Conclusions

Forests & Global Climate Change: Potential Impacts on U.S. Forest Resources

Unlike other sectors, such as agriculture, that are almost exclusively comprised of managed systems, forests are comprised of both natural and managed systems. This makes it more difficult to state with precision what the overall economic impacts of climate change on forests will be. Further, understanding the impacts on forests and timber markets is difficult given the long time lags between the planting and harvesting of trees.

Despite the many practical problems with understanding climate change impacts on forested ecosystems and timber markets, the combination of historical observation, modeling results, and experimental data allows us to draw several conclusions. Future research will certainly revise these conclusions, but the following points summarize the most important findings in the research to date regarding the overall impacts of climate change on forest ecosystems and timber markets over the next century:

1. Tree species generally are expected to migrate northward or to higher altitudes in response to increased temperatures. While species will adapt over time by moving from one region to another, differential rates of change may cause significant differences in the types of natural stands in the future. Rates will depend critically on (a) how fast seeds migrate into new regions that are climatically suitable for a species after a climate change, (b) changes in the spread of insects and disease, (c) the spread of wildfire in different climates, and (d) human interventions to promote species migration.

2. Forest productivity is expected to change, but the changes could be positive or negative. Forests could become more or less productive, depending on how much climate changes (including both temperature and precipitation), how forests respond to higher carbon concentrations in the atmosphere, whether mortality changes, and whether disturbance-induced dieback increases or decreases. Many of these factors are expected to vary from region to region, suggesting that economic impacts are likely to differ among regions in the United States.

3. The effect of additional carbon dioxide in the atmosphere on forested ecosystems (“carbon fertilization”) is complex and uncertain, but it has large implications for understanding how forest productivity will change. Most studies suggest that forest area and productivity will increase if carbon fertilization enhances forest growth, but will decline if carbon fertilization does not occur. Plant level experiments suggest that carbon fertilization will enhance tree growth, at least for some period of time. Scaling these results up to the ecosystem level is complex, but available studies suggest that carbon fertilization will be limited by competition, disturbance, and nutrient limitations. It is important to continue developing a better understanding of carbon fertilization effects, particularly at the ecosystem scale.

4. Changes in the frequency and severity of forest disturbance, such as storm damage, fires, and pests are likely to affect forest structure and function. The impact on markets, while generally negative, can be ameliorated by salvage. At the market level, salvage associated with disturbances can increase timber supply and reduce prices in the short term, which benefits consumers. However, increased disturbance and lower prices generally have negative effects on landowners.

5. United States timber markets have low susceptibility to climate change because of the large stock of existing forests, technological change in the timber industry, and the ability to adapt. The United States currently consumes less timber than grows within the country each year, providing a cushion if climate change has short-term impacts on supply. Further, companies already substitute a wide array of species in end products, so that if particular species are negatively affected by climate change, markets can adapt by changing the types of species used in the production of end products. In addition, landowners can assist natural migration of timber by planting southern species in the North.

6. Economic studies have tended to find small negative to positive overall effects on timber production in the United States. While the studies have looked at a wide range of potential climate change effects across species within the United States, the net productivity effects used by the studies have tended to be positive over the long-term. Higher forest productivity translates into increased timber yield, increased timber inventory, increased supply, and lower prices. Lower prices generate overall net benefits, although they primarily benefit consumers at the expense of landowners. Lower forest productivity has the opposite effect.

7. Northern states may gain from climate change if productivity increases and if southern species move North, while southern states may lose production. Producers in southern regions are the most vulnerable to climate change because they have a large share of the nation’s current timber production capital, and the highly productive species in that region are sensitive to potential drying effects. Northern states are generally predicted to gain productivity and market share during climate change.

8. Understanding the economic effects of climate change on timber production is limited by scientific understanding of several key factors that control the response of natural and managed forests to climate change. Additional research is needed to enable ecologists and foresters to develop a more robust understanding of future changes in U.S. climate, ecosystem responses to climate change, the relationship between forest productivity and timber yield, and adaptation options available to foresters. Future clarification of these uncertainties will permit more informed assessments of the economic impacts of climate change to the forestry sector.