Rodent Occurrence, Habitat Disturbance, and Seed Fall in a Larch‐Fir Forest

Small—mammal population changes were measured for 5 yr (1970—1974) by live trapping on broadcast burned western larch (Larix occidentalis)/Douglas—fir (Pseudotsuga menzielsii) clear—cuts and in uncut timber on a north and a south slope in western Montana. Four species comprised 96% of the 1324 animals caught: deer mice (Peromyscus maniculatus) 42%, red—backed voles (Clethrionomys gapperi) 27%, red—tailed chipmunks (Eutamias ruficaudus) 22%, and long—tailed voles (Microtus longicaudus) 5%. Deer mice and chipmunks were common on clear—cut and timber plots. The red—backed vole and long—tailed voles were associated with moist sites but showed local allopatry. The red—backed voles were present only under tree canopy and the long—tailed vole was found only in absence of tree canopy. A hard burn effect was to eliminate most of the organic mantle and all small mammals except deer mice, who existed as the single species for 2 yr, and as 80—90% of numbers for 5 postburn yr. A light burn which left duff intact was associated with retention of species diversity and a low initial postburn (2 yr) mammal population, followed by the largest increases. Numbers of deer mice varied inversely with numbers of red—backed voles in the timber. Deer mice increased sharply on all plots the first fall after a heavy conifer seed crop, an occurrence reported by other workers. From these pattern observations it is theorized that red—backed voles may dominate deer mice, but a heavy seed crop can temporarily enhance competitive standing of deer mice. An open, xeric, pioneering situation (hard burn) was conducive to persistently high deer mouse populations. The most obvious habitat feature associated with an inverse spatial relationship between the two voles was tree canopy. The long—tailed vole may be further discriminated against by drier habitats. Clear—cutting appeared to be the principal determinant of total population size, but burn intensity seemed to influence species composition.