City water strategy can offset threat to urban forests from hot temperatures, pests

BONN, Germany (Landscape News) — Trees can give a city neighborhood its character — their leafy boughs not only offering awe-inspiring scenery, shade from the hot sun or shelter from rain, but they also mitigate the impact of climate change through carbon storage.

Forests in urban areas also help to filter and regulate water, contributing to high-quality freshwater supplies, while protecting watersheds and preventing flooding due to water stored in tree branches and soil, according to the U.N. Food and Agriculture Organization (FAO) which is highlighting the importance of city trees for International Day of Forests on March 21.

They also contribute to temperature regulation, reducing the need to use heat energy by up to 50 percent and the need to use air conditioning by 30 percent, FAO statistics show. Additionally, forests and trees in and around cities provide homes, food and protection for many plants and animals, helping to maintain and increase biodiversity.

A recent research paper from North Carolina State University (NCSU), titled “Water availability drives urban tree growth responses to herbivory and warming” published in “Journal of Applied Ecology” highlights the benefits of city trees, and sheds light on the role of water and tree preservation.

Climate change and urbanization pose multiple threats to the services trees offer, but how these threats combine to affect urban trees, and how to mitigate their effects, remains largely untested, leading the scientists to undertake their research.

Across many temperate cities worldwide, urban trees grow less than rural trees, and the research results point to water stress as the most likely driver for this pattern.

The researchers found that water stress both reduces tree growth on its own and exacerbates effects of warming and insect pests on tree growth.

However, as long as trees have adequate water, they found that higher temperatures can have a positive effect on tree growth. Additionally, they learned that insects had little or no adverse effect on the trees if the trees were not water stressed.

Water stress limited tree growth, but combined with increased heat and insects a multiplier effect occurred, restricting growth much more than water stress or scale insects alone.

The findings are important because trees need to grow in order to perform valuable ecosystem services, such as removing pollutants from the air and storing carbon, said Steve Frank, an associate professor of entomology at NCSU and co-author of the paper, in a statement.

The scientists aimed to discover why some urban trees thrive despite being covered in insects while others struggle in the same conditions.

“Management strategies aimed at increasing tree hydration in cities may reduce the adverse effects of all three of these key stressors,” said lead author Emily Meineke, a postdoctoral researcher at Harvard and former doctoral student in Frank’s laboratory.

“Urban planners could design urban landscapes that retain storm water in vegetation; invest in hydration strategies, such as appropriate soil quality and soil volume; and plant drought-tolerant tree species and genotypes in the hottest parts of their cities,” Frank added.

Due to the difficulty of designing a field study subject to many uncontrolled factors in an urban environment, the researchers used both field data and controlled laboratory experiments to reach their findings.

They collected detailed data on 40 urban willow oaks (Quercus phellos) in the southeastern United States during a two-year period, including temperature, water-stress level, and density of scale insects (Parthenolecanium species), a common tree pest.

In the lab, the scientists experimented using willow oak saplings by manipulating three variables while growing the willow oaks: temperature; water and the presence of scale insects.

These findings are likely to become increasingly important as water availability, temperature and pest abundance are affected by further urbanization and climate change, Meineke said.

“Moving forward, we’re very curious about the prevalence of water stress in urban trees globally — and whether this leads to similar problems regarding the impact of tree pests,” Meineke said. “If so, improved tree hydration could become a higher priority for urban forestry management.”

Management strategies targeted at increasing tree hydration in cities may reduce effects of these three key stressors that are expected to intensify with further urbanization and climate change.