climatic stress as a probable cause of many historical forest declines (Hepting, 1963; Manion, 1991). For example, a few unusually warm summers were associated with past declines of red spruce (Picea rubens) in eastern North America (Cook and Johnson, 1989).
Extremes of environment are most damaging to the reproductive stages of plants. For example, changes in mast fruiting,5 which are often synchronous over large regions, have strong effects that cascade through all levels of an ecosystem (Koenig and Knops, 2000). One example is the influence that large acorn crops have on increasing the populations of deer, mice, and ultimately ticks (Jones et al., 1998). Thus, climatically induced changes in masting that lead to increased acorn production can result in an enhanced risk of Lyme disease, which then impacts human health. It is likely that the effects of abrupt climate change on mast fruiting will be nonlinear and thus the impacts of these changes will be difficult to predict (Koenig and Knops, 2000).
Drought is also of primary importance to forests. In contrast to earlier predictions that global warming would increase radial growth of trees in boreal forests, white spruce (Picea glauca) tree ring records show recent decreases in radial growth. These decreases are presumed to be due to temperature-induced drought stress, which has implications for forest carbon storage at high latitudes. In the Southern Hemisphere (Patagonia), recent pulses of mortality in Austrocedrus chilensis trees were associated with only 2 to 3 years of drought (Villalba and Veblen, 1998). Not only is the lack of water directly damaging in a drought, but there is increased susceptibility to fire as a forest dries out. Further, there is evidence that drought triggered an ecotonal shift in New Mexico (Allen and Breshears, 1998) where ponderosa pine experienced high mortality rates in less than 5years and the ecotone migrated over 2 km. Woody mortality loss occurs much faster than tree growth gain, which has pervasive and persistent ecological effects on associated plant and animal communities.
Ice-core records from Greenland record changes in the frequency of layers enriched in fallout from forest fires (Taylor et al., 1996). The frequency of occurrence of layers was anomolously high during the abrupt cold event about 8,200 years ago, which in turn is associated with drought in regions upwind of Greenland, likely including the United States mid-continent (Alley et al., 1997). In the southwestern US, synchrony of fire-free and severe-fire years across diverse forests implies that climate forces