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Summary: 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. [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]

Parental effects ( PE) can be adaptive and improve offspring performance when parents and offspring experience similar environmental conditions. However, it is unknown whether adaptive PE exist also in habitats where such similarity is unlikely due to strong temporal variation. In particular, we do not know whether PE can adapt offspring to fluctuating levels of neighbour competition in such habitats., Here, we tested for adaptive PE in terms of two key environmental factors in a semi-arid annual system, competition and drought. While rainfall was stochastic in the study site, the competitive environment was partly predictable: higher plant densities followed after favourable (rainy) years due to high seed production. We therefore expected PE to adapt the offspring's competitive ability to these (predictable) fluctuations in plant densities, rather than to adapt the offspring's drought tolerance to the (unpredictable) occurrence of intensified drought., Parental plants of Biscutella didyma, an annual Brassicaceae, were raised under favourable watering and under drought conditions. Offspring performance was then tested under a full-factorial combination of two neighbour regimes and six watering levels in the glasshouse., Offspring of parents grown under favourable conditions were stronger competitors. This was associated with a small shift in phenology but not with higher parental seed provisioning. Offspring from parents grown under drought showed no improved drought tolerance. Moreover, no PE were detectable when offspring were grown without neighbours., Our results suggest a novel path of adaptive PE: higher competitive ability was induced in offspring that were more likely to experience high neighbour densities. Together with the lack of adaptive PE towards drought tolerance, this emphasizes that a correlation between parental and offspring environment is crucial for adaptive PE to evolve. Our results also call for the inclusion of competitive effects in future PE studies., Synthesis. This study demonstrates the important role of adaptive PE for plant fitness (regarding competition) but also their limits (regarding drought) in temporally variable environments, based on the predictability of the respective environmental factor. [ABSTRACT FROM AUTHOR]

Altered rainfall regimes will greatly affect the response of plant species to climate change. However, little is known about how direct effects of changing precipitation on plant performance may depend on other abiotic factors and biotic interactions. We used reciprocal transplants between climatically very different sites with simultaneous manipulation of soil, plant population origin, and neighbor conditions to evaluate local adaptation and possible adaptive response of four Eastern Mediterranean annual plant species to climate change. The effect of site on plant performance was negligible, but soil origin had a strong effect on fecundity, most likely due to differential water retaining ability. Competition by neighbors strongly reduced fitness. We separated the effects of the abiotic and biotic soil properties on plant performance by repeating the field experiment in a greenhouse under homogenous environmental conditions and including a soil biota manipulation treatment. As in the field, plant performance differed among soil origins and neighbor treatments. Moreover, we found plant species-specific responses to soil biota that may be best explained by the differential sensitivity to negative and positive soil biota effects. Overall, under the conditions of our experiment with two contrasting sites, biotic interactions had a strong effect on plant fitness that interacted with and eventually overrode climate. Because climate and biotic interactions covary, reciprocal transplants and climate gradient studies should consider soil biotic interactions and abiotic conditions when evaluating climate change effects on plant performance. [ABSTRACT FROM AUTHOR]

1. Interactions among neighbouring plants are often mediated by foraging choices of pollinators. For example, the presence of a conspicuous species may increase the number of pollinators attracted to its vicinity, indirectly increasing visitation rates also to neighbouring plants. Because pollinator choices are frequently density-dependent, the presence of a conspicuous species at high densities may also increase competition for pollination services. Additionally, models predict that plant density will interact with spatial distribution in manipulating the pollinator behaviour, yet experimental evidence for this effect is missing. 2. We performed a field experiment in which we introduced a highly conspicuous species in different densities and spatial configurations in a full-factorial manner into a species-rich meadow and studied its effect on neighbouring plants. 3. We showed that the highly conspicuous species strongly contributed to the attractiveness of its local patch and thus benefited its neighbours. However, because of the strong density effect, the conspicuous species changed its role and became a competitor for pollinators when its density increased. 4. We supported our theoretical assumptions and showed that when the introduced conspicuous species was regularly distributed among other plants in the patch, it increased visitation rate, and in some cases also seed set, to conspicuous neighbours relative to when it was aggregated, at least at low densities. 5. Synthesis. We suggest that complex interactions between density and spatial distribution of plant species at the patch scale are highly relevant for the interpretation of pollinator behaviour and therefore should be treated as factors of floral attractiveness in future studies. [ABSTRACT FROM AUTHOR]