The herbaceous layer is increasingly acknowledged for its significant contribution to the structure and function of forest ecosystems, with numerous studies investigating the ecological factors that influence its dynamics, including spatial and temporal variation in species composition and aboveground cover/biomass. Other studies in a variety of forest types have shown that excess nitrogen (N) decreases biodiversity of herb layer communities, which are typically responsive to spatial patterns of soil resource and light availability. The purpose of this study was to examine (1) N-mediated temporal change in herb layer composition over a quarter century, and (2) spatial patterns of herb cover and individual species and how they are potentially influenced by soil resources and canopy structure. This study used two watersheds at the Fernow Experimental Forest, West Virginia, USA: WS4 and WS3 as untreated reference and treatment watersheds, respectively, with WS3 receiving 35 kg N/ha/yr via aerial application. Herb cover and composition was measured in seven permanent plots/WS from 1991 to 2014. In 2011, soil moisture and several metrics of soil N availability were measured in each plot, along with measurement of several canopy structural variables. Relationships between herb cover and individual species versus soil and canopy measurements were determined via linear and backwards stepwise regression.
Results/Conclusions
Herb diversity and composition varied only slightly over time on reference WS4, in contrast to substantial change on N-treated WS3, with notable declines in species diversity and evenness. Total herb cover varied spatially only with soil resources on WS4, but only with canopy structure on WS3. Results support work in many forest types that excess N can decrease plant diversity in impacted stands. Much of this response is likely related to N-mediated changes in the response of the herb layer to soil N and light availability. This pattern was especially evident for Rubus allegheniensis (blackberry), a nitrophilic species which (1) varied positively to soil N on both watersheds, (2) was not correlated with canopy structure on reference WS4, and (3) was most significantly 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.