Asymmetric facilitation can reduce size inequality in plant populations resulting in delayed density‐dependent mortality

Size inequality in plant populations is a ubiquitous feature that has received much attention due to ecological and evolutionary implications. The mechanisms driving size inequality were mainly attributed to different modes of competition (symmetric versus asymmetric), while the potential effects of different modes of facilitation (symmetric versus asymmetric) to this pattern have not yet been fully explored. We employed an individual‐based model to explore the relative roles of both competition and facilitation simultaneously along an environmental stress gradient. Special emphasis was given to the assessment of symmetric facilitation (plants receive benefit from each other equally or proportionally to benefactors’ sizes) and asymmetric facilitation (beneficiary plants receive benefits from benefactor plants that are higher than proportional to the benefactors’ size) in altering plant size inequality. We found that independent of the particular mode of competition, symmetric facilitation generally increased size inequality, whereas asymmetric facilitation decreased it. This pattern was consistent along the stress gradient. Because of their different effects on size inequality, symmetric facilitation accelerated self‐thinning, whereas asymmetric facilitation delayed the onset of density‐dependent mortality, promoting survival under intermediate stress conditions. We compared our model predictions with both 1) a previous modelling study focusing on the effect of (symmetric) facilitation on the size inequality, and 2) re‐analysed data from a published experiment generating asymmetric facilitation of plants against enhanced ultraviolet‐B (UV‐B). Whereas our model predictions and the results of the empirical experiment were consistent, we found that previous theoretical results that solely relied on symmetric facilitation need to be re‐adjusted. Our study showed that combinations of different modes of competition and facilitation can alter size inequality in different ways and with important consequences for the onset of density‐dependent mortality during population development. Explicitly considering different modes and mechanisms of interactions (both facilitation and competition) will improve mechanistic understanding in plant ecology.