Tree species in deciduous forests have been shown to differ in their influence on soil nutrient cycling. However, small-statured forest herbs are often overlooked despite their large contribution to forest plant biodiversity and, potentially, forest nutrient cycling. In this study, we quantify the influence of atmospheric N deposition, land use history, and two congeneric tree species on a) herbaceous community characteristics, b) foliar nitrate reductase activity (NRA), and c) soil water nitrogen. Data was collected from 18 study plots in each of four adjacent forested watersheds (WS) at the Fernow Experimental Forest in West Virginia. Watersheds differed with respect to N deposition level, land-use history, and tree species composition, allowing for case studies of watershed pairs: WS3 vs WS7 (N fertilization vs unfertilized reference; same stand age of 50 yrs); WS7 vs WS13 (stand age: 50 yrs vs ~100 yrs, ambient N deposition); and WS10 vs WS13 (red maple vs sugar maple dominance in watershed, same tree age of ~100 yrs). In each watershed, study plots were centered around paired sugar maple (Acer saccharum) and red maple (A. rubrum) trees. Herb data (cover, species richness), foliage, and soil water were collected from 1-m2 subplots or a tension lysimeter around each tree.
Results/Conclusions
Comparing the reference watersheds (WS10 vs WS13), herb characteristics, NRA, and soil water N were similar, as expected. Maple species did not affect herb characteristics and soil water N but NRA was significantly higher in sugar than red maple (P<0.05). Comparing watersheds of different stand age (WS7 vs WS13), there was an effect of WS (presumably stand age) and an interaction effect (WS × maple species) on herb cover and richness. Red maple NRA and soil water N were higher in the ‘younger’ than the ‘older’ watershed; there was no difference in NRA or soil water N between maple species. Comparing the fertilized with the unfertilized watershed (WS3 vs WS7), there was an effect of WS (presumably a fertilization effect) and an interaction effect (WS × maple species) on herb cover and richness. There was no effect on foliar NRA in either tree species, but several herb species had significantly higher NRA in the fertilized watershed. Soil water nitrate was higher only under sugar maple in WS3, but not under red maple, compared to WS7. Given the nuanced responses and interactions between tree species and the understory, the role of herb layer species in forest functioning deserves further study.