Distribution of Seeds in Sonoran Desert Soils: Implications for Heteromyid Rodent Foraging

We used a hierarchical sampling design to investigate temporal and spatial patterns of variation in the topsoil seedbank at a site on the Santa Rita Experimental Range, 50 km south of Tucson, Arizona. Replicate sets of contiguous 4 ° 4 cm samples to 2 cm depth ("transects") were collected three times per year, from 1980 to 1983, from four microhabitats that are differentiated by desert heteromyid rodents: large and small open spaces, under canopies of large shrubs, and under canopies of small trees. We also analyzed texture and organic content of soils from the four microhabitats. Seeds from 54 plant taxa were encountered, most of them rare. Average seed mass across species was 1.28 mg/seed, but because tiny seeds were most numerous, the abundance-weighted average was 0.16 mg/seed. Total seed abundance was highly variable, ranging from 0 to 256 seeds and from 0 to 146 mg per sample. The fourfold temporal variation in mean mass or number of seeds per sample was largely due to between-year rather than between-season effects. Seed standing crops were positively correlated with rainfall during the previous 6-mo rainy period, and were lowest in fall and winter of 1982 after 2 yr of poor winter rains. Mean seed number per sample, which reflects primarily abundance of tiny seeds, did not vary among microhabitats. Mean per-sample seed mass, which reflects abundance of the large seeds eaten by rodents, did vary among microhabitats, however, with values being higher under shrubs and trees than in open spaces. The low mean abundance in open spaces was due more to a high frequency of seedless samples than to low maximum per-sample abundance. Hence, profitable seed patches do occur in open spaces, but are rarer than under shrubs or trees. Within microhabitats there was significant variation among replicate transects and among samples within transects. On a local scale, seeds occurred in patches 12 cm in diameter in all microhabitats and sampling periods. Seed species composition varied seasonally and among microhabitats, and there was an overall correlation between seed and adult microhabitat distribution for spring ephemerals. Soil from open spaces was finer and had lower organic content than that under shrub or tree canopies. Our results suggest that the resource upon which desert granivores depend is highly variable in time and space. Microhabitat is one of the most conspicuous sources of variation, and heteromyid rodent species differ in use of those microhabitats (open spaces vs. areas under canopies of perennial plants) that differ most noticeably in seed abundances, species composition, and soil characteristics.