Facilitation among plants can accelerate density-dependent mortality and steepen self-thinning lines in stressful environments

The speed and slope of plant self-thinning are all affected by plant-plant interactions across environmental gradients. Possible mechanisms driving the self-thinning dynamics include the relative strength of root vs. shoot competition, and the interplay between competition and facilitation. Although these mechanisms often act in concert, their relative importance has not yet been fully explored. We used both a one-layer and a two-layer zone-of-influence (ZOI) model to examine how competition and facilitation drive self-thinning across stress gradients. As a development of the traditional ZOI model, the two-layer version explicitly models shoot and root growth and neighbor interactions, and thus the overall size-symmetry of competition is regulated by the relative strength of root vs. shoot competition. One-layer model simulations revealed that increasingly asymmetric competition accelerated thinning, and steepened (slope ranged from about -1 to -4/3) and lowered self-thinning lines. Str...