Most invasive plants negatively affect the native communities they invade.  However, invasives can affect natives via different mechanisms, and distinguishing among them is essential for understanding the dynamics of invasions and for directing management strategies.  Two common mechanisms of invasive plant effect are direct competition for limiting resources and alteration of the environment.  We tested these mechanisms in a Great Lakes marsh invaded by Typha x glauca (hybrid cattail), which produces monodominant stands with considerable litter accumulation.  We teased apart the effects of live T. x glauca vs. its litter on the environment and native plant community with a live/litter transplant experiment in a non-invaded part of the marsh.  We also tested whether these effects were reversible by a live/litter removal experiment within the Typha stand.   
Results/Conclusions

Over four years, both transplanted litter and live T. x glauca increased soil inorganic nitrogen, but only litter decreased light.  Only litter addition, not live T. x glauca, affected the plant community, decreasing native diversity and abundance and changing community composition to a more terrestrial species assemblage.  Removal of litter and live T. x glauca from within the dense Typha stand had little effect on soil nutrients, but litter removal greatly increased light.  Litter removal, but not removal of live T. x glauca, resulted in an increase in native diversity and abundance, however it did not restore native marsh species, but rather recruited more terrestrial grasses and forbs.  These results suggest that T. x glauca affects native plants by altering the environment though litter accumulation, not by direct competition.  However, removal of aboveground litter will not restore wetland communities, because altered soil conditions favor colonization by terrestrial plants.