Hemlock Woolly Adelgid Mortality in the Southern Appalachians

The non-native hemlock woolly adelgid decimated eastern hemlock trees across the Southern Appalachians between 2004 and 2010. While some trees have survived up to the present, largely thanks to aggressive treatment efforts, appreciable hemlock canopy and understory decline was common over these first few years. Subsequent decline or mortality is not captured by this time frame, but areas with the highest hemlock concentration are likely included.

From overlooks or from above, the "grey ghosts" of dead hemlock have been starkly notable since their death due to their fine branch structure. Researchers have used this phenomenon to map the distribution of the adelgid's impact using winter/early spring change in Landsat 5 and summer high resolution imagery. Eastern hemlock prefer cooler and wetter lower slope positions and shady aspects, and that becomes less critical at higher elevations.

These maps show the patchiness of the affected forests due to irregularities in the hemlock distribution. That irregularity across watersheds apparently resulted from differences in logging and fire history during the early 20th century, if not earlier. Some of our observed decline may involve understory-only hemlock mortality as some areas lacked overstory hemlock due to the species' slow growth rate and high shade tolerance.

Improved mapping of where this radical change took place over a decade ago can help us understand the long-term impacts of this non-native insect on the biodiversity and hydrology of the region. These maps may also be useful for hemlock treatment prioritization efforts and restoration planning when resistant varieties of hemlock are eventually developed.