Sixty Years of Change in Californian Conifer Forests of the San Bernardino Mountains

Abstract

We revisited 68 plots of forest vegetation in the San Bernardino Mountains that had been quantitatively described in 1929‐1935, from the California Vegetation Type Map (VTM) Survey. By using the same sampling methods, we documented changes—over approximately 60 years and during a period of fire suppression management—in tree density by both species and size class. In general, we found increasing stand densities, a transformation from old‐growth age structure to young growth, and a compositional shift from Pinus ponderosa and P. jeffreyi to Abies concolor and Calocedrus decurrens. Density of trees of more than 12 cm diameter at breast height (dbh) increased by 79%, including three to ten‐fold increases in the youngest cohorts 12–66 cm dbh. The magnitude of change depended upon initial forest composition and local annual precipitation. Monotypic stands of P. jeffreyi or those initially dominated by Abies concolor showed the least change in species composition; the most xeric stands of P. jeffreyi showed the least gain in density; and mesic mixed P. ponderosa stands showed the most dramatic change in composition and density. We compared these data to records of past and present forests in the Sierra Nevada and found parallel trends, but magnified by the increased precipitation of the Sierra Nevada. We also compared VTM data from the San Bernardino Mountains to mixed conifer forests in the Sierra San Pedro Martir of Baja California. These Mexican sites and forests are ecologically similar to those in California, but they still experience unmanaged fire regimes. Californian forests of 60 years ago are remarkably similar to modern forests in the Sierra San Pedro Martir. Thus, we conclude that forest changes in the San Bernardino Mountains are primarily due to lengthening fire intervals. Forest changes as a result of fire suppression have important conservation consequences for bird species diversity in general and for Spotted Owl and Neotropical migrants in particular.