We can’t see bugs from space, but we can see the effects of insect infestation in Landsat imagery. An unprecedented mountain pine beetle epidemic started in the Black Hills of South Dakota in 1996 and has damaged about 430,000 acres of forest land. The pine beetle, about the size of a grain of rice, is killing ponderosa and other pines throughout the Black Hills.
Normally, the mountain pine beetle contributes to the health of a forest by infesting and killing older and stressed trees, which helps make the forest more productive. However, the recent large outbreak is doing more harm than good. It can affect water quality. It can convert the forest from a carbon sink to a carbon source. And where insect outbreaks and forest disturbance caused by wildfire overlap, the effects can actually be both harmful and beneficial. All of the effects of an outbreak need to be monitored.
Mountain pine beetles usually live in small numbers. It’s normal for the population to boom occasionally, but the current epidemic is unprecedented. Beyond the Black Hills, pine beetle outbreaks have occurred extensively in many pine forests throughout western North America, from British Columba in Canada to New Mexico.
In these Landsat images, beetle infestation is typically indicated by a washed-out pinkish color as seen in the top center and lower right of the images. The more pronounced pink region in the lower left of these images is a burn scar from the Jasper Fire, which occurred in 2000.
Mountain pine beetles, which are native to the pine forests of western North America, live most of their lives just beneath the bark of living pine trees. They usually only kill a few trees per year that are already weakened. Healthy trees have defenses that can defeat the beetles. They resist the beetles by releasing pitch, which helps prevent beetles from boring into the tree. This works when it’s only a few beetles on the tree. But the current epidemic is overwhelming their defenses.
Once a pine beetle is on a host tree, it communicates to other beetles by emitting a pheromone that attracts other beetles to mass-attack. They overwhelm the tree’s defenses and tunnel under the bark. Although the tunneling injures the tree by disrupting the movement of food and water from the needles to the trunk and roots, its needles initially remain green, which makes the early stages of infestation difficult to see from the air. A year after the flow of water and nutrients has been cutoff, the tree dies and its needles turn red. A year later the dead trees take on a greyish color as the dead needles fall to the ground.
An outbreak like this begins in dense, and sometimes overcrowded, forest. Climate also plays a role as warm, dry summers are good for beetle development and dispersal, while mild winter temperatures allow more larvae to survive.
Successive years of favorable summer and winter weather can lead to widespread damage, which has been the case in the Black Hills. A more close-up look at the affected region shows a pronounced decrease in the amount of green forest cover between 1992 and 2018.
In 2018, precipitation in the Black Hills was above normal, and the beetles have remained at normal levels. The damage is done, however, and current Landsat imagery still shows the degraded forest. Recovery in the pine forest is very gradual.
Another set of close-up images shows the area around Mount Rushmore National Memorial. The same pattern is apparent here—a decrease in green forest cover surrounding this popular tourist attraction.
Landsat is well suited to studying tree mortality over large areas. The Landsat record covers a long time period. Its infrared wavelengths help make forest disturbance events easier to detect. In most regions of the United States, Landsat provides several clear images per year, which can be useful for tracking the spread of the infestation. Best of all, Landsat data is freely available.
Monitoring a large-scale epidemic like this one is aided by the comprehensive coverage of Landsat imagery. Because field plots are expensive to implement over large areas, Landsat provides forest managers an affordable way to pinpoint where to break up stands of infested trees to try to minimize the spread and reduce fire risk.
The dense historical record that Landsat provides is also important because in some areas, other disturbances such as fire can obscure evidence of past disturbances. For example, the upper right of the 2002 image shows a maroon burn scar from a recent wildfire. Landsat enables scientists to better study the historical interaction between pine beetles and other types of disturbances which occur in complex landscapes.
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