Positive litter feedbacks of an introduced species reduce native diversity and promote invasion in Californian grasslands

Questions: Californian grasslands have a long history of invasion, starting with the introduction of exotic forage species a century ago, and followed by newer waves of invaders. Both exotic and invasive species produce large amounts of litter, but the importance of litter accumulation on the growth and expansion of these species has not been rigorously assessed. We addressed the following questions: (1) do litter type (exotic or invasive species) and litter quantity affect exotic and invasive plant performance; (2) at which plant life stages does litter accumulation effects occur; and (3) is litter accumulation impacting native species recruitment and diversity? Location: Semi-natural grasslands, northern California. Methods: We created litter quantity gradients (0, 62.5, 125, 250 or 500 g.m(-2)) of three litter types (exotic or invasive species litter or a mixture of both). We evaluated litter effects on the germination, recruitment, production of flowers and biomass production of the exotic Avena fatua, the invasive Elymus caput-medusae and a mixture of native species. We used litter depth as a proxy for litter accumulation and measured litter impacts on light availability at ground level, soilmoisture, soil andmicrobial C and N. Results: We found a significant litter depth impact on all species, but no litter type effects. Litter accumulation principally reduced light availability at ground level and positively affected exotic and invasive species at different life-history stages. Above-ground biomass production of the exotic species Avena increased with increased litter depth, suggesting a litter accumulation feedback. The invasive species Elymus responded to increased litter with increased seed production, rather than biomass production. In contrast, litter negatively affected the recruitment and diversity of native species. Conclusion: Overall, our findings point to cascading effects in invasion with positive litter accumulation feedbacks of an exotic species simultaneously reducing native diversity and promoting seed production of a more recent invasive arrival. These results have strong consequences for invasion management and suggest that litter removal practices, such as burning and grazing, would better succeed at limiting the growth and spread of invasive species than herbicide application or seeding.