Response of herbaceous layer species to canopy and soil variables in a central Appalachian hardwood forest ecosystem

Previous work has suggested that excess nitrogen (N) alters the degree to which forest canopy versus soil variables influence forest herb communities. This study tested the hypothesis that excess N would shift this influence on individual herb species from soil N availability to stand structural variables that determine light availability to the forest floor. Two watersheds at the Fernow Experimental Forest, West Virginia, USA were used: WS4 and WS3 as untreated reference and treatment watersheds, respectively. WS3 receives 35 kg N/ha/year via aerial application. Herb cover and composition was measured in seven permanent plots/WS from 1991, currently on-going. In 2011, soil moisture and N availability were measured in each plot, along with several variables of canopy structure. Backwards stepwise regression was used to determine relationships between herb cover/individual species and soil/canopy measurements. Herb cover varied spatially with soil resources on WS4, whereas cover varied spatially with canopy structure on WS3. Although results for total herb layer cover supported this hypothesis, results for individual herb species rejected it. This contrast was especially evident for Rubus allegheniensis (blackberry), a nitrophilic species which increased with increasing soil N on both watersheds, but was not correlated with canopy structure on reference WS4, while being correlated with canopy structure on N-treated WS3. Excess N from atmospheric deposition has been shown to decrease plant biodiversity of impacted forests, especially in its effects on herbaceous layer communities. This work demonstrates that one of the mechanisms of such response is in N-mediated changes in the response of herb communities to soil resources and light availability.