Eastern temperate forest tree seedlings exhibit minimal evidence of conspecific negative density dependence

Benjamin S. Ramage, Avery A. Crowder and Katie J. Tisler, Biology Department, Randolph-Macon College, Ashland, VA, Daniel Johnson, University of Florida

Background/Question/Methods
Tree seedling performance and species coexistence are influenced by several factors, including light availability and conspecific negative density dependence (CNDD; a reduction in performance with high local conspecific abundance). Light-related niche partitioning is well-established, but the importance of CNDD for diversity maintenance is less clear, especially in temperate forests, perhaps in part because complex interactions between light availability and CNDD have been largely ignored. To quantify these effects - in isolation and in interaction with each other - we planted and monitored almost 2000 tree seedlings across 32 plots in central Virginia, in micro-sites that varied substantially in light availability and local conspecific abundance. In spring 2018, we planted seedlings of flowering dogwood (Cornus florida), sweetgum (Liquidambar styraciflua), tupelo (Nyssa sylvatica), red maple (Acer rubrum), and black cherry (Prunus serotina). All seedling locations were mapped, and canopy cover (an inverse proxy for light availability) was recorded above each seedling location in July. Seedling performance (survival, height, and foliar damage) was measured in the summers of 2018 through 2020. Performance was then analyzed, via generalized linear mixed effects models, as a function of light, local abundance (inverse distance-weighted basal area) of conspecifics and heterospecifics, and the interaction between light and conspecific abundance.
Results/Conclusions
Overall, light availability was a much better predictor of seedling performance than local conspecific abundance. Light was positively associated with total height growth, at all measurement times, for four of five species (excepting red maple), while local conspecific abundance had no relationship with height growth for any species; we also looked for interactions between these two predictors but found nothing significant. Survival, which was approximately 80% in 2019 and 55% in 2020, across all species, was unrelated to either light or conspecific abundance, excepting sweetgum, for which we found a positive dependence on light and a borderline negative effect of conspecifics. Total foliar damage in was higher in brighter locations for sweetgum, tupelo, and cherry, and also higher in areas with greater abundance of conspecifics for dogwood and sweetgum (in some but not all years); again, no interactions between predictors were detected. While our results provide some limited evidence of CNDD, effects were weak and inconsistent across species, generally dwarfed by the effects of light, and not altered by light availability. Accordingly, it seems unlikely that CNDD - at least at the seedling stage - contributes substantially to local diversity maintenance in our study system.