Study Finds Urban Design Impacts Trees’ Ability to Suck Up Water

A new University of Maryland study found that the amount of rainwater a tree absorbs varies dramatically depending on where it’s planted, and whether it is alone or with other trees.

Because trees reduce rainwater runoff and improve and protect water quality, they play a key role in many urban stormwater management designs that give credits for planting urban trees and forests.

Understanding how different factors influence a tree’s ability to absorb rainwater is important for developing effective incentive programs. The study, a collaboration with researchers at the U.S. Forest Service and the nonprofit Center for Watershed Protection, waspublished in the journal Urban Forestry & Urban Greening.

Most of the data used for developing stormwater management plans comes from non-urban settings or arid cities like Los Angeles, while little is known about how different tree species absorb water in urban settings in the mid-Atlantic and elsewhere. This knowledge gap is especially problematic in the Chesapeake Bay region, where five states are bound by U.S. Environmental Protection Agency laws to reduce rainwater runoff into waterways.

“We were partly surprised that there weren’t strong differences between species,” said Mitchell Pavao-Zuckerman, an associate professor in the Department of Environmental Science and Technology, and senior author of the study. “The differences really were more connected to the size of the trees, where larger trees basically had more capillary tissue to move water up through their trunks and into their canopies."

The researchers were also surprised to discover that individual trees take up much more water when they are alone than when they are planted in a patch of other trees, even though clusters of trees absorb more total water—the way a cluster of drinking straws can draw more water than one large milkshake straw. This information means street trees and urban forests can be more accurately incorporated into developing and refining stormwater management programs.