Thursday, May 24, 2018

Low Water Stresses Urban Trees

Hot Wings Tatarian Maple, photo courtesy Denver Post

Information excerpted from: Lack of Water is Key Stressor for Urban Trees, North Carolina State University. Click on the link for the article in its entirety.

A recent study found that urban trees can survive increased heat and insect pests fairly well - unless they are thirsty. Insufficient water not only harms trees, but allows other problems to have an outsized effect on trees in urban environments.

"We would see some vibrant urban trees covered in scale insects, but we'd also see other clearly stressed and struggling urban trees covered in scale insects," says Emily Meineke, a postdoctoral researcher at Harvard and first author of a paper on the study. "We wanted to know what allowed some trees to deal with these pests so much more successfully."

"This is important because trees need to grow in order to perform valuable ecosystem services, such as removing pollutants from the air and storing carbon," says Steve Frank, an associate professor of entomology at North Carolina State University and co-author of the paper.

The researchers collected detailed data on 40 urban willow oaks (Quercus phellos) over the course of two years. The data included temperature, how water-stressed the trees were, and the density of scale insects (Parthenolecanium species), well-known tree pests. The researchers also conducted laboratory experiments using willow oak saplings.

The researchers found that higher temperatures could actually have a positive effect on tree growth, as long as the trees had adequate water. And scale insects had little or no adverse effect on the trees if the trees were not water stressed. They also found that water stress limited tree growth all by itself. But the presence of increased heat and/or scale insects, when combined with water stress, had a multiplier effect - curtailing growth far more than water stress or scale insects alone.

Recommendations for urban planners include: designing urban landscapes that retain stormwater in vegetation; investing in hydration strategies, such as appropriate soil quality and soil volume; and planting drought-tolerant tree species and genotypes in the hottest parts of their cities. These recommendations are also relevant for home gardeners.

The paper, "Water availability drives urban tree growth responses to herbivory and warming," was published March 13 in the Journal of Applied Ecology. 

Additional resources from CSU:

Watering, from Colorado State Forestry Service