Two critical drivers of forest dynamics are disturbance and conspecific negative density dependence (CNDD; a reduction in performance with high conspecific abundance). These processes have been researched extensively in isolation, but little is known about how they interact. This is a surprising research gap since several lines of evidence suggest disturbance may alter CNDD strength (e.g. if a species-specific pathogen thrives only in shade). To investigate, we planted and monitored tree seedlings in disturbed and undisturbed plots (32 total) in central Virginia. Prior to disturbance (selective tree cutting in fall 2017), all trees (> 5cm DBH) in each plot (17.9m radius; 1/10 ha) were identified, measured, and mapped. In spring 2018, we planted over a thousand seedlings of flowering dogwood (Cornus florida), sweetgum (Liquidambar styraciflua), and tupelo (Nyssa sylvatica). All seedling locations were mapped, and canopy cover (a proxy for light availability) was recorded above each seedling location in July. Seedling height, leaf number, and foliar damage were measured in both June and August. Seedling performance was analyzed, via mixed effects models, as a function of light, local abundance (inverse distance-weighted basal area) of uncut conspecifics and heterospecifics, and the interaction between light and conspecific abundance.
Results/Conclusions
Height growth and leaf production generally responded positively to greater light availability, but there was considerable variation across species for the other response variable (foliar damage) and with the other predictors. Negative effects of conspecific adults were detected for dogwood (greater foliar damage) and tupelo (reduced leaf production and greater foliar damage), but we found no evidence of positive conspecific effects for any species or response variable. Heterospecific abundance was generally non-significant after accounting for light, with the sole exception of tupelo height growth, which was (very weakly) negatively associated with heterospecific abundance. Interactions between light and conspecific abundance were non-significant for all species and response variables, but in several cases both variables were predictive in isolation. Our results contribute to mounting evidence that CNDD is important in temperate forests, although they do not demonstrate any interactive effects of CNDD and disturbance. However, these conclusions relate only to the first season of data collection, and we intend to measure seedling performance for at least one more year. Given that height growth and leaf production are difficult to quantify with precision over short intervals, a longer monitoring period may be necessary to reveal interactions between CNDD and disturbance.