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

0 We tested predictions about how the e}ect of vegetation and litter on seedling establishment varies among sites and herbaceous community types "sand barrens\ prairies\ fens#[ For both vegetation and litter\ we also separated direct interactions from indirect interactions and interaction modi_cations along the gradient[ 1 Although the intensity of the e}ects varied across sites\ the direct e}ects of vegetation or litter alone were consistently facilitative along the productivity gradient[ Pre! dominance of facilitative e}ects may be due to the focus on the seedling establishment phase[ 2 However\ inclusion of indirect interactions and interaction modi_cations caused the net e}ects of both vegetation and litter to become largely negative[ While one layer of biomass may be advantageous to ameliorate some moisture stress\ the addition of another layer may be disadvantageous if this layer limits light proportionally more than it relieves moisture stress[ 3 One exception to this pattern occurred at high productivity when the net e}ect of vegetation\ even in the presence of litter\ remained facilitative[ The net e}ect of vegetation was competitive at low productivity and grew increasingly facilitative with productivity[ Thus\ indirect e}ects of litter may alter interaction patterns across this gradient[