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. 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 self-thinning 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.

植物自然稀疏（plant self-thinning）的速率与斜率，均会在环境梯度下受到植物间相互作用的影响。驱动自然稀疏动态的潜在机制包括地下竞争（root competition）与地上竞争（shoot competition）的相对强度，以及竞争与促进作用（facilitation）之间的相互作用。尽管这些机制通常协同发挥作用，但其相对重要性尚未得到充分探究。本研究采用单层与双层影响区（zone-of-influence, ZOI）模型，探究在胁迫梯度（stress gradients）下竞争与促进作用如何驱动植物自然稀疏过程。作为传统影响区模型的改进版本，双层模型可显式模拟地上、地下生长以及邻体相互作用，因此竞争的整体大小对称性会受地下与地上竞争的相对强度调控。单层模型的模拟结果显示，竞争的不对称性越强，稀疏过程越快，且会使自然稀疏线更陡峭（斜率范围约为-1至-4/3）、位置更低。当竞争未完全对称时，胁迫（stress）会显著减缓密度制约性死亡过程。胁迫会显著降低自然稀疏的截距，而促进作用仅会抵消胁迫的影响。胁迫与促进作用对斜率的影响均极小。在双层模型中，当竞争未完全对称时，胁迫与促进作用对死亡过程的影响与单层模型一致。与单层模型不同的是，双层模型显示，胁迫与促进作用对自然稀疏斜率的影响由竞争不对称性介导。随着胁迫程度升高，由于地下竞争的相对强度提升，竞争的整体不对称性会从不对称转向对称。因此高胁迫会显著拉平自然稀疏线，而引入促进作用则会抵消胁迫的影响，使自然稀疏线更陡峭。本研究的双层模型基于当前对植物间相互作用的认知，能更好地反映生态学现实，可用于设计相关实验，以检验竞争与促进作用在驱动植物种群动态中的作用。