Effects of climatic variability and change on forest ecosystems: a comprehensive science synthesis for the U.S

Abstract

This report is a scientific assessment of the current condition and likely future condition of forest resources in the United States relative to climatic variability and change. It serves as the U.S. Forest Service forest sector technical report for the National Climate Assessment and includes descriptions of key regional issues and examples of a risk-based framework for assessing climate-change effects. By the end of the 21st century, forest ecosystems in the United States will differ from those of today as a result of changing climate. Although increases in temperature, changes in precipitation, higher atmospheric concentrations of carbon dioxide (CO2), and higher nitrogen (N) deposition may change ecosystem structure and function, the most rapidly visible and most significant short-term effects on forest ecosystems will be caused by altered disturbance regimes. For example, wildfires, insect infestations, pulses of erosion and flooding, and drought-induced tree mortality are all expected to increase during the 21st century. These direct and indirect climate-change effects are likely to cause losses of ecosystem services in some areas, but may also improve and expand ecosystem services in others. Some areas may be particularly vulnerable because current infrastructure and resource production are based on past climate and steady-state conditions. The ability of communities with resource-based economies to adapt to climate change is linked to their direct exposure to these changes, as well as to the social and institutional structures present in each environment. Human communities that have diverse economies and are resilient to change today will also be prepared for future climatic stresses. Significant progress has been made in developing scientific principles and tools for adapting to climate change through science-management partnerships focused on education, assessment of vulnerability of natural resources, and development of adaptation strategies and tactics. In addition, climate change has motivated increased use of bioenergy and carbon (C) sequestration policy options as mitigation strategies, emphasizing the effects of climate change-human interactions on forests, as well as the role of forests in mitigating climate change. Forest growth and afforestation in the United States currently account for a net gain in C storage and offset approximately 13 percent of the Nation’s fossil fuel CO2 production. Climate change mitigation through forest C management focuses on (1) land use change to increase forest area (afforestation) and avoid deforestation, (2) C management in existing forests, and (3) use of wood as biomass energy, in place of fossil fuel or in wood products for C storage and in place of other building materials. Although climate change is an important issue for management and policy, the interaction of changes in biophysical environments (e.g., climate, disturbance, and invasive species) and human responses to those changes (management and policy) will ultimately determine outcomes for ecosystem services and people. Although uncertainty exists about the magnitude and timing of climate-change effects on forest ecosystems, sufficient scientific information is available to begin taking action now. Building on practices compatible with adapting to climate change provides a good starting point for land managers who may want to begin the adaptation process. Establishing a foundation for managing forest ecosystems in the context of climate change as soon as possible will ensure that a broad range of options will be available for managing forest resources sustainably.