The Janzen-Connell hypothesis predicts that high densities of specialist enemies surrounding established trees leads to higher mortality of conspecific seeds and seedlings. Where conspecific seedlings fail, heterospecific seedlings have a higher probability of survival, leading to greater species coexistence in forests. Although this conspecific negative density dependence (CNDD) can lead to biodiversity in temperate forests, not all tree species exhibit recruitment patterns indicative of CNDD. It is unclear whether plant traits can predict whether a tree species is vulnerable to CNDD. Fast-growing species such as gap specialists are generally shade intolerant, have lower investment in tissue defense, have greater vulnerability to pathogens, and therefore may be more susceptible to CNDD. Similarly, the type of fungal mutualist (mycorrhiza) with which a species associates may decrease susceptibility to pathogen infection by surrounding root tips with a thick hyphal barrier. We tested these hypotheses using a linear mixed modeling approach across several mapped forest plots in mixed hardwood forests in the Piedmont of North Carolina (Orange County, NC). Our intention was to determine: 1) whether CNDD is apparent for certain tree species; and 2) whether two key plant traits (shade tolerance and mycorrhizal status) influence CNDD susceptibility.
Results/Conclusions
We found evidence of CNDD acting at the sapling stage via decreased growth rates and survival. Furthermore, mycorrhizal status and shade tolerance both influenced CNDD susceptibility, and the former was more strongly correlated with growth and survival than the latter. Species associated with ectomycorrhizae were less susceptible to CNDD than those associated with arbuscular mycorrhizae (p<0.001). At relatively large spatial scales (15-20m), both shade tolerant and intolerant saplings experienced a decrease in growth when the density of conspecifics was high; however, only shade tolerant saplings had lower growth rates under these conditions at the smallest spatial scale (5m) (p<0.001). Conversely, shade tolerant saplings had much higher survival than shade intolerant saplings under high conspecific pressure (p<0.001). Our results imply that certain plant traits may allow us to predict which species are susceptible to CNDD in temperate forests. The implication is that as tree species composition changes (e.g. through natural succession or anthropogenic change), the composition of tree traits may also shift, altering processes that contribute to the maintenance of biodiversity.