America's Forests
1999 Health UpdateOverview MapIntroduction
Issues and Examples of Forest Ecosystem Health Concerns
V. Air Pollution

Air pollution, particularly acid rain and ground-level ozone, impacts certain forest ecosystems in the Eastern United States. Forested ecosystems in the Great Lakes States, Northeast, and mid-Atlantic sections of the United States are receiving high levels of soil-acidifying pollutants. Acid rain mobilizes elements such as aluminum, depleting essential nutrients such as calcium and magnesium and reducing their availability to trees. Another concern is that deposition of nitrogen-based ions can fertilize plant communities with a resultant shift in species composition toward nitrogen-loving plants. Ozone has been shown to alter forest ecosystems in areas of high deposition in the mountains of southern and central California, but has fewer effects on forests in other areas of the country. The USDA Forest Service, in cooperation with partners, continues to monitor air pollution impacts across the country.


Ozone and Nitrogen Damage—At high ozone levels, sensitive trees show ozone-related injury; while lower ozone levels have been shown to reduce photosynthesis of trees—affecting tree health. The Forest Health Monitoring (FHM) Program currently monitors ozone effects in forests in 33 States. FHM ozone monitoring indicates that ozone is at extremely high levels in the Southwestern United States and is at high levels in parts of the Eastern United States. Ponderosa pine and Jeffrey pine in southern and central California show needle injury from ozone. Monitoring of sensitive tree, shrub, and herb plant species in forested areas of the Northeast, Great Lakes, and Mid-Atlantic States also shows ozone injury to leaves. USDA Forest Service scientists have identified how ozone damages trees, and they have successfully screened tree varieties to identify trees that are less susceptible to ozone damage. Ongoing studies will help forest managers identify ozone-sensitive trees in areas of ozone exposure, increasing our understanding of how to manage resources.

Figure 1. FHM Ozone Map.

In California, two major pollutant types threaten natural resources: photochemical oxidants, of which ozone is the primary compound, and nitrogen pollutants. Ozone is toxic (plant-killing) to sensitive plant species, such as ponderosa and Jeffrey pine, and some understory species. Nitrogen is unique in that, while it is the primary growth-limiting nutrient, it is also a pollutant when in excess. Forests are simultaneously exposed to a growth enhancer and a toxic pollutant. The combined effects of ozone and nitrogen, in the highest pollution sites in the San Bernardino Mountains in southern California, reduce fine root biomass in mature ponderosa pine trees. Ozone causes older needles to drop prematurely, while nitrogen enrichment results in greater foliage production. As a result, thick litter layers accumulate on the forest floor; carbon is sequestered in the litter and aboveground biomass, making it unavailable to trees. Compared to trees in low pollution sites that could retain foliage for 5 to 7 years, many of the pine trees exposed to high ozone retain foliage only 1 to 2 years. With more foliage, the trees are capable of manufacturing higher levels of carbohydrates (food).

Air pollution damages trees
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