The role of fire refugia in the distribution of Pinus sabiniana (Pinaceae) in the southern Sierra Nevada

Although widespread throughout the interior foothills of central and northern California, Pinus sabiniana Dougl. has a disjunct distribution in the southern Sierra Nevada, where it is abundant in the Kern River and Tule River watersheds, but is absent from the Kaweah River watershed between 36° and 37°N. This gap in the pine's distribution has long intrigued botanists and ecologists and has elicited a number of hypotheses for this anomalous biogeographical pattern. Here we propose a new hypothesis that couples unique features of the southern Sierra Nevada topography with unique features of P. sabiniana's response to fire. This low elevation pine is widely distributed in grassland and chaparral, and where it occurs with the latter vegetation, it is extremely vulnerable to high intensity wildfires. Under these conditions, meta-populations persist over time in refugia in riparian areas and during fire-free intervals expand outwards re-colonizing shrubland dominated slopes. The lack of such refugia in the very steep and narrow Kaweah drainage is hypothesized to explain the absence of this pine in that area. To test this hypothesis, we studied the age-structure of P. sabinianain the area of the 2002 McNally Fire in the Kern drainage to compare age distributions of trees and tree skeletons along a gradient up slope away from riparian zones. Maximum age declined significantly with distance from riparian areas, suggesting that past fires have eliminated P. sabinianafrom the slopes and that the pines have re-colonized during fire-free intervals. The relationship was strongest when our data were restricted to areas that had a previously recorded fire. We also found that the riparian areas in the Kern drainage were significantly wider than those in Kaweah drainage, suggesting that fewer such fire refugia exist in the latter watershed, and providing an explanation for the lack of P. sabiniana between 36° and 37°.