The geography of saguaro cacti (Carnegiea gigantea) in Arizona

摘要

Most studies on saguaro (Carnegiea gigantea (Engelm.) Britt. and Rose) ecology have been conducted at Saguaro National Park East, Saguaro National Park West, and at Organ Pipe Cactus National Monument. As a result, our understanding of saguaro ecology is greatly limited to the environmental conditions that occur specifically at these three locales. The purpose of this research was to analyze saguaro populations throughout their Arizona range and to make observations about saguaro ecology under a variety of conditions to determine which variables control saguaro populations, and which variables are coincidental to the location where they are studied. Thirty populations are studied in Arizona, and vegetation and climate data were collected. Three findings result from this study: (1) Higher proportions of juvenile saguaros were better predicted by lower maximum summer temperatures than by increased summer moisture. The importance of summer rains has been emphasized and, and in fact, summer precipitation limits the saguaro's range to the west. However, once ample moisture is available, lower maximum temperatures that decrease desiccation rates between rain events may explain this relationship. (2) Branching, which directly controls reproductive output, is controlled by winter and not summer or annual precipitation as has been suggested by Steenbergh and Lowe. Winter rains are more predictable and non-monsoonal precipitation falls in greater quantities, and with cooler temperatures, evaporates more slowly. Thus, it is not surprising that winter precipitation is a controlling factor of saguaro distribution. (3) Saguaro stem diameter is thought to be primarily controlled by cold winter temperatures that help protect the apical meristem from freezing temperatures. This study found that any relationship of stem diameter with temperatures, although undoubtedly beneficial, is coincidental. Winter or non-monsoonal precipitation was again the best predictor of saguaro stem diameter. This further suggests that winter precipitation is critical in shaping saguaro populations.