. Age-class structure of old growth ponderosa pine/Douglas-fir stands and its relationship to fire history. Ponderosa pine Montana; Douglas fir Montana; Old growth forests Montana; Forest fires Montana. competition from the more shade-tolerant Douglas- fir. This age-class structure was associated with low-to-moderate intensity surface fires that occasion- ally killed groups of overstory trees. The nearly all- aged structure of pre-1900 stands is similar to that of pure ponderosa pine forests in Arizona (Cooper 1960; Covington and Moore 1994; White 1985) and eastern Oregon (Weidman 1921). The A
. Age-class structure of old growth ponderosa pine/Douglas-fir stands and its relationship to fire history. Ponderosa pine Montana; Douglas fir Montana; Old growth forests Montana; Forest fires Montana. competition from the more shade-tolerant Douglas- fir. This age-class structure was associated with low-to-moderate intensity surface fires that occasion- ally killed groups of overstory trees. The nearly all- aged structure of pre-1900 stands is similar to that of pure ponderosa pine forests in Arizona (Cooper 1960; Covington and Moore 1994; White 1985) and eastern Oregon (Weidman 1921). The Arizona pine forests had shorter pre-1900 fire intervals (2 to 10 years) than our dry-site pine/fir stands (Swetnam 1990). However, several dry-site ponderosa pine/fir forests at lower elevations in the northern Rockies and inland Pacific Northwest also had average fire intervals of 7 to 10 years (Arno 1988; Kilgore 1987). On the Bitterroot National Forest moist site (B-4), fires at a mean interveJ of 13 years helped maintain a nearly pure ponderosa pine overstory despite com- petition fi"om both Douglas-fir and grand fir (fig. 8). The pine were multiaged and long-lived. Douglas-fir survived fires but did not attain great longevity, per- haps as a result of decay hastened by fire injury. Only a few grand fir predate the most recent fire (ca. 1908) and these have fire scars accompanied by severe heart rot. In contrast, on the Flathead National Forest moist sites, serai ponderosa pine stands became es- tablished in even-aged classes after patchy and in- frequent stand-replacing disturbances—evidently fire and bark beetle epidemics. Low-intensity sur- face fires at the rate of three to four per century then maintained the stands in ponderosa pine-larch domi- nance with open understories. Other pre-1900 fire patterns probably existed in serai ponderosa pine stands. Some patterns were probably intermediate to those we have described. Some probably were beyond the range of the
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