Oecologia (Berlin) (1986) 70:555-558 Oeco/og/a 9 Springer-Verlag 1986 Biomass accumulation and resource utilization in co-occurring grassland annuals H.A. Mooney 1, R.J. Hobbs 2, J. Gorham 1, and K. Williams x 1 Department of Biological Sciences, Stanford University, Stanford, CA 94305, USA 2 Dr. R.J. Hobbs, Division of Wildlife and Rangelands Research, CSIRO, Locked Bag No. 4, P.O. Midland, W.A. 6056, Australia Abstract. Mediterranean-climate annuals growing on ser- pentine soils in central California differ greatly in their life spans and reproductive periods dependent on their access to soil moisture. The longer-lived annuals accumulate a greater lifetime biomass, have a higher total, but lower pro- portional, reproductive output, and produce leaves with a higher C/N ratios at the time of reproduction. Key words: Serpentine - Annuals - Water-use - Nitrogen- use - Reproduction In coastal California many areas underlain by serpentine rock develop grassland communities which are rich in na- tive annual species. These communities have been described by MacNaughton (1968) and others. These annual plants germinate more or less synchronously with the initial fall rains, with most completing their life cycle as the annual drought is initiated around May. A few however persist into the summer drought period. Gulmon (et al. 1983) studied three annual species which represent the total range of life spans found in one of these communities. They noted that the longer-lived annuals were able to extend their growth period and attain considerably greater biomass by tapping deeper soil water reserves than could the shorter-lived annuals. In this study we extend these observations to a broader base to determine if the relationships among life span, bio- mass accumulation, and resource availability are general. Materials and methods Study site. All observations were made at the Jasper Ridge Biological Preserve of Stanford University which is located in the interior foothills of the Santa Cruz Mountains in San Mateo County, California. Phenological and Water Potential Observations. 10-1 m 2 plots were established at regular intervals along a 50 m long transect within the serpentine grassland com- munity. At approximately weekly intervals during 1983-85 the number of plots containing annuals at given reproduc- tive states were noted. Species encountered in the quadrants are listed in Table 1. Xylem water potential was also mea- sured on several individuals of selected species during 1983 utilizing a Scholander presure bomb. Offprint requests to: H.A. Mooney Biomass. During 1985, at the time of apparent maximum biomass, generally during the fruiting stage before leaf loss, the above-ground portions of 10 individuals of each species were harvested and dried at 70 C for weight analysis. Sam- ples were partitioned into reproductive (buds, flowers, seeds) and vegetative portions. Leaves of a number of spe- cies were additionally collected just at the initiation of re- production and dried and ground for carbon and nitrogen content analyses utilizing a Perkin-Elmer Elemental Ana- lyzer. Results Phenology. Over 20 annual species were encountered in the study plots. There was a large spread in the flowering dates of the various species of annuals which were studied, some flowering as early as January and others not starting until August (Fig. 1). This spread occurred even though there was little difference among species in their initial germina- tion date which commenced with the first significant fall rains (Hobbs and Mooney 1985). Differences in the flower- ing dates were noted between years for the various species with most, but not all, flowering several weeks earlier in the dry years of 1984 and 1985. Some species varied greatly between years in time of onset of flowering (e.g. Evax, Mi- cropus, Bromus), while others varied little (e.g. Plantago, Lasthenia, Calycadenia). The later-blooming species gener- ally had a more protracted flowering period than the earlier ones. Water availability. Measurements of dawn water potential of two early and two late blooming species indicated dissim- ilar water availabilities (Fig. 2). The late blooming species did not experience the same degree of water stress as the early blooming ones. These dissimilarities relate to differ- ences in their rooting depths (Gulmon et al. 1983). Biornass allocation. There was a large range in the size of the annuals studied (Gilia and Trifolium biomass data not available) with later blooming species attaining nearly an order of magnitude greater biomass than the earlier ones (Fig. 3). The absolute reproductive output per individual generally followed these trends (Fig. 4A) although the pro- portional effort significantly declined with plant size (Fig. 4 B). The variance in the biomass attained at flowering was much greater for the later blooming species than the earlier ones (Fig. 3). This can be attributed to the attain-