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

Abstract

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.

High and low temperature tolerances and their relationships to distribution of agaves

“…Simulations of the effeet of leaf morphology on leaf surface temperature were made using a thermal model previously validated for A. deserti (Woodhouse et al, 1983), Particular emphasis was placed on leaf length and volume. Also eonsidered was the influence of leaf shortwave absorptance, whieh was determined in the field using an integrating sphere radiometer (Dunkle et al, 1960;Smith & Nobel, 1977),…”

“…Figure 2 shows the effect of leaf volume per unit length on the diurnal maximum and minimum leaf surface temperatures, as determined from simulations in whieh the leaf thiekness was varied. When the leaf thickness was inereased from TU of that for the A. deserti used to validate the model (which had a volume per unit length of 8,6 cm^) to 10 times its leaf thickness, the maximum average surfaee temperature rose markedly from 33,4°C to 50,8°C and the minimum fell slightly from 8,6°C to 7,0°C (Fig, 2), Although all the terms in the energy budget affect surface temperature and the various parts of the leaf interaet (Nobel, 1978;Woodhouse et al, 1983), the shapes of the curves refleet the domination of eonvective exchange for the thin leaves and heat storage for the thick leaves.…”

“…Interspecific variation for mature plants was more than 10-fold in leaf length and approximately 1000-fold in leaf volume (Table 1), In general, speeies from more northern latitudes exhibited smaller leaf volumes (e,g,, A. utahensis, A. schottii), while the species with the largest leaf volumes {A. americana, A. bovicornuta, and A. rhodacanttia) are of more southern distribution (Gentry, 1982), Agave deserti, for which the computer model predicting surface temperature was validated (Woodhouse et at., 1983), was intermediate in size. Leaf absorptance for shortwave irradiation was considerably less variable than leaf size, ranging from 0,49 to 0,65, The two species which exhibited the lowest absorptances {A. deserti and A. lecheguilla) were the only species occurring in open desert scrub habitats.…”

“…Previous simulations (Woodhouse et al, 1983) examined the effect of parameters such as leaf length and absorptance on the extremes of leaf surface temperature. However, the effect of average leaf volume (or mass) per unit length was not tested, but it eould affect leaf temperatures and it varied substantially for the species eonsidered here (Table 1), For instance, volume per unit length varied from 0,3 cm^ (0,3 cm^/cm) for A. schottii to 71 cm^ for A. americana.…”

“…In contrast to the cacti, the thermal relations of agaves have received little attention, and tissue tolerances to extreme temperatures are not known. Modelling efforts predieting the surface temperatures of cacti (Lewis & Nobel, 1977;Nobel, 1978) have recently been extended to the eonsiderably more complex geometry of agaves (Woodhouse, Williams & Nobel, 1983), These models differ from typieal leaf energy budget models in that conduction and storage terms can be of considerable importance for the relatively massive cacti and agaves, resulting in plant surface temperatures that diverge considerably from air temperature under eertain eonditions. As a result, interspecific morphologieal differenees among agaves, such as rosette size, leaf number, and leaf volume per unit length, may significantly infiuence stem or leaf temperatures and hence the range boundaries of individual speeies.…”