SPATIAL AND TEMPORAL VARIABILITY IN CALIFORNIA ANNUAL GRASSLAND - RESULTS FROM A LONG-TERM STUDY

We present data from the first 11 years of a long-term study of the dy namics of an annual grassland on serpentine soil in Jasper Ridge Biolo gical Preserve, Northern California. Annual rainfall amounts and distr ibutions varied greatly over the period 1982-1993, as did the amount a nd distribution of gopher disturbance. Temporal variation in gopher di sturbance showed no relationship with rainfall, but spatial variation in disturbance frequency was related to soil depth, The disturbance re gime experienced by the grassland is complex, both spatially and tempo rally, and most of the area is disturbed at least once every 3-5 years . Plant species abundances showed a variety of responses to climate va riation and disturbance. Abundances of individual species in any given year could not be linked directly to rainfall amount due to hysteresi s effects and other interactions. The grassland composition changed ma rkedly over the study. Exclusion of gophers suggested that changing ab undances of several species were linked to gopher disturbance. In part icular, perennial species' abundances increased greatly in the years f ollowing exclosure, but then subsequently declined. Data on plant dens ities on gopher mounds disturbed at different times of year and in dif ferent years indicate that the local species composition remains disti nct for a number of years following disturbance. Disturbance history i s hence a major factor controlling local community variation. Changing species importances, a complex disturbance regime and the importance of disturbance history make prediction and modelling of this system di fficult. It is suggested that the same is probably true for many plant communities, and that long-term studies must be an essential part of ecological research programs. This study illustrates the practical pro blems inherent in maintaining long-term field experiments and in analy zing complex time series data which suffer from inadvertent deviations from the original experimental design.