UC Riverside

Conifer populations in the Peninsular Range of southern California and Baja California form isolated biogeographic "sky-islands" on mountains with orographic enhanced precipitation. Fire suppression management altered burning patterns in southern California since ca. 1900. Significant changes in tree composition, density, and diameters are documented over a 75-year period in mixed conifer forest at Cuyamaca Mt. by replicating ground-based measurements sampled in 1932 for the Weislander Vegetation Type Map survey. Average conifer density more than doubled, from 271 trees ha-1 ± 82 (standard error) to 716 ha-1 ± 79. Repeat aerial photographs for 1928 and 1995 show significant increase in canopy cover from 47% ± 2 to 89% ± 1. Changes comprise mostly ingrowth of shade-tolerant Calocedrus decurrens in the smallest stem diameter class (10 cm to 29.9 cm dbh). The 1932 density of overstory conifer trees (> 60 cm dbh) and 1928 canopy cover were similar to modern mixed conifer forest in the Sierra San Pedro Mártir, ~200 km S in Baja California, Mexico, where fire-suppression had not been practiced.

The winter of 2001-2002 was the driest in southern California since instrumental records began in 1850. Whole-stand die-off of a serotinous pine (Pinus coulteri) occurred over much of the northern Peninsular and eastern Transverse Ranges, before an outbreak of wildfires in 2003 extensively burned drought-killed stands. I investigated four regeneration condition classes: (1) live unburned stands (control group) and those (2) fire-killed (i.e., drought surviving but burned), (3) posthumously burned (i.e., burned after death by drought), and (4) stands with mixed mortality by drought and fire. Posthumously burned stands had an average tree to seedling ratio of 13:1, and the systematic census failed to detect any Coulter pine seedlings among two-thirds of plots. Crown-fire in drought-killed posthumously burned stands is sufficiently intense to destroy the canopy seedbank of serotinous cones. In contrast, an isolated population of Cuyamaca Cypress (Cupresses arizonica var. stephensonii) successfully reproduced in King Creek and at a nearby plantation site. A significant decline in seedling and tree size was measured with distance from King Creek, suggesting that soil moisture is a likely factor limiting growth.