Preparing for the Onset of Hemlock Mortality in Great Smoky Mountains National Park: An Assessment of Potential Impacts to Riparian Ecosystems

Currently the Hemlock Woolly Adelgid (Adelges tsuga) is spreading across forests in eastern North America, causing the decline and mortality of the eastern hemlock and the Carolina hemlock. Investigation into the impact of hemlock mortality on ecosystem processes has only recently begun and is not yet fully understood. The loss of hemlock from riparian forests in Great Smoky Mountains National Park (GSMNP) could reasonably be expected to result in significant alterations to stream environments. The goal of this study was to assess the influence of riparian hemlock stands on stream conditions and estimate possible impacts from hemlock loss in GSMNP. I paired hardwood- and hemlock-dominated streams that were similar in topography, geology, land use, and disturbance history using Geographic Information Systems (GIS) analysis and statistical techniques. I then monitored each stream pair for water temperature, nitrate concentrations, pH, and discharge over eleven months. I found that differences between hemlock- and hardwood-dominated streams could not be explained by dominant forest type alone; the presence of hemlock or hardwood riparian forest does not appear to exert a consistently dominant signal on measured conditions of headwater streams in GSMNP. The variability in the results suggests that other landscape variables, such as the influence of understory Rhododendron species, may exert more control on stream conditions than differences between hemlock and hardwood canopies. For example, Rhododendron was found to reduce light levels reaching the forest floor and streambeds in both hemlock- and hardwood- dominated forest stands.

Evidence from recent peer-reviewed literature suggests that short-term stream condition impacts from forest disturbances can be severe. However, research also indicates that conditions can return to pre-disturbance levels within five to ten years. In GSMNP, the return to long-term stability of stream conditions after hemlock mortality will depend on the type of replacement species and how quickly the replacement species can establish in disturbed sites. There is evidence that deciduous hardwood species are most likely to replace hemlock. The results of this study suggest that hemlock and hardwood stream conditions are similar in GSMNP. Therefore, if hardwood species are able to replace hemlock in GSMNP and streams are able to recover from short term impacts, the long term impacts from hemlock mortality on stream conditions will be minimal. However, the presence of Rhododendron in riparian hemlock forests in GSMNP may prevent hardwood species from effectively replacing hemlock, which could hinder the return to long-term stability.