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Invasive plants typically escape specialist herbivores but are often attacked by generalist herbivores in their introduced ranges. The shifting defense hypothesis suggests that this will cause invasive plants to evolve lower resistance against specialists, higher resistance against generalists, and greater tolerance to herbivore damage. However, the duration and direction of selective pressures can shape the evolutionary responses of resistance and tolerance for invasive plants. Two critical factors are (1) residence time (length of time that an invasive species has been in its introduced range) and (2) specialist herbivore reassociation (attack by purposely or accidentally introduced specialists). Yet, these two factors have not been considered simultaneously in previous quantitative syntheses. Here, we performed a meta-analysis with 367 effect sizes from 70 studies of 35 invasive plant species from native and invasive populations. We tested how the residence time of invasive plant species and specialist reassociation in their introduced ranges affected evolutionary responses of defenses against specialists and generalists, including herbivore resistance traits (physical barriers, digestibility reducers and toxins), resistance effects (performance of and damage caused by specialists or generalists) and tolerance to damage (from specialists or generalists). We found that residence time and specialist reassociation each significantly altered digestibility reducers, specialist performance, generalist damage, and tolerance to specialist damage. Furthermore, residence time and specialist reassociation strongly altered toxins and generalist performance, respectively. When we restricted consideration to invasive plant species with both longer residence times and no reassociation with specialists, invasive populations had lower resistance to specialists, similar resistance to generalists, and higher tolerance to damage from both herbivore types, compared with native populations. We conclude that the duration and direction of selective pressure shape the evolutionary responses of invasive plants. Under long-term (long residence time) and stable (no specialist reassociation) selective pressure, invasive plants generally decrease resistance to specialists and increase tolerance to generalist damage that provides mixed support for the shifting defense hypothesis. [ABSTRACT FROM AUTHOR]

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Heavy metal hyperaccumulation (MH) is a rare trait found in plant species that inhabit metal‐contaminated soils. Two main hypotheses proposed to explain the selective advantage of MH are the elemental defence hypothesis and elemental allelopathy hypothesis. The elemental defence hypothesis suggests that MH functions as defence against herbivores while the elemental allelopathy hypothesis suggests that MH acts to inhibit the growth of neighbours. Nevertheless, these hypotheses are not likely to be mutually exclusive. Here, we present the first study to test both hypotheses simultaneously. We examined these hypotheses with the Cd hyperaccumulator, Arabidopsis halleri, which inhabits both metalliferous and non‐metalliferous soils, thus providing an opportunity to test the hypotheses of both habitats.Arabidopsis halleri plants originating from several populations in both metalliferous and non‐metalliferous soils were grown in a greenhouse in soils with or without cadmium (Cd). Their leaves were used in a feeding experiment with a specialist herbivore and in a set of leaf‐leachate experiments that tested their effect on seed germination and seedling establishment of species co‐occurring with A. halleri. Finally, a field survey in several A. halleri populations was conducted to compare herbivore load between A. halleri and neighbours from metalliferous versus non‐metalliferous soils.Results of the feeding experiment and field survey suggest that Cd accumulation in A. halleri leaves could provide it with defence against herbivores. Results of the leaf‐leachate experiments reveal that Cd accumulation has no effect on seed germination of neighbouring species but inhibits seedling establishment, particularly of plant species originating from non‐metalliferous soils.Our results suggest that both herbivores and competing neighbours may jointly select for MH in plants. Moreover, MH could provide a selective advantage particularly in non‐metalliferous soils, where neighbouring plants lack metal tolerance. These results highlight the importance of including different origins and populations of both the target species and its neighbouring plant species when studying the ecological role of MH. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Abstract: Admixture is the hybridization between populations within one species. It can increase plant fitness and population viability by alleviating inbreeding depression and increasing genetic diversity. However, populations are often adapted to their local environments and admixture with distant populations could break down local adaptation by diluting the locally adapted genomes. Thus, admixed genotypes might be selected against and be outcompeted by locally adapted genotypes in the local environments. To investigate the costs and benefits of admixture, we compared the performance of admixed and within‐population F1 and F2 generations of the European plant Lythrum salicaria in a reciprocal transplant experiment at three European field sites over a 2‐year period. Despite strong differences between site and plant populations for most of the measured traits, including herbivory, we found limited evidence for local adaptation. The effects of admixture depended on experimental site and plant population, and were positive for some traits. Plant growth and fruit production of some populations increased in admixed offspring and this was strongest with larger parental distances. These effects were only detected in two of our three sites. Our results show that, in the absence of local adaptation, admixture may boost plant performance, and that this is particularly apparent in stressful environments. We suggest that admixture between foreign and local genotypes can potentially be considered in nature conservation to restore populations and/or increase population viability, especially in small inbred or maladapted populations. [ABSTRACT FROM AUTHOR]

Summary: Global warming and reduced precipitation may trigger large‐scale species losses and vegetation shifts in ecosystems around the world. However, currently lacking are practical ways to quantify the sensitivity of species and community composition to these often‐confounded climatic forces. Here we conducted long‐term (16 yr) nocturnal‐warming (+0.6°C) and reduced precipitation (−20% soil moisture) experiments in a Mediterranean shrubland. Climatic niche groups (CNGs) – species ranked or classified by similar temperature or precipitation distributions – informatively described community responses under experimental manipulations. Under warming, CNGs revealed that only those species distributed in cooler regions decreased. Correspondingly, under reduced precipitation, a U‐shaped treatment effect observed in the total community was the result of an abrupt decrease in wet‐distributed species, followed by a delayed increase in dry‐distributed species. Notably, while partially correlated, CNG explanations of community response were stronger for their respective climate parameter, suggesting some species possess specific adaptations to either warming or drought that may lead to independent selection to the two climatic variables. Our findings indicate that when climatic distributions are combined with experiments, the resulting incorporation of local plant evolutionary strategies and their changing dynamics over time leads to predictable and informative shifts in community structure under independent climate change scenarios. [ABSTRACT FROM AUTHOR]

Plant individuals rely on pollinator services for their reproduction and often have to share these services with co-occurring neighbours, creating complex indirect plant-plant interactions. Many current theoretical models focus on the effect of floral resources' density on the net outcome of these indirect plant-plant interactions, often neglecting the identity of plant species in the communities and especially the species' spatial pattern. To fill this gap, we created a spatially explicit model whose goal was to study the interplay between relative densities and spatial distribution patterns of two plant species differing in their attractiveness for pollinators. Since theory predicts that pollinator behaviour strongly governs the outcome of pollination, we allowed the pollinators to systematically change their plant preferences based on their foraging experience. Thus the interplay between density and spatial patterns of plants was tested over a continuum of behaviours from specialists to generalists. Our most striking finding was that reproductive success of the less attractive species was affected in an opposite way by spatial patterns depending on whether the species had relatively low or high densities. Namely, when the less attractive species was highly abundant, its survival was higher when aggregated in large monospecific patches than when uniformly distributed. On the other hand, when the attractive species was more abundant, the less attractive species survived better when uniformly distributed. These results were consistent as long as the scale of the plant spatial aggregation was similar to or larger than the pollinators' detection range. Our results suggest that aggregated plant spatial patterns manipulate pollinator behaviour by trapping them within monospecific patches. This effect was sufficiently strong to enhance the survival of a competitively inferior species and hence to act in a way similar to the more familiar niche or temporal separation among plant species. [ABSTRACT FROM AUTHOR]

Seed dormancy is thought to be a key mechanism allowing annual plants to spread extinction risk in unpredictably varying environments. Theory predicts increasing germination fractions with increasing probability of reproductive success but solid empirical evidence is scarce and often confounded with environmental factors. Here we provide an empirical test of bet-hedging via delayed germination for three annual plant species along a 'predictability gradient' in Israel. We excluded confounding environmental and maternal effects by raising inbred seed families and germinating them under controlled conditions. Additionally, we germinated field-collected seeds in three consecutive seasons to compare their germination with inbred families where maternal effects were removed. Risk of reproductive failure was quantified using demographic data from the field and from second-generation inbred lines raised in a rainfall gradient in the greenhouse. Our findings were consistent with bet-hedging theory in that germination fraction was negatively related to species- and site-specific risk of reproductive failure. Both field and hand-raised seeds of one species exhibited higher dormancy with increasing risk of reproductive failure across sites, and hand-raised seeds of another species showed the same pattern. The third species exhibited a rather random pattern of germination between years and sites, corresponding to the lack of site-specific risk of reproductive failure. Species-specific patterns of dormancy and risk could be related to alternative risk-spreading strategies such as high adult survival, but were also affected by phylogeny. We provide strong empirical evidence for seed dormancy being a mechanism to reduce the risk of reproductive failure in highly variable environments, but a larger number of rigorous experimental tests of bet hedging germination are needed. Specifically, the genetic basis of bet-hedging must be shown in species with different life histories, for demonstrating that dormancy is adaptive and how it is modified by other risk-spreading traits. [ABSTRACT FROM AUTHOR]

Abstract: Positive interactions among plants have rarely been investigated with respect to their evolutionary consequences and vice versa. The outcome of facilitative interactions depends on the competitive ability and stress tolerance of the species. We tested whether this also applies to populations of conspecifics that are locally adapted to different environments and thereby differ in these traits. We hypothesised that ecotypes from less stressful environments experience a greater effect of facilitation when grown in stressful environments compared to populations adapted to these conditions. Seeds of two ecotypes of the annual grass species, Brachypodium distachyon, were collected from Mediterranean and arid origins and transplanted at an arid environment within the species’ distribution range. To examine the effect of biotic interactions on these ecotypes, we transplanted the individuals with and without the presence of the shrub Gymnocarpos decander (underneath or away from the shrub), and with and without the presence of annual vegetation (removal experiment). We examined the effect of these interactions on the two B. distachyon ecotypes by comparison of emergence success, biomass, and survival to reproduction. The presence of shrubs had a positive effect on all three variables in both ecotypes. Facilitation by shrubs enabled individuals from Mediterranean origin to grow and reproduce in arid conditions. Unlike the locals, they failed to survive to reproduction away from the shrubs, because of the markedly shorter growing season in open areas. The annual vegetation did not affect emergence or survival to reproduction in either ecotype; however, the positive effect of shrubs on biomass was reduced in the presence annual vegetation in the Mediterranean ecotype. This demonstrates that ecotypes adapted to arid conditions respond differently to these biotic interactions compared to Mediterranean populations. We argue that facilitation may have important evolutionary consequences by enabling maladapted ecotypes to invade and colonize stressful habitats. [Copyright &y& Elsevier]

Under global climate change, adaptation to new conditions is crucial for plant species persistence. This requires the ability to evolve in traits that are correlated with changing climatic variables. We studied between-year seed dormancy, which correlates with environmental variability, and tested for clinal trends in its evolvability along an aridity gradient in Israel. We conducted a germination experiment under five irrigation levels with two dryland winter annuals ( Biscutella didyma, Bromus fasciculatus) from four sites along the gradient. Species differed in means and evolvability of dormancy. Biscutella had high dormancy, which significantly increased with aridity but decreased with higher irrigation. In Bromus, dormancy was low, similar among populations, and only marginally affected by irrigation. Evolvability in Biscutella was high and varied among populations, without a clinal trend along the gradient. Conversely, in Bromus, trait evolvability was low and declined with increasing aridity. We argue that changes in evolvability along climatic gradients depend on the relative intensity of stabilizing selection. This may be high in Bromus and not only depends on environmental stress, but also on variability. Our findings point to the importance of measuring evolvability of climate-related traits across different natural and artificial environments and for many coexisting species. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100, 924–934. [ABSTRACT FROM AUTHOR]

1 Interactions among plants strongly influence the structure and dynamics of plant populations and communities. However, most empirical studies of plant–plant interactions failed to make repeated measures of responses to neighbouring individuals and thereby neglected possible changes in interactions throughout the life history of the plants. 2 We tested the hypothesis that competition between annual species intensifies from early to late life-history stages, by sequentially measuring interactions in neighbour-removal experiments at three study sites located along a rainfall gradient in Israel. 3 Two annual species, Biscutella didyma and Hymenocarpos circinnatus, grew with and without neighbours in their natural habitats. Five response variables representing consecutive life-history stages (seedling survival, juvenile biomass, adult survival, number of seeds and final biomass) were recorded throughout the whole growing season. 4 The direction and intensity of interactions varied considerably between environments and life stages. On average, growth-related response variables indicated higher competition intensity at the productive end of the climatic gradient, while survival indicated either facilitation at the dry end or no trend along the gradient. 5 Temporal changes occurred, with moderate facilitation soon after germination shifting to strong competition at the end of the growing season. 6 Our results demonstrate that the outcome of experimental studies on plant–plant interactions may depend not only on the environmental productivity but more so on the life stage at which the target plant is studied. [ABSTRACT FROM AUTHOR]

Environmental gradients represent an ideal framework for studying adaptive variation in the life history of plant species. However, on very steep gradients, largely contrasting conditions at the two gradient ends often limit the distribution of the same species across the whole range of environmental conditions. Here, we study phenotypic variation in a winter annual crucifer Biscutella didyma persisting along a steep gradient of increasing rainfall in Israel. In particular, we explored whether the life history at the arid end of the gradient indicates adaptations to drought and unpredictable conditions, while adaptations to the highly competitive environment prevail at the mesic Mediterranean end. We examined several morphological and reproductive traits in four natural populations and in populations cultivated in standard common environment. Plants from arid environments were faster in phenological development, more branched in architecture and tended to maximize reproduction, while the Mediterranean plants invested mainly in vertical vegetative growth. Differences between cultivation and field in diaspore production were very large for arid populations as opposed to Mediterranean ones, indicating a larger potential to increase reproduction under favorable conditions. Our overall findings indicate two strongly opposing selective forces at the two extremes of the aridity gradient, which result in contrasting strategies within the studied annual plant species. [ABSTRACT FROM AUTHOR]

Abstract Climate warming enables plant species to migrate to higher latitudes and altitudes. Within Europe, the Mediterranean harbours many species that might expand their ranges towards Western Europe. Small seed size may facilitate dispersal, however, it may impair establishment of the range-expanding plant species in the novel vegetation. In a greenhouse experiment, we examined effects of average seed size of Mediterranean plant species on their establishment in a mixed community of Western European plant species. Applying two levels of densities of the natives and a herbivory treatment, we tested how seed size is linked to response in plant growth and fitness in novel vegetation. While all non-native plant species showed a negative response to increased planting density, species with small seeds showed a less negative response. This effect persisted under herbivory. Our data suggest that small-seeded non-native plant species may tolerate competitive pressure from novel plant communities better than large-seeded species, so that small seed size may confer a higher probability of establishment of non-native species in novel communities. Highlights • Native plant communities can have a large role in the success of non-native plants. • Competition arising from high plant densities was more important than herbivory. • All non-native plant species showed a negative response to increased planting density. • Small-seeded species coped better than large-seeded species with high densities. [ABSTRACT FROM AUTHOR]

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

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