Variation in size class distributions along elevation gradients can indicate localized spatial expansion by tree populations inhabiting mountaintops. We searched for this pattern in northern foxtail pine (Pinus balfouriana Grev. & Balf. ssp. balfouriana) stands located in the Klamath Mountains of northern California. Data from 32 belt-transects placed within 16 foxtail pine stands were used to test the hypothesis that this species is expanding downslope locally. Nonmetric multidimensional scaling (NMS) illustrated that size class distributions could be grouped into four types: (1) those at low elevations on north and east aspects; (2) those at low elevations on south and west aspects; (3) those at high elevations on north and east aspects; (4) those at high elevations on south and west aspects. Low elevation transects had greater abundances of small trees, while high elevation transects had greater abundances of large trees. We interpreted this pattern as support for the hypothesis that foxtail pine is locally expanding downslope in the Klamath Mountains. Further analyses showed that NMS dimensions were correlated with estimates of tree density, species diversity, the importance of shade-tolerant conifers, and boulder cover. Moreover, the importance of shade-tolerant conifers was related to aspect, substrate type, boulder cover, and interactions among these variables. The pattern of those correlations suggests that several ecological and environmental factors affect the ability of foxtail pine to expand downslope through alleviation or alteration of competitive interactions. Those results provide an ecological context for climate-mediated range expansions and retractions in northern foxtail pine.
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Vol. 54 • No. 2