60 YEARS OF CHANGE IN CALIFORNIAN CONIFER FORESTS OF THE SAN-BERNARDINO MOUNTAINS

We revisited 68 plots of forest vegetation in the San Bernardino Mount ains that had been quantitatively described in 1929-1935 from the Cali fornia Vegetation Type Map (VTM) Survey. By using the same sampling me thods, 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 tran sformation from old-growth age structure to young growth, and a compos itional shift from Pinus ponderosa and P. jeffreyi to Abies concolor a nd Calocedrus decurrens. Density of trees of more than 12 cm diameter at breast height (dbb) increased by 79%, including three to ten-fold i ncreases in the youngest cohorts 12-66 cm dbh. The magnitude of change depended upon initial forest composition and local annual precipitati on. Monotypic stands of P. jeffreyi or those initially dominated by Ab ies 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 composit ion 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 Bernar dino Mountains to mixed conifer forests i n the Sierra San Pedro Martir of Baja California. These Mexican sites and forests are ecologically similar to those in California but they s till experience unmanaged fire regimes. Californian forests of 60 year s ago are remarkably similar to modern forests in the Sierra San Pedro Martir. Thus, we conclude that forest changes in the San Bernardino M ountains are primarily due to lengthening fire intervals. Forest chang es as a result of fire suppression have important conservation consequ ences for bird species diversity in general and for Spotted Owl and Ne otropical migrants in particular.