Hemlock death transforms Appalachian forests

Across the Southern Appalachians, Eastern Hemlock has long created an important ecological niche. As these trees are evergreen, they provide year-long cover for wildlife and they cool riparian areas where they tend to grow. Since the mid 2000s, these forests have been in decline due to a lethal non-native insect known as the Hemlock Woolly Adelgid. Like its kin that devastated Fraser Fir forests at higher elevations during the 1960s and 1970s, the Hemlock Woolly Adelgid has been transformational; however while fir were able to re-establish, hemlock may not without aggressive management.

What surprised many was the rate at which the Adelgid was able to kill both Eastern Hemlock and its much less common cousin, Carolina Hemlock, once it arrived in the South. In the Northeast, where winter temperatures are colder and this exotic insect arrived decades earlier, the Adelgid impacts on Eastern Hemlock have been much slower. This rapid pace of change has afforded managers little time to take preventative action, such as use of pesticides and it means that large portions of their forests may be in need of remedial action.

Through its cross seasonal phenology-based monitoring design, ForWarn is able to show the onset, rate and severity of evergreen decline. The loss of evergreenness is most obvious in winter because summer deciduous growth can compensate for this loss, however where hemlock decline has been severe, the decline in greenness occurs during both the dormant and growing seasons, as shown on Slide 5. These are areas where the loss of hemlock has resulted in a substantial loss in canopy cover.

If we had detailed species maps to manage forests with, we could use them to anticipate where the loss of hemlock would be greatest, but most maps are vegetational and they only predict the likely occurrence of individual species like Eastern or Carolina Hemlock. Although belatedly, the loss of hemlock that ForWarn readily detects shows us where hemlock once were. When the onset of decline is caught early, intervention may still be possible, especially when decline is less rapid than it has been in the Southern Appalachians. These maps can help managers prioritize areas for monitoring or management and improve our understanding of how invasive insects and diseases can impact our forests.

The last slide shows non-wildfire evergreen decline from 2000 to 2011 for the entire southern Appalachian region based on a seasonal separation of the annual deciduous and evergreen portions of the MODIS history. The evergreen contribution to the cells is indicated by the year's winter condition while the deciduous fraction is inferred based on the summer (leaf on) portion of the NDVI condition. This gives us maps of the linear trends in evergreen and deciduous vegetation mapped as thrive and decline based on whether the slopes are positive or negative. These evergreen-deciduous thrive-decline maps are available for the conterminous US on the ForWarn viewer (look under the Physiography menu on the left).

Evergreen decline outside the Appalachians is mostly from logging of pine forests and most of that inside the National Forests and National Park has resulted from loss of hemlock. Note that Great Smoky Mountains National Park has been particularly hard hit as has the westernmost part of the Nantahala National Forest NC and the area around Mount Mitchell to the east. Where hemlock occurs only in narrow strips along riparian areas, it is often hard to see at this map's resolution.