Branch direction in Carnegiea gigantea (Cactaceae): Regional patterns and the effect of nurse plants

Abstract. Branch direction in Carnegiea gigantea (saguaro) cacti affects PAR interception and CO₂ uptake, and may influence reception of photomorphogenic cues. Branch direction data were collected in 29 saguaro populations over Arizona to determine if the southerly bias observed in previous studies at two locales can be generalized to the American range of the saguaro, and to determine the effect of shading by nurse plants on branch direction. Locally observed S bias from previous studies can be extended and generalized to include the American saguaro range. Branches were statistically uniform across environmental variability regarding temperature and precipitation. The S bias was not found in saguaros shaded by large nurse plants, supporting the assertion that branch direction occurs, in part, to maximize receipt of insolation.