Variation in the effects of vegetation and litter on recruitment across productivity gradients

1 We tested predictions about how the effect of vegetation and litter on se edling establishment varies among sites and herbaceous community types (san d barrens, prairies, Fens). For both vegetation and litter, we also separat ed direct interactions from indirect interactions and interaction modificat ions along the gradient. 2 Although the intensity of the effects varied across sites, the direct eff ects of vegetation or litter alone were consistently facilitative along the productivity gradient. Predominance of facilitative effects may be due to the focus on the seedling establishment phase. 3 However, inclusion of indirect interactions and interaction modifications caused the net effects of both vegetation and litter to become largely neg ative, While one layer of biomass may be advantageous to ameliorate some mo isture stress, the addition of another layer may be disadvantageous if this layer limits light proportionally more than it relieves moisture stress. 4 One exception to this pattern occurred at high productivity when the net effect of vegetation, even in the presence of litter, remained facilitative . The net effect of vegetation was competitive at low productivity and grew increasingly facilitative with productivity. Thus, indirect effects of lit ter may alter interaction patterns across this gradient.