Facilitation can maintain clustered spatial pattern of plant populations during density-dependent mortality: insights from a zone-of-influence model


G.-X. Wang, College of Life Sciences, Zhejiang Univ., CN–310058 Hangzhou, PR China. E-mail: fzstsys2@zju.edu.cn


Recent years have seen a growing body of evidence showing that plant competition and facilitation usually operate simultaneously to drive population dynamics, community structure and ecosystem functions. However, the potential role of facilitation in spatial patterning of plant populations has rarely been explicitly examined. We used a ‘zone-of-influence’ model to explore how facilitation interacts with competition and abiotic stress to determine the spatial patterning of populations during density-dependent mortality. Model simulations revealed that started with the same clustered pattern, the final pattern of simulated populations depended strongly on the interaction among facilitation, stress level and size-symmetry of competition. Asymmetric competition consistently led to immediate and non-random mortality towards regularity, thus rapidly decayed the initially clustered pattern to final patterns of small-scale regularity and large-scale randomness. The role of symmetric competition in decaying the clustered pattern increased with abiotic stress because stress-induced reductions in plants’ growth rates can make individuals in high-density clusters more likely to die even from symmetric competition. Facilitation played a clear role in counteracting the effect of stress, thus tended to maintain the degree of clustering of the pattern during density-dependent mortality. This is because the amelioration of harsh conditions by neighboring plants relieved the reductions in plant growth due to competition, thus slowed down and reduced the mortality inside clusters (relative to that outside clusters). Moreover, the effect of facilitation appeared to increase with abiotic stress. Our results indicate that facilitation among neighboring plants should partially be responsible for clustered population spatial patterns observed in stressful environments, even though its contribution relative to other factors (e.g. local dispersal and environmental heterogeneity) remains to be evaluated. In addition, the potential influence of facilitation on self-thinning trajectory should be explicitly examined in future modeling and experimental studies considering its effects on density-dependent mortality.