ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011 Box 4.UNDERSTANDING THE CRITICAL LOADS APPROACH Critical loads and other effect de tools that esource ma s and p way that alo anagers.Critical loads are mos ystem (e.g.,gr land)or biotic c munity (e.g.,understory plants or tree-dw ina lic ns) acid ne ritical loads may be a ted with biological thresholds for different negative Ce2aalhrngtldeteetargloeoeaieyhresholdgbasodontheeveofooysiomprotectiondesrod.economiccon Forests U.S.researchers use models to develop critical the critical loads is quite high,but has de s in to the 2000(E criti the Appalachian Mountain Range and orida and in the lakes recreationally valu nd n gen are expressed in terms of ionic charge bal- of streams evaluated exceed the critical load itical cid load bu at leas 25%including much of New England,West in SO,emissions is related to con- Virginia,and parts orth C 1 ina.By low ini and Mg n,wh (Ontario..New Brunswick.Nova f2053 lakes in six north ast mn states and four n canadian Scotia and Newfoundland). Surface Waters A米r ults ogen a critical load in the ation with Adirondack Mountains of New York and in imated that the critical load is exceeded in or a targe s In hese of l studi ies point to t pe c fot asses sing the impact of emissions A 8 esa The Ecological Society of America esahq@esa.org ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011 8 esa © The Ecological Society of America • esahq@esa.org Forests U.S. researchers use models to develop critical loads for forest soil acidification (Box 4). A recent study estimated the critical acid loads for forest soils across the conterminous U.S. The critical acid loads for S and N throughout the Appalachian Mountain Range and Florida are estimated to be less than 1,000 eq/ha/yr (critical loads for combined sulfur and nitro￾gen are expressed in terms of ionic charge bal￾ance as equivalents per hectare per year). This study estimated that about 15% of U.S. forest soils exceed their critical acid load by at least 25% including much of New England, West Virginia, and parts of North Carolina. By comparison, critical load modeling in Canada estimated that 30 to 40% of upland forest areas in Canada are in exceedance of the criti￾cal load for acidification, while more than 50% are in exceedance in eastern Canada (Ontario, Quebec, New Brunswick, Nova Scotia and Newfoundland). Surface Waters Regional critical loads for surface waters have been developed for acidifying deposition of sulfur and nitrogen in sensitive regions of the Adirondack Mountains of New York and in the central Appalachians of Virginia and West Virginia. The median critical load for a target ANC of 50 µeq/L is 129 milliequivalents per square meter per year (meq/m2 /yr) in the Adirondacks and 45 meq/m2 /yr in the central Appalachians with values ranging from less than 0 to over 1,000 meq/m2 /yr in relatively insensitive ecosystems. The number of aquatic ecosystems exceeding the critical loads is still quite high, but has declined with decreases in acid deposition from the early 1990s to the late 2000s (Figure 4). Currently, 44% of Adirondack lakes evaluated exceed the criti￾cal load and in these lakes recreationally valu￾able fish species such as trout are missing due to acidification. In the Shenandoah area, 85% of streams evaluated exceed the critical load resulting in losses in fitness in fish species such as the blacknose dace. The persistence of criti￾cal load exceedances despite significant decreases in SO2 emissions is related to con￾tinued high inputs of acidifying NOx, low ini￾tial ANC conditions, and soil depletion of nutrient cations (Ca+2 and Mg+2) that have left many watersheds more sensitive to acid deposition over time. A similar study of 2053 lakes in six north￾eastern states and four eastern Canadian provinces estimated critical loads for acidify￾ing deposition of sulfur and nitrogen for a tar￾get ANC of 40 µeq/L. Results show that 28% of the lakes studied have a critical load in the categories of ≤20 and 20–40 meq/m2 /yr, sug￾gesting vulnerability to acidification with rela￾tively moderate atmospheric deposition. It is estimated that the critical load is exceeded in 12% of the study lakes, based on deposition levels in 2002. These studies point to the importance of long-term monitoring and research for assessing the impact of emissions control programs on deposition and ecological recovery (Box 5). Box 4. UNDERSTANDING THE CRITICAL LOADS APPROACH Critical loads, and other approaches that use models or empirical observations to link deposition with effects, provide tools that enable resource managers and policymakers to evaluate tradeoffs between the costs of more stringent emissions controls and the benefits of ecosystem services provided by healthy ecosystems. A critical loads approach can be used to synthesize scientific knowledge about air pollution thresholds that cause adverse impacts or ecosystem change. Describing air pollutant effects on ecosystems in critical load terms quantifies estimates of “cause and effect” in a way that allows researchers to communicate science to air quality regulators and natural resource managers. Critical loads are most commonly applied to evaluate the effects of nitrogen and sulfur pollutants and their associated acidity or the eutrophying effects of nitrogen. When critical loads are exceeded there is increased risk for a range of problems including ecosystem acidification, excess nitrogen effects, declines in forest health, and changes in biodiversity. Critical loads are typically expressed as deposition loading rates of one or more pollutants in amount per area per year (e.g., kilo￾grams per hectare per year (kg/ha/yr)). Critical loads are based on changes to specific biological or chemical indicators such as species composition of a given ecosystem (e.g., grassland) or biotic community (e.g., understory plants or tree-dwelling lichens) or acid neu￾tralizing capacity (ANC) in soils, streams or lakes. Because different sensitive receptors (e.g., forest soils, high elevation lakes, species of lichen) or species may have varying sensitivities to air pollutant loads, multiple critical loads can be used to describe a continuum of impacts with increasing deposition at a given location (See Figure 5). In addition, even for the same organism, multiple critical loads may be associated with biological thresholds for different negative effects, such as stunted growth, reduced reproduction, and increased mortality. Several different threshold levels may therefore be included in a critical load assessment. The policymaker, air regulator, or land manager can assess all the critical loads (science-driven ecological thresholds) and select target loads (policy thresholds) based on the level of ecosystem protection desired, economic con￾siderations, and stakeholder input at a given location