Priority effects are an important mechanism promoting dominance of some communities by non-native species. Previous studies support that early establishment by non-native grasses can confer competitive advantages over natives, but mostly focusing on perennial species. For native annuals, flexibility in germination timing can differ between species, potentially affecting which persist in invaded communities. The costs and benefits of different germination strategies are also likely to vary substantially between years, with important implications for competitive interactions. We studied three native annual forbs (Amsinckia mensiezii, Phacelia distans and Clarkia purpurea ssp. quadrivulnera) in a type-converted California grassland dominated by Bromus spp. Control plots were compared with those where non-native grass cover was reduced through regular clipping. We quantified seedling abundances, survival and final size of early compared to late native germinators for three years (2009, 2010 and 2011). Our analysis asked: 1) How is seedling emergence for each species allocated between early (October) and late (December) bouts? 2) How do seedling survival and size differ between early and late germinators? 3) How does non-native grass clipping affect seedling emergence and survival, and do the effects differ for early and late germinators? 4) Do these patterns vary across years?
Results/Conclusions
Significantly more A. mensiezii and P. distans seedlings emerged in response to early (October) rains than late (December) ones, in all years. In contrast, the third and least abundant native, C. purpurea, germinated at lower rates and almost exclusively in December. Only P. distans emergence responded to non-native grass clipping, but weakly, with positive effects in one year and negative in another. For both A. mensiezii and P. distans, early germinators gained a strong survivorship advantage in spring, but the mortality cost of late germination was more than halved by grass clipping. Likewise, for later emerging C. purpurea, clipping significantly increased survival. Early germination did not lead to significantly greater final size or flower production, except in 2010 for A. mensiezii. Early germinators experienced substantial fall seedling mortality prior to emergence of late germinators, ranging from 70% to 95% in P. distans and 60% to 90% for A. mensiezii. Clipping of non-native grasses did not affect early season mortality. These results support that competition with non-native grasses increases the relative fitness of early-germinating native annuals compared to those emerging later. Nevertheless, early germination comes at a substantial mortality cost for native annuals in some years.