Plant and Animal Response The effect of a more mesic environment in an arid region should be a significant increase in photosynthetic rate, and thus primary production. Because of reduced evaporative demand during periods of greatest water stress, and increases in water vapor pressure, plants should respond by maintaining greater stomatai aperture for longer periods of time than would be possible in the open desert. Increases of stomatai aperture in sheltered crops are well documented (Rosenberg 1966a, Brown and Rosenberg 1971, Miller et at. 1973, 1974), as well as increases in water use efficiency (Rosenberg 1966a, b, Rosenberg et at. 1967, Miller et at. 1973). A combination of increased water use efficiency and increased soil moisture could result in substantial increases in primary production. Furthermore, nocturnal respiration rates may be lower in sheltered areas due to cooler leaf temperatures (Miller et at. 1973, Rosenbert 1974). A combination of cooler nighttime temperatures and increased daytime transpiration rate (and thus increased heat loss) would result in the cooler leaf temperatures. It has been suggested that, at low wind speeds, the photosynthetic rate in a sheltered field may be decreased because of a shortage of carbon dioxide in the air surrounding the leaves (Lemon 1970). In a thorough study of this, Brown and Rosenberg (1972) concluded that if decreases occurred they would be slight, and may limit photosynthesis only in dense crop canopies. In the desert, with its sparse vegetation, carbon dioxide is not considered a limiting factor to plant production. In conclusion, the influence on plant growth of reduced soil and atmospheric moisture stress and reduced mechanical motion should be beneficial in the water stressed desert habitat. The response of animal populations to a more mesic environment with less advective water loss potential should increase the desirability of the habitat, especially in warm, dry seasons. Since certain desert species are highly adapted to extremely xeric conditions (Edney 1967), shifts in relative abundance of animals may occur in mesic microhabitats. Increases in plant production will also be conducive to increase in animal population numbers, as has been previously discussed.
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