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 fire ef- 

 fects described here. A high degree of variation and 

 complexity in pre- 1900 mixed severity fire patterns 

 has now been documented in diverse western and 

 northern coniferous forests including western larch 

 and lodgepole pine (Arno and others 1993; Barrett 

 and others 1991), coastal Douglas-fir (P. m. var. 



Bitterroot plot 4 

 development of 

 current structure 



PP DF LP GF SAF 



• O a ♦ * 



Plot size is 2.36 acres 



1600 



1700 



1800 1900 1993 



Figure 8 — Chronological development of current stand structure on plot B-4. Only trees that 

 were alive in 1993 (and >4 inches d.b.h.) are shown. Ponderosa pine (PP); Douglas-fir (DF); 

 lodgepole pine (LP); grand fir (GF); and subalpine fir (SAF). 



21 



