Why are native annual abundances low in invaded grasslands? Testing the effects of competition and seed limitation

Abstract

Competitive suppression of native species has long been considered among the most important mechanisms allowing exotic plants to dominate some communities, but recent work has focused attention on the potential role of native seed limitation. We tested effects of competition from exotic annual grasses and seed limitation on the abundances of four native annual plants in invaded southern California grassland for two different years. Both initial responses and effects still present 1–3 years later were measured. In the wetter year of 2006, all four native species (Amsinckia menziesii var. intermedia, Phacelia distans, Camissonia bistorta and Clarkia purpurea ssp. quadrivulnera) increased in abundance with exotic grass removal, and two (Clarkia and Camissonia) responded significantly to seed addition. We observed limited treatment effects during the much drier growing season of 2007. By 2008–2009, any initial effects of exotic grass removal had disappeared. However, for both the 2006 and 2007 experiments seeding significantly increased native cover in 2009, independent of initial grass removal. Abundances of both Amsinckia and Phacelia, but not Camissonia or Clarkia, were significantly higher in seeded plots during 2008–2009. These delayed responses may have resulted from seed dormancy. Our results support the importance of both competition with exotics and seed limitation in determining native annual community composition of California grasslands. An interesting future question is whether traits related to resource use or germination phenology may predict differences in native species interactions with exotic grasses.