By Steven G. McNulty and John D. Aber

The forest sector assessment is nearing completion.  As previously reported (Acclimations November/December 1998), the sector considered four categories of climate change impacts: forest processes, biodiversity change, disturbance interactions, and socioeconomic change. These categories represent key interactions between a changing climate, forest structure or function, and human interactions with forests. 

A combination of experimental and long-term monitoring data, biogeochemical and biogeographic modeling outputs, and literature reviews were used to develop the final assessment products.  In addition to the NAST foundation and summary documents, the assessment findings are currently in review for special editions of BioScience and Science of the Total Environment.  Conclusions from the assessment are listed by forest sector category.

Forest Processes

Forests provide water, timber and pulp for residential and industrial use, and are an important sink of atmospheric carbon dioxide (CO₂).   Long-term impacts of changes in air temperature, precipitation, atmospheric CO₂, and ozone (O₃) and their variation could significantly impact these forest processes during the next century.  Examination of national and regional scale forest process models, combined with two transient and five static climate change scenarios, suggested that forests will experience slight to moderate (5 to 30%) increases in forest productivity.

Although most of the climate scenarios suggest a generally more productive environment, certain regions may experience significant reductions (>20%) in forest productivity, especially if other stresses such as ozone impacts are included in the analysis. Carbon accumulation may increase or decrease, depending on the extent of and severity of fire and climate change model predictions of precipitation change.  Forest water use efficiency may increase due to increased atmospheric CO₂ concentrations, but in some parts of the US, leaf area and associated evapotranspiration from forests may increase, resulting in decreased water flow from forests. 

Reductions in water flow could be most pronounced in the Plains States where severe reductions in stream volume could negate the use of the Mississippi River inter-coastal waterway.  Largely due to increased precipitation, the western U.S. may experience a 10% to 60% increase in water flow.

Biodiversity Change

Climate and land use are the two major factors controlling biological diversity. Species richness generally increases with increasing air temperature and precipitation.  As climate changes during the next century, biological diversity will also change. Under all of the climate scenarios, many of the northern U.S. forest types adapted to cooler temperatures will migrate northward, while isolated communities of other species such as red spruce may become extinct within their current region.

As the range of northern species moves further north, southern mixed pine and hardwood ecosystems will expand northward, greatly increasing the geographic distribution of southern forest communities.  Amphibians and reptiles are expected to increase in species richness across the nation. Birds and mammals may experience reduced species richness across the southern U.S. and increase in species richness across the warming northern section of the country.

 However, ecosystems are complex communities and current models only associate biodiversity with existing environmental conditions.  If climate change occurs faster than functioning ecosystems can be developed, then the historic relationships between plant, animal and climatic conditions may not be reestablished and biological diversity will be reduced.

Disturbance Interactions

Fire, drought, introduced species, insect and pathogen outbreaks, hurricanes, wind storms, and ice storms are the most important natural disturbance mechanisms for forest change in the U.S.  Some disturbances such as hurricanes and ice storms, may be infrequent (i.e., one major event every 3 to 10 years), but have extreme (i.e., near complete destruction), periodic impact on large forests areas (> 104 km²). Other disturbances such as wind storms may be more frequent (i.e., hundreds per year) but individually impact a smaller area (< 102 km2). 

While much has been learned about the impacts of individual disturbances on forest structure and function, there is a lack of research on the interactions of climate and disturbance.  From our current understanding, some disturbances will likely increase in severity (e.g., insect and pathogen outbreaks), shift in geographic region  (e.g., ice storms), or shift in frequency (e.g., fire). There is insufficient information to predict if the frequency or severity of some disturbances such as hurricanes and drought will increase or decrease. 

However, climate change will likely shift forest ecosystem distribution across much of the nation. During the transition forests may be more predisposed to disturbance.  The amount of forest area burned may increase by 25 to 50% as increased forest productivity initially builds fuel loads and subsequent droughts increase fire occurrence.

Socioeconomic Change

Long-term forest productivity is important to sustained economic development of many states and to national economic growth.  Climate change is generally expected to increase forest productivity more than 20% at the national scale.  Assuming there are no major shifts in timber demand, forest timber volume will increase, market welfare will increase, and timber prices will decrease during the next century.   In regions such as the southern U.S. where forest products are either the first or second most important crop, there may be serious losses in revenue and jobs. 

Climate change may also impact the recreation industry.  The southern U.S. may lose important economic revenues as cold water fisheries move northward and marginally successful southern winter recreation industries are driven out of business by higher operating costs and shorter seasons.  The northern U.S. may be negatively impacted through a reduction in tourism associated with fall foliage change as more colorful species such sugar maples are replaced with duller colored oaks and hickories.  

However, the western U.S. may benefit from the additional precipitation increasing the availability of water based recreation.  In addition to examining climate change impacts on these four forest categories, we also addressed potential coping strategies that could be used to reduce the impacts of climate change.   From our analysis, we concluded that future research should focus on interactions between climate and other stresses, and how changes in forest structure and function interact with socioeconomic change. Only through better understanding can future forest managers cope with our changing environment.

Assessment Publications

The sector has produced papers on forest processes, biodiversity, socioeconomic and disturbance, which were submitted to BioScience and are currently being reviewed for a special issue. Seven papers providing more information on disturbance impacts in association with climate change were also sent to Science and Total Environment to be published as a special issue. The set of papers received very favorable reviews and will soon be in press pending minor revisions.

For more information, contact:

Steven McNulty, USDA Forest Service; 920 Main Campus Dr., Venture Center II, Suite 300 Raleigh, NC 27606; phone: (919) 515-9489; email: steve_mcnulty@ncsu.edu