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 a t a sharp ecotone between a sandy area with abundant Indian ricegrass and a rocky area where Indian ricegrass was scarce. Two hypotheses wer e tested: 1) Soil factors limited the recruitment rate of Indian riceg rass in the rocky habitat; and 2) Heteromyid rodents limited the distr ibution 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 t he 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 fi rst winter in the rocky habitat as in the sandy habitat, although. sur viving plants were taller in the sandy habitat. Analysis of the soil s eed bank indicated that fewer seeds were dispersed to the rocky habita t than to the sandy habitat. To determine whether scatterhoards were b eing made in the rocky habitat, seeds dyed with fluorescent powder wer e 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 ecoton e in the sandy habitat. The results suggest that both soil characteris tics and heteromyid rodents influence the distribution of Indian riceg rass in western Nevada.