Intraspecific facilitation: A missing process along increasing stress gradients - Insights from simulated shrub populations

In recent years many field studies have been conducted to assess the relative importance of facilitation and competition in structuring vegetation communities in different environments. Herein, we present a simulation model which systematically explores the relative importance of intra-specific facilitation and competition between adult shrubs and seedlings for spatial pattern formation. A grid-based simulation model was constructed and calibrated using data collected in the field from Sarcopoterium spinosum populations in Israel to simulate population dynamics along a rainfall gradient. A series of simulation experiments was conducted in which manipulations of seedling survival probabilities were carried out to assess the relative importance of these processes in generating spatial patterns. Increased survival probabilities of first-year shrubs in open areas were used to simulate competition effects, while increased survival probabilities in the vicinity of shrubs were used to simulate facilitation effects. Simulation results were then compared to shrub spatial patterns observed in the field. The results indicate that facilitation is not an important process in generating intra-specific spatial patterns. Rather, in mesic environments with high precipitation, competition is the dominant process generating spatial patterns, resulting in regular spacing of shrubs, similarly to the patterns observed in the field (L(h) values<0). In arid sites, where precipitation values are lower, and stress conditions are higher, the dominant process generating spatial patterns was random mortality due to drought conditions. The resulting spatial patterns in this case are random (L(h)∼0), whereas observed field populations exhibited clumped patterns (L(h)>0). We conclude that as stress conditions increase, the importance of intraspecific neighborhood interactions decrease whereas the importance of environmental factors increase in dictating intra-specific spatial pattern formation. Consequently in mesic environments intra-specific competition among adults determines the emerging patterns, while intraspecific facilitation is a negligible process.