What city trees and their pests can tell us about climate change

Planting trees in cities is a great thing!  Trees regulate temperatures, filter air, and reduce runoff. And a tree-lined cityscape is beautiful!

But anyone who works as a tree health specialist knows that if you want to find a sick tree, go to the city. Many urban trees are plagued with a number of insults: reduced space for roots, pollution and other factors can reduce the growth and vigor of trees. What about higher temperatures?

Cities are known to be “heat islands” and can be several degrees warmer than surrounding rural environments, especially at night. This is a result of heat produced by dense motor traffic and the concrete and asphalt re-radiating the sun’s heat. If there is inadequate water heat can be devastating to trees. The stomates, minute holes in leaves that allow for gas exchange, stay open to evaporate water and cool the tree down, but this requires water. If there is no water, the plant begins to wilt.

It appears that heat can affect tree health in more subtle and indirect ways.

Recent work from the lab of Dr. Steven Frank at North Carolina State University has shown that scale pests of some urban trees are more abundant on trees in hotter parts of the city and that these insects have a negative impact on the infested tree’s health.

Red maples and willow oaks are popular trees in the urban landscape. Look closely at specimens of these trees in cities and you are likely to find scale insects: gloomy scale (Melanaspis tenebricosa) on maple, and oak lecanium (Parthenolecanium quercifex) on oaks. You would be hard-pressed to find these scale insects on their hosts during a walk in the woods. The leafless limbs on many maples and willow oaks planted in parking lots are often a result of these pests.

Oak lecanium scales on willow oak.

Gloomy scale on the bark of a maple.

 

Why are they more common on urban specimens? Stressed trees are often more susceptible to pests. They are too busy trying to survive and allocate fewer resources to defending themselves. But this is not true for sap-sucking insects like scales and aphids which are often more common on the healthiest plants. Greenhouse experiments showed that increased temperature, not lack of water or fertilizer, was the only measured variable accounting for the increase in scale insects.  

Like all insects, scales are cold-blooded and reproduce faster at higher temperatures (just a note: this is true within an optimum range of temperatures that varies according to the species; above a species’ maximum their survivorship rapidly declines).

But if it was simply a matter of faster reproduction as a result of warmer temperatures we would expect other insect pests to increase on trees in warmer areas, too, but that is not the case. Although willow oaks and red maples have a number of insect pests associated with them, including several other scale insect species, oak lecanium and gloomy scale stand out as pests of oaks and maples in warmer environments.

What does this tell us about the impacts climate change will have on our forests? It tells us that it’s complicated. There will be winners and there will be losers. There are many trees that do very well in urban environments. In fact, oaks and maples are great urban trees if planted in the right place. It tells us that some insects are predisposed to take advantage of warmer environments and these may become considerable pests in warmer environments.