Removal of Woody Riparian Vegetation Substantially Altered a Stream Ecosystem in an Otherwise Undisturbed Grassland Watershed

Abstract

Riparian zones are key interfaces between stream and terrestrial ecosystems. Yet, we know of no whole-watershed experiments that cut only woody vegetation in the riparian zone in an otherwise intact watershed to isolate the role of riparian zones on stream ecology. We removed all of the woody riparian vegetation (from 10- and 30-m-wide buffers in headwaters and main channels, respectively) for 5 km of stream in a single watershed while leaving the remainder of the grassland watershed un-impacted. We assessed water chemistry changes 3 years before and 3 years after riparian wood removal and in two neighboring control watersheds with a before–after, control-impact design and analysis. Riparian woody removal caused 10–100-fold increases in mean stream water nitrate concentrations and pulses of high nitrate for 3 years thereafter. Other nutrients and total suspended solids increased 2–25 times for the 3 years of post-removal. In-stream rates of gross primary production, ecosystem respiration, and net ecosystem production had large treatment effect sizes but also high variance among samples. Past studies of whole-watershed deforestations showed similar water quality responses to our riparian deforestation. Riparian zones of grassland streams are sensitive to disturbance and likely impart relatively greater influence on stream structure and function than the upslope of the watershed. Our results further emphasize the role of riparian zones in biogeochemically linking aquatic and terrestrial habitats.