about 1525 and 1550, which suggests that a fire or 

 other event opened the overstory (fig. 3a). Coinciden- 

 tally, two approximately 800-year-old pines on this 

 site grew slowly (mean annual radial increments of 

 about 0.02 inch) for >100 years prior to about 1525, 

 followed by accelerated growth (two to three times as 

 fast) for several decades. This kind of growth pattern 

 is often associated with fire-caused thinning (Arno 

 and Sneck 1977). Fire scar records did not extend 

 back before 1630 in these plots. 



Plot L-3 had a distinctively different age structure; 

 most of the stand became established during a 70-year 

 period (1700 to 1770) despite intervening surface fires 

 (fig. 3a). This major age group established after sur- 

 face fires in about 1687 and 1698 and its rapid early 

 growth suggests that these fires (and perhaps associ- 

 ated bark beetle attacks) may have created openings 

 by killing many of the overstory trees. Still, plot L-3 

 had 15 live trees and about eight dead trees (post-1900 

 mortality) as well as several older dead trees (1800's 

 mortality) that were established before the late-1600's 

 fires. Thus, an open "shelterwood" overstory of at 

 least 12 trees per acre survived the ~1698 fire, and 

 the 1700-1770 age class developed beneath it. Some 

 of these pines became suppressed and today are only 

 about 12 inches in diameter and 40 ft tall at >250 years 

 of age, less than half the diameter or height of the 

 dominant trees in this age group. 



Moist Sites 



The moist-site plot on the Bitterroot National For- 

 est had an age class pattern similar to those on the 

 dry sites. In contrast, the Flathead National Forest 

 plots exhibited a distinctively even-aged ponderosa 

 pine structure, despite relatively short fire intervals. 



The Bitterroot National Forest moist site (B-4) had 

 a continuous representation of pine trees dating from 

 the mid-1400's until 1680 (fig. 3c). Thereafter, only 

 three trees (all pines) date fi-om the next 120-year pe- 

 riod. After 1800, however, both pines and Douglas-fir 

 became successfully established. Today, with fire ex- 

 clusion after about 1908, there is heavy mortality and 

 damage in Douglas-fir due to root rot, western spruce 

 budworm, dwarf mistletoe, and perhaps other insects 

 and diseases. The oldest Douglas-fir in the plot was 

 only 180 years, and very few large (>200-year-old) 

 Douglas-fir are seen in the surrounding stand. This 

 contrasts with the dry-site plots, all of which had old 

 Douglas-fir, and also with the one Flathead National 

 Forest plot (F-1) that supported >200-year-old trees, 

 including several Douglas-fir. 



The high frequency of fires (13-year mean interval) 

 in plot B-4 presumably favored maintenance of nearly 

 pure ponderosa pine among >200-year-old trees. But 

 why is there such a plurality of Douglas-fir establish- 

 ment during the mid and late 1800's (fig. 3c)? Perhaps 



Douglas-fir tended to succumb to root or bole rot on 

 this site type in a frequent underbum scenario, just as 

 the species is doing today with fire exclusion. Frequeni 

 underburning evidently did not predispose a serai 

 pine-dominated stand to pathogen mortality, since 

 pines commonly achieved ages in excess of 400 years. 



The Flathead National Forest moist-site plots had 

 ponderosa pine establishment confined largely to one 

 or two distinct episodes immediately after a major 

 fire (fig. 3c). The dates of fire/pine establishment epi- 

 sodes were similar among some of the four Flathead 

 plots and different among others (fig. 4). Because of 

 these episodes, pre- 1900 ponderosa pine stand struc- 

 ture tended to be even-aged. For example, plot F-1 is 

 primarily ponderosa pine that became established after 

 a stand-replacement fire in about 1601 (fig. 3c), with 

 a few additional pines estabhshed after an underbum 

 in about 1759. The 1601 fire was dated from a fire 

 scar followed immediately by growth release on one 

 surviving tree and a pronounced growth release at 

 that time on two other pre- 1600 trees in the stand. 

 The post- 1601 stand maintained an open structure 

 apparently without development of a Douglas-fir un- 

 derstory until the early 1900's, presumably as a re- 

 sult of the eight surface fires between 1660 and 1850. 

 Numerous lodgepole pine became established in the 

 late 1800's, not clearly post-dating any given fire. 



Plot F-2 is a dense, nearly even-aged ponderosa pine/ 

 western larch stand post-dating a replacement fire 

 in about 1817; it also includes additional age classes 



Plot Regeneration episode and fire year 



A A & A A 



F-3 A A 



F-4 A A 



1 i 1 1 



1500 1600 1700 1800 



Year 



^ Regeneration episode A Underbum 



Figure 4 — Approximate dates of stand- 

 replacement fire/ponderosa pine regen- 

 eration episodes in the vicinity of the Flat- 

 head plots. Episodes on F-3 and F-4 are 

 based on pine age classes, since site- 

 specific fire history was not obtained, al- 

 though Freedman and Habeck (1985) 

 provide data from the general vicinities. 



9 



