Stand-replacing fire renews successional processes in forest ecosystems,shaping composition and structure. The 1988 Yellowstone fires provided an opportunity to examine post-fire forest community development, whichhas been well-studied in lodgepole pine forests on the subalpine rhyolite plateau in Yellowstone National Park (YNP). However, other community types within the Greater Yellowstone (GY), especially those including whitebark pine, a species of management concern, have received less attention.In two areas within the GY that differed climatically and topographically, we examined successional processes, conifer establishment timeframes, and regeneration densities nearly 30 years post-fire.
In 1990, 250 permanent plots, stratified into study sites by relative moisture availability (xeric, mesic), were established in areas that experienced stand-replacing fire during the 1988 Yellowstone fires and in adjacent unburned forest: a south-facing slope on Henderson Mtn. (HM), Custer Gallatin National Forest and north- and west-facing slopes on Mt. Washburn (MW), Yellowstone National Park. At its lower elevational limit, whitebark pine regeneration may be most vulnerable to warming trends. Both locations experienced increasing annual temperatures since 1988. We surveyed plots for conifer regeneration during seven remeasurements between 1990 and 2017, estimated regeneration density by species over time, and determined stand age-structure as of the most recent assessment.
Results/Conclusions
Whitebark pine (Pinus albicaulis), subalpine fir (Abies lasiocarpa), Engelmann spruce (Picea engelmannii), lodgepole pine (Pinus contorta), and Douglas fir (Pseudotsuga menseizii) comprised the post-fire communities at both study locations. Preliminary results indicate regeneration density varied with time, species, and relative moisture regime. Whitebark pine densities increased over time on MW, with higher rates of increase in mesic than xeric areas. In contrast, on HM, whitebark pine densities converged between moisture regimes at densities lower than those on MW. Densities of other conifer species increased over time in both moisture regimes on MW, with the exception of subalpine fir, which declined after an initial post-fire regeneration pulse. On HM, densities were lower overall than on MW, but similarly, all species increased in density over time. The highest densities as of 2017 varied by moisture regime and area. Subalpine fir density was higher than other species in both moisture regimes at HM, whereas lodgepole pine and Engelmann spruce densities were higher than other species in the xeric and mesic sites at MW, respectively. Lower regeneration densities on HM may have resulted from moisture stress, exacerbated by a south aspect, and/or by competition for moisture with an increasingly dense understory community.