Establishment, competition and the distribution of native grasses among Michigan old-fields

1 In this study the potential role of competition in influencing the distri bution of three displaced native perennial grasses across complex gradients of plant productivity and species composition was investigated in Michigan old-fields. To do this plant removal and propagule addition experiments we re conducted at nine old-held sites to examine the effects of living plant neighbours and litter on seedling establishment and growth of target specie s in relation to community biomass. 2 For two target species, Andropogon gerardi and Schizachyrium scoparium, l iving plant neighbours suppressed establishment from seed at most sites, an d suppressed the growth of transplants at all sites. 3 Plant litter strongly inhibited the seedling establishment of both Androp ogon and Schizachyrium at sites of high community biomass and litter accumu lation, but had little impact on the growth rate of transplants at any of t he sites. 4 The total suppressive effect of the plant community on seedling establish ment and transplant growth of both Andropogon and Schizachyrium increased i n magnitude in a non-linear fashion with community biomass. These effects i ncreased in magnitude more rapidly across sites of low to medium biomass th an sites of medium to high biomass. 5 The results suggest that these native grasses maybe restricted to low pro ductivity habitats within this landscape because of strong competitive inte rference with establishment by the existing vegetation in the most producti ve sites.