Synthesis
Riparian zones help maintain regional biodiversity,4 and we hypothesize that alterations to these ecotones could have large impacts on aquatic ecosystem function. Two examples from LTER sites illustrate this hypothesis. Current declines in eastern hemlock (Tsuga canadensis) resulting from woolly adelgid (Adelges tsugae) infestations are altering riparian tree community composition5 and ecosystem function6 in eastern U.S. forests. It is predicted that losses of eastern hemlock will result in regional decreases in forest plant diversity as existing species become more dominant.7 For example, rhododendron (Rhododendron maximum), a dominant evergreen shrub, is becoming increasingly dominant at Coweeta LTER.8 Rhododendron can inhibit canopy seedling survival and suppress associated ectomycorrhizae in forest soils through root exudates9,10 and its leaf litter has antimicrobial effects on other leaf litter species in streams (Kominoski, unpublished data). In Puerto Rico, (Luquillo LTER) the non-native ornamental African Tulip tree (Spathodea campanulata) has become one of the most common trees on the island.11 In feeding trials with several of species of common aquatic shrimp and crabs (Crowl and Friola, unpublished data), leaves from the African Tulip tree were consumed faster than other leaf species. In contrast, common terrestrial herbivores suffer high mortality when feeding on leaves of the African Tulip tree (Prather and Jansen, unpublished data). Changes in riparian plant community composition may alter consumer dynamics and organic matter processing rates in adjacent aquatic ecosystems. Our objective is to explore broad-scale trends in changes in terrestrial plant communities and effects on aquatic ecosystem function throughout the LTER network.