Virginia H. Dale, Linda A. Joyce, Steve McNulty, Ronald P. Neilson, Matthew P. Ayres, Michael D. Flannigan, Paul J. Hanson, Lloyd C. Irland, Ariel E. Lugo, Chris J. Peterson, Daniel Simberloff, Frederick J. Swanson, Brian J. Stocks, and B. Michael Wotton
INTRODUCTION Analyses of climate change effects on forests have focused on species' tolerances to temperature and moisture changes and the potential for dispersal but often ignore the effects of climate on disturbances (e.g., Ojima et al. 1991). Yet, modeling studies indicate the importance of climate effects on disturbances regimes (He et al. 1999). Local, regional, and global changes in temperature and precipitation can influence the occurrence, timing, frequency, duration, extent, and intensity of disturbances (Baker 1995, Turner et al. 1998). Because trees can survive from decades to centuries and take years to become established, climate change impacts are expressed in forests, in part, through alterations in disturbance regimes (Franklin et al. 1992). Disturbances, both human-induced and natural, shape forest systems. Forests are routinely subject to various disturbances that influence their composition, structure and functional processes. Indeed, the forests of the United States (US) are molded by their land-use and disturbance history. For example, pine forests in the southeastern US experience sporadic southern pine beetle outbreaks that result in patches of tree mortality. Within the US, disturbances having the greatest effects on forests include fire, drought, introduced species, insect and pathogen outbreaks, hurricanes, windstorms, ice storms, and landslides (Figure 1). Each disturbance affects forests differently. Some cause large-scale tree mortality, whereas others affect community structure and organization without causing massive mortality (e.g., ground fires). Forest disturbances influence how much carbon is stored in trees or dead wood. All these disturbances interact with human-induced effects on the environment such as air pollution and land-use change (e.g., as a result of resource extraction, agriculture, urban and suburban expansion, and recreation). Some disturbances are purely natural, others are also instigated by humans; for example, forest fire ignition and spread is a function of both natural and human conditions (Figure 2). Each disturbance has both social and economic effects (Table 1). Estimating the costs of each of these disturbances is very difficult; these esitmates are illustrative only. Of the eight forest disturbances considered, ice storms are the least costly annually averaging about $10 million and greater than 180,000 ha (Michaels and Cherpack 1998), and insect and pathogens are the most expensive with costs exceeding $2 billion/year and 20,400,000 ha (USDA 1997). The socioeconomic aspects of these damages are only part of the cost. Costs of impacts to ecological services can be large and long term. This article examines how eight disturbances influence forest structure, composition, and function, and how climate change may influence the severity, frequency, and magnitude of disturbances to US forests. We also consider options for coping with disturbance under changing climate. This analysis points to specific research needs that should improve our understanding of how climate change affects forest disturbances. This paper is one in a series that were developed by the Forest Sector of the U.S. National Climate Change Assessment. In examining how forests may be affected by climate change, the Forest Sector Committee divided the topic into four areas (processes, diversity, disturbances, and socioeconomics) that are each a focus of papers in this issue of BioScience.