Morphology, Surface Temperatures, and Northern Limits of Columnar Cacti in the Sonoran Desert

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Abstract
Interspecific morphological differences and intraspecific morphological changes with latitude were evaluated to help examine the distributional ranges of Carnegiea gigantea, Lemaireocereus thurberi, Lophocereus schottii, Pachycereus pecten—aboriginum, and P. pringlei in the Sonoran Desert (USA and Mexico). A computer model, which predicted the average surface temperature of the stem within 1°C of that measured hourly throughout a 24—h period, was particularly useful in studying the thermal relations of the stem apex, where the lowest surface temperature occurred. Simulated increases in stem diameter raised the minimum apical temperature for C. gigantea and may help account for the extension of its range to higher latitudes than the other species studied. However, diameter increases led to a slight decrease in minimum apical temperatures for Lophocereus schottii. The immature stems of L. schottii are morphologically distinct from the mature stems which caused minimum apical temperatures to be 1.6°C lower for the immature stems under given environmental conditions; thus, freezing damage to the immature stems could limit the northward extension of the range of this species. As the apical pubescence in the simulations was increased up to the normal amount (10 mm), the minimum apical temperature for the stem of C. gigantea increased 2.4°C. Simulated increases in spine shading of the apex also raised the minimum apical temperatures, again indicating the influence of morphological features on the temperature of the meristematic region. Under the same environmental conditions the minimum apical temperatures were 7.7°C, 5.9°C, and 3.9°C for C. gigantea, Lemaireocereus thurberi, and immature stems of Lophocereus schottii, respectively, which is the same relative order as their northernmost limit (34°56'N, 32°38'N, and 31°55'N respectively). The northern limit for these three species, but not for Pachycereus pectenaboriginum and P. pringlei, may well be determined by the low temperatures occurring at the stem apex.

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