Mechanisms underlying the suppression of forb seedling emergence by grass (Poa pratensis) litter

We investigated the mechanism by which grass litter (dead Poa pratensis L. shoots) suppressed the emergence of seedlings of four old-field forbs (Centaurea nigra L., Dipsacus sylvestris Huds., Hypericum perfioratum L., Verbascum thapsus L.) by determining the effect of litter on their seed germination and shoot extension. When seeds were placed beneath litter (715 g m -2 ) that had been collected from an old-field, the germination of all species except Verbascum was reduced significantly by 26% to 41% compared to a no-litter control. When seeds were placed in plastic dishes containing a leachate solution made from litter (7 g dish -1 ), the germination of two species (i.e. Centaurea and Dipsacus) was reduced significantly by 10% to 34% compared to the distilled water control. When germinated seeds were placed beneath litter, the emergence of seedlings of all species was reduced significantly by 95% to 100% compared to a no-litter control. These results indicate that grass litter may suppress forb seedling emergence by reducing seed germination and (or) by preventing shoot extension and that these effects are species dependent.