SPATIAL AND TEMPORAL VARIATION IN THE SEED BANK OF A SEMIARID GRASSLAND

The spatial and temporal variability in the seed bank of a semiarid grassland in Colorado was evaluated using soil cores. Spatial variability in the soil storage of germinable seeds was assessed by sampling two shortgrass plant communities on sites with the same climatic conditions but differing in soil texture. Differences between communities were largely the result of annual plant seeds. Eight sampling dates over two years were used to assess temporal variability, which was more important to the storage of germinable seeds than spatial variability. Differences in the numbers of seeds stored were found between the two sampling years, and seasonally within years. The number of seedlings that emerged from the samples ranged from 122-2,748/M2. A poor correspondence was found between the species composition of the plant communities and the storage of germinable seeds; however, the species composition of the seeds produced on the sites tended to have a high similarity with the seedlings that emerged. Most of the species had a transient rather than a persistent presence in the seed bank. THE SEED BANK of a plant community represents the "memory" of previous conditions (Templeton and Levin, 1979), and it is an important component of the potential of the community to respond to conditions in the present and future. The recovery of a plant community after disturbances is related to the germination and establishment of seedlings, although in some communities vegetative reproduction by perennial plants is another important process (e.g., Platt, 1979). Characteristics of disturbances and the storage of germinable seeds vary in both space and time (King, 1976; Thompson and Grime, 1979; Sousa, 1984; Thompson, 1986). Patterns of recolonization on disturbed areas are a function of the characteristics of the disturbance and the dynamics of the pool of propagules of which the seed bank is an important part. Few studies have been conducted on the storage of seeds in semiarid grasslands, and particularly in shortgrass communities (Weaver and Mueller, 1942; Lippert and Hopkins, 1950). The variability of the seed bank through space and time has not been evaluated. The 1 Received for publication 10 February 1988; revision accepted 5 May 1988. We thank Bob Shaw, Daniel Milchunas, D. D. Briske, and Jack Major for constructive comments on the manuscript, Bob Dougherty for allowing us to use the data on seed production, and Tom Kirchner for helpful discussions. Dieter Wilkin and Bob Shaw provided plant identifications. This study was supported by the National Science Foundation, Shortgrass Steppe Long-Term Ecological Research Project (BSR-8 114822), and the Colorado State University Experiment Station Project, Long-Term Grassland Ecosystem Research. interaction between the effects of soil texture on plant community composition (Anderson, 1983), and the low and variable patterns of precipitation found in semiarid regions (NoyMeir, 1973; Sala and Lauenroth, 1982) leads to the potential for spatial and temporal variability in processes important to the storage of germinable seeds. The objectives of this study were to evaluate the spatial variability in the seed bank of shortgrass plant communities by considering sites with the same climatic conditions but differing in soil texture, and to characterize the seasonal and between-year dynamics of the seed bank. MATERIALS AND METHODS-This study was conducted at the Central Plains Experimental Range (CPER). The CPER is located in north central Colorado approximately 60 km northeast of Fort Collins (40?49'N latitude, 1 07?47'W longitude). Mean annual precipitation over the past 45 years is 311 mm (SD = 79 mm) and mean monthly temperatures range from -5 C in January to 22 C in July. Moderate grazing by cattle occurs throughout the site. Two locations differing in soil texture were chose to study the germinable seeds in the seed bank. A coarse-textured site on a sandy loam soil and a fine-textured site on a clay to clay loam soil were selected. Plant communities at both sites are dominated by the perennial grass, blue grama [Bouteloua gracilis (H.B.K.) Lag. ex Griffiths]. Nomenclature follows McGregor (1986). Canopy cover of all plants ranges from 40-45% of which 25-35% is B. gracilis. A number of grasses, succulents,