Whitebark pine communities in the Olympic Mountains: microclimates, physical and competitive environments, and effects of white pine blister rust

Abstract

We examined the relation of an Olympic Mountain (Washington, U.S.A.) whitebark pine (Pinus albicaulis) population to its physical and biological environments. We analyzed temperature, light, soil moisture, foliar nutrients, tree size and age, white pine blister rust (Cronartium ribicola), plant species canopy cover, slope stability, and snow cover data to assess the dynamic nature of the whitebark pine population and plant communities since the end of the Little Ice Age around 1850 Common Era. We found that high summer temperatures, drought, and slope stability were important in controlling tree growth on south-facing slopes. Soil temperatures low enough to limit tree root growth existed only in microsites; and the tree line, as defined by cold, root growth-limiting soil temperature, was mostly higher in elevation than our study area, even though much of the area was dominated by tundra. We found evidence that there had been an increase in the elevation of vegetation zones following the Little Ice Age when the tree line likely fell within the study zone. Whitebark pine suffered high mortality as a result of white pine blister rust. Shade-tolerant subalpine fir (Abies lasiocarpa) is increasing in density, and the combination of its competition with whitebark pine and blister rust mortality places the long-term survival of the whitebark pine in question. Although there was little evidence of past fire, the current trend is toward increasing fuels and greater wildfire risk.

Citation

Peter, David H.; Harrington, Timothy B. 2025. Whitebark pine communities in the Olympic Mountains: microclimates, physical and competitive environments, and effects of white pine blister rust. Gen. Tech. Rep. PNW-GTR-1026. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 364 p. https://doi.org/10.2737/PNW-GTR-1026.