Hemlock loss affects nitrate fluxes and stormflow in southern Appalachian headwater streams

Background/Question/Methods

Few studies have examined how insect outbreaks affect landscape-level hydrologic and biogeochemical processes. Here, we report the hydrologic and biogeochemical effects of the invasive, exotic Hemlock Woolly Adelgid (Adelges tsugae Annand; HWA) in a headwater catchment in the southern Appalachian mountains. The study area has experienced high mortality of the evergreen eastern hemlock (Tsuga canadensis (L.) Carr.) due to HWA infestation first detected in 2004. We used a paired watershed approach to quantify changes in nitrate nitrogen (NO₃-N) fluxes and watershed yield using stream flow and chemistry data from two nearby watersheds with similar elevation, slope and soil properties but differing in pre-infestation hemlock abundance with the infested watershed having ~6% hemlock basal area and the reference watershed having almost no hemlock. We used the same paired approach to reconstruct storm hydrographs and compare expected and observed storm flow parameters, including peakflow and event duration, based on 5-minute streamflow records.

Results/Conclusions

Annual watershed NO₃-N fluxes increased significantly after HWA infestation, with the greatest increases during late spring. The response peaked with a 232% annual increase in NO₃-N fluxes during the second year and then recovered to near pre-HWA levels in the fifth year following infestation indicating that water quality effects are transient. Annual watershed yield did not increase, although some months showed significant increases in yield immediately after infestation. We did find significant changes in the storm hydrograph following infestation. Mean flow event duration, peakflow and total stormflow increased 2, 20 and 21%, respectively, for storms exceeding 2.0 cm precipitation during the dormant season. Changes in stormflow parameters were more consistent across years during the dormant season (Nov-Apr) than during the growing season indicating that dormant season changes will persist while changes during the growing season are likely to be transient because hemlock is being rapidly replaced by deciduous species. While water quantity effects were not observable over longer time scales, the observed increases in stormflow suggest that there may be important effects on water quantity during extreme events. Streams draining watersheds where a significant proportion of hemlock has been lost may experience significantly higher stream flows during large storms, particularly in the dormant season.