Conclusions

Improved understanding of the effects of climate factors on P. gaeumannii abundance now helps to clarify the underlying causes for recently observed increases in Swiss needle cast in the Western Coast Range. Previous observations on the regional distribution of P. gaeumannii in the Pacific Northwest have suggested a connection between P. gaeumannii abundance and spring rainfall (Hood 1982). In the Western Coast Range of Oregon, where spring precipitation is abundant, winter temperature has been found to be a highly reproducible predictor of the spatial variation in abundance of P. gauemannii and resulting Swiss needle cast severity, presumably because of its effect on fungal growth. Winter temperature alone is not a satisfactory predictor of P. gauemannii abundance regionwide or in areas where spring precipitation is not as abundant, such as the Willamette Valley, or Oregon Cascade Range, as shown by Manter and others (2005). The predictive disease model described here, therefore, is applicable for predicting spatial variation in P. gaeumannii abundance and Swiss needle cast severity only for the western slope of the Oregon Coast Range. Within this area, there have been few historical reports of Swiss needle cast, and the disease has been considered an insignificant forest health issue. The natural distribution of Douglas-fir in the Western Coast Range has undoubtedly been influenced by P. gaeumannii and Swiss needle cast, along with other disturbance agents. The effect of chronic, profuse P. gaeumannii colonization of Douglas-fir foliage is to reduce growth rates of affected trees relative to competing species, such as spruce and hemlock. Normally faster growing than western hemlock, Douglas-fir is an inferior competitor where Swiss needle cast disease pressure is high. In the coastal lowlands and interior valleys of the Western Coast Range, seasonal climatic conditions are the most favorable for P. gaeumannii growth and reproduction. In these areas, a distinct natural forest type has historically been dominated by western hemlock and Sitka spruce, with Douglas-fir occurring only sporadically. Douglas-fir gradually becomes more abundant in natural forests at higher elevations and further inland, as the Sitka spruce zone gradually merges into the western hemlock vegetation zone, where Douglas-fir is a successional dominant, and where climatic conditions are less favorable for P. gaeumannii growth. This leads to the conclusion that the Sitka spruce vegetation zone occurs as a consequence not only of favorable habitat for Sitka spruce and western hemlock, but also because of the inhibition of their main competitor, Douglas-fir, due to Swiss needle cast disease. This scenario also suggests that the severity of Swiss needle cast in the region may be the result of recent forest management in the Western Coast Range, where Douglas-fir has been strongly favored in forest plantations because of its greater economic value, increasing the abundance of the host species in the area most favorable for growth and reproduction of the pathogen. As noted above, however, recent climate trends also are likely to have contributed to current Swiss needle cast severity. Furthermore, forecasts of future climate trends for the Pacific Northwest suggest a probable expansion of the area affected by severe Swiss needle cast beyond the Western Coast Range as winter temperatures and spring precipitation continue to increase, resulting in greater disease pressure on Douglas-fir stands further inland.