Allelopathic dominance by a tree-associated herb in a California annual grassland

Summary In some parts of southern California, Pholistoma auritum dominates understory vegetation beneath isolated Quercus agrifolia trees occurring in annual grasslands. Pure stands of P. auritum are maintained in successive years although adjacent to grasses with easily dispersed propagules. Lack of establishment by the grasses in areas covered with previous season's P. auritum litter suggested a germination inhibition mechanism. Laboratory and field tests indicated allelopathic inhibition of grass seedlings by the herb litter. Dominance by an understory species is evaluated in reference to community organization and stability.