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

Plant productivity in deserts is extremely variable from year to year. While much of the variation can be attributed to variation in rainfall (Noy-Meir 1973), plant productivity at many desert sites is much more variable than rainfall (Le Houerou et al. 1988). There may be a number of reasons for this, but observations suggest that much of the variation may be associated with responses of desert annuals (seasonal ephemerals, short-lived forbs) to short term pulses of water and nutrients (Beatley 1969). These species are important resources for desert tortoises and other herbivores. Our ability to understand and predict availability of these plant resources depends upon determining how a variety of plant and environmental factors interact in controlling plant productivity in desert ecosystems. To explore how some of these factors may impact plant productivity, we have used a mechanistic model of plant production associated with a group of plants that occupy a small patch (1-10 m2) of desert landscapes. The model - Patch Arid Land Simulator (PALS) - includes mathematical descriptions of evapotranspiration, soil water distribution, decomposition of litter, soil nutrient availability, and phenological/physiological responses of principal desert plant functional types (life forms): shrubs, subshrubs, grasses, short-lived forbs, and winter- and summer-seasonal annuals.

We carried out simulations of plant production over long-term periods at various desert sites using long-term rainfall records from these sites. Model results show that on both a seasonal basis and an annual basis, the greatest variation in plant production is associated with annual species. The lion’s share of this variability is related to phenological responses of annual plants to rainfall. For example, productivity is low where rainfall events occur outside the period of seed germination or where rainfall is smaller than the threshold necessary to induce germination, and productivity is high where rainfall events are sequenced to stimulate maximum germination and length of the growth period. Other factors that affected productivity of annuals, such as competition from other plant functional types and soil nutrient levels, had much less impact than rainfall. Variability in plant productivity differed somewhat among desert sites, with greatest variability associated with sites that had the greatest variability in seasonal components of rainfall. Sites with greatest variability in productivity of desert annuals and forbs appear to be among those favored as desert tortoises habitat. Thus, desert tortoises seem well adapted to the natural variability in their native annual and forb resources. Using a mechanistic model such as PALS may prove useful in helping to evaluate how changes in the tortoises’ habitat (such as introduction of alien species, nitrogen deposition, and resource removal by competing herbivores) might impact future resource availability for desert tortoises.

Literature Cited

Beatley, J. C. 1969. Biomass of desert winter annual plant populations in southern Nevada. Oikos 20:261-273.

Le Houerou, H. N. R. L. Bingham, and W. Skerbek. 1988. Relationship between the variability of production and the variability of annual precipitation in world arid lands. Journal of Arid Environments 15:1-18.

Noy-Meir, I. 1973. Desert ecosystems: environment and producers. Annual review of Ecology and Systematics 4:25-41.