Response of the herbaceous layer of forest ecosystems to excess nitrogen deposition

Summary1 This review brings into focus what is known about the response of the herbaceous layer of forest ecosystems to increasing nitrogen deposition. The emphasis on forests in general is important for two reasons. First, forests often occupy areas receiving high rates of atmospheric deposition of N. Second, compared with herb‐dominated communities, about which much is known regarding response to excess N, forests generally display greater biological and structural complexity. The more specific focus on the herbaceous layer – here defined as all vascular (herbaceous and woody) plants ≤ 1 m in height – is warranted because most of the species diversity of forests occurs in the herb layer. 2 Most responses of forest ecosystems to N saturation, defined here as ecosystem supply of N exceeding ecosystem demand for N, can be characterized by two complementary hypotheses, each emphasizing different facets of ecosystem structure and function. One focuses on ecosystem processes, such as net primary productivity, whereas the other addresses seasonal patterns of concentrations of in stream water. Although neither hypothesis considers the effects of chronically elevated N deposition on forest herbs, both share a prediction – a dramatic increase in availability of – that is relevant to forest herb response. Such a shift towards dominance has important implications for changes in herbaceous layer dynamics. For example, increases in soil pools can increase invasibility by exotic species. Also, because preferential use of vs. can be highly species‐specific among forest herbs, predominance of availability can bring about further changes in herb layer species composition. Also important to forest herbs is the tendency for enhanced mobility of to increase mobility and decrease availability of essential cations, such as Ca 2+ . 3 The response of plant species of the herbaceous layer of forest ecosystems to excess N availability can arise from N‐mediated changes in several processes. Here I identify six – competition, herbivory, mycorrhizal infection, disease, species invasions and exotic earthworm activity – and discuss how they respond to excess N and how this response affects the herb layer of forests. With the exception of the activity of exotic earthworms (which is correlated with, but not necessarily caused by, high N deposition), there is compelling evidence that all processes respond sensitively to N deposition and that this response generally leads to drastic shifts in species composition and decreases in biodiversity of forest herb communities. 4 A hypothesis is suggested – the nitrogen homogeneity hypothesis – that predicts loss of biodiversity in forest ecosystems experiencing chronically elevated N deposition. This hypothesis is based on the decrease in spatial heterogeneity of N availability that is typically high in forest soils under N‐limited conditions.