differences of plants among the study areas were more 

 pronounced when only green plants were considered 

 (table 8). Our data confirm that moisture content 

 is important to plant nutrient and mineral content 

 (Black 1968). 



Moisture content was highest in the spring for all 

 plant life forms except succulents (table 5), significantly 

 so for annual and perennial grasses, perennial forbs, 

 and shrubs. On an individual species basis, spring 

 moisture content was significantly higher for 10 of the 

 20 species of appendix B (omitting from consideration 

 the special flower, fruit, and leaf collections). For in- 

 dividual species and for plant life forms, spring was 

 also the season of highest content for several other nu- 

 trients and minerals. For the plant life forms shrubs 

 and succulents, nitrogen, phosphorus, potassium, and 

 zinc were all highest and ADF was lowest in spring 

 (table 5). For individual species (appendix B), spring 

 values were significantly higher than at least one of 

 the other seasons for potassium (10 of 20 species, ex- 

 cluding the special parts collections), nitrogen and 

 phosphorus (9 species), zinc (5 species), sodium (2 spe- 

 cies), and manganese, sulfur, and TNC (1 species). 



In general, values for all measured parameters de- 

 crease from a spring high through summer and fall 

 (increasing ADF represents declining quality). There 

 are, however, exceptions for individual species (appen- 

 dix B). For Bromus rubens, new plants in the fall 

 cause moisture content to rebound. For Krameria 

 parvifolia, calcium, magnesium, and sodium are high 

 in the fall. Desert tortoise activity is controlled by 

 moisture and heat so these animals are generally ac- 

 tive in foraging and drinking in the spring and fall 

 when food and habitat conditions are best for them 

 (Nagy and Medica 1986; Woodbury and Hardy 1948). 

 They seek shelter underground and become inactive 

 during winter cold and summer heat and drought. 



Plant Mineral and Nutrient Parameters 



Our discussion is limited to the growing season when 

 desert tortoises may be feeding on the plants we stud- 

 ied. We recognize that these plants have other eco- 

 system values, such as providing food and habitat for 

 other animals. For discussion of winter values of vari- 

 ous plant classes and some individual species, please 

 see Tueller (1979) and Welch (1989) and the references 

 they cite. The values obtained in this study seem to 

 be within the normal range for plants growing in semi- 

 arid sites (Krausman and others 1990; Seegmiller and 

 others 1990). As figure 2 illustrates, the elements 

 sampled often covary. Potassium, for example, is sig- 

 nificantly correlated with the content of all 10 other 

 elements, manganese with all but sodium and sulfur, 

 nitrogen with all but calcium and magnesium, and 

 copper with all but calcium, sodium, and sulfur. 



The very low sodium values for both the soils and 

 plants of our study deserve comment. The soil values 

 for sodium are for water soluble sodium only; even so, 

 the values are low. The sandy nature of our study 

 sites (table 11) no doubt contributed to these low val- 

 ues. The plant sodium values are low but not unprec- 

 edented for semiarid sites (Jones and Hanson 1985; 

 Kincaid 1988). 



The various mineral and nutrient values were often 

 significantly different among plant classes (table 4). 

 During the sampling seasons of our study it would 

 seem that forage from a mixture of plant life forms 

 would be desirable for foraging animals. Water con- 

 tent is highest in succvdents (cacti), also high in peren- 

 nial and annual forbs and shrubs, and low in annual 

 and perennial grasses. Grasses dry out through the 

 summer and fall. Annual forbs tend to disapp>ear after 

 the spring season unless new cohorts are stimulated 

 by precipitation. Nitrogen (protein) is higher in shrubs, 

 annual and perennial forbs, and annual grasses than 

 it is in perennial grasses and succulents. Phosphorus, 

 zinc, and iron are highest in annual forbs; values are 

 high in perennial forbs as well and in shrubs (phos- 

 phorus), succulents (zinc), and annual grasses (iron). 

 Potassium is highest in succulents; values are also 

 high in perennial forbs and shrubs. Manganese and 

 calcium are highest in succulents and annual forbs. 

 Copper is highest in annual forbs and annual grasses. 

 Magnesium is highest in succulents and shrubs. Sul- 

 fur is highest in shrubs but is also high in perennial 

 and annual forbs. Sodium is highest in perennial forbs. 

 The ADF is lowest in succulents and highest in peren- 

 nial grasses. Fat is highest in shrubs and perennial 

 forbs. 



Some plants in our study deserve special comment. 

 The introduced annuals, Bromus rubens, Schismus 

 barbatus, and Erodium cicutarium, are by far the most 

 common and readily available herbaceous plants in 

 the study areas (Baird 1990; Esque and others 1991; 

 McArthur and Sanderson 1992a). These plants are 

 also common desert tortoise foods in the study areas 

 under present conditions (Esque and others 1991; 

 Woodbury and Hardy 1948). Schismus barbatus is 

 exceptionally high in TNC (13.64 percent) and E. cicu- 

 tarium is high in nitrogen (2.03 percent N = 12.69 

 percent protein), potassium (1.76 percent), and TNC 

 (8.96 percent) (appendix B). Other species with notably 

 high values for mineral and nutrient parameters in- 

 clude Plantago patagonica (calcium and TNC), Baileya 

 multiradiata (calcium), Eriogonum inflatum (potas- 

 sium), Atriplex canescens (nitrogen, potassium, calcium, 

 and low ADF), Ceratoides lanata (nitrogen and potas- 

 sium), Hymenoclea salsola (nitrogen, fat and low 

 ADF), Eriogonum fasciculatum and Krameria parvi- 

 folia (TNC), and Opuntia basilaris and O. erinacea 

 (water content, potassium, and calcium) (appendix B). 



12 



