Effects of excess nitrogen deposition on the herbaceous layer of temperate hardwood forests

Background/Question/Methods

The herbaceous layer carries with it a disproportionately great significance, relative to its diminutive physical stature, to the structure and function of forests of the eastern United States, representing up to 90% of the plant diversity of these ecosystems.  Of concern to conserving forest biodiversity are the myriad anthropogenic threats to ecological integrity of herb layer communities, such as past agricultural practices, invasions by exotic species, forest harvesting, and vertebrate herbivory.  An additional threat is that of excess atmospheric deposition of nitrogen (N).  Predictive models suggest that N deposition should continue to increase sharply in the future, arising from anthropogenic N emissions of reactive N (e.g., NH_y and NO_x) from several sources.  Evidence indicates that excess N typically decreases herb layer diversity, although the specific mechanisms for this can vary greatly between forest ecosystem types.  For example, in tropical forests, which are typically phosphorus-limited with herb layer communities dominated by seedlings of overstory dominants, excess N can rapidly reduce cover and richness via direct changes in soil conditions, such as decreased pH and increased aluminum mobility. 

Results/Conclusions

In temperate hardwood forests, the following response often occurs in response to increasing N availability: (1) initial increases in cover of the herb layer as a whole, (2) decreases in species richness from loss of relatively numerous N-efficient species; (3) decreases in species evenness from increasing dominance of relatively few high N-requiring species, and (4) declines in forest biodiversity as a result of decreases in both richness and evenness. Several mechanisms potentially interact to cause this, including N-mediated decreases in mycorrhizal fungi and competitive abilities of N-efficient herbaceous dominants, and N-mediated increases in herbivory, fungal pathogens, and invasions by exotic species. Superimposed on these responses is that increased N deposition lowers the typically high degree of spatial heterogeneity in N processing and availability of forest soils, heterogeneity that contributes to the maintenance of high species diversity of the herbaceous layer.  The N homogeneity hypothesis predicts that N-mediated decreases in spatial heterogeneity of N availability will lead to decreases in biodiversity of the herb layer of impacted forests.  Finally, a relatively new area of research emphasis in North America is that of establishing critical loads for N, defined as the deposition of N to an ecosystem below which no harmful effects of N saturation occur.  Recent evidence suggests that critical loads of N for the herb layer are being exceeded for some eastern hardwood forests.