Surface Temperatures of Cacti‐‐Influences of Environmental and Morphological Factors
An energy budget model predicted the average stem surface temperatures of a Mam- millaria dioica 9.4 cm tail and a Carnegiea gigantea 5.64 m tail within 1?C of the hourly measured temperatures over 24-h periods. For the ranges of various environmental factors considered in a sensitivity analysis, the average surface temperature of the stem was most influenced by air temper? ature. Greater diurnal extremes of stem temperature occurred at lower wind velocities. Asymmetrical heating of the stem by direct solar irradiation was clearly evident in the measurements and the simulations. The model was used to study the effect of changes in spine coverage, apical pubescence, stem absorptance, tissue thermal properties, plant height, and diameter on stem surface temperatures. In the absence of spines the average surface temperature of Mammillaria became 1 to 2?C cooler at night and 2 to 6?C warmer during the day. The effect of spines on the simulated diurnal temperature variation was even greater for the apical meristem, which had the most dense coverage of spines. A mat of pubescence additionally protected the apical meristem of Carnegiea and Ferocactus acan- thodes, substantially reducing diurnal temperature extremes there. Simulated decreases in the di? ameter of Carnegiea increased the daily range of temperatures of the apical meristem, suggesting that some adaptive optimization of plant diameter may have occurred. Simulations for the 3 species under the same environmental conditions showed that the lowest stem and apical temperatures occurred for Carnegiea, consistent with its restriction to lower (presumably warmer) elevations than Ferocactus and Mammillaria.