Effect of Paloverde (Cercidium) Trees on the Radiation Flux at Ground Level in the Sonoran Desert in Winter

Abstract

The radiation flux at winter solstice is reported for desert microenvironments under Cercidium crowns and in open desert. In addition to the net radiation flux and that of the upper and lower hemispheres, hourly radiation temperatures (equivalent black—body temperatures) were measured for soil surfaces, sky, and tree crowns. Radiation temperature—time as a limiting factor for organisms in the sense of resistance time was investigated. Winter environmental modification provided by Cercidium of 4—5 m height is large, and greater than expected. The effective (net) incoming radiation at ground level at midday in the open is more than twice that under the paloverde, and the effective (net) outgoing radiation in the open at night is more than twice as great as that under the tree. For these data, a computer model of the energy budget of a 1—kg hypothetical homoiotherm predicts (i) on the ground surface under the paloverde at night, an effective body—surface temperature of 7 degrees C with an increse of metabolic rate by only 2.6 times its standard rate; this is in sharp contrast to (ii) in the open under the night sky and on a colder ground surface, where the metabolic rate of the same animal exhibiting Newtonian cooling would have to be increased 6.2 times to balance its heat budget. The model also predicts little or no presence in the open on the ground during winter nights in the desert for small homoiotherms greatly less than 1 kg body weight, e.g., weights on the order of 100—200 g or less, unless for rapid forays of short duration.