Direct interactions among plant species may be highly modified by indirect or diffuse effects within a multispecies community. We investigated the direct and diffuse effects of two salt marsh perennials, Monanthechloe littoralis and Arthrocnemum subterminale, on winter annuals and the perennial herb Limonium californicum in a salt marsh in central California. In permanent plots, Monanthechloe had expanded substantially in the upper marsh over the past 13 yr, while Arthrocnemum and all annual species had decreased. These dynamics suggest that Monanthechloe may directly outcompete most other species in the upper marsh. In contrast, Arthrocnemum is known to facilitate some annual species. In our field experiments, Monanthechloe strongly suppressed Arthrocnemum, all four common annual species, and Limonium in direct interactions. In contrast, Arthrocnemum directly facilitated the winter annuals Parapholis incurva and Lasthenia glabrata, competed with Spergularia marina, and did not have a significant effect on Limonium. However, when the combined effects of Monanthechloe and Arthrocnemum were tested, Arthrocnemum ameliorated the negative effect of Monanthechloe on all four species. Although isolated Arthrocnemum competed with Spergularia and had no direct effects on Limonium, Arthrocnemum in the presence of Monanthechloe facilitated both species. We hypothesize that Arthrocnemum buffered the strong competitive effects of Monanthechloe on Lasthenia and Parapholis via direct positive effects and benefited Spergularia and Limonium through its competitive effect on Monanthechloe. These findings add to the growing body of literature emphasizing the importance of diversity and interdependence in the functioning of plant communities
