Factors Associated with Establishment and Growth of Pinus coulteri and Pinus sabiniana in California's Central Coast Bioregion

Abstract

Forest inventory indicated recent decline in a P. coulteri population at its northern extent but live stems in sympatric P. sabiniana increased. Patterns of cone production, localized factors associated with regeneration, relations between wildfire and tree age distributions, and correlations between climate, seedling establishment, and tree growth were analyzed as potential mechanisms underlying unequal demographics between the two species. Four findings emerged: (1) P. sabiniana had a higher frequency of open cones at the time of observation, but cone production was size-biased, and cone production rates were similar for both species; (2) P. sabiniana seedlings were associated with a higher proportion of bare ground cover and westerly aspects, whereas establishment of P. coulteri seedlings was correlated with warm growing seasons and high vapor-pressure deficits, and seedlings occurred on northernly aspects; (3) age distributions of stands inside and outside a wildfire perimeter did not differ for either species, but annual growth increments of P. coulteri were greater within the fire perimeter; and (4) P. coulteri was even-aged, establishment was episodic, and occurred approximately 10 years post-wildfire. In contrast, the age distribution of P. sabiniana was highly variable, consistent with continuous recruitment, and establishment was not correlated with a climate signal.