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Responses of Northern U.S. Forests to Environmental Change
ISBN 0-387-98900-5

Chapter 8: Atmospheric Deposition Effects on Surface Waters, Soils, and Forest Productivity

Gregory B. Lawrence, Kristiina A. Vogt, Daniel J. Vogt, Joel P. Tilley, Philip M. Wargo, and Margaret Tyrrell

Research during the 1980's yielded conclusive evidence that acidic deposition had acidified poorly buffered surface waters causing loss of fish populations and other aquatic organisms. Although acidic deposition affected soil chemistry, effects on forest health were not apparent, with the exception of stand dieback of high elevation red spruce. The National Acid Precipitation Assessment Program (NAPAP) provided much information about natural processes within aquatic and forest ecosystems, and initiated baseline monitoring of deposition rates and chemical changes in ecosystems. NAPAP also highlighted the importance of interactions between multiple stressors, which together threatened the long-term structure, function, and productivity of ecosystems by changing chemical composition and nutrient cycling. In particular, it was noted that acidic deposition could decrease nutrient retention in forest ecosystems and cause imbalances in the availability of nutrients.

Research in the 1990's began to address the issue of recovery following declining acid deposition rates. Researchers demonstrated the complex interactions between precipitation, soil characteristics, and downstream processes. For example, release of nitrate from watersheds is controlled by biologcal processes that determine N mobility within soils, not hydrologic transport of atmospherically deposited N directly through soils into surface waters. Also, the importance of the forest floor in supplying Ca for root uptake was noted, along with the regional decline of available Ca that had occurred in the second half of the 20th century. Important mechanisms causing Al mobilization were discovered. Acidic deposition lowers the pH of the mineral soil, causing mobilization of Al that can then be transported into the forest floor, reducing the amount of available Ca. There seems to be a strong connection between soil Ca availability and the health and long-term growth rates of sugar maple; however, this is part of a complex of factors rather than a single factor effect.

Deposition of N may cause ecosystem saturation and a possible shift in species composition because of differential utilization by deciduous and coniferous species. Growth responses to N additions depend not only on species but on tree health and availability of nutrients in the soil, in particular, the availability of Ca. N additions cause decreases in C allocation to fine roots, with implications for uptake of other nutrients. However N may also increase the retention of nutrients on the site through a series of processes involving litter quantity and quality, microbial activity, and water retention capacity.

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