A test of the relationship between seasonal rainfall and saguaro cacti branching patterns

Reproductive output, as well as photosynthetically active radiation interception and CO₂ uptake, increase as saguaro cacti Carnegiea gigantea (Engelm.) Britt. and Rose branch, and branching increases with increasing moisture. The Sonoran Desert experiences distinct summer and winter precipitation regimes that vary in both geography and scale. Many aspects of saguaro ecology are known to depend on the summer rains, which has resulted in an emphasis on summer rains in the literature. Similarly, branching studies have been limited geographically to areas that receive relatively high amounts of summer rainfall. These studies, therefore, attribute branching patterns to the summer (or possibly annual) rains, and conclusions reflect the summer precipitation bias.
Environmental variability in space was explored in the present study to investigate saguaro branching patterns. I collected height and branching data in thirty saguaro populations across their American range. Stepwise regression was used to determine which climate, vegetation and soil variables best predict branching. Contrary to the literature, this study found that winter precipitation, particularly from January to April, was the best predictor of branching, not summer or annual rain. Surprisingly, the relationship between the summer monsoons (July and August precipitation) and branching was negative. This is likely due to the fact that summer and winter rainfall patterns are geographically distinct. Winter precipitation appears to play a key role in branching, and thus in seed production. This suggests that saguaros benefit from moisture during the winter, possibly utilizing cold-season rains for increasing their reproductive output through branching, and challenging the view that the summer rains dominate virtually every aspect of the saguaro life-cycle, and creating a more balanced view of saguaro ecology.