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Non-native plant removal and high rainfall years promote post-fire recovery of Artemisia californica in southern California sage scrub

Citation

Thomson, Diane; Meyer III, Wallace (2021), Non-native plant removal and high rainfall years promote post-fire recovery of Artemisia californica in southern California sage scrub, Dryad, Dataset, https://doi.org/10.5061/dryad.hdr7sqvjb

Abstract

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.

Methods

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 Erodium 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 (N = 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 A. californica 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 A. californica 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 (N = 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 N = 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 cm3 in canopy volume were assumed to be seedlings. This classification system correctly predicted 98.4% of records for confirmed seedlings (N = 128), and 97.0% of those for confirmed one-year old plants (N = 34).

Artemisia californica 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.

Usage Notes

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:

Adult survival records.csv

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 cm3.

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.

Seedling survival records.csv

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.

Seedling emergence data.csv

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 m2. 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 m2.