- Driscoll, Charles
Air pollution effects on ecosystems have been an important environmental concern in the Northeast since the 1970s. The concept of critical loads (CLs)/target loads (TLs) has been widely used in Europe for air quality management since the 1980s. A critical load is the input of air pollutants below which adverse ecological effects do not occur. Agencies in the U.S., such as the Forest Service, the Park Service and the Environmental Protection Agencies, and states are interested in using critical loads to guide air quality management to protect ecosystems from acidic deposition and nitrogen deposition. Three approaches are commonly used to establish critical loads: empirical observations and experiments; the application of steady-state (time-invariant) models; and application of dynamic (time-dependent) models. This research involved three projects on the development and application of critical loads to forest and aquatic ecosystems in the Northeast and beyond. The application of a dynamic model, PnET- BGC, was used to determine critical loads of lake/watershed ecosystems in the Adirondack region of NY. PnET-BGC was applied to a watershed at the Hubbard Brook Experimental Forest, NH to examine how changing climate influences the values of critical loads. Empirical critical loads for effects of atmospheric nitrogen deposition were developed for ecosystem types and attributes based on ecoregions for the U.S. We found that critical loads of acidity and nitrogen are routinely exceeded in forest ecosystems of the northeastern U.S. Under changing climate at the Hubbard Brook Experimental Forest in New Hampshire for acidity and nitrogen, the sensitivity of the forest ecosystem to acidic and nitrogen deposition increases, decreasing the critical loads.