The Janzen-Connell mechanism is hypothesized to be an important contributor to maintaining plant diversity. It operates when species-specific natural enemies limit the density of their host species by consuming seeds or seedlings near parent trees. Soil-borne pathogens are a known contributor to this process, but the relative contribution of their life cycle stages are poorly understood. To determine the role of soil-borne pathogens’ life cycle stages in the development of Janzen-Connell recruitment patterns, we constructed a multi-scale simulation model of the interaction between tree populations and soil-borne pathogens. We characterized seedling recruitment patterns based on the resulting spatial distribution of surviving seedlings and adult trees and related these to tree and pathogen life history traits using classification tree analysis. We also determined which attributes of plants and pathogens affected tree and pathogen population size and spatial pattern with regression tree analyses. The explanatory variables we included in the analysis were dormant oospore mortality, pathozone radius, oospore fecundity, oospores produced per infected host, zoospore dispersal distance, seed dispersal distance, and tree fecundity.
Results/Conclusions
Tree fecundity and seed dispersal distance were found to be the most important attributes determining tree population size and seedling density in relation to adult tree locations (i.e. seedling recruitment patterns), while pathozone radius and oospores produced per infected host affected the locations and populations of pathogens. The effect of pathogens on tree populations and seedling survivorship were not based on pathogen attributes. Only 1/4 of our simulated forests resulted in Janzen-Connell patterns, and those patterns were primarily created by low tree fecundity or seed dispersal at an intermediate range. Pathogen attributes differentiated between seedling recruitment patterns other than the Janzen-Connell pattern, indicating that pathogen life history traits are important for seedling spatial patterns, but may have little influence on species diversity. The abundance and distribution of pathogens are expected to shift with climate change. Previous research has suggested that future declines in precipitation may reduce pathogen dispersal. Although we did not find evidence that pathogen dispersal distances influence recruitment patterns, severe reductions in pathogen populations and their ability to spread could reduce tropical tree species diversity.