Removing riparian Rhododendron maximum in post-Tsuga canadensis riparian forests does not degrade water quality in southern Appalachian streams

Abstract

In the past century, the evergreen woody shrub, Rhododendron maximum, has experienced habitat expansion following foundational tree species die-off in eastern US deciduous forests. Rhododendron can potentially alter stream chemistry, temperature, trophic dynamics, and in-stream decomposition rates, given its dominance in riparian areas. Here we conducted two operational-scale (3 ha) riparian treatments that removed rhododendron through cutting alone (CR, canopy removal), or removing both the rhododendron canopy and forest floor using cutting and prescribed fire (CFFR, canopy and forest floor removal). We expected that rhododendron shrub removal, with or without soil organic horizon removal, would increase soil nutrient availability and subsequently alter stream pH, acid neutralizing capacity (ANC), inorganic nitrogen (NO₃-N, NH₄-N), total dissolved inorganic nitrogen, dissolved organic carbon (DOC), calcium (Ca), potassium (K), and magnesium (Mg). We hypothesized that responses would occur more quickly in the CFFR treatment. Treatments reduced shrub-, but not tree basal area. Treatments lowered soil N, but not C. Stream chemistry responses to treatments varied between CR and CFFR and were transient, generally with pH, N, and some cations declining, and aluminum (Al) and DOC showing a pulse increase. By removing rhododendron, the remaining deciduous trees likely accelerated N uptake as soil moisture availability increased. This could partially explain why we observed lower than expected stream nutrients (NO₃-N, Ca, and Mg) after treatments. Initial rhododendron slash on the forest floor coupled with incomplete consumption of the O-horizon on the CFFR treatment likely elevated DOC in the upper soil horizons and mobilized Al. From a management perspective, using these treatments to restore structure and function to riparian forests in the wake of eastern hemlock mortality, with or without fire, would most likely not result in short-term diminished water quality that is common when overstory trees are harvested and may even lower stream NO₃-N concentrations long term.

Keywords: Deciduous forest; Disturbance; Evergreen shrub; Headwater streams; Southern Appalachian Mountains; Tsuga canadensis.