Postfire species shifts, and the growth and regeneration of western red cedar (Thuja plicata), a range margin species, in Glacier National Park, Montana

McKenzie, David

The distribution of species is determined primarily by climate, topography, and biotic interactions, and the overlay of species distributions determines local community composition. Populations at the range-margin may be particularly susceptible to changes in environmental variables, resulting in redistribution of species on the landscape over time. Disturbances such as forest fires provide opportunity for rapid changes in species composition. I characterized changes in tree species composition from before to after stand-replacing fires in Glacier National Park (GNP), Montana. Stands were dominated almost exclusively by two tree species: Pinus contorta (lodgepole pine) and Populus tremuloides (quaking aspen) following fire in six of seven sites. Similarly, changes in relative-densities of life-history groups of tree species were primarily driven by changes in P. contorta and P. tremuloides. Other species that were more frequently encountered before fire were significantly reduced in relative-density following fire in the seedling layer. One such species was Thuja plicata (western red cedar) which, in GNP, also represents a range-margin population, and its life-history characteristics suggest that it may be susceptible to climate change. To determine the state of the T. plicata population in GNP, I identified the degree to which it is sensitive to climatic and topographic variability, as well as its successional status within stands and preferred locations of regeneration. Dendroecological analyses revealed that T. plicata and other range-margin species were more sensitive to minimum winter temperatures than other climate variables, while non-range-margin species were not. The effects of topography on tree sensitivity to climate were not discernible. Within the relatively small portion of the park in which T. plicata occurs, it was frequently a codominant with Tsuga heterophylla (western hemlock) at all successional stages, including following fire. Similarly, T. plicata appears to establish both beneath the closed canopy of a mature forest as well as in canopy openings. Within both locations, it shows a preference for establishment on soil, rather than decayed logs. Rather than appearing to be in a state of decline, the characteristics of the T. plicata population in GNP suggest that it may tolerate future changes, and potentially even expand in range.

Postfire species shifts, and the growth and regeneration of western red cedar (Thuja plicata), a range margin species, in Glacier National Park, Montana

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