Urban trees are stressed owing to lack of water, study finds

Trees in urban areas are affected by a number of stress factors, but lack of water is one of the most important stress factors, researchers have shown.

Urban tress are able to withstand the increasing heat and insect pests fairly well, but only until there is no lack of water. It turns out that urban trees are unable to handle other stress factors if lack of water becomes a common thing. Insufficient water not only harms trees, but allows other problems to have an outsized effect on trees in urban environments.

It’s extremely difficult to design a field study that addresses these questions about the role of various environmental variables, given all of the uncontrolled factors in an urban environment. So researchers of a new study used both field data and controlled laboratory experiments.

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. Scale insects (Parthenolecanium species) are well-known tree pests.

But the researchers also conducted laboratory experiments using willow oak saplings. In these experiments, the researchers manipulated three variables while growing the willow oaks: temperature, water and the presence of scale insects.

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.

The researchers 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.