Effects of Variable Precipitation on the Strucutre and Diversity of a California Salt Marsh Community. Ragan M. Callaway; Craig S. Sabraw.
Journal of Vegetation Science:
1994
Notes
We measured the effects of annual variation in climate and experimentally augmented rainfall on patterns of distribution and above-ground productivity in annual plant communitiesa t CarpinteriaS alt Marsh in central California. In the driest year, Hutchinsia procumbens was codominant throughout much of the upper marsh; however, Hutchinsia was very rare or not present in the wetter years. Conversely, Juncus bufonius was common in the wettest year and absent in the driest year. Elevational distributions of other annual species also differed among years with different total precipitation. In 1989-1990, an exceptionally dry season, supplemental water decreased soil salinity, increased above-ground productivity of annuals, and caused significant changes in spatial patterns and relative density. In the lowest zone, Hutchinsia occurred only in watered plots and supplemental water increased the density of Spergularia marina. At intermediate elevations Lasthenia glabrata occurred only in watered plots and supplemental ater increased the density of Spergularia, Hutchinsia, and Parapholis incurva. At upper elevations, Juncus occurred only in watered plots, and Lasthenia was the only species that increased significantly in density with watering. Unlike natural shifts in species abundance, no species declined significantly in cover in any zone in the watered treatment. Although climatic variation has complex affects on annual plant communities, our experiments isolated important affects of total annual rainfall on the structure of annual plant communities that were similar to those that occurred with natural variation in rainfall.W e conclude that variation in total annual precipitation promotes dynamic community composition and spatial distributions among years, and thus increases overall species diversity in the salt marsh.
Spatial and Temporal Variability in California Annual Grassland: Results from a Long-Term Study. R.J. Hobbs; H.A. Mooney.
Journal of Vegetation Science:
1995
Notes
We present data from the first 11 years of a longterm study of the dynamics of an annual grassland on serpentine soil in Jasper Ridge Biological Preserve, Northern California. Annual rainfall amounts and distributions varied greatly over the period 1982-1993, as did the amount and distribution of gopher disturbance. Temporal variation in gopher disturbance showed no relationship with rainfall, but spatial variation in disturbance frequency was related to soil depth. The disturbance regime experienced by the grassland is complex, both spatially and temporally, and most of the area is disturbed at least once every 3-5 years. Plant species abundances showed a variety of responses to climate variation and disturbance. Abundances of individual species in any given year could not be linked directly to rainfall amount due to hysteresis effects and other interactions. The grassland composition changed markedly over the study. Exclusion of gophers suggested that changing abundances of several species were linked to go er disturbance. In particular, perennial species' abundances increased greatly in the years following exclosure, but then subsequently declined. Data on plant densities on gopher mounds disturbed at different times of year and in different years indicate that the local species composition remains distinct for a number of years following disturbance. Disturbance history is 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 difficult. 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 problems inherent in maintaining long-term field experiments and in analyzing complex time series data which suffer from inadvertent deviations from the original experimental design.
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