DOI: 10.5061/dryad.pk0p2ngz6
title: Data from: Contrasting demographic processes underlie uphill shifts in a desert ecosystem

description: Climate change is projected to cause extensive plant range shifts, and in
 many cases such shifts already are underway. Most long-term studies of
 range shifts measure emergent changes in species distributions but not the
 underlying demographic patterns that shape them. To better understand
 species’ elevational range shifts and their underlying demographic
 processes, we use the powerful approach of rephotography, comparing
 historical (1978-82) and modern (2015-16) photographs taken along a 1000 m
 elevational gradient in theColorado Desert of Southern California. This
 approach allowed us to track demographic outcomes for 4263 individual
 plants of 11 long-lived, perennial species over the past ~36 years. All
 species showed an upward shift in mean elevation (average = 45 m),
 consistent with observed increasing temperature and severe drought in the
 region. We found that varying demographic processes underlaid these
 elevational shifts, with some species showing higher recruitment and some
 showing higher survival with increasing elevation. Species with faster
 life history rates (higher background recruitment and mortality rates)
 underwent larger elevational shifts. Our findings emphasize the importance
 of demography and life history in shaping range shift responses and future
 community composition, as well as the sensitivity of desert systems to
 climate change despite the typical ‘slow motion’ population dynamics of
 perennial desert plants.
description: We utilized photos originally taken by Dr. Wilbur Mayhew between
 1977 and 1982 (Mayhew 1981), which we digitized from 35 mm slides stored
 at Philip L. Boyd Deep Canyon Desert Research Center
 (doi:10.21973/N3V66D). We relocated permanently marked sites where
 historical photos had been taken and rephotographed them using a Canon 5D
 Mark II camera and tripod in 2015 and 2016. We took one additional set of
 photos in April 2017 after the end of a multi-year drought, so that we
 could distinguish dormant from dead individuals of two drought-deciduous
 species (brittlebush, <em>Encelia farinosa</em> and white
 bursage, <em>Ambrosia dumosa</em>). We approximated the
 original view of the original photos as closely as possible in modern
 photos. For each photo view, we chose a single historical and modern photo
 for analysis based on resolution, contrast and coloration. The mean
 timespan between paired historical and modern photos was 36 years. We
 perfected the alignment between the paired historical and modern photos in
 Photoshop by making one photo semi-transparent, then rotating and resizing
 it while maintaining original aspect ratios.
 <em>Data extraction</em> We
 extracted data on 11 perennial species that appeared in 5+ sites. We
 extracted data from the photos in ArcGIS, arranging the paired photos as
 map layers. We created polygons to delimit a survey area close enough to
 the camera to identify species; these polygons serve as the “sites” in our
 subsequent analysis. In some cases, we collected data on larger-bodied or
 particularly conspicuous species, such as ocotillo (<em>Fouquieria
 splendens</em>) and creosote (<em>Larrea
 tridentata</em>), in a larger area including locations farther from
 the camera than for smaller, less conspicuous species. We recorded whether
 each plant underwent recruitment (absent historical, alive modern),
 mortality (alive historical, dead modern) or survival (alive both). We
 excluded plants that were dead in the historical period or with main stems
 outside the site polygon. In some cases we consulted other historical and
 modern photos of the same site to determine species identity or assess
 whether an individual was alive. We counted and measured clusters of agave
 (<em>Agave deserti</em>) and Mojave yucca (<em>Yucca
 schidigera</em>) as single individuals. Rarely, we may have
 misidentified pygmy cedar (<em>Peucephyllum schottii</em>) for
 creosote where these species co-occur on steep slopes, since they have
 similar morphology and are difficult to distinguish from a
 distance. We measured individual relative change in
 plant size by measuring the height (perpendicular to the ground) and width
 (the largest horizontal extent of the plant perpendicular to the camera,
 i.e. canopy width) of surviving plants in both time periods, using the
 ruler tool in ArcGIS and focusing on woody biomass. When dead agave
 rosettes were surrounded by live rosettes, we did not include the width
 that was dead if it was &gt;20% the total width. We calculated the
 relative change in height of each plant as
 (<em>H</em><sub>1</sub>–<em>H</em><sub>0</sub>) / <em>H</em><sub>0</sub>, where <em>H</em>  indicates plant height and the subscripts <sub>0</sub> and <sub>1</sub> indicate the historical and modern period, respectively. We used an equivalent equation for relative change in width. For some species at some sites, we could not track the fate of individuals between the two time periods. This most often occurred for narrow-bodied and relatively short-lived species (e.g. teddy bear cholla, <em>Cylindropuntia bigelovii</em>) in photo pairs that were difficult to perfectly align, thereby making it difficult to tell whether plants either survived, or died and were replaced by recruits. It also occurred when a large plant died and a new plant “appeared” in a spot that was previously hidden, such that we were unable to determine whether the second plant was a recruit or a surviving plant. We therefore designated two site types for each species: “trackable” sites – those where we could track the fate of at least one third of individuals of a given species over time, and “count-only” sites – those where we could track fewer than one third of individuals, and instead only counted individuals. Count-only sites were retained for analyses of mean elevation shifts but not demographic rates.  <em>Geophysical data</em> We used Google Earth Pro “ground level view” to draw polygons matching the extent of the site polygons outlined in the photos. To do so, we first “stood” at the camera’s locality and angle, then used corresponding features (e.g. washes, large creosote, hills) to find the exact site, and finally dropped pins to mark polygon vertices. We used these polygons to extract data on each site’s size, as well as its mean elevation, aspect, slope and annual solar radiation (“insolation”) using USGS NED Contiguous US 1/3 arc-second digital elevation model (2013) in ArcGIS. We took the cosine of aspect to create linear values ranging from -1 (South) to 1 (North). <em>Additional details </em> Additional details on how these data were collected and processed can be found in the Methods and Supplementary Materials of Skikne et al. 2024. Contrasting demographic processes underlie uphill shifts in a desert ecosystem.
description: # Contrasting demographic processes underlie uphill shifts in a desert
 ecosystem
 [https://doi.org/10.5061/dryad.pk0p2ngz6](https://doi.org/10.5061/dryad.pk0p2ngz6) ## Description of the data and file structure ### Files and variables #### File: individ\_plant\_outcomes.csv **Description:** Data on demographic outcomes for individual plants extracted from paired historical-modern photos taken along the Deep Canyon Transect in Riverside County, California.  ##### Variables * site: site name. Map of sites can be found in the Supplementary Materials of Skikne et al. 2024. Contrasting demographic processes underlie uphill shifts in a desert ecosystem. *Ecology*. * spp: species * extant_t1: whether the plant existed in the historical photo (1) or not (0) * alive_t1: whether the plant was alive in the historical photo (1) or not (0) * height_t1: height of the plant in the historical photo (unitless, see below) * width_t1: width of the plant in the historical photo (unitless, see below) * extant_t2: whether the plant existed in the modern photo (1) or not (0) * alive_t2: whether the plant was alive in the modern photo (1) or not (0)  * height_t2: height of the plant in the modern photo (unitless, see below) * width_t2: width of the plant in the modern photo (unitless, see below) * outcome: whether the plant recruited, survived, or died between the two periods * d.height: relative change in height between the two periods, calculated as *(height_t2 – height_t1) / height_t1* * d.width: relative change in width between the two periods, calculated as *(width_t2 – width_t1) / width_t1* * area: area of the site (km2) * slope: mean slope of the site (degrees) * elev: mean elevation of the site (m) * insol: mean insolation of the site (Wh/m2) * asp: mean cosine(aspect) of the site, where aspect is in radians. This creates values ranging from -1 (South) to 1 (North). * date_t1: date of historical photo * date_t2: date of modern photo * trackable_site: whether this site was "trackable" or "count-only" for this species.  See Methods of Skikne et al. 2024. Contrasting demographic processes underlie uphill shifts in a desert ecosystem. *Ecology*. Notes:  * NA indicates that we did not collect the data, which could happen for a variety of potential reasons, including plants that were blocked from view in one time period, or cases where we could not tell if a plant was alive or dead. * Height and widths were measured using the ArcGIS ruler tool and unitless for our purposes.