Air pollution emitted from a variety of sources is deposited from the air into ecosystems. These pollutants may cause significant harmful ecosystem effects when exceeding a threshold, known as a critical load (CL). When a CL is exceeded, negative ecosystem effects are likely to occur, immediately or over time. Similarly, when deposition is reduced and exceedances eliminated, recovery occurs at different timescales. In some cases, improvements are unlikely even when deposition is eliminated; mitigation and restoration together might be necessary to enable ecosystem improvements. The Clean Air Act and the Forest Service 2012 Planning Rule require an understanding of the potential negative ecosystem effects of air pollution, as well as an understanding of how to preserve and protect ecosystem health in the face of these threats.
Air quality assessments are used to inform managers in national forests and grasslands about critical load exceedances within their boundaries, and to help them set target loads (TLs) when appropriate. This information can be used to communicate the risk of air pollution effects on various resources to Forest Service personnel, to the public, and to the air regulators who issue permits to air pollution sources. This document outlines some of the management options available to reduce critical load exceedances and mitigate the effect of air pollution on national forests and grasslands.
When discussing management alternatives with line officers seeking to minimize the ecosystem effects associated with air pollution, it is helpful to highlight the relationship between critical load exceedances and subsequent impacts to ecosystem services. Ecosystem services are defined as the benefits people obtain from ecosystems. Sullivan (2012) integrates the principles of ecosystem services with the use of critical loads for public land management and natural resource policy decision-making. Specifically, Sullivan suggests that a CL exceedance indicates an increased likelihood that there will be a loss of one or more ecosystem services. Examples of ecosystem services that can be affected by critical load exceedances are available in Fenn et al. (2011), summarized in Table 1 and separated into potential acidification and nitrogen saturation impacts. Ecosystem services potentially affected by critical load exceedances in terrestrial ecosystems include decreased soil fertility leading to potential reductions in timber available for harvest. Similarly, aquatic ecosystems may exhibit decreased water quality causing reduced fish populations used for sustenance and recreation. The reductions in timber harvest and fishery populations are secondary effects (resulting from primary effects including loss of soil base cations, degraded water quality, etc.) but serve to highlight the potential costs associated with failing to address critical load exceedances. Sullivan concludes that, based on the estimated loss of ecosystem services, management and/or policy decisions might be made to change emissions regulations (by air regulators), and/or implement remediation or restoration actions (by resource managers).
A series of management options are presented that can be implemented to eliminate or reduce critical load exceedances or mitigate the effect of air pollution on national forests. Management options are presented by pollution effect: acidification or nitrogen saturation.