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Über dieses Buch

This volume offers a scientific assessment of the effects of climatic variability and change on forest resources in the United States. Derived from a report that provides technical input to the 2013 U.S. Global Change Research Program National Climate Assessment, the book serves as a framework for managing U.S. forest resources in the context of climate change. The authors focus on topics having the greatest potential to alter the structure and function of forest ecosystems, and therefore ecosystem services, by the end of the 21st century. Part I provides an environmental context for assessing the effects of climate change on forest resources, summarizing changes in environmental stressors, followed by state-of-science projections for future climatic conditions relevant to forest ecosystems. Part II offers a wide-ranging assessment of vulnerability of forest ecosystems and ecosystem services to climate change. The authors anticipate that altered disturbance regimes and stressors will have the biggest effects on forest ecosystems, causing long-term changes in forest conditions. Part III outlines responses to climate change, summarizing current status and trends in forest carbon, effects of carbon management, and carbon mitigation strategies. Adaptation strategies and a proposed framework for risk assessment, including case studies, provide a structured approach for projecting and responding to future changes in resource conditions and ecosystem services. Part IV describes how sustainable forest management, which guides activities on most public and private lands in the United States, can provide an overarching structure for mitigating and adapting to climate change.



Seeking the Climate Change Signal


Chapter 1. Recent Changes in Climate and Forest Ecosystems

In recent decades, gradual warming over the long term has been punctuated by droughts that facilitate widespread ecological disturbances. Although no single event can be attributed to climate change, it is reasonable to infer that a permanently warmer climate will escalate disturbances, causing a much faster change in ecosystem structure and function than a gradual increase in warming. Various species of pine beetles have spread across large land areas in Alaska, the western United States, and southern United States, in some cases attacking tree species that have not experienced previous outbreaks. Area burned by wildfire has been especially high during the 2000s. A reduction in the quantity and persistence of snow in mountainous regions is affecting the hydrology of forest ecosystems and downstream water supply. Interactions of multiple disturbances and stressors may result in new combinations of species and ecosystem conditions for which there is no precedent in historical or paleoecological records. Rapid shifts in climate and disturbance may strain both the resilience of forest ecosystems and the capacity of social systems and management institutions. In the future, shifting the management focus from restoring systems to building resilience will be a more viable strategy for retaining key ecological functions and ecosystem services.
David L. Peterson, Kailey W. Marcinkowski

Chapter 2. Projected Changes in Future Climate

Temperature in the United States has warmed over the past 100 years, with high rates of warming in Alaska (∼4.5 °C) and the West (∼1.5 °C), whereas precipitation has increased in the East and South and decreased in the Southwest. Global climate models project a steady increase in future temperature through the end of the twenty-first century. Compared to 1971 through 2000, average annual air temperature will likely increase from 0.8 to 1.9 °C by 2050, from 1.4 to 3.1 °C by 2070, and from 2.5 to 5.3 °C by 2099, where the range is bounded by the B2 (low) and A2 (high) greenhouse gas emission scenarios. Temperature increases will be higher in northern and interior areas of the United States, especially during the winter, and extreme droughts are expected to increase. Changes in precipitation are expected to be small (higher in some regions, lower in others), although potential changes in timing and spatial distribution of extreme precipitation events may occur. Sea level may rise by as much as 2 m, affecting coastal forests and human communities. Most climate models project similar climatic trends until around 2050, but diverge considerably after that. Users of climate information often represent future climate with a range of output from different climate models and emission scenarios. Given that greenhouse gas emissions will likely increase unabated for at least the next few decades, using a high emission scenario will provide a more accurate future climate for forest management and planning.
Chelcy F. Miniat, David L. Peterson

Effects of Climatic Variability and Change


Chapter 3. Forest Processes

During the twenty-first century, tree mortality from forest disturbances may switch the United States from a current carbon sink (offsetting 13 % of U.S. fossil fuel greenhouse gas emissions) to a source. Carbon losses from disturbances in western U.S. forests (insects, wildfire) may be partially offset by increased growth in the East, where water is sufficient and elevated atmospheric carbon dioxide (CO2) and N deposition promote tree growth. Habitat for some tree species will likely move northward and upward in elevation, and the movement of suitable habitat may be faster than species can disperse to the new habitats. Direct and indirect effects of climate change will affect the hydrologic cycle. The effects of elevated CO2 on transpiration will likely be less than ± 10 %, a relatively small change compared to the effects of precipitation variability on transpiration. More frequent droughts will probably reduce streamflow, and concentrating precipitation in intense storms will likely increase the risk of erosion and landslides. Tree mortality from disturbances will likely increase runoff, and decreased snow cover depth, duration, and extent will advance the timing of runoff. Some effects, like the response of mature trees to elevated CO2, are difficult to project because current empirical data and modeling are inadequate. The effects of climate change on forest ecosystems can be projected with some confidence at regional scales, although projecting changes at smaller spatial scales (e.g., watersheds) or for individual tree species will be challenging because of the complexity and variability of biophysical interactions at specific locations.
Michael G. Ryan, James M. Vose, Paul J. Hanson, Louis R. Iverson, Chelcy F. Miniat, Charles H. Luce, Lawrence E. Band, Steven L. Klein, Don McKenzie, David N. Wear

Chapter 4. Disturbance Regimes and Stressors

The effects of climate change on insect outbreaks, wildfire, invasive species, and pathogens in forest ecosystems will greatly exceed the effects of warmer temperature on gradual changes in forest processes. Increased frequency and extent of these disturbances will lead to rapid changes in vegetation age and structure, plant species composition, productivity, carbon storage, and water yield. Insect outbreaks are the most pervasive forest disturbance in the United States, and rapid spread of bark beetles in the western United States has been attributed to a recent increase in temperature. Wildfire area burned has increased in recent decades, although frequency and severity have not changed, and is expected to greatly increase by 2050 (at least twice as much area burned annually in the West). More frequent occurrence of fire and insects will create landscapes in which regeneration of vegetation will occur in a warmer environment, possibly with new species assemblages, younger age classes, and altered forest structure. Increased fire and insects may in turn cause more erosion and landslides. Invasive plant species are already a component of all forest ecosystems, and a warmer climate will likely facilitate the spread of current and new invasives, particularly annuals that compete effectively in an environment with higher temperature and frequent disturbance. The interaction of multiple disturbances and stressors, or stress complexes, has the potential to alter the structure and function of forest ecosystems, especially when considered in the context of human land-use change. Occurring across large landscapes over time, these stress complexes will have mostly negative effects on ecosystem services, requiring costly responses to mitigate them and active management of forest ecosystems to enhance resilience.
Matthew P. Ayres, Jeffrey A. Hicke, Becky K. Kerns, Don McKenzie, Jeremy S. Littell, Lawrence E. Band, Charles H. Luce, Aaron S. Weed, Crystal L. Raymond

Chapter 5. Climate Change and Forest Values

Interactions between changes in biophysical environments (climate, disturbance, and ecological function) and human responses to those changes (management and policy) will determine the effects of climate change on human communities. Effects of climate change on forests could result in a ripple effect of policy and economic response on economic sectors and human communities. The United States produces more timber than any other nation, and although timber volume nearly doubled between 1945 and the late 1980s, production since then has declined. Per capita consumption of wood products has declined since the late 1980s, but population growth has continued to increase consumption to 0.57 billion m3 in the 2000s. Increased production will be concentrated on a smaller land base with a projected net loss of 9.3 million ha of forest land in the United States over the next 50 years, mostly on private lands subject to urbanization. In natural resource-based communities, socioeconomic relationships based on commodities (e.g., timber) or amenities (e.g., recreation) will be disproportionately affected by climate-forest interactions. Anticipated climate changes, coupled with population growth, strongly increase the value of urban trees in providing ecosystem services and for mitigating climate change impacts at fine scales. Policies targeting climate mitigation directly influence forest extent and use, and responses may include more harvesting (a result of new product markets such as biofuels) and altered forest management (responding to demands for forest-based C storage). Preparation for future climate stresses in rural, urban, and wildland-urban interface communities will be enhanced by ensuring that present-day communities have diverse economies and are capable of adapting to change.
David N. Wear, Linda A. Joyce, Brett J. Butler, Cassandra Johnson Gaither, David J. Nowak, Susan I. Stewart

Chapter 6. Regional Highlights of Climate Change

Climatic extremes, ecological disturbance, and their interactions are expected to have major effects on ecosystems and social systems in most regions of the United States in the coming decades. In Alaska, where the largest temperature increases have occurred, permafrost is melting, carbon is being released, and fire regimes are changing, leading to a transition from conifers to hardwoods in some forests. In Hawaii and the U.S.-affiliated Pacific islands, an altered climate and sea level rise are changing hydrology and fire regimes, affecting both forest ecosystems and human communities. In the Northwest, insect outbreaks (already prominent) and increased area burned, in combination with declining snowpack, are expected to have a major effect on dry, interior forests. In the Southwest, recent large wildfires and forest dieback in pinyon pine exemplify the kinds of changes that may occur in arid and semi-arid forests if droughts become more common in the future. In the Great Plains, where trees currently occupy only a small portion of the landscape, warmer temperature and non-native insects could reduce the amount of forested area and alter species distribution. In the Midwest, warmer temperature is expected to affect the distribution and abundance of many tree species, associated habitat, and human use of forests in a region where private lands are mixed with public lands. In the Northeast, warmer temperature is expected to affect the distribution and abundance of many tree species, although the productivity of hardwood species may increase significantly. In the Southeast, biodiversity and productivity may be affected by a combination of warmer climate, altered fire regimes, and invasive plants and insects.
David L. Peterson, Jane M. Wolken, Teresa N. Hollingsworth, Christian P. Giardina, Jeremy S. Littell, Linda A. Joyce, Christopher W. Swanston, Stephen D. Handler, Lindsey E. Rustad, Steven G. McNulty

Responding to Climate Change


Chapter 7. Managing Carbon

In the United States, net carbon (C) has increased in forests and harvested wood product stocks since the 1950s. Annual C storage currently accounts for 13 % of U.S. fossil fuel C emissions, and increased C storage is attributed to reforestation, regrowth of harvested forests, and use of forest products. In a warmer climate, U.S. forests could become a net C emitter of tens to hundreds of Tg C year−1 within a few decades. Carbon mitigation through forest management focuses on (1) increasing afforestation, avoiding deforestation, or both, (2) C management in existing forests, and (3) use of wood as biomass energy or in wood products. The mitigation potential of these strategies differs in timing and magnitude. Longer harvest intervals extend the time that C is stored in biomass, whereas increased growth rates accelerate uptake of C per unit time. Carbon can be stored in wood products for a variable length of time, oxidized to produce heat or electrical energy, or converted to liquid transportation fuels and chemicals that would otherwise come from fossil fuels. Wood products can also be used to substitute for other products that emit more greenhouse gases in manufacturing (e.g., concrete and steel). Life cycle assessment is used to evaluate C management strategies by focusing on the change in C storage or emissions over time. Public lands contain 37 % of the land area of the United States (76 % of which are managed by federal agencies), representing an important component of long-term C storage, although managing these lands for C benefits involves multiple jurisdictions, social objectives, and political factors. Management of C on private lands, and to some extent public lands, is affected by markets, regulations, taxes, and incentives that have only recently evolved to address climate change and C.
Kenneth E. Skog, Duncan C. McKinley, Richard A. Birdsey, Sarah J. Hines, Christopher W. Woodall, Elizabeth D. Reinhardt, James M. Vose

Chapter 8. Adapting to Climate Change

Federal agencies have led the development of adaptation principles and tools in forest ecosystems over the past decade. Successful adaptation efforts generally require organizations to: (1) develop science-management partnerships, (2) provide education on climate change science, (3) provide a toolkit of methods and processes for vulnerability assessment and adaptation, (4) use multiple models to generate projections of climate change effects, (5) incorporate risk and uncertainty, (6) integrate with multiple management objectives, (7) prioritize no-regrets decision making, (8) support flexibility and adaptive learning, and (9) incorporate adaptation in planning and projects. Resistance, resilience, response, and realignment strategies help to identify the scope of appropriate adaptation options at broad spatial scales in forest ecosystems. At the local scale, it is necessary to: (1) define management objectives, spatial extent, and timeframes, (2) analyze vulnerabilities, (3) determine priorities, (4) develop local tactics associated with strategies, (5) implement plans and projects, and (6) monitor, review, and adjust. The best examples of vulnerability assessment and adaptation planning in forests have occurred in national forests, where science-management partnerships have been established across multiple institutions. Although strategic planning for adaptation has been increasing, implementation of on-the-ground adaptation projects has been rare, primarily because of a lack of budget, personnel, and mandate for action. No one agency or organization can fully meet the challenge of adaptation, but this task is within reach if willing partners work collaboratively toward sustainable management grounded in knowledge of climate science and dynamic ecosystems.
Constance I. Millar, Christopher W. Swanston, David L. Peterson

Chapter 9. Risk Assessment

Climate change will generally increase the risk of negative consequences to forests and associated biosocial systems. Risk management identifies risks, estimating their probability of occurrence and magnitude of impact. A risk framework provides a means to quantify what is known, identify where uncertainties exist, and help resource managers develop strategies with better knowledge of risks. Case studies were used to illustrate how risk assessment can be used to assess critical resources and functions. For water, the ratio of precipitation (P) to potential evapotranspiration (PET) was examined across the United States as a means of assessing risk; forests where PET is expected to exceed P (especially in the Southwest) in the future will be most vulnerable. For carbon (C), the transition between being a C sink to being a source was a critical feature of risk assessment; vegetation change from forest to non-forest created the biggest risk for C emissions (loss of C storage). For wildfire in the West, risk assessment focused on frequency, severity, and annual area burned; the latter component of fire regimes is expected to increase greatly in future decades, creating rapid shifts in forest structure and function. For forest species distribution, bioclimatic envelopes were used to assess future changes; in Eastern forests, risk of altered distribution will be relatively high for some hardwood species and will depend on how disturbances change. For bird species distribution, species vulnerabilities were based on empirical models that include habitat, physiology, phenology, and biotic interactions; habitat specialists will generally be more vulnerable to climate change than generalists. Risk assessment must consider the biosocial context of the system being evaluated, reflecting contributions of ecosystem services and the capability of forest systems to withstand stress.
Dennis S. Ojima, Louis R. Iverson, Brent L. Sohngen, James M. Vose, Christopher W. Woodall, Grant M. Domke, David L. Peterson, Jeremy S. Littell, Stephen N. Matthews, Anantha M. Prasad, Matthew P. Peters, Gary W. Yohe, Megan M. Friggens

Scientific Issues and Priorities


Chapter 10. Research and Assessment in the Twenty-First Century

Because thresholds for climatic triggers of environmental change are poorly understood, additional empirical data are needed to confirm thresholds, and research is needed to improve the accuracy of process modeling at different spatial scales. Understanding of stress complexes in forest ecosystems needs to be expanded to additional ecosystems, including better quantitative descriptions of stressor interactions. Priorities for future research and development include: (1) develop new approaches to understand the effects of elevated carbon dioxide in mature and diverse forests, (2) develop a standard approach for tracking carbon dynamics over space and time, (3) identify appropriate uses and limitations of remote sensing imagery for detecting climate change effects, (4) determine which long-term measurements are useful for tracking climate change effects, (5) identify standard approaches for evaluating uncertainty and risk, and (6) evaluate recently developed processes and tools for vulnerability assessment and adaptation to identify which ones are effective for “climate smart” management. These topics need to be framed at the appropriate spatial and temporal scales for different climate change issues, and the effects of land use on climate-ecosystem relationships need to be quantified at large spatial and temporal scales. Inferences about climate change effects will be more relevant if various land uses, including evaluation of future alternatives, are considered in a context that incorporates people and ecosystem services. Resource assessments will be more powerful if the work of stakeholders and scientists across the United States is integrated in an ongoing, continuous process by tracking climatic stressors, observing and projecting climate change effects within regions, and delivering data and Web-based products for decision making.
Toral Patel-Weynand, David L. Peterson, James M. Vose


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