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.
Since 2004, southwestern Colorado's Englemann spruce forests have experienced a severe outbreak of spruce beetles (Dendroctonus rufipennis). Such outbreaks occur episodically, multiple times per century (Anderson et al. 2010). Early research in northern Colorado linked a prior outbreak that occurred in the 1940s and 1950s to wind damaged trees that favored growth in the population of the spruce beetle.
By late 2012, forest health monitors in western New York and Pennsylvania knew that they were about to experience a severe outbreak of the non-native Gypsy Moth. Surveys showed an unusually high density of egg masses on the branches and trunks of trees, but neither egg densities, nor the distribution of primary host trees are ever perfectly known across the forest, so translating general predictions to a map remains a challenge. Using the best information available, forest managers prioritized their lands for spraying, then they watched this event unfold.
Over the last 15 years, Mountain Pine Beetles have killed large tracks of ponderosa pine across the Black Hills of South Dakota. Beetles thrive with continuous stands of dense trees, and decades of fire exclusion have led to that landscape and stand-level condition. That favorable habitat may help explain why the current epidemic of this native insect has been so severe. In this summer 2010 aerial photograph, the patchy structure of recent and older tree mortality shows up well around Harney Peak--the highest elevation in the Black Hills.
Spring often brings defoliating insects to the forests of Louisiana. In this image from early May 2010, defoliations from forest tent caterpillars and baldcypress leafrollers create an erupting "measles-like" pattern. Note the subtle differences in severity outward from the centers of several of the blotches. The insets show the baldcypress leafroller (left: photo by Gerald Lenhard, LSU: bugwood.com 0014219) and the forest tent caterpillar (right; photo by Stephen Katovich, USFS; bugwood.com 1398248).
Outbreaks of the defoliating pine butterfly are rare. In eastern Oregon, outbreaks occurred in 1908-11, 1940-43, 1982 and from 2008 to 2011. Sometimes defoliations can lead to mass mortality of ponderosa pine—the primary host, but not always. This current outbreak on the Malheur National Forest is largely responsible for the forest change anomalies in ForWarn for September 29, 2011 compared to 2010. According to aerial detection surveys, areas inside and outside these pine butterfly areas were also affected by spruce budworm.
This fall 2011 webworm outbreak in the Hickory Creek Wilderness Area of the Allegheny National Forest, Pennsylvania stands out against the near normal (blue) background that dominated the state in early fall. There were minimal indications of defoliation in mid-August, but clear patterns of severity were clear by mid-September. This anomaly persisted through November, when leaflessness is the normal pre-winter condition for deciduous trees.
A sizable portion of the Wenatchee National Forest, Washington shows less vegetational vigor than it did at its peak of the last decade. While such declines in NDVI often result from tree death due to wildfires in the west, no large fire has occurred here as is shown by the distribution of MODIS hotspots (shown as white triangles) between 2000 and 2011. Much of this reduction through September 13, 2011 may have been caused by the cumulative effects of defoliating insects on tree mortality, particularly from spruce budworm.