Characterization of riparian zone vegetation and litter production in tropical, montane rainforest along an environmental gradient

Riparian zones are dynamic areas adjacent to flowing water that connect aquatic and terrestrial environments throughout the landscape. Riparian zones in temperate, arid, and floodplain systems have distinct vegetation composition, as well as high species diversity and productivity. In particularly wet environments, such as tropical montane rainforests, it is unclear whether the environment is different between riparian and upland areas. In this study, I determined whether distinct vegetation communities are found in riparian zones of a tropical montane rainforest by comparing vegetation composition and leaf-litter production in riparian and upland areas within Tabonuco forest, and then along an elevation gradient. Within Tabonuco forest I analyzed tree species composition (≥ 10 cm dbh), understory vegetation (≥ 25 and ≤ 200 cm), and leaf-litter production relative to two permanent streams in the Luquillo Forest Dynamics Plot (LFDP). No tree species had a restricted range of < 20 m from the nearest stream. I found significantly higher tree stem density near the streams, but found no relationship between species richness or diversity (H') and distance to streams. However, in understory vegetation, ferns and vines were more abundant in riparian areas, while upland areas had more small trees. Total leaf-litter production rates were higher in the riparian area compared to the upland area. Large tree species were not associated with riparian areas, but riparian zones had greater amounts of ferns and vines, higher values of small tree species richness, and higher leaf-litter production, as compared to upland areas Tabonuco forest. When comparing vegetation in riparian and upland areas along an elevation gradient, we observed greater numbers of stems per plot in riparian areas. In all forest types along the gradient, there was a tendency for greater species richness in riparian sites as compared to upland areas within each forest type. However, species associations and productivity were not different between riparian and upland areas among forest types along the elevation gradient. We conclude that in this tropical montane rainforest landscape, as elevation and precipitation increase, there is no difference between riparian and upland areas, and thus, no riparian vegetation community is found.