Growth in Four Species of Sonoran Desert Trees

Trees of moist, warm tropical areas where contrasting seasons are lacking pass from one season to the next with little synchronization of phenologies among species. This is in direct contrast with trees of temperate regions where conditions during one or more seasons drastically curtail growth phenomiiena. Many detailed studies of tree growth in temperate regioiis (see bibliography by Kozlowski 1956) have slhown (lefinite periods of growth quiescenice that can be correlated with insufficient soil moisture, unfavorable temperatures, variations in duration of darkness, or some combination of these factors. Studies of tree growth in arid subtropical regions (Phillips 1927, MacDougal 1920, 1936, 1938, Fahn 1955, 1958) are few in nuumber, and relatively little is kno,wn of the periodicity of stemradius change where drouth periods are often sharply defined and( where winter temperatures are mild. This paper presents the results of a study of stemi-ra(litus chatnges and season of leaf, flower, anid fruit dlevelopmeint in four species of small leguminous trees, eaclh of whiclh has a widlespread distribution in the arid subtropical portion of North America referred to as the Sonoran Desert (sensu Shreve 1951). The four species studied were Olneva tesota A. Gray (ironwood), Prosopis juliflora (Swartz) DC var. velittina (Woot.) Sarg. (velvet mesquite), Cercidin ni viicrophyliumtut (Torr. ) Rose & Johnston (foothill palo-verde), and C. floriduiwPi Beluth. (blue palo-verde). Trees, mostly legumes, occur throtughout much of the Sonorani Desert anid mlay be founid under regimes of predominanitlv winter or summer precipitation or with precipitation divided betweenl winiter and sumiimer. Temperature conditions vary as well. In the north, winter minima frequently fall below 320 F (00 C), while a gradual amelioration of wiinter temperatures toward the south produces large areas of the deserts that are often frost-free for several consecutive years. Tree growth was studied in the northeasterln portioln of the Sonoraii Desert in the viciniity of TuLcson, Arizona, where the average total annual precipitationi is al)out 11 inches (280 mm) and is abotut eqtually divided between winter and summer. W\Vinter miiinimial temperatures frequently fall below freezing here. The trees selected for study vary 1 Presenit address: U.S. Geological Survey, Institute of Atmospheric Physics, Uiniversity of Arizolna, Tucson, Ariz. considerably in their growth habits and include an evergreen species (0. tesota) and species with foliage appearing only during periods of favorable combinations of moisture and temperature (C. floridum, C. microphyllum, and P. juliflora var. velutina). The stems of two of the species (C. floridum and C. microphyllum) have smooth, greeni bark apparently allowing for a high level of photosyntlhetic activity even in the absence of leaves. The topographic preference of these trees varies throughout their ranges but within the study area two of the species (P. juliflora. var. velutina and C. floridum) are found typically along the margins of intermittent stream courses whereas the other two occur on the adjacent uplands. This study was aided by funds from National Science Foundation Grant G-3218. I should like to express my appreciation to Mr. S. Clark Martin of the U.S. Forest Service, who made available the precipitationi data. I am indebted to Dr. R. Daubenmire, Dr. H. C. Fritts, and Dr. Henry Tucker for reviewing the manuscript.