Litter removal increases plant diversity by promoting both native and exotic forbs in heavily invaded coastal sage scrub in Southern California, USA

Litter produced by exotic grasses is known to negatively impact native species through multiple mechanisms. While litter removal is a potential restoration tool to recover native species, the effects of litter removal on overall plant community structure are not well understood as most studies focus on native species recovery. In a litter removal experiment in Southern California, U.S.A., we found that plant communities receiving litter removal treatment differed in species composition from control plots and had 4.4% higher native forb relative abundance and 7.0% higher exotic forb relative abundance, on average. These changes in exotic and native forb abundance with litter removal altered community structure by increasing overall Shannon diversity, while only having a modest positive effect on overall species richness. In contrast, we found no effect of litter removal on the species richness or Shannon diversity of native species. Taken together, these findings suggest that in herbaceous systems litter removal is likely to alter community composition primarily by increasing the abundance of native and exotic forbs already present in the community, rather than by allowing the establishment of new native species. Our study adds to a growing body of literature demonstrating that litter produced by exotic grasses inhibits native forbs in California grasslands and coastal sage scrub and highlights a potential trade-off between controlling exotic grass invaders and controlling exotic forb invaders. If managing for increased species diversity is the goal, litter removal may be an appropriate intervention to move plant communities toward a more diverse state.