Plant community composition in response to litter removal in Riverside, California, USA, 2021-2023

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.

Methods
Study Site

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 Artemisia californica (California sagebrush; Asteraceae), Encelia farinosa (brittlebrush; Asteraceae), Salvia mellifera (black sage; Lamiaceae), Salvia apiana (white sage; Lamiaceae), Acmispon glaber (deerweed; Fabaceae), and Eriogonum fasciculatum (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 Bromus diandrus (ripgut brome; Poaceae) with interspersed exotic and native forbs.

Experimental Design

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).