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williamsandliebhold_barkbeetle

williamsandliebhold_barkbeetle - Agricultural and Forest...

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Climate change and the outbreak ranges of two North American bark beetles David W. Williams and Andrew M. Liebhold* USDA Forest Service, North-eastern Research Station, 11 Campus Boulevard, Suite 200, Newtown Square, PA 19073, U.S.A., and *USDA Forest Service, North-eastern Research Station, 180 Canfield Street, Morgantown, WV 26505, U.S.A. Abstract 1 One expected effect of global climate change on insect populations is a shift in geographical distributions toward higher latitudes and higher elevations. South- ern pine beetle Dendroctonus frontalis and mountain pine beetle Dendroctonus ponderosae undergo regional outbreaks that result in large-scale disturbances to pine forests in the south-eastern and western United States, respectively. 2 Our objective was to investigate potential range shifts under climate change of outbreak areas for both bark beetle species and the areas of occurrence of the forest types susceptible to them. 3 To project range changes, we used discriminant function models that incor- porated climatic variables. Models to project bark beetle ranges employed changed forest distributions as well as changes in climatic variables. 4 Projected outbreak areas for southern pine beetle increased with higher temperatures and generally shifted northward, as did the distributions of the southern pine forests. 5 Projected outbreak areas for mountain pine beetle decreased with increasing temperature and shifted toward higher elevation. That trend was mirrored in the projected distributions of pine forests in the region of the western U.S. encompassed by the study. 6 Projected outbreak areas for the two bark beetle species and the area of occurrence of western pine forests increased with more precipitation and decreased with less precipitation, whereas the area of occurrence of southern pine forests decreased slightly with increasing precipitation. 7 Predicted shifts of outbreak ranges for both bark beetle species followed general expectations for the effects of global climate change and reflected the under- lying long-term distributional shifts of their host forests. Keywords Climate change, Dendroctonus frontalis , Dendroctonus ponderosae , discriminant analysis, geographical range shift, insect outbreak. Introduction General circulation models (GCMs) of global climate pre- dict temperature increases from 1 ± C to over 5 ± C in central North America under doubled CO 2 levels (Giorgi et al ., 1998). Such increases could produce profound changes in the forests of the continent (Shriner & Street, 1998). The geographical ranges of many tree species are predicted to shift northward (Davis & Zabinski, 1992; Dyer, 1995; Iver- son et al ., 1999), and the extent to which they do so is likely to alter stand composition across forested landscapes. Insect herbivores of tree species will also shift ranges to follow their hosts (Williams & Liebhold, 1997), and altered trophic interactions under changed climate may further confound predictions of distributions over long periods of time (Franklin et al ., 1992; Davis et al ., 1998; Harrington et al ., 1999).
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