Each year, an individual mature large saguaro cactus produces about one million seeds in attractive juicy fruits that lure seed predators and seed dispersers in a three-month feast. From the million seeds produced, however, only a few will persist into mature saguaros. A century of research on saguaro population dynamics has led to the conclusion that saguaro recruitment is an episodic event that depends on the convergence of suitable conditions for survival during the critical early stages. Because most data have been collected in Arizona, particularly in the surroundings of Tucson, most research has relied on a limited amount of environmental variation. In this study, we upscaled this knowledge on saguaro recruitment to a regional scale with a new method that used the inverse-growth modeling of 1,487 saguaros belonging to 13 populations in a latitudinal gradient ranging from arid desert to tropical thornscrub forest in Sonora, Mexico. Using generalized linear and additive mixed models, we created two 110-year-long saguaro recruitment curves: one driven only by previous size, and the second driven by size, drought, and soil structure. We found evidence that saguaro recruitment is indeed episodic with periodicities of 20–30 years possibly related to strong El Niño Southern Oscillation events. Our results suggest that saguaros rely on multidecadal periodic pulses of good beneficial years to incorporate new individuals into their populations. Inverse-growth modelling can be used in a wide variety of plant species to study their recruitment dynamics.

We sampled contiguous 20 x 20 m quadrats measuring every living saguaro until reaching 100 individuals. We ended up with differently sized plots depending on the density of saguaros in each site. Plots ranged from 0.48 ha to 21.36 ha. Larger plots corresponded to sites with wider spacing between individuals while smaller plots corresponded to sites with more aggregated individuals. Each saguaro was tagged with a unique ID number attached to its main stem and georeferenced for subsequent surveys. On each site, we also carefully searched for saguaro seedlings in the whole plot, searching thoroughly under adult saguaros, tree canopies, and rocky outcrops. During the winter seasons of 2014, 2015 and 2016, we recorded the heights (cm) of all individuals and the incorporation of new individuals. Height of saguaros taller than 200 cm were measured from a marked height (about 130 cm) using a telescoping rod with a terminal crossbar (Crain Enterprises, Inc., Illinois). Smaller individuals were measured with a tape measure to the nearest cm.

Size and SizeNext variables are related to the transition from time at t (Size) to time at t+1 (SizeNext). They are saguaro heights of the main stem measured in cm.

Sites are abbreviated in the dataset while not in the manuscript.

SWS is the abbreviation of "soil water content at saturation" and is a proportion per individual per site.

PDSI_Anual is the mean monthly "Palmer drought severity index" extracted for each site and each year.