Whitebark pine community types and their patterns on the landscape

Abstract

Within whitebark pine's (Pinus albicaulis) relatively narrow zone of occurrence-the highest elevations of tree growth from California and Wyoming north to British Columbia and Alberta-this species is a member of diverse plant communities. This paper summarizes studies from throughout its distribution that have described community types containing whitebark pine and the habitat types (environmental types based on potential vegetation) it occupies. Whitebark pine is most abundant and widespread in the semiarid inland mountain ranges of the northwestern United States and southwestern Canada, where it occurs in a continuum of environmental situations. It can be (1) a fire-dependent, early seral component of spruce-fir forests on moist sites; (2) a persistent seral or minor climax associate in drier forest habitats; (3) a major climax species or the only tree under still drier or more wind-exposed conditions; or (4) a major component or sole dominant of krummholz communities above tree line. The timberline landscape is a mosaic of cover types including windswept fellfields and grassy balds, wet meadows, snowdrift communities, and krummholz (shrub-like conifers) and forest communities with various proportions of whitebark pine. Four factors explain much of the variation in cover types: (1) rugged topography, through its influence on microclimate; (2) differences in surface rockiness, ranging from boulder piles to moderately well-developed soils; (3) differences in substrate composition, with especially noteworthy changes occurring between calcareous and noncalcareous geologic parent materials; and (4) a patchwork of different disturbance histories in the aftermath of fires, bark beetle epidemics, blowdowns, or snow avalanches. Whitebark pine communities also vary regionally, with changes in both climate and competing species. For example, in maritime mountain regions whitebark pine is unable to compete in the closed upper subalpine forest; it is, therefore, restricted to tree islands in the open heath parklands at timberline.