Nitrogen Fertilization, Stand Age, and Overstory Tree Species Impact the Herbaceous Layer in a Central Appalachian Hardwood Forest

Lacey J Smith,Kirsten Stephan

doi:10.3390/f12070829

Abstract

Research Highlights: Herb-layer community composition, abundance, species richness, and Shannon–Wiener diversity index are shaped by nitrogen fertilization, disturbance history, and the overstory tree species in its immediate vicinity. Background and Objectives: While the herbaceous layer in deciduous forests is increasingly recognized for its importance in various aspects of forest ecosystem function, this study sought to describe the factors impacting the herbaceous layer. Specifically, this study’s objective was to quantify and compare herb-layer species composition, cover, and other community indices in watersheds with (a) different levels of N deposition, (b) different stand ages due to differing disturbance histories, and (c) different watershed aspects. This study also tested the hypothesis that herb-layer characteristics vary beneath tree species with contrasting nutrient dynamics (i.e., red and sugar maple). Materials and Methods: At the Fernow Experimental Forest in West Virginia (USA), the cover of all herb-layer species was recorded directly under nine red maple and nine sugar maple trees in each of four watersheds (WS): long-term fertilized WS3 and unfertilized WS7, both with a stand age of about 50 years, and two unmanaged watersheds with 110-year-old stands and opposite watershed aspects (south-facing WS10, north-facing WS13). Community composition and plot-level indices of diversity were evaluated with multivariate analysis and ANOVA for watershed-level differences, effects of the maple species, and other environmental factors. Results: In the fertilized watershed (WS3), herb-layer diversity indices were lower than in the unfertilized watershed of the same stand age (WS7). In the unfertilized watershed with the 50-year-old stand (WS7), herb-layer diversity indices were higher than in the watershed with the 110-year-old stand of the same watershed aspect (WS13). WS10 and WS13 had similar herb-layer characteristics despite opposite watershed aspects. The presence of sugar maple corresponded to higher cover and diversity indices of the herb-layer in some of the watersheds. Conclusions: Despite the limitations of a case study, these findings bear relevance to future forest management since the forest herb layer plays important roles in deciduous forests through its influence on nutrient cycling, productivity, and overstory regeneration.

Highlights

Comparing the two younger stands, half of the cover in fertilized Watershed 3 (WS3) was due to shrubs/vines and less than 3% of all cover was due to herbs, whereas the contribution to cover from these two plant types in watershed 7 (WS7) was about equal (Figure 3B)
This study indicates that a sugar maple effect on the herb layer may not manifest under some conditions such as those in the younger unfertilized stand (WS7) and the older unfertilized stand with a southerly watershed aspect (WS10)
This study demonstrated (1) the possible existence of a sugar maple effect, i.e., sugar maple having a positive effect on herb-layer cover, (2) that long-term N enrichment can reduce ecosystem biodiversity by favoring nitrophilic plant species, (3) that herb-layer characteristics can be influenced by stand age, in that lower litter quality in later-successional stands may lead to lower herb-layer abundance and diversity than in earlier successional stands dominated by arbuscular mycorrhizal fungi (AM) trees, and (4) that watershed aspect did not influence herb-layer indices