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BOWERS, J. E. AND R. M. TURNER. (U. S. Geological Survey, Tucson, AZ 85745). Dieback and episodic mortality of Cercidium microphyllum (foothill paloverde), a dominant Sonoran Desert tree. J. Torrey Bet, Sec. 128:000-000. 2001. - Past and current dieback of Cercidium microphyllum a dominant, drought-deciduous tree in the Sonoran Desert, was investigated at Tumamoc Hill. Tucson, Arizona, USA. Logistic regression predicted that the odds of a Ceracidium plant being alive should decrease with increasing circumference, association with the columnar cactus Carnegiea gigantea, and occurrence on steep slopes. Slope azimuth. parasitization by Phoradendron californicum, and distance to nearest Ceracidium within 5 m did not significantly affect the odds of survival. Ceracidium was a source of background mortality rather than a primary cause of dieback. Of the >1,000 living and dead plants sampled, 7.7% had died within the past 5 to 7 years. An additional 12.8% died in the more distant past. Diebacks tended to occur during severe deficits in annual, especially summer, min. More than half of the dead plants in the sample were greater than or equal to 50 cm in girth. In current and past diebacks on Tumamoc Hill, it seems likely that severe drought interacted with natural senescence of an aging population. weakening large, old trees and hastening their deaths.