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

The scarcity and unpredictability of precipitation presents a challenge to all desert plants. Perennial plants such as shrubs and cacti cope with soils in which moisture is seasonally very low, but winter annuals are conventionally considered drought avoiders that complete reproduction while soil moisture remains high. This physiological strategy includes: 1) seeds germinate only after heavy rains, 2) photosynthetic and growth rates are very high, 3) the stomata (surface pores) remain open during the day to maximize CO2 influx, 4) transpirational water loss is consequently high, 5) photosynthetic enzyme levels are high, and 6) the capacity for osmotic adjustment to low soil water is limited (Smith et al. 1997). True drought avoiders are unable to tolerate low soil moisture. Species shown to fit this pattern include an evening primrose (Camissonia claviformis) and a lupine (Lupinus arizonicus).

The combination of high photosynthesis, high water use and limited osmotic adjustment is associated with plants that are high in water and protein but low in potassium concentration. Plants of four species of desert evening primroses (Camissonia spp.) contained 78-84% water, 10-18% protein (on a dry matter basis, DMB) and 80-180 mmol/L potassium when in flower or immature fruit. Similarly, seven species of desert lupines (Lupinus spp.) contained 80-86% water, 18-24% protein (DMB), and 70-150 mmol/L potassium. Such species appear to be of high nutritional value for desert tortoises, having calculated Potassium Excretion Potential (PEP) indices of about 3-5.

However, other annual species are more tolerant of low soil moisture. Low tissue water potentials (<-3.0 MPa) in some annuals and in herbaceous perennials reflect adaptation to drying soils and may indicate greater water use efficiency and reduced photosynthetic rates. Not surprisingly many of these plants had lower water (<75%) and protein (<12% DMB) and higher potassium (>300 mmol/L) concentrations as soils dried, so that PEP indices were very low (<1) or negative.

It is likely that tortoises rely disproportionately on annuals with high photosynthetic rates that require relatively high soil moisture. In drier years such plants may not be present, and available plants, having adjusted to low soil moisture, have low nutritional value (PEP).

Literature Cited

Smith, S. D., R. K. Monson, and J. E. Anderson. 1997. Physiological Ecology of North American Desert Plants. Springer-Verlag, New York.