Herbaceous plants, displaying several differing growth forms, constitute a characteristic and diverse element of the flora for warm desert regions of North America (Chapter 3), and the Mojave Desert in particular. More than 85% of herbaceous species found on the Nevada Test Site and adjacent parts of central-southern Nevada are annuals, including those that complete their life cycles in the spring or in the summer, and herbaceous perennials and biennials (53%), which also may be either spring or summer active (Beatley 1976a; Mulroy & Rundel 1977). Whereas within a single week the standing biomass of herbaceous plants is generally small in comparison with that of the shrubs, net primary productivity by annuals during a favorable rain-year may approach 50% of the total for shrubs. Desert annuals tend to have relative high protein concentration in their tissues, but generally low levels of complex structural carbohydrates and toxic secondary compounds, so that these plants are important food resources for desert animals. Nitrogen buildup from annual plants may also serve as an important buffer for ecosystem nitrogen pools in these desert regions.

GENERAL ATTRIBUTES OF MOJAVE DESERT ANNUALS

Germination requirements

The physiological controls of germination in desert annuals have important ecological implications for their establishment and reproductive success. Many of the experimental scientific studies of desert annuals have been conducted on species of the Mojave Desert, beginning with early quantitative studies on germination requirements (Went 1948, 1949, 1955; Went & Westergaard 1949; Juhren, Went, & Phillips 1956).