Simulation of plant temperature and water loss by the desert succulent, Agave deserti.

A simulation model has been developed to describe the thermal relations of individuals of an important group of desert succulents, the agaves, similar to previous modeling efforts on cacti. The model utilizes an energy budget approach to evaluate the effect of various morphological and microclimatic parameters on plant temperature and water loss. For an Agave deserti 0.5 m tall with a basal rosette of 60 leaves, the predicted surface temperatures differed by an average of only about 1°C from those measured in the field in the western Sonoran Desert. Stimulations indicated that leaf and stem temperatures as well as plant water loss were especially sensitive to changes in air temperature. Nocturnal stomatal opening reduced leaf surface temperatures by only 1.4°C. Increasing the shortwave absorptance from the measured value of 0.45 to 0.80 caused the maximum leaf surface temperature to increase 8°C. Stimulated increases in plant size markedly reduced the diurnal range of stem tissue temperatures, and simulated decreases in size reduced the diurnal range in leaf surface temperatures. The small stature of A. utahensis would result in higher minimum leaf temperature and may account for its survival at a cold site in Nevada. Water loss per plant varied approximately as the square of the linear dimensions, which may help explain the decreasing height of agave species with increasing aridity from central Mexico northward. Thermal buffering of the meristematic region in the stem apex by the surrounding massive leaves may also be quite important for the growth and distribution of agaves.