Air pollution is altering ecosystems across the United States in many ways. It reduces tree growth and survival, which affects the health, aesthetics, and productivity of forests. It decreases lichen species richness and abundance—lichens are a fundamental part of the food-chain for moose and deer in these sensitive forests. Air pollution causes harm to fish and other organisms in sensitive lakes and streams.

Long-term monitoring, mostly by the USDA Forest Service Forest Inventory and Analysis program, has made it possible to measure the past responses to air pollution and model future response. The lichen data, for example, represent the world’s largest and most consistent dataset for lichen community surveys, spanning three decades. There is so much new data about how ecosystems respond to air pollution that it is hard for resource managers to keep track. The CLAS (Critical Loads Assessment by Site) tool was developed to give easy access to the state-of-the science data. CLAS allows resource managers to visualize the extent of the area at risk from air pollution, which tree species are affected, and how much the community is expected to change. CLAS includes National Forests, National Parks, and Tribal lands among the types of areas that can be mapped.