Individuals vary a lot and are rarely identical. Conspecific individuals can differ in anti-predator defenses, parasite resistance, abiotic tolerances, resource use or competitive ability. In plant-soil pathogen interactions which caused density-dependent mortality, tree-to-tree variation is usually ignored in previous researches. People noticed that the strength of density dependence varied considerably among individual trees and sites, but few studies have investigated the regularity of these intraspecific variations and the underlying mechanisms. Considering that pathogens cause a great majority of density-dependent mortality, and intraspecific trait variation would influence pathogen spread and reservoir potential as well as disease resistance, it’s reasonable and important for us to investigate how density dependent effects vary among individuals and how the variation correlate with functional traits.
Herein, we experimentally studied seedling survival of two subtropical tree species (Castanopsis fissa and Canavium album): we collected seeds and root soils from individuals of 5 different populations for each species, then conducted a 6-month shadehouse experiment, where the seedlings from each population were transplanted into the soil of all 5 populations, to investigate how seedling survival changed when they were planted in the soil of their own and other populations. We measured several important functional traits of these adult individuals and seedlings, to see whether trait variation can explain the difference of density dependent strength.
Consistent with our prior hypothesis, the strength of density dependence varied a lot among different individuals for both species. Seedlings suffered increased mortality when they were planted in their own soil, and those effects disappeared after fungicide treatment of the soil. When seedlings were grown in the soil collected from other populations, the strength of negative density-dependent effect was significantly correlated with the conspecific adult density within a same population and the spatial distance between seed and soil sources. The strength was also correlated with trait differences, populations with more similar specific leaf area and wood density had stronger Janzen-Connell effects on each other.
Our results suggest that tree species could prevent excessive local advantage through intensive Janzen-Connell effect among trait similar individuals. Meanwhile, species can maintain the overall richness through dispersing to other populations, where they would escape the inhibition effect from conspecific adults. These results improve our understanding of how species maintain conspecific richness at appropriate levels and hence maintain high species diversity in the tropics.