Sugar Maple Decline after Defoliation by Forest Tent Caterpillar

more likely after early season defoliation (Allen 1987). Defoliation and subsequent refoliation also increase tree susceptibility to invasion by secondary organisms (Wargo 1972). Because of these stress-induced changes, defoliated trees can exhibit crown dieback and loss in vigor and may eventually die. These changes have been referred to as symptoms of decline. In some cases, a single stress such as defoliation may trigger these decline symptoms (Houston 1999, Horsley et al. 2002). The FTC was strongly related to sugar maple decline in Ontario in the mid-1970s (Hendershot and Jones 1989), although there were probably other contributing factors (Gross 1991). Some sugar maple stands in the Northeast have been less resilient to the recent FTC defoliation than others. A common hypothesis is that the number or severity of defoliations is related to the amount of damage, but stands with similar defoliation histories have differed in their response to this stress. Also, some stands have significant crown dieback and mortality after only one season of FTC defoliation. It is likely that there are some predisposing, inciting, or contributing factors (Manion 1991, Houston 1992) interacting with defoliation in these stands that explain their different responses. Extreme climatic conditions have been associated with past sugar maple declines (Horsley et al. 2002). Growing season drought and resulting soil moisture deficiency are common inciting and predisposing factors. Declines in the 1950s in Wisconsin (Skilling 1964), in the late 1970s and 1980s in Pennsylvania (Kolb and McCormick 1993), and in the 1980s in New York (Allen 1987) and southern Québec (Payette et al. 1996) were associated with prolonged periods of dry weather. Stand structure has been an important predisposing factor to sugar maple declines in Wisconsin, Vermont, and New York (Skilling 1964, Bauce and Allen 1991, Allen 1996). In Vermont, mortality and dieback were especially prevalent in pole and small sawtimber–sized sugar maple stands (Teillon et al. 1982). Gross (1991) found that damage was significantly greater on intermediate and suppressed trees in even-aged stands. Similarly, average annual mortality in sugar maple stands over a 9-year period was significantly greater in intermediate and suppressed trees (Allen et al. 1999). Intermediate and suppressed trees on even-aged northern hardwood stands are usually weak trees of poor vigor and poor genetic potential (Nyland 2002). Stand history and anthropogenic disturbance are important to forest health. Some cases of sugar maple decline occurred on or near the fringes of the natural range for northern hardwoods (i.e., northwestern Pennsylvania and Wiconsin; Nyland 1999, Whitney 1999), where other forest types had previously dominated. A poor adaptation of sugar maple to marginal sites in these cases may have predisposed sugar maple to decline (Houston 1999, Nyland 1999). Soil chemistry has been an important factor in many of the previous sugar maple declines. In northwestern Pennsylvania, declining sugar maple stands were deficient in important base cations (Ca, Mg, and K), had excesses of antagonistic base cations (Al and Mn), and had experienced heavy insect defoliation (Kolb and McCormick 1993, Wilmot et al. 1996, Long et al. 1997, Horsley et al. 2000, Bailey et al. 2004). Soil chemistry is undoubtedly critical to sugar maple health. We did not include soil chemistry in this study, however, because forest managers commonly do not have this information when they forecast the health of their stands. The recent outbreak of FTC offered the opportunity to test climatic and site factors and their interactions with FTC defoliation within the natural range of northern hardwoods using stands monitored under the North American Maple Project (NAMP). The NAMP is a regional program established in 1988 that monitored sugar maple health across 10 states and 4 Canadian provinces (Millers et al. 1991). Using new and previously monitored NAMP stands, the objectives of this study were to (1) examine the health and condition of sugar maple stands in New York and Vermont, (2) determine if damage was significantly greater in stands defoliated by FTC, and (3) identify factors that may exacerbate forest damage after defoliation by FTC. Important factors were incorporated into a vulnerability rating index that can be used by forest managers to predict forest damage. Tables such as these can provide managers with a basis for prioritizing management tasks, scheduling and implementing treatments, and preventing timber value loss (Hyland 1983).