Journal Article

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

Xin Jia, Wei-Ping Zhang, Gen-Xuan Wang

2017

Overview

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 versus 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 versus 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. Stress slowed down density-dependent mortality considerably when competition was not completely symmetric. Stress significantly decreased the self-thinning intercept, while facilitation simply counteracted stress effects. Both stress and facilitation showed little effect on the slope. In the two-layer model, both stress and facilitation affected mortality in the same way as in the one-layer version when competition was not completely symmetric. Different from the one-layer model, the two-layer version showed that the effects of stress and facilitation on the self-thinning slope were mediated by the asymmetry of competition. As stress increased, the overall asymmetry of competition shifted from asymmetric to symmetric due to increased relative strength of root competition. High stress thus dramatically flattened self-thinning lines, whereas the inclusion of facilitation counteracted stress and led to steeper selfthinning lines. Our two-layer model is based on the current knowledge of plant–plant interactions, and better represents ecological realities. It can help elaborate experiments for testing the role of competition and facilitation in driving plant population dynamics.

• Publisher: Nordic Society Oikos,Blackwell Publishing Ltd
• Subject: Asymmetry / Biological competition / Competition / Computer simulation / Environmental gradient / Flattening / Gradients / Growth / Interactions / Lines / Mortality / Plant growth / Plant population / Plant populations / Population dynamics / root growth / roots / shoots / simulation models / Strength / Stress / Stresses / Symmetry / Testing / Thinning