California grasslands experience a Mediterranean climate, with an extended dry season. With climate change, they will likely encounter extended droughts, and precipitation delivered in fewer storms, with longer dry periods between storms, even in the wet season. Resilience in these conditions will rely on soil water holding capacity, as well as plants that can maintain cover during dry periods. Water holding capacity (WHC) can be influenced by plant composition, through differences in root biomass and buildup of soil organic matter. Clipping (to simulate mowing or grazing) may also influence WHC, through changes in plant community composition and root growth, and different plants may increase or decrease root growth in response to clipping. We investigated how soil WHC is influenced by an interaction of vegetation composition, rainfall, and clipping. In 2008, experimental plots (1.5m x 1.5m) received seed mixtures of: naturalized, early-season exotic annual grasses, late-season, noxious exotic annual grasses, and native perennial grasses, and mixes of these different groups. In the mixed-group treatments, clipping treatments occurred in either fall or spring. Starting in the 4th growing season, rainfall manipulations (wet, dry, control) were implemented. Plant composition and biomass were measured each year and soil WHC was measured in 2020.
Results/Conclusions
In plots where native seeds weren’t planted, it took 4-5 years for them to establish from neighboring plots. Under wet treatments with spring clipping, native plants were able to reach co-dominance ( >25% cover) three years sooner than in dry or control treatments. When natives were included in the initial seeding mix, wet treatments and clipping increased native plant cover.
The 3-way interaction of clipping, precipitation treatment, and initial species composition had a significant effect on WHC (p< 0.01, Χ2=10.6, df=2). In plots with all functional groups included in the initial seed mix, WHC did not differ in the three precipitation treatments. Spring clipping increased WHC except in dry treatments. In plots seeded with noxious and naturalized plants only, WHC in the dry treatment was significantly lower than in control plots (52-56% and 62-66% respectively). Clipping had negative effects on WHC in each precipitation treatment. These results suggest that the presence of native grasses is critical to maintain WHC under dry conditions, and that clipping (often used to decrease fire fuels) can further decrease water holding capacity in exotic annual-dominated grasslands. Understanding effects of clipping under different community structures and with variable precipitation will be needed to properly maintain WHC.
