The saguaro cactus (Carnegiea gigantea) is a long-lived columnar cactus that is among the most well-studied plants in the world. Long-term research indicates that saguaro establishment is generally episodic and strongly influenced by precipitation and temperature. Drought conditions can reduce survivorship of recently germinated saguaros up to 100%. Long-term drought will likely cause saguaro populations to decline as older or vulnerable saguaros die without new cohorts replacing them. Most large-scale saguaro studies have been located on small, non-randomly selected plots that are assumed to be representative of suitable habitat in the area. However, the relative importance of local habitat variability and the coinciding drought impacts on establishing populations remains largely unexplored. We addressed these unknowns by studying saguaro establishment on 36 4-ha plots located randomly across a wide range of elevations and in a variety of habitats (bajadas, foothills, and slopes) in the two disjunct districts of Saguaro National Park in southern Arizona, USA. We examined relationships between drought and saguaro establishment over a >50 year period across and between districts of the park. Further, we modelled saguaro establishment in response to drought in various habitat and soil types.
Recent, severe drought coincided with drastic declines in saguaro establishment that has seen few saguaros establishing in recent years. Overall, there was a negative relationship between establishment and drought in both districts of the park. However, the Tucson Mountain District exhibited steeper declines in saguaro establishment with increasing drought than the Rincon Mountain District did. Saguaro establishment was best explained by the interaction of drought and habitat type. The best model identified bajada and foothill plots as responding somewhat similarly to drought regardless of severity but foothill plots outperformed bajada plots regardless of drought severity. Furthermore, the predicted number of saguaros to establish in bajada or foothill plots dropped to near zero under the most severe drought but remained higher in slope plots, suggesting that the most suitable habitat type for establishing saguaros has shifted during the recent drought. These results reveal that saguaro establishment strongly correlates with drought but that the impact of drought varies with local habitats and their associated physical characteristics. Overall, our study shows that incorporating the range of local variability in how plants may respond to past climate conditions is important for predicting future response to climate change.