As the global climate change conversation intensifies and nations look to minimize environmental impacts in their own backyards, nature-based solutions are garnering new levels of interest.
Trees are widely recognized for their role in sequestering carbon, and capturing and storing rainfall in their canopy to manage stormwater runoff, but to date there has been minimal research and clarity around how urban forests in particular can be used as practical stormwater management tools. Members of the academic community speculate that urban trees can help mitigate stormwater flows, but the actual amount of stormwater that trees remove through functions like transpiration, infiltration, and storage is not well established.
To address this gap, University of Maryland researchers have conducted an empirical field study and concluded that single urban trees, such as street trees, function differently than trees grown in clusters featuring significantly greater transpiration rates. This result offers a new understanding of how to manage the landscape in urban settings to reduce the harmful effects of stormwater runoff.
The findings are published in Scientific Reports, with authorship from Mitch Pavao-Zuckerman and Sarah Ponte in UMD's Department of Environmental Science and Technology in partnership with the Center for Watershed Protection and the United States Forest Service.
"This work is important because urban trees are increasingly being considered as a stormwater management practice, but we don't have much information about how trees function in different parts of the landscape," explains Deb Caraco, senior watershed engineer with the Center for Watershed Protection. "Quantifying the impacts of urban trees affect different parts of the water balance, such as the evapotranspiration component discussed in Mitch and Sarah's paper, gives us a better understanding of the benefits of urban trees, and knowing where and how to plant and preserve them to achieve the greatest benefit."
To better understand how the relationships between transpiration and environmental influences change within different tree management contexts, Pavao Zuckerman's team evaluated three distinct urban settings -- single trees over turfgrass and a cluster of trees over turfgrass in Montgomery County, and a closed canopy forest with a leaf litter layer in Baltimore, Md. They built and used sap flux sensors - which give a clearer picture of how trees access groundwater - installed in 18 mature red maple trees to continually monitor transpiration rates during the growing season.
They also measured soil water content, air temperature, relative humidity, and precipitation at each site. Single trees had a much greater transpiration rate, and were more responsive to climate influences than closed canopy or cluster trees. This data presents important implications for the future.
"This work explores how trees function in different urban contexts, say street trees vs. a forest patch, where their environments are very different than non-urban trees," explains Pavao-Zuckerman. "Cities can be hotter and drier for example. Our data can help make tree crediting policies better reflect the actual benefits of trees in urban landscapes because they interact with water and their environment differently in cities than they do outside cities. Our next step is to take this data set on how each tree functions and scale it up to see how an entire stand or patch of trees mitigates stormwater flows."
Some may envision a tree having the same characteristics regardless of where it is growing but due to Pavao-Zuckerman's work, we now see that the same tree species will function differently in different urban settings, and can help mitigate stormwater in cities which affects flooding and water quality which are becoming increasingly important public-facing issues.
"This work emphasizes the importance of thinking about cities as not a homogenous thing that we're trying to manage, but that environmental outcomes and benefits are going to vary within a city," says Pavao-Zuckerman. "A tree along a street isn't the same as a tree in a patch or woodlot. Considering this variability is important in our future research – we are now modeling how these different settings may mitigate runoff from different sized rain storms for example."
Pavao-Zuckerman emphasizes that these findings can serve as helpful guidelines for those managing urban stormwater runoff. And that the current method of relying on data gathered from non-urban locations should be put to rest.
"Practitioners are now able to better integrate urban trees into their stormwater green infrastructure network. These findings suggest that approaches to use urban trees and forests to mitigate urban stormwater runoff should rely on data that is derived from urban settings, rather than non-urban locations."
The Chesapeake Bay Trust's Pooled Monitoring Initiative that provided funding for this project supports research for key restoration questions such as this study to guide future restoration efforts.
""The importance of trees to clean water, clean air, and provide shade resonates now more than ever as we look for ways to reduce urban heat islands, clean stormwater before it enters streams, and provide habitat for our wildlife," said Jana Davis, Executive Director of the Chesapeake Bay Trust.
This paper, titled "Transpiration rates of red maple (Acer rubrum L.) differ between management contexts in urban forests of Maryland, USA" is published in Scientific Reports