutionary responses to drought had opposing directions, likely limiting the potential for adaptive change. Our study contributes to a more detailed understanding of the evolutionary potential of species with different adaptive strategies in response to climate change and may help to inform future scenarios for semi-natural grassland ecosystems.

A Case for the "Competitive Exclusion-Tolerance Rule" as a General Cause of Species Turnover along Environmental Gradients

AbstractClosely related, ecologically similar species often segregate their distributions along environmental gradients of time, space, and resources, but previous research suggests diverse underlying causes. Here, we review reciprocal removal studies in nature that experimentally test the role of interactions among species in determining their turnover along environmental gradients. We find consistent evidence for asymmetric exclusion coupled with differences in environmental tolerance causing the segregation of species pairs, where a dominant species excludes a subordinate from benign regions of the gradient but is unable to tolerate challenging regions to which the subordinate species is adapted. Subordinate species were consistently smaller and performed better in regions of the gradient typically occupied by the dominant species compared with their native distribution. These results extend previous ideas contrasting competitive ability with adaptation to abiotic stress to include a broader diversity of species interactions (intraguild predation, reproductive interference) and environmental gradients, including gradients of biotic challenge. Collectively, these findings suggest that adaptation to environmental challenge compromises performance in antagonistic interactions with ecologically similar species. The consistency of this pattern across diverse organisms, environments, and biomes suggests generalizable processes structuring the segregation of ecologically similar species along disparate environmental gradients, a phenomenon that we propose should be named the competitive exclusion-tolerance rule.

Effects of microtopographic patterns on plant growth and soil improvement in coastal wetlands of the Yellow River Delta

INTRODUCTION

To clarify the effects of microtopography on plant growth and soil water, salt and nutrient characteristics of saline soils in mudflats within muddy coastal zones and explore suitable microtopographic modifications.

METHODS

Six microtopographic modification patterns, namely, S-shaped, stripe-shaped, pin-shaped, stepshaped, dense stripe-shaped and crescent-shaped patterns, were established in the coastal mudflats of the Yellow River Delta. The soil water, salt, ion, total carbon, total nitrogen, and total phosphorus contents and their ecological stoichiometric characteristics were measured and analyzed after theimplementation of different microtopographic modification patterns, with bare mudflats as the control.

RESULTS

The results showed that microtopographic modification significantly changed the soil water and salt contents and the soil total carbon, total nitrogen and total phosphorus contents. Compared with the bare ground, microtopographic transformation significantly promoted the growth of the pioneer plant Suaeda salsa, significantly increased the soil water and nutrient contents, and significantly decreased the soil salinity. The soil salinity was mainly reduced by Na⁺ and Cl^- ions. The soil salinity and nutrient contents gradually decreased with increasing soil depth, indicating the occurrence of surface aggregation. Compared to that of the bare ground, the soil C/N was significantly lower and the N/P was significantly higher in the microtopographic treatments, and the overall performance suggested soil N limitation. The ions contained in the saline soil were dominated by Na⁺ and Cl^-, followed by Mg²⁺ and SO₄ ^2-, with lower contents of K⁺, Ca²⁺ and HCO₃ ^-. Among the six microtopography modification patterns, the crescent-shaped pattern best promoted vegetation restoration. This pattern was the most effective in reducing soil salinity, with a 98.53% reduction in soil salinity compared with that of bare ground, followed by the pin-shaped pattern. Compared with that in the bare ground samples, the nutrient content in the samples from the step-shaped modification increased by 23.27%; finally, the S-shaped, step-shaped and dense stripe-shaped patterns performed poorly in terms of plant restoration and soil improvement.

DISCUSSION

It is suggested that a crescent-shaped pattern should be considered first when carrying out microtopographic transformation on the beaches of the Yellow River Delta, followed by stripe-shaped and pin-shaped patterns. The dense strip-shaped should not be adopted.

Drying Shapes Aquatic Fungal Community Assembly by Reducing Functional Diversity

Aquatic fungi are highly diverse organisms that play a critical role in global biogeochemical cycles. Yet it remains unclear which assembly processes determine their co-occurrence and assembly patterns over gradients of drying intensity, which is a common stressor in fluvial networks. Although aquatic fungi possess drying-specific adaptations, little is known about how functional similarity influences co-occurrence probability and which functional traits are sorted by drying. Using field data from 15 streams, we investigated how co-occurrence patterns and assembly processes responded to drying intensity. To do so, we determined fungal co-occurrence patterns, functional traits that best explain species co-occurrence likelihood, and community assembly mechanisms explaining changes in functional diversity over the drying gradient. Our results identified 24 species pairs with positive co-occurrence probabilities and 16 species pairs with negative associations. The co-occurrence probability was correlated with species differences in conidia shape and fungal endophytic capacity. Functional diversity reduction over the drying gradient is generally associated with non-random abiotic filtering. However, the assembly processes changed over the drying gradient, with random assembly prevailing at low drying intensity and abiotic filtering gaining more importance as drying intensifies. Collectively, our results can help anticipate the impacts of global change on fungal communities and ecosystem functioning.

Contrasting effects of two phenotypes of an alpine cushion plant on understory species drive community assembly

In alpine systems, cushion plants act as foundation species by ameliorating local environmental conditions. Empirical studies indicate that contrasting phenotypes of alpine cushion species have different effects on understory plant species, either facilitative or competitive. Furthermore, dependent species within each community type might also exhibit different responses to each cushion phenotype, which can be clustered into several "response groups". Additionally, these species-groups specific responses to alpine cushion species phenotypes could alter community assembly. However, very few studies have assessed responses of dependent communities at species-group levels, in particular for both above- and below-ground communities. Here, we selected a loose and a tight phenotype of the alpine cushion species Thylacospermum caespitosum in two sites of northwest China, and use the relative intensity of interactions index to quantify cushion plant effects on subordinate communities of plants and soil fungi and bacteria. We assessed variations in responses of both above- and below-ground organisms to cushion plant effects at species-group level. Species-group level analyses showed that the effects of the phenotype varied among groups of each of the three community types, and different species-groups were composed by unique taxa. Additionally, we found that loose cushions enhanced stochastic processes in community assembly, for plants and soil fungi but not for soil bacteria. These variations of phenotypic effects on different species-group induced contrasting taxonomic composition between groups, and alter community assembly thereby. Our study highlights the occurrence of contrasting effects of two phenotypes of a foundation cushion plant on understory plants, soil fungi and bacteria community composition, but not necessarily on their richness. We also showed that assessing responses of understory species at the species-group level allows a more realistic and mechanistic understanding of biotic interactions both for above- and below-ground communities.

Shrub-mediated effects on soil nitrogen determines shrub-herbaceous interactions in drylands of the Tibetan Plateau

INTRODUCTION

Shrub promotes the survival, growth and reproduction of understory species by buffering the environmental extremes and improving limited resources (i.e., facilitation effect) in arid and semiarid regions. However, the importance of soil water and nutrient availability on shrub facilitation, and its trend along a drought gradient have been relatively less addressed in water-limited systems.

METHODS

We investigated species richness, plant size, soil total nitrogen and dominant grass leaf δ¹³C within and outside the dominant leguminous cushion-like shrub Caragana versicolor along a water deficit gradient in drylands of Tibetan Plateau.

RESULTS

We found that C. versicolor increased grass species richness but had a negative effect on annual and perennial forbs. Along the water deficit gradient, plant interaction assessed by species richness (RII_species) showed a unimodal pattern with shift from increase to decrease, while plant interaction assessed by plant size (RII_size) did not vary significantly. The effect of C. versicolor on soil nitrogen, rather than water availability, determined its overall effect on understory species richness. Neither the effect of C. versicolor on soil nitrogen nor water availability affected plant size.

DISCUSSION

Our study suggests that the drying tendency in association with the recent warming trends observed in drylands of Tibetan Plateau, will likely hinder the facilitation effect of nurse leguminous shrub on understories if moisture availability crosses a critical minimum threshold.

Sex-specific competition differently regulates the response of the rhizosphere fungal community of Hippophae rhamnoides-A dioecious plant, under Mn stress

In this study, we investigated the soil physicochemical parameters and responses of rhizospheric fungal communities of Hippophae rhamnoides to Mn stress under different sexual competition patterns. The results showed that competition significantly affects soil physicochemical properties, enzyme activity, and rhizosphere-associated fungal community structures. Under Mn stress, soils with intersexual competition had higher levels of N supply than those with the intrasexual competition. Moreover, fungal communities under intersexual interaction were more positive to Mn stress than intrasexual interaction. Under intrasexual competition, female plants had higher total phosphorus content, neutral phosphatase activity, and relative abundance of symbiotic fungi in soils to obtain phosphorus nutrients to alleviate Mn stress. In contrast, male plants had relatively stable fungal communities in soils. In the intersexual competition, rhizosphere fungal diversity and relative abundance of saprophytic fungi in male plants were significantly higher than in female plants under Mn stress. In addition, female plants showed greater plasticity in the response of rhizosphere microorganisms to their neighbors of different sexes. The microbial composition in soils of female plants varied more than male plants between intrasexual and intersexual competition. These results indicated that sex-specific competition and neighbor effects regulate the microbial community structure and function of dioecious plants under heavy metal stress, which might affect nutrient cycling and phytoremediation potential in heavy metal-contaminated soils.

Effects of shrub encroachment on grassland community and soil nutrients among three typical shrubby grasslands in the alpine subhumid region of the Qinghai-Tibet Plateau, China

Introduction

The alpine meadows are distributed widely and play a vital role in ecosystem service functions on the Qinghai-Tibet Plateau (QTP). Under the combined effect of climate change and overgrazing, shrubs display an apparent expansion trend, leading to the shrinking of alpine meadows, and directly affecting the structure and function of grassland ecosystems. However, the effects of shrub encroachment on the plant community and soil nutrients of alpine grassland ecosystems still need to be clarified.

Method

We aimed to determine differences in vegetation characteristics and nutrient distribution along the soil profile between shrub patches and their adjacent grassland at three sites, which were three typical types of shrub-encroached grassland, including Spiraea alpina Pall. (SA), Lonicera tubuliflora Rehd. (LT), and Salix cupularis Rehd. (ST).

Results

The results showed that shrub invasion changed the plant community structure of alpine grassland ecosystems, and shrub type was the critical factor driving this alteration. The expansion of the three shrubs reduced grassland species diversity. Shrub encroachment in SA positively impacted vegetation biomass but significantly decreased the soil organic content (SOC) and total nutrients. Shrub invasion in the ST had the most substantial impact on vegetation and soil, resulting in significantly lower nutrient content in shrubs than in grassland patches. The effect of LT was a significant reduction in vegetation biomass but no significant changes in biodiversity or soil nutrients. Grassland patches were more strongly correlated than shrub patches for SA and LT, while the opposite was true for ST. Vegetation characteristics were correlated least with soil nutrients for SA, while ST was most correlated, and LT was between them. Soil nutrients show more positive correlations with vegetation, enzyme activity, and microbial biomass in deeper soils (20–100 cm) than in shallow soils (0–20 cm). The deeper the soil layer is, the more significant the positive correlations in the shrub patches. Our findings indicated that shrubs play critical roles in the dynamics of vegetation patterns and soil environments for managing and sustainable utilization of shrubby alpine grasslands.

Biocrusts mediate the plant community composition of dryland restoration ecosystems

Degradation and expansion are current threats associated with drylands. During natural or artificial restoration, dryland ecosystems tend to contain a unique community, namely, biocrusts. Studies have shown that biocrusts serve multiple functions and have the potential to accelerate or inhibit degradation. However, the mechanisms by which biocrusts mediate dryland plant community structure and composition remain largely unknown. In this study, we performed a series of pot experiments under greenhouse conditions at three disturbance levels (strong, moderate and no disturbance) and at three rainfall gradients (- 50 %, normal, + 50 %) for different biocrust types. Our objective was to examine how biocrusts influence the introduction and establishment of different functional forms of plant species during the different succession stages of dryland recovery. Our results showed that biocrusts have significantly different effects on the seed germination and survival of the tested plant species. Although strong disturbances to the biocrusts and increasing rainfall alleviated the inhibitory effects of biocrusts on the perennial herb (S. glareosa) and subshrub (C. latens) species, these factors could only promote the temporary survival of these two plants in the revegetated area. These results indicate that biocrusts in revegetated systems play important sieving functions during plant species establishment. Additionally, the survival of S. viridis, a plant species with small seeds adapted to the current habitat, was promoted in the community, but two plant species that are currently unsuitable, S. glareosa and C. latens, were filtered out by the biocrusts. Our conclusions suggest that biocrusts play an ecological filtering role in plant species introduction and establishment in revegetated ecosystems and demonstrate the scientific significance of biocrusts in maintaining the health and stability of dryland ecosystems.

Positive effects of exotic species dampened by neighborhood heterogeneity

It is well known that species interactions between exotic and native species are important for determining the success of biological invasions and how influential exotic species become in invaded communities. The strength and type of interactions between species can substantially vary, however, from negative and detrimental to minimal or even positive. Increasing evidence from the literature shows that exotic species have positive interactions with native species more often than originally thought. Gaps in our theory for how population growth is limited when interactions are positive, however, restrict our understanding of the mechanisms by which exotic "facilitators" contribute to diversity maintenance in invaded systems. Here, we quantified interactions between seven native and four exotic (established nonnative) common annual plant species in the highly diverse, York Gum woodlands of Western Australia. We used a Bayesian demographic modeling approach that allowed for interaction coefficients to be positive or negative, and explored key sources of variation in species responses to native and exotic neighbors at per capita (individual) and neighborhood levels. We observed positive per capita effects from exotic neighbors on exotic focal species as well as on several native focal species. However, all focal species were, on average, inhibited by their interaction neighborhood, when the variance in identity and abundance of observed neighbors was considered. At the neighborhood scale, exotic species were found to suppress all focal species, particularly those with high intrinsic fecundity. Our study demonstrates that within-neighborhood heterogeneity can regulate per capita positive effects of invaders, limiting runaway population growth of both natives and exotic invaders.

Interspecific interactions alter plant functional strategies in a revegetated shrub-dominated community in the Mu Us Desert, China

BACKGROUND AND AIMS

Previous studies investigating plant-plant interactions have focused on plant growth, context dependence and shifts in interactive outcomes. However, changes in functional traits in the context of interactions have been inadequately explored; few studies have focused on the effects of interactions on the plasticity of functional strategies.

METHODS

We conducted a 4-year removal experiment for the xeric shrub Artemisia ordosica and perennial graminoids (PGs) in the Mu Us Desert, northern China. Soil nutrient content, biomass and 12 functional traits related to plant morphology and nutrient status were measured for the shrub species and a dominant PG species (i.e. Leymus secalinus) in the presence and absence of shrubs and PGs.

KEY RESULTS

Shrubs affected the functional traits of L. secalinus, reducing leaf dry matter content and increasing plant height, which probably promoted the functional strategy of L. secalinus towards a more resource-acquisitive and competitive strategy. In contrast, when the shrubs were affected by PGs, they shifted towards a resource-conservative and stress-tolerative strategy, by increasing leaf dry matter content and decreasing specific leaf area. Moreover, the shrub species relied more on internal nutrient recycling (higher nitrogen resorption efficiency) rather than on external nitrogen uptake under nitrogen competition; instead, L. secalinus tended to exhibit higher external nitrogen uptake from soil during nitrogen shortages.

CONCLUSIONS

This study indicated that the functional strategies and nutrient cycling of the shrub species and the dominant PG were altered by each other. The shifts in functional traits may help plants to coexist in the community for a relatively long time. Our findings highlighted that interspecific interactions alter plant functional strategies and provided new insights into community assembly and succession mechanisms in a revegetated shrubland for ecological restoration of drylands.

An Integrative Perspective on the Mechanistic Basis of Context Dependent Species Interactions

It has long been known that the outcome of species interactions depends on the environmental context in which they occur. Climate change research has sparked a renewed interest in context dependent species interactions because rapidly changing abiotic environments will cause species interactions to occur in novel contexts and researchers must incorporate this in their predictions of species' responses to climate change. Here we argue that predicting how the environment will alter the outcome of species interactions requires an integrative biology approach that focuses on the traits, mechanisms, and processes that bridge disciplines such as physiology, biomechanics, ecology, and evolutionary biology. Specifically, we advocate for quantifying how species differ in their tolerance and performance to both environmental challenges independent of species interactions, and in interactions with other species as a function of the environment. Such an approach increases our understanding of the mechanisms underlying outcomes of species interactions across different environmental contexts. This understanding will in turn help determine how the outcome of species interactions affects the relative abundance and distribution of the interacting species in nature. A general theme that emerges from this perspective is that species are unable to maintain high levels of performance across different environmental contexts because of trade-offs between physiological tolerance to environmental challenges and performance in species interactions. Thus, an integrative biology paradigm that focuses on the trade-offs across environments, the physiological mechanisms involved, and how the ecological context impacts the outcome of species interactions provides a stronger framework to understand why species interactions are context dependent.

Net plant interactions are highly variable and weakly dependent on climate at the global scale

Although plant-plant interactions (i.e. competition and facilitation) have long been recognised as key drivers of plant community composition and dynamics, their global patterns and relationships with climate have remained unclear. Here, we assembled a global database of 10,502 pairs of empirical data from the literature to address the patterns of and climatic effects on the net outcome of plant interactions in natural communities. We found that plant interactions varied among plant performance indicators, interaction types and biomes, yet competition occurred more frequently than facilitation in plant communities worldwide. Unexpectedly, plant interactions showed weak latitudinal pattern and were weakly related to climate. Our study provides a global comprehensive overview of plant interactions, highlighting competition as a fundamental mechanism structuring plant communities worldwide. We suggest that further investigations should focus more on local factors (e.g. microclimate, soil and disturbance) than on macroclimate to identify key environmental determinants of interactions in plant communities.

The salt secretion of leaves promotes the competitiveness of Reaumuria soongarica in a desert grassland

BACKGROUND

For better understanding the mechanism of Reaumuria soongarica community formation in a salt stressed grassland ecosystem, we designed a field experiment to test how leaves salt secretion changes the competitive relationship between species in this plant communities.

RESULTS

Among the three species (R. soongarica, Stipa glareosa and Allium polyrhizum) of the salt stressed grassland ecosystem, the conductivity of R. soongarica rhizosphere soil was the highest in five soil layers (0-55 cm depth). The high soil conductivity can increase the daily salt secretion rate of plant leaves of R. soongarica. In addition, we found the canopy size of R. soongarica was positively related to the distance from S. glareosa or A. polyrhizum. The salt-tolerance of R. soongarica was significantly higher than the other two herbs (S. glareosa and A. polyrhizum). Moreover, there was a threshold (600 µS/cm) for interspecific competition of plants mediated by soil conductivity. When the soil conductivity was lower than 600 µS/cm, the relative biomass of R. soongarica increased with the soil conductivity increase.

CONCLUSIONS

The efficient salt secretion ability of leaves increases soil conductivity under the canopy. This leads the formation of a "saline island" of R. soongarica. Meanwhile R. soongarica have stronger salt tolerance than S. glareosa and A. polyrhizum. These promote the competitiveness of R. soongarica and inhibit interspecies competition advantage of the other two herbs (S. glareosa and A. polyrhizum) in the plant community. It is beneficial for R. soongarica to establish dominant communities in saline regions of desert grassland.

Building on 150 Years of Knowledge: The Freshwater Isopod Asellus aquaticus as an Integrative Eco-Evolutionary Model System

Interactions between organisms and their environments are central to how biological diversity arises and how natural populations and ecosystems respond to environmental change. These interactions involve processes by which phenotypes are affected by or respond to external conditions (e.g., via phenotypic plasticity or natural selection) as well as processes by which organisms reciprocally interact with the environment (e.g., via eco-evolutionary feedbacks). Organism-environment interactions can be highly dynamic and operate on different hierarchical levels, from genes and phenotypes to populations, communities, and ecosystems. Therefore, the study of organism-environment interactions requires integrative approaches and model systems that are suitable for studies across different hierarchical levels. Here, we introduce the freshwater isopod Asellus aquaticus, a keystone species and an emerging invertebrate model system, as a prime candidate to address fundamental questions in ecology and evolution, and the interfaces therein. We review relevant fields of research that have used A. aquaticus and draft a set of specific scientific questions that can be answered using this species. Specifically, we propose that studies on A. aquaticus can help understanding (i) the influence of host-microbiome interactions on organismal and ecosystem function, (ii) the relevance of biotic interactions in ecosystem processes, and (iii) how ecological conditions and evolutionary forces facilitate phenotypic diversification.

Plant community responses to grassland restoration efforts across a large-scale precipitation gradient

Identifying how plant species diversity varies across environmental gradients remains a controversial topic in plant community ecology because of complex interactions among putative factors. This is especially true for grasslands where habitat loss has limited opportunities for systematic study across broad spatial scales. Here we overcome these limitations by examining restored plant community responses to a large-scale precipitation gradient under two common Conservation Reserve Program (CRP) restoration approaches. The two restoration strategies examined were CP2, which seeds a relatively low number of species, and CP25, which seeds a higher number of species. We sampled plant communities on 55 CRP fields distributed along a broad precipitation gradient (410-1,170 mm mean annual precipitation) spanning 650 km within the grassland biome of North America. Mean annual precipitation (MAP) was the most important predicator of plant species richness and had a positive, linear response across the gradient. To a lesser degree, restoration practices also played a role in determining community diversity. The linear increase in species richness across the precipitation gradient reflects the species pool increase from short to tallgrass prairie communities and explained most of the richness variation. These findings provide insight into the diversity constraints and fundamental drivers of change across a large-scale gradient representing a wide variety of grassland habitats. Across a broad environmental gradient, initial planting differences between restoration practices had lower effects on plant diversity than expected. This suggests that new strategies are needed to effectively establish diverse plant communities on large-scale restorations such as these.

Environmental stress responses in sympatric congeneric crustaceans: Explaining and predicting the context-dependencies of invader impacts

The role of ecophysiology in mediating marine biological pollution is poorly known. Here we explore how physiological plasticity to environmental stress can explain and predict the context-dependencies of invasive species impacts. We use the case of two sympatric skeleton shrimps, the invader Caprella scaura and its congener C. equilibra, which is currently replaced by the former on the South European coast. We compare their physiological responses to hyposalinity stress under suboptimal low and high temperature, while inferring on hypoxia tolerance. We use an energy-redox approach, analyzing mortality rate, the energetic balance and the consequent effects on the oxidative homeostasis. We found that decreased seawater salinity and/or oxygen levels can weaken biotic resistance, especially in females of C. equilibra, leading to periods of heightened vulnerability to invasion. Our approach provides mechanistic insights towards understanding the factors promoting invader impacts, highlighting the potential of ecophysiology for improving invasive species management.

Conditional love? Co-occurrence patterns of drought-sensitive species in European grasslands are consistent with the stress-gradient hypothesis

AIM

The stress-gradient hypothesis (SGH) postulates that species interactions shift from negative to positive with increasing abiotic stress. Interactions between species are increasingly being recognized as important drivers of species distributions, but it is still unclear whether stress-induced changes in interactions affect continental-to-global scale species distributions. Here, we tested whether associations of vascular plant species in dry grasslands in Europe follow the SGH along a climatic water deficit (CWD) gradient across the continent.

LOCATION

Dry grasslands in Europe.

TIME PERIOD

Present.

MAJOR TAXA STUDIED

Vascular plants.

METHODS

We built a context-dependent joint species distribution model (JSDM) to estimate the residual associations (i.e., associations that are not explained by the abiotic environment) of 161 plant species as a function of the CWD based on community data from 8,660 vegetation plots. We evaluated changes in residual associations between species for pairs and on the community level, and we compared responses for groups of species with different drought tolerances.

RESULTS

We found contrasting shifts in associations for drought-sensitive and drought-tolerant species. For drought-sensitive species, 21% of the pairwise associations became more positive with increasing CWD, whereas 17% became more negative. In contrast, only 17% of the pairwise associations involving drought-tolerant species became more positive, whereas 27% became more negative in areas with a high CWD. Additionally, the incidence of positive associations increased with drought for drought-sensitive species and decreased for drought-tolerant species.

MAIN CONCLUSIONS

We found that associations of drought-sensitive plant species became more positive with drought, in line with the SGH. In contrast, associations of drought-tolerant species became more negative. Additionally, changes in associations of single species pairs were highly variable. Our results indicate that stress-modulated species associations might influence the distribution of species over large geographical extents, thus leading to unexpected responses under climate change through shifts in species associations.

How does interspecific competition modify the response of grass plants against herbicide treatment? A hierarchical concentration-response approach

Ecological interactions are rarely taken into account in environmental risk assessment. The objective of this work was to assess how interspecific competition affects the way plant species react to herbicides and more specifically how it modifies the concentration-response curves that can be built using ecotoxicological bioassays. To do this, we relied on the results of ecotoxicological bioassays on six herbaceous species exposed to isoproturon under two conditions: in presence and in absence of a competitor. At the end of the experiments, eleven endpoints were measured. We modelled these data using a hierarchical modelling framework designed to assess the effects of competition on each of the four parameters of the concentration response curves (e.g. the level of response at the control or the concentration at the inflection point of the curve) simultaneously for the six species. The modelled effects could be of three types, 1) competition had no effect on the parameter, 2) competition had the same effect on the parameter for all species and 3) competition had a different effect on the parameter for each species. Our main hypothesis was that different species would react differently to competition. Results showed that about a half of the estimated parameters showed a modification under competition pressure among which only a fourth showed a species-specific effect, the three other fourth showing the same effect between the different species. Our initial hypothesis was thus not supported as species tended to react in the same way to competition. The competition effect on plants was mainly negative, thus showing that they were more affected by isoproturon under competition pressure. This study therefore establishes how competition modifies plant responses to chemical stress and how this interaction varies from one species to the other.

Impacts of invasive annual grasses and their litter vary by native functional strategy

Invasive species may act as a functional filter on native communities by differentially affecting species with different trait values. Across environments, invasive plants typically display traits associated with high resource acquisition and fast growth. Conversely, native plants, especially those in water-limited environments, tend to adopt one of two functional strategies: fast growth during high resource availability to avoid stress (resource-acquisitive), or slow growth during resource-poor conditions to tolerate stress (resource-conservative). While invasive competition can be a strong filter on these groups, many invaders also alter the structure of native communities through their accumulation of litter. How fast-growing invaders with litter shift native functional communities remains unknown. To elucidate these functional shifts, I manipulated invasive annual grasses and their litter in an annual grassland and followed the demographic rates of six native annual forb species that varied in their functional strategy. Live grass competition alone decreased per capita growth rates of resource-acquisitive natives and had no effect on resource-conservative natives. The presence of litter, however, decreased growth rates in both functional types of natives, with stronger declines in resource-acquisitive species through differential effects on seed set and germination. Invaders in this system thus create an unfavorable environment for natives through litter, limiting the capacity of both resource-acquisitive and resource-conservative native forbs to maintain high population growth. These findings suggest that grass invasions have the potential to dramatically shift the functional composition of native communities through the time-lagged effects of their litter.

Phylogenetic structure of understorey annual and perennial plant species reveals opposing responses to aridity in a Mediterranean biodiversity hotspot

Aridity is a critical driver of the diversity and composition of plant communities. However, how aridity influences the phylogenetic structure of functional groups (i.e. annual and perennial species) is far less understood than its effects on species richness. As perennials have to endure stressful conditions during the summer drought, as opposed to annuals that avoid it, they may be subjected to stronger environmental filtering. In contrast, annuals may be more susceptible to interannual climatic variability. Here we studied the phylogenetic structure of the annual and perennial components of understorey plant communities, along a regional aridity gradient in Mediterranean drylands. Specifically, we asked: (1) How do species richness (S) and phylogenetic structure (PS) of annuals and perennials in plant communities respond to aridity? (2) What is the contribution of other climatic and topo-edaphic variables in predicting S and PS for both components? (3) How does the taxonomic and phylogenetic turnover of annuals and perennials vary with spatial and environmental distances? We assessed annuals' and perennials' species richness, the phylogenetic structure at deep and shallow phylogenetic levels, and taxonomic and phylogenetic turnover along spatial and environmental distances. We found no relationship between annuals' richness and aridity, whereas perennials' richness showed a unimodal pattern. The phylogenetic structure of annuals and perennials showed contrasting responses to aridity and negatively correlated with topo-edaphic variables. We found phylogenetic clustering at intermediate-to-higher aridity levels for annuals, and at lower aridity levels for perennials. Both taxonomic and phylogenetic turnover in annuals and perennials correlated with the environmental distance rather than with spatial distance between communities, suggesting adaptation to local factors. Overall, our results show a decoupling in the response of the phylogenetic structure of annual and perennial components of plant communities to aridity in Mediterranean drylands. Our findings have significant implications for land management strategies under climate change.

Frequency-dependent interactions determine outcome of competition between two breast cancer cell lines

Tumors are highly dynamic ecosystems in which diverse cancer cell subpopulations compete for space and resources. These complex, often non-linear interactions govern continuous spatial and temporal changes in the size and phenotypic properties of these subpopulations. Because intra-tumoral blood flow is often chaotic, competition for resources may be a critical selection factor in progression and prognosis. Here, we quantify resource competition using 3D spheroid cultures with MDA-MB-231 and MCF-7 breast cancer cells. We hypothesized that MCF-7 cells, which primarily rely on efficient aerobic glucose metabolism, would dominate the population under normal pH and low glucose conditions; and MDA-MB-231 cells, which exhibit high levels of glycolytic metabolism, would dominate under low pH and high glucose conditions. In spheroids with single populations, MCF-7 cells exhibited equal or superior intrinsic growth rates (density-independent measure of success) and carrying capacities (density-dependent measure of success) when compared to MDA-MB-231 cells under all pH and nutrient conditions. Despite these advantages, when grown together, MCF-7 cells do not always outcompete MDA-MB-231 cells. MDA-MB-231 cells outcompete MCF-7 cells in low glucose conditions and coexistence is achieved in low pH conditions. Under all conditions, MDA-MB-231 has a stronger competitive effect (frequency-dependent interaction) on MCF-7 cells than vice-versa. This, and the inability of growth rate or carrying capacity when grown individually to predict the outcome of competition, suggests a reliance on frequency-dependent interactions and the need for competition assays. We frame these results in a game-theoretic (frequency-dependent) model of cancer cell interactions and conclude that competition assays can demonstrate critical density-independent, density-dependent and frequency-dependent interactions that likely contribute to in vivo outcomes.

Direct and indirect facilitation affect community productivity through changes in functional diversity in an alpine system

BACKGROUND AND AIMS

Facilitation is an important ecological process for plant community structure and functional composition. Although direct facilitation has accrued most of the evidence so far, indirect facilitation is ubiquitous in nature and it has an enormous potential to explain community structuring. In this study, we assess the effect of direct and indirect facilitation on community productivity via taxonomic and functional diversity.

METHODS

In an alpine community on the Tibetan Plateau, we manipulated the presence of the shrub Dasiphora fruticosa and graminoids in a fenced meadow and a grazed meadow to quantify the effects of direct and indirect facilitation. We measured four plant traits: height, lateral spread, specific leaf area (SLA) and leaf dry matter content (LDMC) of forbs; calculated two metrics of functional diversity [range of trait and community-weighted mean (CWM) of trait]; and assessed the responses of functional diversity to shrub facilitation. We used structural equation modelling to explore how shrubs directly and indirectly drove community productivity via taxonomic diversity and functional diversity.

KEY RESULTS

We found stronger effects from herbivore-mediated indirect facilitation than direct facilitation on productivity and taxonomic diversity, regardless of the presence of graminoids. For functional diversity, the range and CWM of height and SLA, rather than lateral spread and LDMC, generally increased due to direct and indirect facilitation. Moreover, we found that the range of traits played a primary role over taxonomic diversity and CWM of traits in terms of shrub effects on community productivity.

CONCLUSIONS

Our study reveals that the mechanism of shrub direct and indirect facilitation of community productivity in this alpine community is expanding the realized niche (i.e. expanding range of traits). Our findings indicate that facilitators might increase trait dispersion in the local community, which could alleviate the effect of environmental filters on trait values in harsh environments, thereby contributing to ecosystem functioning.

Tussocks facilitate their neighbours mainly by ameliorating extreme temperatures in tropical high mountains

Facilitation by tussocks is common in high-altitude tropical environments. It is thought that facilitation results from stress amelioration, but it is unclear which of the many stressors acting in these environments is ameliorated. We aimed at determining the relative importance of different stressors as drivers of facilitation by the tussock Festuca tolucensis in Mexico. We employed eight experimental treatments to manipulate five stressors in the field: minimum temperatures by using electric radiators that kept plants warm; maximum temperatures by means of reflective sand that precluded temperature build-up during the day; UV radiation by using screens opaque to UV; poor soil properties by comparing soils from beneath tussocks and from bare ground; and low water availability by adding vermiculite to the soil. The performance (survival and growth) of Mexerion sarmentosum (a plant usually associated with Festuca) in these treatments was compared to that recorded under tussocks and in bare ground. Amelioration of extreme temperatures had the largest positive effects on Mexerion survival. UV radiation and increased soil humidity did not affect survival, although humidity increased growth rates. Nevertheless, tussocks reduced the growth of Mexerion, which is consistent with observations of competition between plants and soil microorganisms favoured by tussocks. Our results highlight the importance of the extreme daily fluctuations in temperature that characterise tropical mountains as fundamental drivers of their dynamics.

Melatonin modulates photosynthesis, redox status, and elemental composition to promote growth of Brassica juncea-a dose-dependent effect

Melatonin (MEL) is an antioxidant molecule, present throughout plant kingdom, animals, and microbes. It is a well-known free radical scavenger and modulates growth and development in plants against various abiotic and biotic stresses. The present study was done to investigate the role of MEL as a foliar spray on the morphological, physiological, and biochemical parameters in Brassica juncea cv. Varuna. Five different doses (10, 20, 30, 40, or 50 μM) of MEL were applied as foliar spray to the leaf of plant at 25 days after sowing (DAS) and continued up to 30 DAS once in a day. The plants were sampled at 30, 45, and 60 DAS to assess various parameters. The present results indicate that most of the parameters, i.e., growth, photosynthetic, nutrients, and enzyme activities increased in a concentration dependent manner. MEL application reduced the accumulation of reactive oxygen species (ROS) by enhancing the antioxidant enzyme activities. Microscopic examinations further revealed a significant increase in the size of the stomatal aperture in the presence of MEL. Out of the various concentrations tested, 40 μM of MEL proved best and can be used for further studies.

Density-dependence tips the change of plant-plant interactions under environmental stress

Facilitation studies typically compare plants under differential stress levels with and without neighbors, while the density of neighbors has rarely been addressed. However, recent empirical studies indicate that facilitation may be density-dependent too and peak at intermediate neighbor densities. Here, we propose a conceptual model to incorporate density-dependence into theory about changes of plant-plant interactions under stress. To test our predictions, we combine an individual-based model incorporating both facilitative response and effect, with an experiment using salt stress and Arabidopsis thaliana. Theoretical and experimental results are strikingly consistent: (1) the intensity of facilitation peaks at intermediate density, and this peak shifts to higher densities with increasing stress; (2) this shift further modifies the balance between facilitation and competition such that the stress-gradient hypothesis applies only at high densities. Our model suggests that density-dependence must be considered for predicting plant-plant interactions under environmental change.

Do shrubs improve reproductive chances of neighbors across soil types in drought?

Plant reproduction is highly sensitive to stress from severe weather. While facilitation has been shown to buffer negative impacts along stress gradients, less is known about facilitating plant reproduction in drought periods. Because intensity and frequency of drought are predicted to increase, plant reproductive facilitation has important implications for a species ability to adapt to changes in climate. Our primary study objective is to test if nurse shrubs act as reproductive micro-refugia across soil types, by improving reproductive potential of perennial bunchgrass neighbors subjected to severe drought. To investigate this objective, we designed a fully factored study testing direct interaction between shrub and bunchgrasses in eastern Oregon sagebrush steppe, at two sites with different soil types. The study consisted of six simple effect treatments combining three moisture regimes (moist, ambient, and drought) with two shrub conditions (shrub intact or shrub removed). Our results indicate when facilitation of reproductive potential occurs, it occurs strongly and particularly in drought, consistent with the stress gradient hypothesis (SGH), where several species produced at least 54% more inflorescences in the presence of shrub neighbors. In addition, we found facilitation to be consistent with the SGH at the species level likely reflecting differences in plant strategy and perception of strain, but to follow alternative SGH models more closely at the site level where facilitation declined on the drier soil. Ultimately, our findings highlight the importance of facilitation in improving plant reproductive potential in drought, and support the role of nurse shrubs as micro-refugia in a changing climate.

Shrub facilitation of tree establishment varies with ontogenetic stage across environmental gradients

Plant-plant interactions are important drivers of ecosystem structure and function, yet predicting interaction outcomes across environmental gradients remains challenging. Understanding how interactions are affected by ontogenetic shifts in plant characteristics can provide insight into the drivers of interactions and improve our ability to anticipate ecosystem responses to environmental change. We developed a conceptual framework of nurse shrub facilitation of tree establishment. We used a combination of field experiments and environmental measurements to test the framework with a shrub (Artemisia tridentata) and a tree (Pinus monophylla), two foundation species in a semiarid environment. Shrub microsites allowed trees to overcome an early population bottleneck and successfully establish in areas without tree cover. Shrubs facilitated trees at multiple ontogenetic stages, but the net outcome of the interaction shifted from strongly positive to neutral after the transition of P. monophylla from juvenile to adult foliage. Microhabitat conditions varied across a broad elevational gradient, but interaction outcomes were not strongly related to elevation. Favorable microsites provided by A. tridentata cover are crucial for P. monophylla recovery after stand-replacing disturbance. Models of vegetation response to rapid global environmental change should incorporate the critically important role of nurse shrub interactions for ameliorating population bottlenecks in tree establishment.

Arabidopsis species deploy distinct strategies to cope with drought stress

BACKGROUND AND AIMS

Water limitation is an important determinant of the distribution, abundance and diversity of plant species. Yet, little is known about how the response to limiting water supply changes among closely related plant species with distinct ecological preferences. Comparison of the model annual species Arabidopsis thaliana with its close perennial relatives A. lyrata and A. halleri, can help disentangle the molecular and physiological changes contributing to tolerance and avoidance mechanisms, because these species must maintain tolerance and avoidance mechanisms to increase long-term survival, but they are exposed to different levels of water stress and competition in their natural habitat.

METHODS

A dry-down experiment was conducted to mimic a period of missing precipitation. The covariation of a progressive decrease in soil water content (SWC) with various physiological and morphological plant traits across a set of representative genotypes in A. thaliana, A. lyrata and A. halleri was quantified. Transcriptome changes to soil dry-down were further monitored.

KEY RESULTS

The analysis of trait covariation demonstrates that the three species differ in the strategies they deploy to respond to drought stress. Arabidopsis thaliana showed a drought avoidance reaction but failed to survive wilting. Arabidopsis lyrata efficiently combined avoidance and tolerance mechanisms. In contrast, A. halleri showed some degree of tolerance to wilting but it did not seem to protect itself from the stress imposed by drought. Transcriptome data collected just before plant wilting and after recovery corroborated the phenotypic analysis, with A. lyrata and A. halleri showing a stronger activation of recovery- and stress-related genes, respectively.

CONCLUSIONS

The response of the three Arabidopsis species to soil dry-down reveals that they have evolved distinct strategies to face drought stress. These strategic differences are in agreement with the distinct ecological priorities of the stress-tolerant A. lyrata, the competitive A. halleri and the ruderal A. thaliana.

Can competitive asymmetries maintain offspring size variation? A manipulative field test

Offspring sizes vary within populations but the reasons are unclear. Game-theoretic models predict that selection will maintain offspring-size variation when large offspring are superior competitors (i.e., competition is asymmetric), but small offspring are superior colonizers. Empirical tests are equivocal, however, and typically rely on interspecific comparisons, whereas explicit intraspecific tests are rare. In a field study, we test whether offspring size affects competitive asymmetries using the sessile marine invertebrate, Bugula neritina. Surprisingly, we show that offspring size determines whether interactions are competitive or facilitative-large neighbors strongly facilitated small offspring, but also strongly competed with large offspring. These findings contradict the assumptions of classic theory-that is, large offspring were not superior competitors. Instead, smaller offspring actually benefit from interactions with large offspring-suggesting that asymmetric facilitation, rather than asymmetric competition, operates in our system. We argue that facilitation of small offspring may be more widespread than currently appreciated, and may maintain variation in offspring size via negative frequency-dependent selection. Offspring size theory has classically viewed offspring interactions through the lens of competition alone, yet our results and those of others suggest that theory should accommodate positive interactions in explorations of offspring-size variation.

Shrubs indirectly increase desert seedbanks through facilitation of the plant community

The mechanisms supporting positive ecological interactions are important. Foundation species can structure desert biodiversity by facilitating seedbanks of annual plants, but the direct and indirect mechanisms of shrub effects on seedbank have not been experimentally decoupled. We conducted the first test of shrubs increasing seedbank densities through direct effects on the seedbank (i.e. shrub seed-trapping, animal-mediated dispersal) and indirect effects by facilitating the annual plant community (i.e. seed deposition, annual seed-trapping). Two distinct desert ecosystems were used to contrast transient seedbank densities in shrub and open microsites by manipulating annual plant density and the presence of the persistent seedbank. We measured transient seedbank densities at the end of the growing season by collecting soil samples and extracting seeds from each respective treatment. Transient seedbank densities were greatest in shrub canopies and with relatively higher annual plant densities. The persistent seedbank contributed to transient seedbank densities only in one desert and in the open microsite. Shrubs indirectly increased seedbank densities by facilitation the seed production of the annual plants. Therefore, shrubs are increasing seedbank independently of the annual plant community, likely through trapping effects, and dependently by facilitating seed production of the annuals. These findings provide evidence for a previously undescribed mechanism that supports annual seedbanks and thus desert biodiversity. We also identify shrubs as being significant drivers of desert plant communities and emphasize the need to consider multiple mechanisms to improve our ability to predict the response of ecosystems to change.

Functionally distinct assembly of vascular plants colonizing alpine cushions suggests their vulnerability to climate change

BACKGROUND AND AIMS

Alpine cushion plants can initially facilitate other species during ecological succession, but later on can be negatively affected by their development, especially when beneficiaries possess traits allowing them to overrun their host. This can be reinforced by accelerated warming favouring competitively strong species over cold-adapted cushion specialists. However, little empirical research has addressed the trait-based mechanisms of these interactions. The ecological strategies of plants colonizing the cushion plant Thylacospermum caespitosum (Caryophyllaceae), a dominant pioneer of subnival zones, were studied in the Western Himalayas.

METHODS

To assess whether the cushion colonizers are phylogenetically and functionally distinct, 1668 vegetation samples were collected, both in open ground outside the cushions and inside their live and dead canopies, in two mountain ranges, Karakoram and Little Tibet. More than 50 plant traits related to growth, biomass allocation and resource acquisition were measured for target species, and the phylogenetic relationships of these species were studied [or determined].

KEY RESULTS

Species-based trait-environment analysis with phylogenetic correction showed that in both mountain ranges Thylacospermum colonizers are phylogenetically diverse but functionally similar and are functionally different from species preferring bare soil outside cushions. Successful colonizers are fast-growing, clonal graminoids and forbs, penetrating the cushion by rhizomes and stolons. They have higher root-to-shoot ratios, leaf nitrogen and phosphorus concentrations, and soil moisture and nutrient demands, sharing the syndrome of competitive species with broad elevation ranges typical of the late stages of primary succession. In contrast, the species from open ground have traits typical of stress-tolerant specialists from high and dry environments.

CONCLUSION

Species colonizing tight cushions of T. caespitosum are competitively strong graminoids and herbaceous perennials from alpine grasslands. Since climate change in the Himalayas favours these species, highly specialized subnival cushion plants may face intense competition and a greater risk of decline in the future.

Facilitation from an intraspecific perspective - stress tolerance determines facilitative effect and response in plants

Plant-plant interactions are reciprocal and include effects on and response to neighbours. Distinct traits confer competitive effect and response ability, but how specific traits determine effect and response in facilitative interactions has not been studied experimentally. We utilized the model species Arabidopsis thaliana to test for trait dependence of facilitative interactions. Salt-sensitive (sos) mutants or salt-tolerant wild-types were exposed to an experimental salinity gradient with and without intraspecific neighbours and the intensity of plant-plant interactions was measured for three performance variables. We tested whether salt tolerance can predict facilitative effect and response and whether a tradeoff exists between competitive ability and tolerance to stress. Interactions shifted very clearly from negative to positive with increasing stress. Salt-sensitive genotypes were less negatively affected by competition but more dependent on facilitation than were wild-types, indicating a tradeoff between competitive ability and stress tolerance. Surprisingly, sensitive genotypes imposed stronger facilitative effects, despite being much smaller under stress, probably because they retrieved more salt from the soil. Stress tolerance defined facilitative effect and response via distinct mechanisms. We advocate more controlled experiments with model species to advance our understanding of the trait dependence of biotic interactions and their consequences for community organization.

Seed mass equalises the strength of positive and negative plant-plant interactions in a semi-arid grassland

The interplay and balance between positive and negative interactions are important and recurrent topics in plant ecology. If facilitation occurs because benefactors ameliorate stress, such balance may be driven by seed mass because stress tolerance and competitive ability of plants, particularly seedlings, are positively correlated with seed mass. Thus, small-seeded, stress-intolerant species may require facilitation, but not large-seeded ones. This would equalise the magnitudes of opposite-signed interactions because in small-seeded species, positive effects of facilitators and negative effects of competitors should be strong, while both effects should be weak in large-seeded species. To test this idea, we assessed the effects of interactions with four associated species on different components of the performance of ten focal species. As expected, the largest facilitative and competitive effects were recorded in small-seeded species, and positive interactions had similar magnitudes to negative ones for any given seed mass, especially when performance was integrated into lifelong fitness. Furthermore, the fact that small-seeded species seem to be strongly facilitated may explain why they are not outcompeted by large-seeded species. This is an alternative to other hypotheses explaining the coexistence of plants with different-sized seeds. The close balance between opposite-signed interactions in the presence of stress may also explain why interactions have strong effects on individuals (that interact with only a few species), but seemingly weaker effects on populations, where interactions between many species would cancel out.

Feedbacks between Biotic and Abiotic Processes Governing the Development of Foredune Blowouts: A Review

This paper reviews the initiation, development, and closure of foredune blowouts with focus on biotic-abiotic interactions. There is a rich body of literature describing field measurements and model simulations in and around foredune blowouts. Despite this abundance of data there is no conceptual framework available linking biotic and abiotic observations to pathways of blowout development (e.g., erosional blowout growth or vegetation induced blowout closure). This review identifies morphological and ecological processes facilitating the transition between blowout development stages and sets them in the context of existing conceptual frameworks describing biotic-abiotic systems. By doing so we are able to develop a new conceptual model linking blowout development to the dominance of its governing processes. More specifically we link blowout initiation to the dominance of abiotic (physical) processes, blowout development to the dominance of biotic-abiotic (bio-geomorphological) processes and blowout closure to the dominance of biotic (ecological) processes. Subsequently we identify further steps to test the proposed conceptual model against existing observations and show possibilities to include it in numerical models able to predict blowout development for various abiotic and biotic conditions.