DOI: 10.5061/dryad.hdr7sqvjb
title: Non-native plant removal and high rainfall years promote post-fire recovery of Artemisia californica in southern California sage scrub

description: Non-native plant invasions, changes in fire regime, and increasing drought
 stress all pose important threats to biodiverse mediterranean-climate
 shrublands. These factors can also interact, with fire and drought
 potentially creating opportunities for non-native species to establish
 dominance before native shrubs recover. We carried out post-fire
 demographic monitoring of the common native shrub Artemisia californica in
 a southern California sage scrub fragment for 7 years, including several
 with very low rainfall. Experimental removals of non-native plants were
 included for the first 4 years. We quantified A. californica post-fire
 crown resprouting and seedling emergence, and tested effects of
 precipitation, non-native plants, and their interactions on seedling and
 adult survival. Only 7 A. californica were confirmed as resprouts; almost
 all individuals established after the fire from seedlings, with 90% of
 emergence occurring in the second growing year after fire (spring 2015).
 Higher spring precipitation increased both adult and seedling survival.
 Non-native grasses and forbs rapidly recolonized control plots, but the
 removal treatment reduced non-native cover by nearly 60%. For seedlings,
 non-native removal reduced the probability of dropping leaves by start of
 summer drought and increased survival both directly and through positive
 interactions with rainfall. Non-native removal also reduced mortality in
 smaller adult plants. By 2020, mean A. californica canopy area was nearly
 four times greater in non-native removal plots. These findings reinforce
 the high vulnerability of sage scrub habitat to post-fire loss of shrub
 cover and potential type conversion, particularly with increasing drought
 frequency and in stands and species with limited crown resprouting. Yet
 they also illustrate the potential for targeted management of non-natives
 immediately after fire to promote recovery of native shrubs in this
 increasingly endangered community.
description: This study was conducted at the Robert J. Bernard Field Station
 in Claremont, CA, USA. This reserve supports fragments of intact sage
 scrub on 34 hectares embedded in a suburban landscape. In September, 2013
 (late dry season),  an accidental fire burned 6.9 hectares containing two
 patches of sage scrub separated by a road We established 12 plots spanning
 the west to east axis of burned sage scrub area, each 10 m by 10 m. Plots
 were separated by a 5 m wide buffer. Eight plots were in the western and
 four plots in the eastern patch (Fig 1). Treatments were paired to control
 for the west to east pre-fire gradient in shrub and non-native grass
 cover. We randomly assigned one plot to each treatment within neighboring
 pairs, starting with the westmost boundary. For the
 first four years after fire (spring 2014-2017), non-native grasses and
 forbs were hand weeded from plots assigned to the removal treatment.
 Removal began each year in mid-January and continued until collection of
 cover data started in the last week of March. Once per week during that
 time period, a team of two to three volunteers with experience in basic
 identification of local weedy species spent 20 minutes per treated plot
 removing non-natives. All non-native species were included, except that
 <i>Erodium</i> spp. proved difficult to control and were
 therefore targeted less. We measured vegetative cover in all plots during
 years when treatments were applied (2014-2017), using point-intercept
 sampling on a grid. Nine transects were established from west to east in
 each plot, spaced at 1 m intervals from south to north. Along each
 transect, we sampled points at 0.5 m intervals (<i>N</i> = 162
 per plot) between the last week in March and the first week in May. The
 identities of all species. touching a straight line running up and down
 from the soil surface were recorded for each sample point (foliar cover);
 as a result, total cover values can exceed 100%.
 Demographic data for <i>A. californica</i> were
 collected from 2014 to 2020 at an annual census between June 14 and June
 30, early in summer drought. In the first census after fire (2014), we
 individually tagged and mapped the locations of all <i>A.
 californica </i>in 10 plots. For one control and one removal plot
 adjacent to each other (6 and 7), high numbers of plants precluded tagging
 all individuals. All plants in the western halves of these two plots were
 tagged in June 2014, and any surviving plants in the eastern halves in
 November 2014. Plants were treated as distinct individuals if their stem
 bases were separate where entering the soil. At every annual census, leaf
 condition of each plant was classified as either good, if leaves were
 still present and appeared to be photosynthetically active, or deciduous,
 if the plant had begun to shed leaves. We quantified size for all
 individuals as canopy volume, by recording plant height, plant diameter on
 the longest axis, and a second diameter perpendicular to the
 first. All seedlings also were assigned individual tag
 numbers, mapped, and measured in most years after 2014 (2016-2018, 2020).
 In 2019, we counted all seedlings and mapped, tagged, and measured up to 6
 per plot. Subsampling was used to assess seedling density and sizes in
 2015, because of high emergence rates. We searched for seedlings in 2015
 using a grid of points spaced at 2 m intervals within each plot
 (<i>N</i> = 16). At each sample point, seedlings within a 0.5
 m radius were counted, scored for condition, and marked with a twist tie
 loop help in place by a plastic fruit fork. The seedling closest to the
 sampling point was measured. If no seedlings were found within 0.5 m, we
 repeated the same protocol at a 1 m radius. We then systematically
 searched any plot where no seedlings were detected at the sampling points.
 If fewer than five seedlings were measured from the sampling grid,
 additional sizes were recorded until reaching at least <i>N
 </i>= 5 per plot, or until all seedlings were measured. Seedling
 density data were collected in the same June census used for adult
 plants. In 2015, we also marked seedlings in a subset
 of plots earlier in the spring to assess survivorship from germination to
 the June census. The two westernmost plots (one control, one removal) were
 surveyed between February 20 and March 1, and the next pair of plots
 between March 12 and April 3. A second control plot was added to the early
 April tagging, to increase sample sizes for control seedlings.
 In some cases, seedlings were missed and appeared as untagged
 individuals in the following year. We used size records from known
 seedlings and first year plants to create criteria for assigning untagged
 individuals to a recruitment cohort. Untagged plants that were either less
 than 20 cm in height or smaller than 3000 cm<sup>3</sup> in
 canopy volume were assumed to be seedlings. This classification system
 correctly predicted 98.4% of records for confirmed seedlings
 (<i>N</i> = 128), and 97.0% of those for confirmed one-year
 old plants (<i>N </i>= 34).
 <i>Artemisia californica</i> likely resprout from
 aboveground organs [36], but definitively identifying resprouting
 individuals in the first year after fire would have required excavating
 roots [41]. We classified plants tagged in June 2014 as likely seedlings
 unless they exceeded the size thresholds used in other years or could be
 confirmed as resprouts by the presence of dead stems. Keeley and Keely
 (1984) report May sizes for sage scrub shrubs that resprouted after fire
 well over our criteria (e.g., mean heights of 50-52 cm), although these
 values were combined across species.
description: This Dryad entry includes
 the core data files for Thomson et al.  2021, Non-native plant removal and
 high rainfall years promote post-fire recovery of Artemisia californica in
 southern California sage scrub, PLOS ONE. See also supporting Tables
 S1-S4. The files
 posted on Dryad include: <b>Adult survival
 records.csv</b> This file contains the
 survivorship records for all adult (one year old or greater) plants
 observed over June to June annual transitions. The data fields/columns are
 as follows: Year_
 record: The first year (June census) of the transition. For example,
 Year_record=2014 indicates a record for survival from June 2014 to June
 2015. Year_recruited: The year (June)
 in which the plant first recruited as a seedling. Plot: Plot identity
 (1-12). Treat:
 Treatment (R=non-native removal, C=control). All records from 2018-2019
 were designated controls, because the experimental removals had
 ended. Tag:
 Individual plant tag number. Surv: Survival (0=died,
 1=survived) Cvol_cm3: Plant size, measured by
 canopy volume in cm<sup>3</sup>. Growyear: The growing (rainfall)
 year preceding the observed transition. The rainfall year extends from
 onset of the winter rainy season in fall until the beginning of summer
 drought in June. Grow_year=2013 indicates the growing year beginning in
 fall of 2013 and ending in June of 2014. Precipmm: The total spring
 rainfall (January to June) for the growing year preceding the
 transition. Norm_precip: Total spring
 rainfall normalized as the percent deviation from the average over the
 time period 2000-2020. <b>Seedling survival
 records.csv</b> This file contains the
 survivorship records for all seedlings (plants tagged in the June census
 after their emergence) until the following year (next June census). The
 data fields/columns are as follows: Year_ record: The spring of the
 growing year in which seedlings emerged. Year_record=2014 indicates the
 seedlings emerged in spring of the 2013-2014 growing year and were checked
 for survival in June of 2015. Plot: Plot identity
 (1-12). Treat:
 Treatment (R=non-native removal, C=control). All records from 2018-2019
 were designated controls, because the experimental removals had
 ended. <b>Seedling emergence
 data.csv</b> Year_ record: The spring of the
 growing year in which seedlings emerged. Year_record=2014 indicates the
 seedlings emerged in spring of the 2013-2014 growing year. Plot: Plot identity
 (1-12). Treat:
 Treatment (R=non-native removal, C=control). Note that this file labels
 all plots originally assigned to the non-native removal treatment as R,
 even in 2018-2019 when non-natives were no longer being removed.
 Density_m2: The density of
 seedlings per m<sup>2</sup>. For years (2015 and 2019) and
 plots within those years counted on a grid system, this value is the mean
 across all grid points. Sqr_root_density: The
 square-root-transformed density per m<sup>2</sup>.