as hairy leaves, which increases albedo of the leaf (Billings and Morris 1951) , are the product of evolution and may not be expected to be plastic in response to environmental changes. However, physiological adaptations also occur, such as the "behavioral" adaptation in which desert shrubs use diurnal stomatai control so that metabolic activities (related to open stomates) occur mostly, or exclusively, in times of low evaporitivity (Caldwell et at, 1972, Evenari et at, 1972, Kappen et at. 1972), such as the early morning, or at times of more favorable soil water potential (Syvertsen et at, 1975). In these situations, radiation input could possibly be a limiting factor to primary production (Noy-Meir 1973). If soil moisture conditions were substantially improved under a collector array, and stomatai rhythms were indeed a plastic response, then stomatai opening could occur at more optimal times (i.e., when solar radiation was at maximum levels). With overall reduced solar radiation input in collector arrays, and occasional periods of high radiation input between collectors, stomatai rhythms appear important. Meinel and Meinel (1972) feel that increased moisture conservation under collector arrays may improve the ability of an arid land to support growh of grasses of commercial importance to ranchers. These grasses, perennial bunch grasses, are well known to respond only to adequate soil moisture levels in the warm summer months (Schreiber and Sutter 1972, Mott 1973, Cable 1975). At the present, perennial grasses are scarce or nonexistent in areas earmarked for large solar collection facilities. Thus, without major seeding efforts, it appears that many years of significantly increased soil moisture in the summer would be required for such a change to take place. If grasses were unsuccessful, desert shrubs of economic importance, such as Jojoba or Guayule, could potentially be farmed. If a change in vegetation occurs due to shading, the most immediate vegetation response will be an increase in ephemeral forbs and grasses. Desert annuals are well known to respond with pulses of biomass after significant inputs of precipitation (Went 1949, Noy-Meir 1973, Beatley 1974b, Slade et at, 1975). Furthermore, their tendency to accumulate underneath shrubs (Shreve 1931, Muller 1953, Patten 1975) indicates a certain degree of tolerance to shading, or perhaps a requirement for shading. Even in a wet season, Patten (1975)
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