Effects of rainfall variability and gopher disturbance on serpentine annual grassland dynamics

We studied the dynamics of a serpentine annual grassland in northern Cal- ifornia in relation to disturbance by pocket gophers (Thomomys bottae) and interannual variation in rainfall over the period 1982-1988. Mapping of gopher mound formation indicated that the probability that gophers would disturb any given area during the 6-yr study period had a near normal distribution with a peak at two disturbances during that period. Disturbance levels varied considerably from year to year and spatially, and thus the disturbance regime is complex. Exclosure experiments indicated that gopher disturbance had a significant effect on the abundances of many of the grassland annuals, perennial grasses, and cormaceous species. Aboveground herbivores had no detectable effects. Over the period of the study annual rainfall varied threshold and plant community dynamics were strongly affected by this. In particular, Plantago erecta decreased and Las- thenia californica increased in abundance with increasing rainfall. Abnormally high rainfall in 1982-1984 allowed the buildup of populations of Bromus mollis, the only nonnative species that invades the serpentine significantly. B. mollis was, however, subsequently virtually eliminated from the serpentine grassland by two consecutive years of severe drought. Invasion of B. mollis was found only on gopher mounds formed in 1983. In subsequent years recolonization of gopher mounds was predominantly by other species, in particular Lotus subpinnatus. Interannual variability in species abundances was most apparent on disturbed microsites. Our results provide a longer term perspective from which to view shorter term exper- imental and modelling work and highlight the need for long-term observations of ecological systems.