The biogeography and ecology of foxtail pine, Pinus balfouriana (Grev. and Balf.), in the Sierra Nevada of California.

Abstract

The biogeography and ecology of P. balfouriana, a subalpine conifer endemic to California, were studied. Direct gradient-analysis, classification, and ordination were combined to identify the primary factors controlling the tree community in the southern Sierra Nevada. Competition, disturbance, and temperature were inferred as the most important factors regulating the tree community. Pinus balfouriana responded strongly to variation in soil drainage, distrubance, and temperature. Wildfire was the most important chronic disturbance agent in southwestern Inyo National Forest. Stochastic models of wildfire probability in space and time were developed. Evidence of thunderstorm genesis zones in the vicinity of Overlook Mountain and Ball Mountain in the Golden Trout Wilderness was found. A gradient in wildfire ignition probability was identified. Wildfire ignition is most likely at 2700 m elevation on submesic sites. Wildfire return interval is long enough to permit the coexistence of P. balfouriana and P. murrayana; and, short enough that it has important fitness consequences for P. balfouriana. Wildfire disturbance may limit the geographic range of the species in the southern Sierra Nevada. There was a close negative correlation between the abundance of P. balfouriana near the lower-forest-border and the wildfire disturbance gradient. However, the northern range boundary is probably not determined solely by wildfire disturbance. The canopies of mature P. balfouriana were found to be highly elliptic (when viewed from above) with the major axis of the ellipse oriented exactly north-to-south. This trait permits maximum photosynthesis in the early morning and late afternoon. It simultaneously minimizes evaporative demands during midday. Elliptic canopies minimize solar interception during winter and maximize it during summer. This is important for carbon-balance since the photosynthetic apparatus is only active for a brief period during mid-summer. The lower-forest-border was accurately predicted with a carbon-balance model strongly suggesting carbon-balance limitations. The dispersal potential of P. balfouriana, in the absence of vertebrate seed dispersers, was studied. Anemochory under modern climatic conditions would permit closure of the disjunction in P. balfouriana within a single (120,000 year) glacial cycle.