Forest ecosystems in the United States (U.S.) in 2100 will differ from those of today as a result of a changing climate, superimposed on the human imprint of land use. This assessment focuses on effects of climatic variability and change, based on natural resource data, experiments, and modeling, with emphasis on issues relevant for sustainable forest management. Because it is difficult to infer if some recent trends in ecological phenomena are the result of human-caused climate change or climatic variability, we emphasize biophysical responses to climatic patterns of the past few decades that are similar to climatic phenomena expected for the rest of the 21st century.
The fastest and most significant effects of climate change will be caused by altered disturbance regimes. A warmer climate, including more frequent droughts, will increase area burned by wildfire and area affected by insects, phenomena that are already occurring. These disturbances, individually, in combination, and as components of broader stress complexes (e.g., invasive species), may lead to altered species composition, distribution of forest age and structure, and spatial patterns across large landscapes. Elevated CO2 may increase water use efficiency of some tree species, thus reducing evapotranspiration, but hydrologic responses will be driven primarily by direct changes in climate (i.e., temperature and precipitation) and interactions with altered disturbance regimes. Warmer temperature may accelerate the rate of nutrient cycling in some systems, promoting increased forest growth and elevated nitrogen levels in streams. Any change in forest ecosystems that affects water resources will result in a significant loss of ecosystem services. Forest growth and afforestation currently account for a net gain in carbon in the U.S., a trend projected to continue to 2040, at which point U.S. forests could become a net emitter of carbon. Eastern forests are expected to continue to sequester carbon through favorable response to elevated CO2 and higher temperature, whereas Western forests may begin to emit carbon through expanded fire and insect disturbance. Management approaches that anticipate and respond to change by guiding development of forest ecosystem structure and function will be needed to sustain ecosystem services across large landscapes and multiple decades. Federal agencies have made significant progress in developing scientific principles and tools for adapting to climate change, and land managers are already using some “climate smart” practices. Building on practices compatible with climate change adaptation will provide a foundation for managers who want to start the adaptation process.