VEGETATION DYNAMICS, FIRE, AND THE PHYSICAL-ENVIRONMENT IN COASTAL CENTRAL CALIFORNIA

Current concepts of vegetation dynamics include that of the shifting l andscape mosaic, but evidence for shifting mosaics in disturbed and un disturbed systems is primarily based on negative spatial relationships among adults and recruits, and not on measurements of actual shifts o ver time. We used aerial photographs to measure transition rates as ev idence for mosaic shifts among grassland, coastal sage scrub, chaparra l, and oak woodland communities in central coastal California between 1947 and 1989. In unburned plots without livestock, transition from gr assland to coastal sage scrub was 0.69% per year, coastal sage scrub t o oak woodland was 0.30% per year, and oak woodland to grassland was 0 .08% per year. These transition rates, considered together, indicate t hat vegetation patterns may be dynamic on landscapes dominated by thes e communities. In burned plots without livestock, and in unburned plot s where livestock were not excluded, transition rates were lower, exce pt for the conversion of oak woodland to grassland. In burned plots, a high rate of transition of coastal sage scrub to grassland was measur ed. Markov chain models predicted much less directional change in comm unity proportions in either grazed or burned conditions than in ungraz ed, unburned conditions. Some transition rates varied with substrate a nd topographical position, indicating that fire, grazing, and the phys ical environment interacted to determine direction and rate of vegetat ion change. Variation in transition on different substrates suggests t hat only portions of the vegetation of these landscapes may be dynamic , with some patches in certain combinations of environment and disturb ance that change rapidly, and other patches that remain static as edap hic or topographic climax communities.