Flowering phenology of six woody plants in the northern Sonoran Desert

BowERs, JAIcE E. (U.S. Geological Survey, 1675 W. Anklam Road, Tucson, AZ 85745) AND MARK A. DIMMITr (Arizona-Sonora Desert Museum, 2021 N. Kinney Road, Tucson, AZ 85743). Flowering phenology of six woody plants in the northern Sonoran Desert. Bull. Torrey Bot. Club 121: 215-229. 1994.-Climatic and flowering data from a site in the northern Sonoran Desert of southern Arizona were used to define flowering triggers and developmental requirements for 6 woody plants. These formulations were then used to predict flowering dates at a second northern Sonoran Desert site. It was determined that flowering is triggered by rain in Larrea tridentata (DC.) Cov., Fouquieria splendens Engelm., Encelia farinosa A. Gray, Ambrosia deltoidea (A. Gray) Payne and Acacia constricta Benth., and that flowering is triggered by photoperiod in Cercidium microphyllum (Torr.) Rose & Johnst. The base temperature for floral development in L. tridentata, F. splendens, E. farinosa, A. deltoidea and C. microphyllum is about 10?C. Their mean degree-day requirements range from 414 to 719. Acacia constricta requires 522 degree-days above 15?C. Minimum rainfall triggers varied from 9 mm forAmbrosia to 20 mm for Encelia. Flowering time in C. microphyllum may reflect phylogenetic constraints, while flowering time in F. splendens may be strongly influenced by pollinator availability. Flowering times of the remaining species seem constrained more by climate than by biotic considerations such as phylogeny, seed germination and competition for pollinators.