Many ecosystems are currently being invaded by species that have a phenology that is asynchronous with the existing vegetation. These differences in phenology may offer potential strategies for managing invaders by: (1)competitively displacing invaders by restoring natives with overlapping phenology: (2) exploiting differences in phenology between desirable and weed species by timing grazing to benefit the desired species over the invaders. California’s grasslands provide an ideal setting to test how phenological traits may be used to manage invasive plants. Early season non-native annuals (Avena, Bromus, etc.) have dominated these grasslands for the past two to three centuries, but are currently threatened by two new invaders (Taeniatherum caput-medusae and Aegilops triuncialis) with later phenology. These new invaders threaten livestock production and grassland diversity. Native grassland species have similar phenology to these noxious weeds, and their restoration has the potential to limit invasion through resource competition. We planted monotypic, two-way and three-way mixtures of these weed, annual, and native species and exposed them to fall, spring, or no clipping treatments to simulate grazing. These manipulations test how restoration plantings and timing of grazing impact the prevalence of natives, naturalized exotic annuals, and invasive noxious weeds.
In the first year, contrary to expectations, the groups with overlapping phenology (natives and weeds) had minimal impact on one another, but both were strongly suppressed by annuals. The annuals were dominated by Lolium, a late-season grass with phenological overlap with weeds and natives. The second year had greater late-season precipitation, supporting weed growth after the annual cover senesced. Annuals decreased their prevalence in all mixes, and dominance within the annual community shifted from Lolium to Avena, an earlier season species. Thus, the suppressive effect of annuals on weeds and natives was weaker in the second year. Weeds established competitive dominance over natives during the second growing season. Spring clipping reduced natives and annuals while increasing weeds. Fall clipping increased weeds, but had no effect on annuals and natives.
In conclusion, restoration of native species may enhance weed establishment in the short-term by removing the naturalized annuals that suppress weeds. However, further establishment of these perennial natives may make them more effective competitors. Lolium may effectively compete with weeds although it also negatively impacts natives. Grazing at a coarse seasonal level (spring vs. fall) enhances weed cover, and more refined timing may be necessary to control weeds.