Temporal and Habitat-Based Growth Rate Variability in the Saguaro Cactus (Carnegiea gigantea)

The estimation of year of germination based on saguaro cactus (Carnegiea gigantea) growth rates is essential for understanding population dynamics. To assess how habitat and climate interact to influence growth rate, we resurveyed 614 saguaro cacti on 11 historical study plots in Saguaro National Park (SNP), including the plots where classic saguaro age–height models were developed. We classified plots into four groups based on their topographic position (bajada or rocky slope) and park district (east or west), compared actual to predicted height distributions, assessed the distributions for (dis)similarity using the Jensen-Shannon distance, tested for significance with Fisher's exact test, and calculated growth rate adjustment factors with the Drezner method. We found that saguaro growth rates slowed significantly for three of the four combinations of topographic position and district from 1975 to 2014. Observed height class distributions differed significantly from the predicted for rocky slopes but not for bajada habitats. Variability in saguaro growth rates among populations in different habitats over a short distance through time may have scientific and ecological implications, including decreasing the accuracy of population age calculations and delaying the onset of flowering and branching. We encourage SNP to continue the ongoing long-term studies to quantify the resulting effects of climate change and to incorporate the results of this study into interpretative programs and literature.