Limber pine (Pinus flexilis) forest dynamics across the Intermountain West
Limber pine (Pinus flexilis James) is widely distributed within the Intermountain West and is often considered ecologically similar to other high elevation 5-needle pines. Limber pine is generally described as a seral species expect under extremely stressful high elevation environments where it forms very old, slow growing open stands. Unlike the other 5-needle pines, limber pine is found at both high and relatively low elevations. This bi-modal distribution is hypothesized to result from competitive exclusion on all but extreme sites at high and low elevations. Using the USDA Forest Inventory and Analysis (FIA), we explored the stand structure and stand dynamics of limber pine communities throughout the Intermountain West. We performed a systematic query of all plots within the ten ecoregions of the Intermountain West that contained limber pine, yielding 683 plots across eight states. Elevation and percent basal area of limber pine were grouped into five categories. A chi-square test was used to assess independence of the categories of elevation and percent basal area.
Limber pine ranges in elevation from 1177 m to 3547 m, and occurs with 24 different overstory tree species. Throughout this elevational gradient, limber pine was a consistent component of forest communities, contributing between 12.7% - 20.7% of total stand basal area. There was no significant difference between percent basal across elevational classes (p = 0.11). This consistency is not observed in other coniferous species in the Intermountain West. Lodgepole pine was the only other species observed at all elevation classes but follows a traditional uni-modal distribution, peaking at mid-elevation (2134-2743 m). Limber pine is a common associate in all the forest zones proposed by Daubenmire (1943). For example, at low elevations, limber pine is observed with ponderosa pine, and Rocky Mountain juniper. With increasing elevation, limber pine is associated with Douglas-fir and lodgepole pine; at the highest elevations, limber pine is associated with Engelmann spruce, subalpine fir, and bristlecone pine. In light of its remarkably broad ecological amplitude, limber pine is clearly different than the other 5-needle high elevation white pines in the Intermountain West. These results are also not consistent with the hypothesized bi-modal distribution.