also experienced high levels of overstory mortality of 

 all species in recent decades. 



Understory composition apparently was linked to 

 disturbance history. Only the three plots that had 

 a major disturbance since 1919 contained a sizeable 

 component (>30 percent) of ponderosa pine in the un- 

 derstory (table 3). Plots L-1 and L-2 underbumed in 

 1919, and L-1 underbumed again in 1953. The L-2 

 and B-3 plots were opened up as a result of major 

 overstory mortality from xinidentified pathogens. The 

 six plots that did not have a fire or major pathogen- 

 induced overstory mortality since 1900 contained only 

 minor amounts (<12 percent) of understory ponderosa 

 pine and these trees were generally slow-growing, 

 deformed saplings (table 4b). 



The development of a dense Douglas-fir or grand fir 

 understory is an important compositional and struc- 

 tural change in the formerly open stands of old growth 

 ponderosa pine (Keane and others 1990; Weaver 1943). 

 Most serai ponderosa pine forests that have been used 

 for timber production (Mutch and others 1993; Weaver 

 1967), as well as those that have been protected as nat- 

 ural areas (Habeck 1988, 1990), have developed fir un- 

 derstories or thickets in the absence of fire. These un- 

 derstories and thickets increase risk of stand-replacing 



wildfires (Anderson and Brown 1988; Amo and 

 Brown 1989). 



The Flathead National Forest moist-site plots had 

 been undisturbed since 1900 and understory larch, 

 like ponderosa pine, was scarce and stunted. Our 

 data indicate that regeneration of Igirch was edso Mm- 

 ited by exclusion of surface fires on moist ponderosa 

 pine sites (Amo 1988; Mutch and others 1993). In 

 both dry- and moist-site plots that had not under- 

 bumed since 1900, the understory Douglas-fir were 

 slow-growing (trees 3 to 8 inches in diameter being 

 70 to 90 years old), but had well-developed crowns. 

 The plots (L-1, 2) that had underbumed in 1919 and 

 1953 had younger, more rapidly growing understory 

 trees— for example, compare the ages of <7-inch- 

 diameter trees among dry-site stands in table 4. 



Discussion and Implications for 

 Management 



Our findings indicate that on these rather high- 

 elevation dry sites, fires at mean intervals of 26 to 

 50 years enabled serai ponderosa pine to develop 

 a nearly all-aged structure (except on L-3) despite 



Table 4a-c — Mean ages of understory trees by species and size class. Sample size is shown in parentheses. 



Diameter-class Dry site type Moist site type 



midpoint 



B-l 



B-2 



B-3 



L-1 



L-2 L-3 



F-1 



F-2 



B-4 











(a) Douglas-fir understory 









1 



70 (7) 



68 (5) 



64 (3) 



27 (3) 



31 (2) 52 (6) 



41 (3) 



29 (2) 



52 (3) 



3 



72 (4) 



71 (1) 



50 (2) 



22(1) 



58 (6) 78 (3) 



79 (4) 



41 (7) 



74 (2) 



5 



86(1) 



75 (2) 



71 (2) 



44 (2) 



49 (4) 84 (2) 



66(1) 



50 (2) 



69 (4) 



7 



88 (2) 



86(1) 



77 (4) 





51(2) 86(1) 



79 (7) 



57(1) 



76 (3) 



9 





86 (3) 



70 (4) 



93(1) 



80(1) 



78 (16) 



64 (6) 



88(1) 



11 





78 (3) 



96 (4) 



91 (2) 



98(1) 



87 (4) 



75(1) 



90 (5) 











(b) Ponderosa pine understory 









1 





43(1) 



67 (3) 



36 (5) 



38 (5) 



44(1) 



28 (2) 





3 



71 (2) 





65 (5) 



36 (4) 



44 (11) 



81 (2) 



37(1) 





5 



92(1) 



73(1) 



77 (3) 





41 (6) 









7 







96 (2) 



69 (3) 











9 





87 (3) 



101 (2) 



87(1) 









90(1) 



11 



85 (2) 



80(1) 





86 (3) 











1 









(c) Lodgepole pine understory 









3 













82 (2) 



85 (3) 





5 













82 (8) 



75 (3) 





7 













92 (7) 



92 (2) 



99(1) 



9 













86 (3) 





87(1) 



11 













95(1) 







20 



