Foxtail pine, Pinus balfouriana Grev. & Balf., is an endemic subalpine conifer of California with two allopatric subspecies. The northern subspecies grows in northwestern California and the southern subspecies is found in the southern Sierra Nevada. We studied three northern and three southern populations of P. balfouriana to evaluate the population biology, demography, mortality agents, and environmental conditions in these contrasting regions. Northern populations exist under a mesic climatic regime, a diversity of geological substrates, stands of high tree species richness, lower densities and basal areas, but higher numbers of recruitment, with a relatively mixed size class distribution. Southern foxtail pine populations exist under a xeric climatic regime dominated by granitic substrates, with moderate to high stand densities, less tree species-rich, lower recruitment numbers, but higher basal areas; due to a skewed size class distribution with high representation of large diameter trees. Recruitment in the north averaged 169.3 seedlings/saplings ha−1, compared to 91.3 seedlings/saplings ha−1 in the south, despite the fact that northern populations produce less cones on average (3695 cones ha−1) than populations in the southern Sierra Nevada (7642 cones ha−1). At the stand-level, solar radiation input and foxtail pine density were correlated with fecundity. These factors may correspond with microenvironmental and topographic conditions that favor germination (e.g., warmer microclimate) and propagule pressure (e.g., seed supply). At the local or plot-level, microenvironmental conditions (e.g., litter, substrate type, and microhabitat) and factors corresponding to local seed supply (e.g., density, basal area, number of cones, and number of reproductive adults) were correlated with recruitment, particularly in the southern Sierra Nevada. Foxtail pine is recruiting episodically in higher numbers in the north and lower numbers in the south. Four of six populations appear to be stable, due to low mortality and high survivorship. Low estimates of population growth (λ) at Lake Mountain (north) and Sirretta Peak (south) were due to mortality of large diameter trees and low recruitment. At these locations, mountain pine beetle-mediated mortality and drought stress appear to be important factors contributing to current population trends.
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Vol. 58 • No. 4