Response of the herbaceous layer of contrasting forest ecosystems to excess nitrogen deposition
Challenges to conserving forest biodiversity include numerous anthropogenic threats to ecological integrity of herbaceous layer communities, which carry with them a disproportionately great significance to the structure and function of temperate forests, representing up to 90% of the plant diversity of these ecosystems. Evidence indicates that excess atmospheric N deposition typically decreases herb layer diversity, although specific responses and associated mechanisms for this can vary greatly between forest ecosystem types, especially compared to tropical forests, wherein most plant diversity is generally associated overstory species. In addition to differences in species distribution, tropical forests have soils typically more highly weathered than those of temperate forests. Central questions addressed in this paper are (1) do temperate and tropical forest herb layers differ in their response to increased N? and (2) what mechanisms underlie this response? We compare N response data from one tropical and one temperate forest site. The tropical site was Dinghushan Forest, Guangdong Province, China. The temperate site was Fernow Experimental Forest (FEF), West Virginia, USA. Both studies involved experimental N manipulations.
Experimental additions of N decreased herbaceous layer diversity in both forests. However, there were profound differences among forest types in patterns and mechanisms of response. At FEF, excess N increased herb layer cover and loss of N-efficient species (e.g., Viola spp.) by substantially increasing cover and homogeneity of nitrophilic Rubus spp. These responses were also consistent with the N homogeneity hypothesis, which predicts N-mediated decreases in herb layer diversity via decreases in spatial heterogeneity of soil N. By contrast, decline in herb layer richness at tropical Dinghushan forest was associated with decreases in cover, wherein excess N brought about species loss via decreased pH and increased aluminum mobility, rather than altering competitive dynamics, some of which may have been related to antecedent conditions of N saturation at this site, where annual atmospheric deposition of N can exceed 30 kg N per ha. Thus, although excess N deposition represents a serious threat to plant biodiversity in widely contrasting forest types, mechanisms for these effects can be highly site dependent.