Responses of forest ecosystems to decreasing nitrogen deposition in eastern North America

Historical increases in emissions and deposition of oxidized and reduced nitrogen (N) provided the impetus for global-scale research on the effects of excess N in terrestrial and aquatic ecosystems. Much of the eastern U.S. has been susceptible to negative effects of excess N. The Clean Air Act and associated rules have led to decreases in emissions and deposition of oxidized N, especially in the eastern U.S., representing a research challenge and opportunity for ecosystem ecologists. This chapter predicts changes in the structure and function of North American forest ecosystems in response to decreased N deposition. Hysteresis is a property of a system wherein output is not a strict function of corresponding input, incorporating a time lag, particularly when responses to decreasing input vary from responses to increasing input. A conceptual hysteretic model predicts varying lag times in recovery of soil acidification and nutrient leaching, surface water nitrogen concentrations and export, plant diversity, soil microbial communities, and forest carbon and N cycling toward pre-N-impact conditions. These processes are expected to respond notably to reductions in N deposition, most showing a degree of hysteresis, with the greatest delays in the response occurring in those tightly linked to “slow pools” of N in wood and soil organic matter. Some responses, especially nitrate concentrations in stream flow, have already become apparent in regions of northeastern U.S. Because experimental studies of declines in N loads in forests of North America are lacking and due to the expected hysteresis, it is difficult to generalize from experimental results to patterns expected from declining N deposition. Responses to declining N will be long-term and difficult to distinguish from concurrent environmental changes affecting the N cycle, e.g., elevated atmospheric CO2, climate change, reductions in acidity, invasive species, and vegetation responses to disturbance.