Twenty-Fourth Annual Meeting and Symposium of the Desert Tortoise Council, March 5-8, 1999
Abstracts

I report results from on-going field studies designed to measure the relative importance of a variety of mechanisms of coexistence to the persistence of a group of roughly 40 winter annual plant species at an Arizona field site. The species and genera that make up the local flora at this site are common across the Sonoran and Mojave deserts. I will discuss briefly the importance of animals, both large grazers and smaller granivores, to plant diversity and species composition, but will focus especially on the roles of weather and of spatial variation in soils or other habitat factors.

Desert annual plants produce seeds that are dormant for at least a season before potentially germinating. Longevity of seeds in the seed bank varies considerably among species and each species germinates in response to specific weather conditions, especially temperature and amount of rainfall. Germination patterns vary among species and cause different annual plant species to grow and reproduce during different years. This temporal separation of plant growth makes an important contribution to the persistence of many species of annuals. Thus, the contributions of desert annuals as food for desert tortoises can be expected to vary from year to year. The fluctuations in species composition that are commonly observed reflect an important mechanism by which the diversity of the annual plant flora is maintained. Short-term changes in weather patterns, as well as longer term directional changes in weather as are predicted from human-caused climate change can be expected to alter the abundance and species composition of desert annuals.

In addition to among-year separation of plant growth and reproduction, annual plant species also show some separation in growth phenology within a year. This additional source of temporal separation of growth contributes significantly to the coexistence of annuals, but is about an order of magnitude less important than are the differences that occur among years, caused by annual variation in weather. The diversity of spatial habitats that are present in a desert also affect the abundance and species composition of annual plants, but we do not have comparable precise measurements of their relative importance yet.