The average number of arms on a saguaro (Carnegiea gigantea) increases with saguaro height, but there is considerable variation among saguaros for any given height. This variation has been largely attributed to variation in water availability among individual plants. To test this hypothesis, I recorded the height and number of arms of a sample of 425 saguaros within Organ Pipe Cactus National Monument along with values of three variables that should affect arm production through their effect on water availability: (1) the number of neighboring saguaros under the same nurse tree canopy, (2) the presence of a wash within 5 m, and (3) the presence and identity of any living or dead plant associate. Deviation from the number of arms expected for a given saguaro height was negatively related to the number of saguaro neighbors and positively related to the presence of a nearby wash. Saguaros growing with live associates also had fewer arms than those growing alone, and saguaros growing with dead associates had more arms than those growing alone. Each of these results is consistent with the idea that arm production is affected by water availability.
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