Urban Heat Islands Are Helping Kill Trees

The warmer conditions cities create make plant-eating pests thrive.

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A healthy tree (right) versus one suffering an attack of the "gloomy scales." (Adam Dale/North Carolina State University)

The urban heat island can be both a boon and a curse for plants. On one hand, the toasty conditions asphalt- and concrete-clad cities create has been shown to make trees grow eight times faster. On the other hand, plant-destroying pests also love warmth, and their numbers in some urban areas are on the rise.

The spread of one unpleasant insect, the aptly named gloomy scale insect, is detailed in two new studies from Adam Dale and others at North Carolina State University. Dale's research poses questions about what will happen to plants as cities continue their fierce expansion. (Washington, D.C., to give one example, packed on around 11 square kilometers of impervious surface area every year of the past several decades.) It also raises the issue of what the warming climate is doing to the urban canopy.

Here's Dale speaking on his pestiferous findings:

New research from North Carolina State University shows that urban "heat islands" are slowly killing red maples in the southeastern United States. One factor is that researchers have found warmer temperatures increase the number of young produced by the gloomy scale insect—a significant tree pest—by 300 percent, which in turn leads to 200 times more adult gloomy scales on urban trees.

"We'd been seeing higher numbers of plant-eating insects like the gloomy scale in cities, and now we know why," says Adam Dale, a Ph.D. student at NC State and lead author of two papers describing the work. 'These findings also raise concerns about potential pest outbreaks as temperatures increase due to global climate change."

The gloomy scale is a tiny bugger that takes on a variety of disguises according to its sex and life stage. Legless females hide under armor that looks like fugly bark and jab at trees with "piercing-sucking mouthparts," stealing away nutrients. Large infestations, like the one shown below (ready your smooshing fists), can stunt a tree's growth and eventually kill it. 

(Adam Dale)

The researchers chose to study the gloomy scale and red maples, because they are respectively "the most important pest species of the most common tree species in urban areas of the southeastern United States," says one. In their 26 test sites in Raleigh, they found scales latched onto trees, with more insects appearing in warmer places. The most badly infested zones also had the largest amount of impervious surfaces, like sidewalks, streets, and parking lots. They also found that hotter temperatures were putting stress on the maples by decreasing the amount of water that flows from roots to leaves.

While this experiment was confined to 2,780 maples, it has wider implications, as gloomy scales also munch on silver maple, boxelder, elm, sycamore, and many other tree varieties. The researchers suggest that urban foresters can take several steps to mitigate this problem: Choosing hardier, pest-resistant trees is one, as is increasing the amount of shade (say, by planting even more trees) to put a chill on the heat island.

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