DOI: 10.5061/dryad.rn8pk0p99
title: Saguaro recruitment data obtained by inverse-growth modelling

description: 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.
description: 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.
description: 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.