UMD Study Shows that Even Small Patches of Urban Forest in Baltimore Infiltrate Stormwater Comparably to Green Infrastructure Features Like Rain Gardens

Findings have implications for urban planning and the conservation of urban forest systems

Baltimore, Maryland

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November 25, 2019 Samantha Watters

Researchers at the University of Maryland released a collaborative study that sheds new light on the added value of even small patches of urban forest for managing and infiltrating stormwater. Published in the Journal of Environmental Management, researchers assessed patches of urban forest in Baltimore, Maryland, and found that regardless of the size of the patch, these areas can infiltrate stormwater at similar rates as green infrastructure features like rain gardens, playing a significant role in curbing pollution, runoff, and nutrient damage to the surrounding waterways feeding into the Chesapeake Bay. Since smaller patches of urban forest aren’t often a major focus from a conservation perspective, these findings suggest that conserving these unmanaged green spaces could be more valuable than originally thought.

“There is a perception that urban soils are very heavily degraded and not as useful for some of these environmental purposes, but we found that 60 to 70% of the rainfall could actually be infiltrated on site by these spaces,” says Mitchell Pavao-Zuckerman, assistant professor in Environmental Science & Technology. “And that was a major question mark for us - are these spaces that haven’t been planned and built like a rain garden doing similar things to a rain garden, and if so, what are they actually doing?”

In collaboration with Baltimore Green Space, the U.S. Forest Service, and the geography department at the University of Maryland, Baltimore County (UMBC), Pavao-Zuckerman and his graduate student set out to assess how well patches of urban forest, which are often neglected and thought to have little value as stormwater management features, can actually infiltrate stormwater.

“It really was the partnerships that gave us the opportunity to do this. We started working with UMBC and the Forest Service who had been working with Baltimore Green Space as a community partner to characterize spaces with respect to stewardship activities in the area. We were all trying to find new ways of thinking about forest in cities - what they are, how they are classified, whether there are limits with what would be, is, and could be conserved, and how is the city thinking about protection.”

These partners provided background information to Pavao-Zuckerman about the makeup and characteristics of these urban forest spaces so that they could start assessing their functionality, starting with hydrology and soil infiltration measures to see how they behave as stormwater management systems. In addition to infiltration, researchers assessed how smaller patches of forest may differ from larger patches.

“When people talk about urban forest in the U.S., they aren’t just talking about large clusters of trees,” explains Pavao-Zuckerman. “Many would even consider a single street tree as part of an urban forest system. In our study, we focused on small patches compared to larger patches that would be more typically considered to be forests. Small patches are usually more disconnected from everything else, like strips that are left behind as the geometry of development occurs.”

Findings show that small patches infiltrate at similar rates to large patches of urban forest. “From a conservation perspective, the smaller patches aren’t always looked to as valuable, but these findings suggest that they are,” says Pavao-Zuckerman. 

He adds, “There is always that tension when new development happens between leveling everything and going back to add in rain gardens and green infrastructure versus leaving what’s there and building around it. These results speak to the ability of these spaces, small and large, to act like a rain garden. From an urban planning perspective, it costs more money to put something in then to leave something behind.”

Pavao-Zuckerman hopes to focus future work on modeling stormwater flow patterns and runoff to see the type of impact urban forest systems like the one in Baltimore are having on bodies of water like the Chesapeake Bay.

“We want to be thinking about green infrastructure not just as a basin or a rain garden or a patch of trees, but as an actual network of infrastructure - how things connect in with the rest of that system is really important. We haven’t looked at the runoff pattern yet, but that would be valuable to see to understand how flow happens.”

This paper, entitled “The capacity of urban forest patches to infiltrate stormwater is influenced by soil physical properties and soil moisture,” is published in the Journal of Environmental Management. This work is supported by Chesapeake Bay Trust, the U.S. Forest Service, and the College of Agriculture & Natural Resources’ Hatch Program through the National Institute of Food and Agriculture, United States Department of Agriculture.