Use of an ecotone to test the effects of soil and desert rodents on the distribution of Indian ricegrass

Effects of soil factors and seed-dispersing heteromyid rodents on the distribution of Indian ricegrass Achnatherum hymenoides were studied at a sharp ecotone between a sandy area with abundant Indian ricegrass and a rocky area where Indian ricegrass was scarce. Two hypotheses were tested: 1) Soil factors limited the recruitment rate of Indian ricegrass in the rocky habitat; and 2) Heteromyid rodents limited the distribution of Indian ricegrass because they did not scatterhoard seeds in the rocky habitat. Seed-planting experiments and soil analyses showed that soil characteristics, especially texture and salinity, limited the distribution of Indian ricegrass, but also that microsites existed in the rocky habitat that were suitable for germination and growth of Indian ricegrass. In particular, as many plants survived over their first winter in the rocky habitat as in the sandy habitat, although. surviving plants were taller in the sandy habitat. Analysis of the soil seed bank indicated that fewer seeds were dispersed to the rocky habitat than to the sandy habitat. To determine whether scatterhoards were being made in the rocky habitat, seeds dyed with fluorescent powder were placed along the ecotone and the fates of these seeds were assessed. Twice as many of these seeds were scatterhoarded in the sandy habitat as in the rocky habitat and seeds were cached further from the ecotone in the sandy habitat. The results suggest that both soil characteristics and heteromyid rodents influence the distribution of Indian ricegrass in western Nevada.