Nitrogen fertilization interacts with light to increase Rubus spp. cover in a temperate forest

Nitrogen additions have caused species composition changes in many ecosystems by facilitating the growth of nitrophilic species. After 24 years of nitrogen fertilization in a 40 year-old stand at the Fernow Experimental Forest (FEF) in Central Appalachia, USA, the cover of Rubus spp. has increased from 1 to 19 % of total herbaceous-layer cover. While Rubus spp. are generally associated with high-light conditions that are created after a disturbance event, some species are also known to be nitrophilic. We investigated whether the increase in cover in Rubus spp. was due to either nitrogen, light, or an interaction between these two factors. To test for the effect of nitrogen and light on Rubus spp. cover, we compared the relative cover of Rubus spp. among fertilized and unfertilized watersheds and among fertilized and unfertilized experimental plots, using estimates of canopy openness as a covariate. Rubus spp. plants were also grown ex situ in a field experiment using a 2-way factorial design, measuring leaf area, and using two levels of nitrogen and three levels of light. The effect of nitrogen fertilization on relative Rubus spp. cover depended on canopy openness in the watersheds (F = 17.57, p = 0.0002) and experimental plots (F = 25.04, p = 0.0047). A similar effect for leaf area was also observed among plants grown in the field experiment (F = 4.12, p = 0.0247). Our results confirm that, although Rubus spp. at FEF are nitrophilic, they require sufficient light to increase their cover. Furthermore, the dominance of Rubus spp. in the herbaceous layer likely contributes to the observed decline in species diversity.