Forests

-- An Overview prepared by Janine Bloomfield, Ph.D. (Environmental Defense) --
 

Climate and changes in it – regardless of their cause – matter to people, communities and businesses. Global warming is likely to bring many changes to the nation. The United States as a whole is in a strong economic position to adapt to many of these changes, but adaptation is often expensive, not always possible or successful, and during transitions ecosystems, communities, and individuals could suffer. Moreover, national impact summaries disguise local dislocations and disruptions to the ways we live, work and recreate. Climate change adds a serious stress to our already threatened resources and treasured places. Overall impact statements also mask significant opportunities. To minimize the negative changes and make the most of the positive changes, we need to take a close look at how climate change will affect each region. How will America's forests experience the effects of global warming? And how can we respond?

Summary
With over twenty major forest types across the continental US, forest ecosystems will respond to global warming in a variety of ways.  The projected increase in temperature will likely shift the ideal range for many forest species by about 200 miles to the north. If the climate changes slowly enough, warmer temperatures may enable southern tree species to colonize to the north. Many factors, including the pace at which different species colonize new areas, determine the future composition of forestlands. Where species move into new areas more slowly than other tree species migrate out, the species previously common will still grow, but likely at a different density. In addition, cities, highways, agricultural fields and other human activities limit available habitat and create barriers to the migration of plants and animals. Forests in protected areas like national parks and national and state forestlands were established without considering the possibility of changing climates.  Rapid climate shifts may reduce appropriate native habitats within protected areas while development outside the boundaries of the protected areas makes adjacent new habitat unavailable and limits the creation of migration corridors. In some US temperate forests, rapid climate change and accompanying extreme events, such as droughts, floods, and wind storms could lead to increased disease, insects, landslides and wildfires that could increase tree mortality and, in some cases, replace forests by grasslands.  Some of the models used in the National Assessment indicate that forest productivity overall is likely to increase, leading to increased supply of certain types of timber, though possible interactions with extreme events and other disturbances could reduce these gains.

Key Findings

Wood, pulp, carbon and water
Across a wide range of climate scenarios analyzed in the National Assessment, it appears that modest warming could result in increased carbon storage and forest productivity in most forest ecosystems in the conterminous US.  Yet, under some warmer scenarios, forests, notably in the Southeast and Northwest, could experience drought-induced losses of carbon, possibly exacerbated by increased fire disturbance.  Increasing CO2 in the atmosphere acts as a fertilizer for plants and enables them to use water more efficiently.  While forest productivity seems to increase with increased atmospheric CO2, local conditions such as moisture stress and nutrient availability strongly temper these results.  

Nitrogen oxides are expected to increase with fossil fuel combustion.  Nitrogen deposition that results may further stimulate forest growth but also increase soil and stream acidification ("acid rain") increasing risks to lake fish and decreasing forest health.  In some regions, increases in ground-level ozone are likely, due to warmer temperatures. The air-pollutant tends to cause a decrease in forest health and productivity.

Forests also play a major role in the water cycle.  Many municipal and regional water systems are dependent upon healthy forest ecosystems to catch and filter rain and snow.  In some areas, such as the Great Plains states, increased forest productivity may decrease the water that flows from forests into rivers, streams and reservoirs.  While beneficial to forestry, the downstream impacts could be severe enough to negatively affect the use of the Mississippi River inter-coastal waterways.  In areas where forest ecosystems suffer from increased disturbances, water quality could be affected by increased soil erosion and contamination.

Biodiversity change
Under the scenarios used in the National Assessment, we could lose the maple/beech/birch forests currently found in the Appalachian Range as far south as West Virginia and western Pennsylvania, and all throughout New England.  The upper Great Lakes region could lose its aspen/birch forests as well.  Isolated forest communities such as red spruce may become extinct within their current range.  These forest types would be replaced by oak/pine and oak/hickory forests, provided the warming is gradual.  If the temperature increase is substantial, droughts, insect infestations and fires could become more likely, and forest cover loss may occur and persist while the new forest types migrate north.   If the average global temperature increase is 2 °C over the next 100 years, tree species will have to migrate 1 to 3 miles every year, improbably fast except for trees whose seeds are spread by birds. 

While the effect of changes in tree compositions on other species is difficult to predict with the modeling tools currently available, plants and animals that live in the forest will be affected, both by changing habitat and in direct response to temperature increases and changes in precipitation, fire regimes, and storm events. It is unknown as yet whether biological diversity would be reduced if climate change occurred at a fast rate, but the new composition of species is likely to be one of heat-tolerant fast adapters. Wildlife has been able to adapt to changing climates for millions of years.  But unlike during previous climate changes, roads, development and other changes to the natural environment now block their migration routes or otherwise impede their adaptations.  Parks and nature reserves established to protect certain species may no longer be hospitable to those species. In other words, appropriate climate for currently protected forest ecosystems may only exist outside protected areas.  In many cases, these areas will already be developed and wholly unsuitable to new forests.

Forests, weather and pests 
Forests are strongly affected by a number of disturbances, including fire, drought, insects, diseases, and severe storms. Under the global warming scenarios used in the national assessment, insect and pathogen outbreaks will likely increase in severity. As forest ecosystems change and move in response to climate changes, they will become more vulnerable to disturbances. Fires, for example, may become more frequent.  The amount of forest area burned might increase by 25 to 50%, especially as projected increases in productivity produce large amounts of brush and other fuels and subsequent droughts combine to increase fire occurrence. 

Jobs, tourism and regional development
Americans and visitors from all over the world enjoy our forests' recreational opportunities, like hunting, fishing, hiking, and camping, or just witnessing the dramatic coloration of the fall foliage in places like New England, the upper Midwest and the Rocky Mountains. Moreover, forest-related jobs and revenues dominate the economies of a number of states, like Washington, Oregon, Maine, Georgia and others.  Forest products are the first or second highest value agricultural crop for all states in the South.  If forest productivity indeed increases, the increased output could be a boon to lumber and pulp companies. However, a shift in the type of timber supply (e.g., more hardwood than softwood) would demand substantial technical adaptations. Also, increased output typically reduces prices - which is good for consumers - but can reduce jobs with negative consequences for the local and regional economy. 

Climate change will affect the recreational and tourism industries in different ways: trout and other cold water fishing may end in New England and other northern areas.  New England's fall foliage tourism could decline as sugar maples are replaced by trees with duller fall colors like oaks and hickories.  Summer recreational opportunities may increase in some northern and mountainous areas while downhill skiing is very likely to decrease with fewer cold days and reduced snowpack. 

Adapting to the Impacts of Global Warming
 

Climate scientists agree that further climate change may be inevitable and will therefore require adaptation, although most scientists also think that the pace of climate change can be slowed by substantially reducing greenhouse gas emissions. This would give governments, businesses, and ecosystems around the world more time to respond and adapt to climate change as well as reducing the overall severity of climate change-related impacts, thereby buying "insurance" for an uncertain future.  Another way to buy insurance now is to incorporate climate change into all long-term decisions about natural resources, thereby providing greater resilience.

Based on past experience, markets will adjust to the range of changes likely in the US.  However, natural forests and their biodiversity could suffer.  There is no market mechanism to value and protect forests for their biological diversity, nor do we have tried and tested strategies to maintain important plant and animal species under a changing climate.   

Management to help forests, and those dependent upon them, adapt to climate change must be integrated with other human activities like agricultural and urban encroachment on forest lands, the impacts of multiple use of forests, and air pollution.

Commercial forestry adaptations could include salvaging dead and dying timber and replanting species appropriate to a new climate.  

Where do we go from here?

The National Assessment is an important first step towards understanding the potential effects of global warming on the nation's forest ecosystems.  Still, we have a very limited understanding of the interactions of the multiple stresses on today's forest ecosystems, much less what will happen in a warmer world.  In addition to research to understand the mechanisms associated with climate change impacts on forests, there must be an improved and enhanced emphasis on long-term monitoring of forest composition and growth.

Additional Information

For more detailed information on the potential impacts of global warming on US forests, several experts are available to answer your questions:

Dr. John Aber, University of New Hampshire, Complex Systems Research Center, Durham, NH;
 Tel.: 603-862-3045;  Email: john.aber@unh.edu
Dr. Ron Neilson, USDA Forest Service, Corvalis, OR
 Tel.: (541) 750-7303  Email: neilson@fs.fed.us   or   neilson@fsl.orst.edu

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