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

Chapter 15: Summary of Prospective Global Change Impacts on Northern U.S. Forest Ecosystems

Richard Birdsey, Robert A. Mickler, John Hom, and Linda Heath

Ecosystem responses to changes in multiple environmental factors are exceedingly complex and cannot be predicted with certainty using current experimental results and models. Environmental factors known to cause changes in ecosystem processes include many climatic variables, tropospheric ozone, nitrogen deposition, acid deposition, and anthropogenic factors such as past land use and introduction of exotic species.

Northern forests have heretofore shown remarkable resiliency and adaptability despite high levels of environmental stress. We have documented climate trends, ozone exposure, high levels of acid and nitrogen deposition, and land use pressures, all simultaneously affecting northern forests. Yet northern forests appear healthy as a whole despite evidence of local problems, and regional inventories continue to show accumulation of biomass and low mortality. Evidence suggests that decades of stress may have altered long-term soil processes sufficiently to begin affecting regional indicators of health and productivity. Therefore, an increased level of monitoring, targeted to areas suspected to be sensitive to environmental change, is warranted.

Forest ecosystems that appear vulnerable to past or continued environmental stress include:

High elevation red spruce forests are damaged by acid - These deposition and winter injury and are vulnerable to additional climatic stress. Temperatures fluctuate broadly and Winter injury, common when when foliage is damaged by acid deposition, could become more common. Forests in the Middle Appalachians are High elevation spruce-fir remnants of an earlier, cooler climate now found only at the highest available elevations.

Aging hardwoods in the Mid-Atlantic region- If the theoretical models are correct, then increased levels of various interacting stressors in the Northeast are likely to lead to increased incidence of decline disease. Increasing environmental stress is occurring at the same time as many species reach biological maturity across much of their range, a consequence of past land use impacts. Aging forests are known to be more susceptible to decline disease. Vulnerable types include maple-beech-birch, oak-hickory, and black cherry in Mid-Atlantic states. Mid-Atlantic forests are highly stressed by ozone and acid deposition, and as they age, are showing increased mortality typical of mature forests. For example, mature sugar maple on unglaciated soils in Northwest Pennsylvania are noticeably affected by biotic factors (defoliating insects, borers, and canker fungus), a series of droughts, and acid deposition. Regeneration of some aging forests is very uncertain. Lack of oak regeneration following harvest is considered a major issue for the region, as loss of oak forests coupled with a major expansion of red maple results in lower economic value and reduction of important mast species. However, regeneration in the face of climate change is likely to be more difficult than survival, because the most sensitive stage of a tree’s life is the beginning, when warmth and drought can have strong effects.

Aspen-birch in the Lake States - Aspen-birch forests in the U.S. grow at the southern end of their current range. Aspen is sensitive to ozone, and for some genotypes, the sensitivity is increased by higher CO₂. t is unknown how climatic change might alter aspen-birch forests when coupled with these physiological responses. Over North America, the most significant observed temperature changes over the last 40 years have occurred from the North Central U.S. through Northwestern Canada into Alaska. Thus, it is possible that multiple stresses will converge on Aspen-birch forests in the Lake States with unknown but potentially harmful consequences.

Below: Projected periodic average annual carbon uptake (million metric tons yr^-1) in Northern and all U.S. forests.

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