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

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