Most studies trying to elucidate the mechanisms that allow invasive plants to displace native species have focused on competitive interactions.  However, some recent work has drawn attention to the potential importance of seed limitation, dispersal and germination dynamics in limiting the abundance of native plants in invaded habitats. We assessed the roles of both competition from introduced annual grasses and seed limitation in reducing the abundance of native annual forbs in a highly invaded California grassland.  In 2005-6 and 2006-7, we carried out factorial experiments crossing the removal of invasive grasses with sowing of seeds from four annual forbs: two that currently occur patchily within the grassland (Phacelia distans and Amsinckia menziesii var. intermedia), and two that are relatively rare there (Camissonia bistorta and Clarkia purpurea ssp. quadrivulnera). Abundances of seedlings in February and of adult plants in May were measured in all plots through spring 2008. 

Results/Conclusions Drier than average growing seasons in both spring 2006 and 2007 resulted in low recruitment rates for all four native plants. We found a trend towards increased probabilities of native germination in plots that were both seeded and underwent invasive grass removal in 2006, but no evidence for effects of any manipulations in 2007. However, delayed treatment effects were observed in the wetter spring of 2008. Both P. distans and A. menziesii seedling abundances in 2008 were significantly increased by seeding in the previous year, although these effects did not lead to strong changes in numbers of flowering adults. We observed no carryover effects of competitor removal in previous years into 2008. Overall, we found surprisingly little evidence for suppressive effects of invasive grasses, except that removal plots in 2007 actually experienced higher native seedling mortality rates than those in the control treatment. Our results suggest a great deal of variability in the factors controlling abundances of native annuals in this invaded habitat, both between years and among species. The timing of manipulations with respect to wet years that promote strong recruitment of annuals may be key in determining their effects.