Abstract
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.
Highlights
Facilitation studies typically compare plants under differential stress levels with and without neighbors, while the density of neighbors has rarely been addressed
The most influential model of facilitation, stress-gradient hypothesis (SGH), which predicts dominance of facilitation over competition under intense stress[6], is conceptual instead of quantitative and there is still much debate about its generality[7,8,9,10,11,12,13,14,15]. This hinders our ability to connect plant interactions with environmental change, hampering efforts to predict the response of populations and communities to pressures and disturbance under ongoing global change[16,17,18]
Note that relative interaction indexes (RIIs) is used to quantify the strength of net plant interactions, which ranges from −1 to 1 with negative values indicating competition and positive values net facilitative interactions
Summary
Facilitation studies typically compare plants under differential stress levels with and without neighbors, while the density of neighbors has rarely been addressed. The most influential model of facilitation, stress-gradient hypothesis (SGH), which predicts dominance of facilitation over competition under intense stress[6], is conceptual instead of quantitative and there is still much debate about its generality[7,8,9,10,11,12,13,14,15] This hinders our ability to connect plant interactions with environmental change, hampering efforts to predict the response of populations and communities to pressures and disturbance under ongoing global change[16,17,18]. Most studies focus either on beneficiaries or on benefactors alone[35,36] and so are the few studies investigating density-dependence[24,27]
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