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Critical Issues for Critical Loads Critical issues for critical loads Gary M. Lovett1 Cary Institute of Ecosystem Studies, Millbrook, NY 12545 n the United States and most other This latter issue may force a reevalua- countries, primary air quality stan- tion and redefinition of the critical loads I dards are designed to protect human concept. If there is no level of N pollution health and are based on concentra- that does not cause harm, then the cur- tions of pollutants in the air. However, rent definition is not useful and must be from the perspective of ecosystem health, replaced by a more nuanced policy that a more appropriate metric for the impacts defines a tolerable level of harm. This is of nitrogen (N) and sulfur (S) pollutants closely analogous to the setting of air is the total atmospheric deposition (or quality standards to protect human health, − − “load,” in kg·ha 1·y 1) of N and S, because where the uncomfortable reality is that ecosystem effects are more strongly de- standards are set to protect the bulk of the termined by cumulative annual loading Fig. 1. Diverse plant community in an acid grass- population, with the knowledge that the than by short-term atmospheric concen- land in southern England. Pollutant nitrogen de- most sensitive individuals will still suffer trations. The European Commission has position causes some species to decline, even at pollution-caused illness. adopted the concept of a “critical load,” levels of deposition below the critical load. Image A related issue involves how to focus defined as the load of a pollutant “below courtesy of Jessica Waugh (www.jessicahomer.com). emission reduction efforts. If relatively which significant harmful effects on spec- pristine areas lose sensitive species quickly ified sensitive elements of the environment et al. are able to identify change points for as N deposition increases, then keeping do not occur according to present knowl- 155 species. Each species has its own N deposition low in those areas, even if edge” (1). In PNAS, Payne et al. (2) pro- change point, and about a third of the they are below the critical load, may be − − vide new information on the impact of N change points fall below 10 kg N·ha 1·y 1, more important than trying to reduce de- deposition on European grassland ecosys- and another third fall between 10 and position in more polluted areas where the − − tems and illustrate some of the important 15 kg N·ha 1·y 1. A community-level plant communities have already shifted to complexities involved in quantifying and analysis showed the most significant point N-tolerant species. This would require a implementing critical loads. of plant community change at about 14 kg rethinking of emissions control policies − − Nitrogen deposition affects ecosystems N·ha 1·y 1. Thus, from the whole-com- that seek to reduce the exceedance of the in two ways. It can be an acidifying agent if munity perspective, a critical load between critical load if those policies allow N de- − − the N is nitrified in the soil or nitrate is 10 and 15 kg N·ha 1·y 1 is too high to position to rise in more pristine areas. fi leached from the ecosystem. Nitrogen can protect the system from gross community Payne et al. emphasize that their nd- also cause eutrophication, meaning that change, and more importantly, from an ings do not diminish the usefulness of the the oversupply of this nutrient can stimu- individual-species perspective, roughly critical load as a tool for assessment of air late productivity of certain species (3). 60% of the species were affected at levels pollution effects on ecosystems. The criti- This may sound like a good thing, but in at or below the critical load. Furthermore, cal load provides a framework for orga- many herbaceous plant communities, the the most sensitive species responded at nizing, simplifying, and applying the large excess N deposition can stimulate growth very low N loads, making it nearly impos- volume of information on impacts of air of N-responsive (nitrophilic) graminoids sible to determine a level of N deposition pollution. Critical loads are being used to that crowd out less responsive plants, that does no harm in this plant community. evaluate the effects of S and N pollution in particularly forbs and bryophytes (Fig. 1). The critical load as defined above in- Europe and in Canada, but have so far The soil acidification and increased com- volves elements of both science and policy. seen only very limited use in the United petition can lead to the decline of sensitive Determining the amount of deposition States (10, 11), and there is as yet no US species in high-deposition areas. This and determining the dose–response re- federal policy to establish critical loads for general pattern of species change in favor lationship between deposition loads and sensitive regions. The US Clean Air Act “ ” of nitrophilic or acid-tolerant species has ecosystem responses are clearly scientific requires the setting of primary standards “ been established by previous work in issues. Deciding which elements of an to protect human health and secondary ” grasslands, coastal dunes, heathlands, and ecosystem are to be considered and what standards to protect public welfare, in- alpine tundras, among other ecosystem constitutes significant harm are policy is- cluding the environment. Like the primary types, although the pace and outcome of sues. This paper speaks to both aspects of standards, the secondary standards are the change can be altered by such factors the problem. It describes the application based on atmospheric concentrations and as soil pH, grazing, and disturbance to the of a sensitive statistical technique to de- thus are not easily compatible with the dominant vegetation (4–8). Payne et al.’s termine dose–response relationships from deposition metric that provides the basis analysis takes this one step further by gradient studies, which represents a scien- of the critical loads approach. Even with- focusing on responses of individual plant tific advance for this field. However, it out a regulatory basis in the Clean Air Act, species in a large dataset of plant abun- also causes us to confront key policy the critical load could and should be used dance in acid grasslands occurring across questions: What kinds of information in the United States as an assessment tool a gradient of N deposition in northwest- should be used to set critical loads? What to evaluate the effectiveness of air ern Europe. The critical load for this constitutes “significant harm”? What if ecosystem type has been set at 10–15 kg the critical load is zero or too low to be − − N·ha 1·y 1 (9). Using a statistical pro- quantified? Should policies be set to pro- Author contributions: G.M.L. wrote the paper. cedure capable of identifying points along tect the most sensitive species, or should The author declares no conflict of interest. the N deposition gradient where particular some of those species be compromised for See companion article on page 984. species show significant decline, Payne the sake of achievable emission targets? 1E-mail: [email protected]. 808–809 | PNAS | January 15, 2013 | vol. 110 | no. 3 www.pnas.org/cgi/doi/10.1073/pnas.1219007110 Downloaded by guest on September 30, 2021 COMMENTARY pollution control measures and the risk to plant is being displaced by proliferation of actions between changing plant species sensitive resources from N and S pollution graminoids fueled by elevated levels of and changing soil conditions. This model- (12–14). An integrated national frame- N deposition (7). ing approach will be especially necessary work for setting and using critical loads in Moreover, plant species do not merely for forests, where because of the long life the United States, such as is currently in respond to N availability in the soil; they span of the trees, N-induced changes in place in Europe, would help the United can also partially control it through their species composition play out on a much States assess the threat of air pollution to slower timescale that does not lend itself its ecosystems and could help guide emis- Payne et al. provide to field experiments. Such models will be sions control policy. necessary for evaluating ecosystem im- Globally, there are many plant species new information on pacts and setting critical loads for the long potentially at risk from N deposition, es- term and for predicting the interactions pecially when one considers the recent the impact of N among N deposition, species shifts, and increases in N pollution in the tropics (15). other perturbations such as invasive spe- To make this problem manageable, ecol- deposition on European ogists will need to identify potentially cies and climate change. susceptible community types through grassland ecosystems. Critical loads remain an important tool broad-scale surveys and then zero in on for assessing the impacts of atmospheric the most susceptible species using experi- deposition on ecosystems, but the best use mental and gradient studies. Of course, uptake of N from the soil and through the of this tool requires effective interplay the decline of a plant species ramifies quality of their litter, which regulates de- between science and policy, including pe- through an ecosystem to affect many dif- composition rates (16). A full accounting riodic reexaminations of the policy to ferent plant and animal species, and these of the long-term consequences of N de- incorporate the most recent scientific follow-on effects will need to be consid- position and species change will require advances. In some cases, such as the Payne ered as well. A good example is the Bay ecosystem models that incorporate the et al.
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