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    Author(s): H.D. Safford; M. North; M.D. Meyer
    Date: 2012
    Source: In: North, Malcolm, ed. 2012. Managing Sierra Nevada forests. Gen. Tech. Rep. PSW-GTR-237. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. pp. 23-45
    Publication Series: General Technical Report (GTR)
    Station: Pacific Southwest Research Station
    PDF: Download Publication  (435 KB)


    Increasing human emissions of greenhouse gases are modifying the Earth's climate. According to the Intergovernmental Panel on Climate Change (IPCC), "Warming of the climate system is unequivocal, as is now evident from observation of increases in average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level" (IPCC 2007). The atmospheric content of carbon dioxide (CO2) is at its highest level in more than 650,000 years and continues to rise. Mean annual surface air temperatures in California are predicted to increase by as much as 10 °F (5.6 °C) in the next century, creating climatic conditions unprecedented in at least the last 2 million years (IPCC 2007, Moser et al. 2009). Yet climate change is by no means the only stress on forest ecosystems. Growing human populations and economies are dramatically reducing the extent of the Earth's natural habitats. Land use change has reduced the availability of suitable habitat for native plants and wildlife, and, in many places, fragmentation of habitat has led to highly disconnected natural landscapes that are only weakly connected via dispersal and migration. Biotic response to climate and land use change is further complicated by other anthropogenic stressors, including exotic invasives, altered disturbance regimes, air and water pollution, and atmospheric deposition (Noss 2001, Sanderson et al. 2002).

    Traditionally, restoration and ecosystem management practices depend on the characterization of "properly functioning" reference states, which may constitute targets or desired conditions for management activities. Because human-caused modifications to ecosystems have been so pervasive, fully functional contemporary reference ecosystems are difficult to find, and reference states must often be defined from historical conditions. One of the implicit assumptions of restoration ecology and ecosystem management is the notion that the historical range of variation (HRV) represents a reasonable set of bounds within which contemporary ecosystems should be managed. The basic premise is that the ecological conditions most likely to preserve native species or conserve natural resources are those that sustained them in the past, when ecosystems were less affected by people (Egan and Howell 2001; Manley et al. 1995; Wiens et al., in press). However, rapid and profound changes in climate and land use (as well as other anthropogenic stressors) raise questions about the use of historical information in resource management. In the last decade, as the scale and pace of climate change have become more apparent, many scientists have questioned the uncritical application of historical reference conditions to contemporary and future resource management (e.g., Craig 2010, Harris et al. 2006, Millar et al. 2007, Stephenson et al. 2010, White and Walker 1997). What role can historical ecology still play in a world where the environmental baseline is shifting so rapidly?

    In this chapter, we review the nature of climate change in the Sierra Nevada, focusing on recent, current, and likely future patterns in climates and climatedriven ecological processes. We then discuss the value of historical reference conditions to restoration and ecosystem management in a rapidly changing world. The climate trend portion of this chapter is drawn from a series of climate change trend summaries that were conducted for the California national forests by the U.S. Forest Service, Pacific Southwest Region Ecology Program in 2010 and 2011 (available at The historical ecology portion is based on work the first author contributed to Wiens et al. (in press), especially Safford et al. (in press a and b).

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    Safford, H.D.; North, M.; Meyer, M.D. 2012. Chapter 3: Climate change and the relevance of historical forest conditions. In: North, Malcolm, ed. 2012. Managing Sierra Nevada forests. Gen. Tech. Rep. PSW-GTR-237. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. pp. 23-45.

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