Context dependent effects of nitrogen deposition on forest understory plant communities

Understory communities can dominate forest plant diversity and strongly affect forest ecosystem structure and function. Understories often respond sensitively but inconsistently to nitrogen (N) deposition, varying greatly among species, functional groups and forest types. Here, we characterize such context dependency as driven by differences in the amounts and forms of deposited N, cumulative deposition, the filtering of N by overstories, and available plant species pools. All else being equal, the effects of N deposition on understory trajectories can also vary due to differences in surrounding landscape conditions, ambient browsing pressure, soils and geology, other environmental factors controlling plant growth and historical and current disturbance/management regimes. The number of these factors and their potentially complex interactions complicate our efforts to make simple predictions about how N deposition affects forest understories. Based on a literature review, we summarize evidence for context dependency in the effects of N deposition on forest understories. Our review reaffirms that historical management, and site conditions, such as light and pH-fertility gradients, significantly affect how understory communities respond to N deposition. We conclude that plant species’ and communities’ responses to N deposition vary greatly depending on environmental contexts, implying context dependent values of critical loads for N deposition, and complicating efforts to predict how N deposition will affect forest understories. We need a better understanding of the relationship between the effect of N deposition on an understory community with environmental conditions, including habitat structure and key site historical information. Ideally, this requires a sufficient observational survey, experimental and modeling evidence base in order to make quantified predictions of effects in relation to broad N deposition. In some regions (e.g., temperate forests in Europe and northeastern US), researchers also need to understand context dependency in understory community dynamics in forest ecosystems experiencing lower levels of N deposition than in the recent past.