The Southern Appalachian Mountains have seen Rhododendron maximum L. expand following the loss of chestnut and the more recent loss of eastern hemlock. This expansion has increased understory canopy cover over headwater streams, resulting in dense understory thickets of rhododendron with localized, low-light microclimates. To understand and quantify the impacts these understory thickets have on summer stream temperatures, we investigated the thermal regime of streams prior to and following the experimental removal of rhododendron thickets. We selected four headwater streams in the southern Appalachian Mountains that had significant portions of dead eastern hemlock in their overstory and removed 300m sections of the rhododendron thickets from two, while leaving the other two untreated. In all of the streams, stream temperature was continuously monitored (upstream, downstream, and within the treatment block) during the summer prior to rhododendron removal, and for the two summers following the treatment. Canopy cover was measured at the reach scale, each year. We developed nonlinear regression relationships with noise terms between the temperature in the reference and treatment streams during the pre-treatment year and applied these models post-treatment. We analyzed model deviations as an assessment of the impact of rhododendron removal.
Results/Conclusions
Pre-treatment summer stream temperatures were significantly different across all streams, as well as across sensors within each stream. Additionally, temperatures in the reference streams were significantly different from year to year. Canopy measurements indicated that complete removal of rhododendron dropped the total canopy cover by only around 5%. Deviations from the pre-treatment model occurred both years following rhododendron removal, with larger deviations occurring towards the end of each summer. There were significant deviations from the model for daily average and daily maximum stream temperature. Increases in these metrics were seen in the treatment block but did not persist consistently downstream. We propose that existing overhead canopy provided enough shade to prevent a systemic summer temperature effect, even with higher insolation from rhododendron removal.