USDA Forest Service

Pacific Southwest Research Station
Pacific Southwest
Research Station

800 Buchanan Street
Albany, CA 94710-0011
(510) 883-8830
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Air Pollution and Global Change Impacts on Western Forest Ecosystems

Southern California and California Central Valley are among the most heavily polluted areas in North America. Ozone, acidic gases and particulates are transported long distances to forests of the central and southern Sierra Nevada and the transverse mountain ranges of southern California. Air pollution effects in forested ecosystems is present in other Pacific, Intermountain, and Rocky Mountain states, but at lower levels. Forested areas surrounding Mexico City, and central and eastern European forests have higher levels of industrial air pollution in different combinations and under different climatic regimes: these conditions provide an additional test of our understanding of transport, deposition, its interaction with climate, and biological response of forest ecosystems.

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This research work unit addresses air pollution and climate change effects on forest ecosystems at local, regional, national, and international levels. The responsibility for protecting forest resources with regard to pollution exposure is specified in the Clean Air Act of 1990 and the Forest Ecosystems and Atmospheric Pollution Act of 1988 (PL 100-521). Although pollution control devices required in the state of California on both industrial and mobile sources have significantly reduced the number of days that exceed state and federal standards, overall pollutant exposures are high and are expected in increase due to 1) continued use of fossil fuels and agronomic application of high loads of fertilizer; 2) more modulated release of industrial pollutants, so that although high hourly concentrations are avoided, the background levels of pollutants increase, 3) increasing population; and 4) changes in land use and management practices, including prescribed burning.

Prescribed fire is an important management tool for National Forest managers. There is a critical need to determine whether prescribed burns comply with local and regional air quality regulations, and if not, what conditions lead to non-compliance. Existing models of smoke chemistry, transport and dispersion need to be tested under local and regional conditions. Ground based, low-cost monitoring equipment for measuring smoke emissions has been developed and will be tested to quantify the location, chemistry, concentration, and deposition of air pollutants from prescribed and wildland fire. Knowledge of the interactions between smoke and air pollution transported from urban sources will also be used to support development and application of smoke transport and dispersion models.

The global average tropospheric ozone level has doubled since pre-industrial times, and is expected to double again by 2020, making it the fastest increasing greenhouse gas. Nitrogenous compounds associated with fossil fuel use is less well quantified, but our best numbers suggest that N deposition is exponentially correlated with oxidant pollution. N transport into forest ecosystems from agronomic sources are as yet unquantified. Projected increases in the population of California from 30 to 48 million people (1990 to 2020) continues to translate into increased pollution, and its deposition in forest ecosystems in the Sierra Nevada, the Transverse Range (San Bernardino Mountains), as well as into desert ecosystems to the east.

These pollutants (and a host of other compounds deposited) modify ecosytem function and structure. For example, high ozone and N deposition alters leaf longevity, how much N gets transported out of the leaves before they drop, and how long the leaves take to decompose once they fall to the forest floor. We think these pollutants in high levels increase ecosystem carbon sequestration (until the next wildfire). Trees at sites with high ozone and N deposition have fewer roots, making them more susceptible to drought stress, and subsequent insect infestations. Some species are more susceptible to pollutant exposure than others, and changes in stand composition and structure have been found over the last 30 years in the San Bernardino Mountains. Poor water quality (high in nitrates) has been documented from watersheds in areas of high pollutant exposure.

The most likely climatically-driven environmental change in the west is increased frequency and duration of drought. Although Global Circulation Models (GCMs) predict as much as a 30% increase in precipitation for California, greater evaporative demand imposed by higher water holding capacity of warmer air, and more rain-on-snow events in late fall and early spring are expected to result in an overall increase in drought. Chronic drought in combination with high air pollution exposure is expected to further deteriorate forest health, and alter ecosystem structure and function.

Our project has a long record of using interdisciplinary approaches to solve problems in cooperation with the National Forest System, US Environmental Protection Agency, California Air Resources Board, Air Resource Management Districts, National Park Service, and various national and foreign partners to develop the information base needed for assessing the ecosystem response to pollutant exposure. Long-term studies in local and regional mountains will continue to yield valuable data on forest ecosystem response to atmospheric deposition. Short- to medium-term experiments will be designed or continued to understand leaf, stand, ecosystem, and watershed response to pollutants and climatically-driven stressors. Simulation models will be used to integrate complex phenomena, and to predict long-term responses that are not amenable to field studies.

One of our assignments is to interpret the mechanisms of how forest ecosystems respond to pollutants for land managers. This is done in the form of risk assessments, which are needed to help set realistic state and federal control standards for air quality. Research and monitoring are needed to determine the extent and magnitude of pollutant deposition in areas at risk. In managed forested ecosystems, a better understanding of multiple stressors over long-time periods is important for developing better strategies. In wilderness ecosystems, it is essential to base risk assessment of pollutant/climate influences on sound, carefully constructed scientific criteria. We provide monitoring protocols needed to track forest productivity and the effectiveness of protection in Class I Wilderness areas.

Solutions to these problems are expected to improve our understanding of atmospheric pollutant/ climatic stressor interactions within forest ecosystems, to provide high quality scientific information to manage resources (multiple forest values, water quality), to help set protective air quality standards, and to provide estimates of future changes in forest ecosystems. This knowledge will provide a scientific basis for managing forest resources now and in the future.

Last Modified: Mar 28, 2013 02:54:55 PM