A Landscape-Level Assessment of Whitebark Pine Regeneration in the Rocky Mountains, USA

Whitebark pine (Pinus albicaulis Engelm.) has recently experienced high mortality due to multiple stressors, and future population viability may rely on natural regeneration. We assessed whitebark pine seedling densities throughout the US Rocky Mountains and identified stand, site, and climatic variables related to seedling presence based on data from 1,217 USDA Forest Service Forest Inventory and Analysis plots. Although mean densities were highest in the whitebark pine forest type, 83% of sites with seedlings present occurred in non-whitebark pine forest types, and the highest densities occurred in the lodgepole pine forest type. To identify factors related to whitebark pine seedling presence, we compared the results generated from three statistical models: logistic regression, classification tree, and random forests. All three models identified cover of grouse whortleberry (Vaccinium scoparium Leiberg ex Coville) as an important predictor, two models distinguished live and dead whitebark pine basal area and elevation, and one model recognized seasonal temperature. None of the models identified forest type as an important predictor. Understanding these factors may help managers identify areas where natural regeneration of whitebark pine is likely to occur, including sites in non-whitebark pine forest types

Natural regeneration of whitebark pine: Factors affecting seedling density across Idaho, Montana, and Wyoming

Whitebark pine (Pinus albicaulis) is an ecologically important species in high-altitude areas of the West due to the food source it provides for Clark’s nutcrackers, red squirrels, grizzly bears, and other animals. Whitebark pine stands have recently experienced high mortality due to wildfire, white pine blister rust, and a mountain pine beetle outbreak, leading several researchers and managers to question the species’ long-term viability. This study examined regeneration at over 1,000 Forest Inventory and Analysis (FIA) plots containing a whitebark pine component (i.e., any dead whitebark pine trees larger than 5 inches d.b.h. or live whitebark pines of any size) in the northern Rocky Mountains. Objectives were to characterize the population’s age and size structures, as well as identify factors that influence whitebark pine regeneration. Mean seedling density at FIA plots ranged from zero to over 3,000 seedlings per acre, with a mean density of about 300 seedlings per acre and a median density of about 110 seedlings per acre. At the landscape scale, whitebark pine’s age classes and size classes both show a steep reverse-j distribution. A two-stage modeling approach was used to relate site-specific and climate variables first to presence/absence of whitebark pine seedlings, and then to seedling density. Preliminary results suggest that regeneration is most strongly related to the density of understory vegetation, particularly the shrub Vaccinium scoparium, as well as seedling density of other tree species. Species composition of the overstory was more important than indicators of overstory density, including tree canopy cover and basal area. With respect to temperature and precipitation, the relative importance of mean versus variability metrics differed by season