Saguaro recruitment data obtained by inverse-growth modelling

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.