Ten years (2006-2016) of annual growth rates for 13 of saguaro populations in the Sonoran Desert

Premise of the study: The saguaro cactus is an iconic species of the Sonoran Desert. Its individual growth rates have been investigated for over one hundred years. Its growth dynamics have been studied using phenomenological models intended to estimate growth, but not to understand the underlying biological processes. Most studies have suggested summer rainfall as the sole factor determining saguaro growth overlooking the influence of other factors related to the process of growth.
Methods: We analyzed the annual growth rates for 13 saguaro populations in the Sonoran Desert using non-linear models which are better suited to analyze growth since they consider the fact that maximum growth rates diminish just before the onset of reproduction and related model parameters to the local climate. Key results: We found that the most parsimonious model was the Ricker function that described growth considering its decline with age. We also found that the variance in temperature, rather than precipitation, was more closely related to growth. Higher variance in temperature at the beginning of the warm season was detrimental to saguaro growth. Conclusions: Simple non-linear equations model growth rate with biologically interpretable parameters related to climate factors. As the temperature is projected to increase in both mean and variance by climate change, the population dynamics of this iconic cactus are likely to be affected.