Abstract
Summary The diurnal and seasonal patterns of climate, shoot water potential, stem photosynthesis (Pn), stem conductance, and stem intercellular CO 2 were measured for two legume shrubs in the southern California desert at two elevations at four seasons of the year. One species (Senna armata) , is restricted to the Mojave desert and was studied at 950 m elevation, while the other species (Caesalpinia virgata) is endemic to the Sonoran desert and was studied at 180 m elevation. The Sonoran desert site was characterized by higher temperatures, higher vapor pressure deficits, and more consistent solar radiation than the Mojave desert site. During the summer, the differences between the microclimates of the two sites were maximal. Both species have high predawn and midday shoot water potentials compared with other desert species, most likely because they have vertical stem orientation and low stem conductance. Stem Pn was positive all year, but Pn max decreased for C. virgata during the summer. Stem temperature, and its impact on vapor pressure deficit, was the most important regulator of stem photosynthesis. Although there were large changes in stem Pn between winter and summer, there was little change in intercellular CO 2 among seasons inferring no change in water use efficiency. Stem Pn most likely provides most of the carbon gain for both species because leaves are small and ephemeral, and stems are present and actively gaining carbon all year.
Published Version
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