DOI: 10.5061/dryad.3xsj3txrj
title: Plant community composition in response to litter removal in Riverside, California, USA, 2021-2023

description: Litter produced by exotic grasses is known to negatively impact native
 species through multiple mechanisms. While litter removal is a potential
 restoration tool to recover native species, the effects of litter removal
 on overall plant community structure are not well understood as most
 studies focus on native species recovery. In a litter removal experiment
 in Southern California, USA, we found that plant communities receiving
 litter removal treatment differed in species composition from control
 plots and had 4.4% higher native forb relative abundance and 7.0% higher
 exotic forb relative abundance, on average. These changes in exotic and
 native forb abundance with litter removal altered community structure by
 increasing overall Shannon diversity, while only having a modest positive
 effect on overall species richness. In contrast, we found no effect of
 litter removal on the species richness or Shannon diversity of native
 species. Taken together, these findings suggest that in herbaceous systems
 litter removal is likely to alter community composition primarily by
 increasing the abundance of native and exotic forbs already present in the
 community, rather than by allowing the establishment of new native
 species. Our study adds to a growing body of literature demonstrating that
 litter produced by exotic grasses inhibits native forbs in California
 grasslands and coastal sage scrub and highlights a potential trade-off
 between controlling exotic grass invaders and controlling exotic forb
 invaders. If managing for increased species diversity is the goal, litter
 removal may be an appropriate intervention to move plant communities
 towards a more diverse state.
description: <strong>Study Site</strong> The
 experiment was located at a site on the University of California–Riverside
 campus in Riverside, California, USA (33.967º N, 117.323º W) where the
 mean annual temperature is 19 ºC and the mean annual precipitation is 238
 mm. The site is at 400 m above sea level and is a north-facing slope that
 was formerly composed of Riversidean CSS dominated by native subshrubs
 <em>Artemisia californica </em>(California sagebrush;
 Asteraceae)<em>, Encelia farinosa</em> (brittlebrush;
 Asteraceae)<em>, Salvia mellifera </em>(black sage;
 Lamiaceae)<em>, Salvia apiana </em>(white sage;
 Lamiaceae)<em>, Acmispon glaber </em>(deerweed; Fabaceae), and
 <em>Eriogonum fasciculatum </em>(California buckwheat;
 Polygonaceae). These native subshrubs are present in small, scattered
 stands in the adjacent south-facing slopes but not at the experiment site.
 The experiment site is now an exotic annual grassland dominated by the
 invasive annual grass <em>Bromus diandrus</em> (ripgut brome;
 Poaceae) with interspersed exotic and native forbs.
 <strong>Experimental Design</strong>
 In 2021, we established 80 3 by 1.5 m experimental plots which
 were assigned treatments in a paired design with blocks containing two
 plots each, where one plot received litter removal via raking with soil
 rakes, while the other was left as an unmanipulated control (40 blocks
 total). The plots were arranged in a grid pattern of alternating litter
 removal and control plots. These plots were previously used to conduct a
 litter removal experiment for teaching an undergraduate lab course from
 2008-2015. To assess whether this legacy of use had any persisting
 effects, we collected baseline species composition in the spring of 2021
 prior to conducting our experiment and compared community structure in
 historically raked versus historically unmanipulated plots. Treatments
 were assigned to plots in an identical fashion to those assigned in the
 2008-2015 experiment (i.e., a plot that had litter removed from 2008-2015
 also had litter removed in our experiment). Plots that we designated as
 controls (no litter removal) have never experienced experimental litter
 removal. We acknowledge that due to the past use at the site and the
 potentially long-lived nature of annual forb seeds in this system, there
 is a possibility that the seedbank of historically manipulated plots
 differs from that of the historically unmanipulated plots. However, as the
 plots are small, only 1 m apart, and arranged in an alternating grid
 pattern, it is highly likely that species could disperse among the plots
 during the previous manipulations. Additionally, as all plots were
 unmanipulated for 7 years before our experiment, there has likely been
 sufficient time for equilibration of composition among plots in this
 annual system. We removed litter using soil rakes in
 the fall (September/October) of 2021 and 2022 before the onset of the
 rainy season (late October – May) and took care to avoid soil disturbance.
 Recumbent (defined as litter laying on the soil surface) and standing
 (defined as senesced grass matter still rooted in soil) litter was
 collected and removed from the plots; however, deeply rooted standing
 senesced grasses and small fragments of litter at the soil surface were
 not removed to avoid causing soil disturbance. Species
 composition data was collected in the plots at peak biomass during spring
 (March – April) of 2022 and 2023 using point-intercept methods along three
 transects within each plot. The transects were located at 25 cm, 75 cm,
 and 125 cm distances with respect to the bottom of the 1.5 m axis of the
 plot. We recorded species identities at 10 cm intervals along each
 transect while traversing the 3 m axis of each plot (totaling to 90 points
 per plot). “Hits” were recording by lowering a marking flag and noting all
 species that contacted the wire. For each point, the soil surface cover
 was recorded (any soil surface not covered in litter, meaning senesced
 plant matter of any species, was considered “bare ground” which included
 loose sandy soil, gopher holes, and rock).
description: # Plant community composition in response to litter removal in Riverside,
 California, USA, 2021-2023
 [https://doi.org/10.5061/dryad.3xsj3txrj](https://doi.org/10.5061/dryad.3xsj3txrj) ## Description of the data and file structure This dataset contains our raw data table of species recorded at each sampling point within each plot; as well as several tables containing metrics of species areal cover, species diversity, functional group abundance, and species relative abundances calculated from the raw data, which were used in our analysis. Additionally, we have included a table with all of the species encountered in our study period (species list). ### Files and variables #### File: plot\_species\_relabund\_matrix.csv **Description:** A matrix of plots sampled at a time point and the relative abundance of all plant species observed (i.e., a site by species matrix). ##### Variables * plot_in_year: the plot ID in the year of sampling, as well as the treatment. All plots were unmanipulated prior to measurement in 2021 (i.e., "2021 raked" plots were not actually raked). Treatment plots were raked prior to measurement in 2022 and 2023. * *all other variables:* relative abundances (proportions) for each species (see list of species for abbreviations and species names). #### File: plot\_areal\_cover\_by\_species\_table.csv **Description:** A table of plot areal cover by species. ##### Variables * plot_id: The plot ID from our experiment in the format of row letter, column number, and position with each block. Each block contained two paired plots (U, upper; L, lower) * year: The year the data was collected * treatment: Treatment assigned to plot (raked = plant litter removed with soil rakes, unraked = no manipulation). All plots were unmanipulated prior to measurement in 2021 (i.e., "2021 raked" plots were not actually raked). Treatment plots were raked prior to measurement in 2022 and 2023. * *all other variables:* Areal cover proportions (number of hits of the species divided by the total number of points sampled in the plot) for each species (see list of species for abbreviations and species names). #### File: study\_species\_list.csv **Description:**  ##### Variables * species_code: Species name abbreviated as the first three letters of the genus name and the first three letters of the species name. * species_name: Species binomial. * provenance: Origin of species as native or exotic to the region (California). * growth_form: Growth habit of species as forb, grass, shrub, or succulent. #### File: plot\_div\_comp\_table.csv **Description:** Species diversity and functional group abundance metrics for each plot and time point in the study. ##### Variables * plot_id:  The plot ID from our experiment in the format of row letter, column number, and position with each block. Each block contained two paired plots (U, upper; L, lower) * block: Each block contained two paired plots (U, upper; L, lower). * row: Vertical (y) position on our plot grid relative to the bottom of the hill at our site. * column: Horizontal (x) position on our plot grid relative to the left-most plot. * position: Position of the plot within the block (U, upper; L, lower). * year: The year, one of 2021, 2022, and 2023. All plots were unmanipulated prior to measurement in 2021 (i.e., "2021 raked" plots were not actually raked). Treatment plots were raked prior to measurement in 2022 and 2023. * treatment: Treatment assigned to plot (raked = plant litter removed with soil rakes, unraked = no manipulation). * years_raked: The number of years raked prior to the time the plot was sampled (ranges from 0 years for plots that were never raked to 2 years for plots sampled in the final year of the experiment).  * BG: The fraction of the soil surface in the plot that was bare ground (included loose sandy soil, gopher holes, and rock). * LIT: The fraction of the soil surface in the plot that was litter (defined as senesced plant matter of any species).  * exotic_forb_relabund: Relative abundance of exotic forbs in the plot. * exotic_grass_relabund: Relative abundance of exotic grasses in the plot. * native_forb_relabund: Relative abundance of native forbs in the plot. * native_shrub_relabund: Relative abundance of native shrubs in the plot. * native_succulent_relabund: Relative abundance of native succulents in the plot. * exotic_forb_cover: Areal cover proportion (number of hits of species from the functional group divided by the total number of points sampled in the plot) for exotic forbs in the plot. * exotic_grass_cover: Areal cover proportion (number of hits of species from the functional group divided by the total number of points sampled in the plot) for exotic grasses in the plot. * native_forb_cover: Areal cover proportion (number of hits of species from the functional group divided by the total number of points sampled in the plot) for native forbs in the plot. * native_shrub_cover: Areal cover proportion (number of hits of species from the functional group divided by the total number of points sampled in the plot) for native shrubs in the plot. * native_succulent_cover: Areal cover proportion (number of hits of species from the functional group divided by the total number of points sampled in the plot) for native succulents in the plot. * sprich: Total number of species in the plot. * shannon: Shannon diversity index for the plot. * sprich_native: Total number of native species in the plot. * shannon_native:  Shannon diversity index for only the native subset of the community for the plot. * sprich_exotic_forbs: Total number of exotic forb species in the plot. * shannon_exotic_forbs: Shannon diversity index for only the exotic forb subset of the community for the plot. #### File: raw\_hits.csv **Description:**  ##### Variables * collected_date: Date of sampling. * plot_id: The plot ID from our experiment in the format of row letter, column number, and position with each block. Each block contained two paired plots (U, upper; L, lower) * treatment: Treatment assigned to plot (raked = plant litter removed with soil rakes, unraked = no manipulation). All plots were unmanipulated prior to measurement in 2021 (i.e., "2021 raked" plots were not actually raked). Treatment plots were raked prior to measurement in 2022 and 2023. * row: Vertical (y) position on our plot grid relative to the bottom of the hill at our site. * column: Horizontal (x) position on our plot grid relative to the left-most plot. * position: Position of the plot within the block (U, upper; L, lower). * transect: The number of the transect within the plot; 25 cm (transect 3), 75 cm (transect 2), and 125 cm (transect 1) in distance with respect to the bottom of the 1.5 m axis of the plot. A value of "P" indicates a species was observed in the plot but not hit on a transect. * point: Point on the transect starting from the left side of the plot. We recorded species identities at 10 cm intervals along each transect while traversing the 3 m axis of each plot (totaling to 90 points per plot). A value of "P" indicates a species was observed in the plot but not hit on a transect. * hitnumber: Hits were recorded by lowering a marking flag and noting all species that contacted the wire. The hit number starts counting from the first species contacted (e.g., hit 1 is the tallest species, hit 2 is the second tallest species, etc.). Species that were present in a plot but did not intersect a transect were added as one hit to the total hits for each plot. * species_code: Species name abbreviated as the first three letters of the genus name and the first three letters of the species name. ## Code/software All files are text files (.csv format) that can be viewed in most free text editors.