University of Delaware
Asia Dowtin discusses her work on how rainwater travels in urban forests.

Studying stemflow

UD doctoral candidate investigates how rainwater travels in urban forests

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1:09 p.m., Oct. 22, 2015--Forests cover approximately 30 percent of the Earth’s surface. They play an important role in purifying air, preventing soil erosion and maintaining proper balance in water resources. 

How rain makes its way into a forest’s soil can affect soil chemistry, groundwater recharge and, ultimately, forest health and productivity.

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Asia Dowtin, a doctoral candidate in the University of Delaware Department of Geography’s ecohydrology group, led by professor Delphis Levia, is studying how water and nutrients are transported from the tree canopy to the soil below. 

She is particularly interested in how stemflow — water that travels down tree trunks or plant stems — is generated in urban forests like those located in the city of Wilmington’s Rockford Park, Delaware Art Museum and Alapocas Run State Park.

Urban areas contain high concentrations of impervious surfaces such as rooftops, roads and concrete that prevent rainwater from permeating into the soil. During rainstorms, this can cause rainwater to pool on surfaces leading to flash flooding, combined sewer overflow and overall water management headaches for city dwellers and city water management workers. 

Over the last several years, a lot of work has focused on how to return nature to urban areas, including increasing urban tree cover through urban greening initiatives. Dowtin’s research seeks to contribute to this effort by identifying what trees help manage water naturally and effectively.

“I’m looking specifically at how the volume and chemical properties of water that reaches the forest soil or road surface [via stemflow] changes given different forest fragment characteristics,” explains Dowtin, who also has field locations in Fair Hill, Maryland, and Banning Park in Wilmington. 

“Stemflow provides a spatially localized input of solutes, nutrients and water to the tree base, which could affect tree health. In urban areas, stemflow also is important because it helps manage atmospheric deposits — or pollutants — from cars, industrial emissions and other natural debris,” continues Levia, professor of ecohydrology and chair of geography, which is housed in the College of Earth, Ocean, and Environment, and Dowtin’s doctoral adviser.

Forest ecosystems need nutrients like manganese and sulfur to function properly and grow, but high concentrations of a particular nutrient, or too much at once, can be detrimental to forest health. In urban areas, for example, soils or plants may contain higher levels of sulfur or nitrogen due to air pollution, while plants near the coast have higher exposure to sodium from sea spray. 

With funding from the National Science Foundation, American Water Resources Association and UD’s geography department, Dowtin is investigating how the different species and density of the trees in the fragments affect the amount and chemical properties of water and solutes that cycles through the forest. 

Beech trees, for example, have smooth bark. During strong storms that drop a lot of rain over a short amount of time, water may run in sheets down this tree species and contribute to soil erosion or loss of soil productivity. Oak trees, however, typically have rough bark with deep grooves that slow the flow of water from the canopy to the forest floor.

To collect stemflow, Dowtin mounted vinyl tubing to trees at various depths within the forest field sites to direct precipitation into a container. She makes regular trips to each of her five field sites to measure the depth of the water collected in each container, then converts the depth to a total water volume using a mathematical equation. 

Over the next year, this data will provide a snapshot of how much water reaches the forest floor through stemflow in each forest fragment. Analyzing stemflow samples will allow Dowtin to measure concentrations of ions present in the water, such as chlorine, potassium, magnesium, nitrogen and phosphorous.

Historically, scientists have said stemflow represents less than 5 percent of the gross precipitation entering the soil. Levia, an expert in hydrology, contends that number is higher for some forest types. 

“While statistics have typically been calculated over the entire canopy area of the tree, measuring stemflow at the base of the tree trunk, as Asia is doing, may provide a more accurate reflection of stemflow’s contribution to the overall forest ecology,” he says.

Levia recently published a review article on stemflow’s role in the hydrology and biogeochemistry of forests and shrublands in Reviews of Geophysics, a journal of the American Geophysical Union. Co-authored with Sonja Germer, a research scientist at Leibniz Institute for Agricultural Engineering in Potsdam, Germany, it is the first review article on stemflow published outside of a hydrology journal, Levia says. 

In the paper, he and Germer provide an overview of current and historical research in this area and identify future research opportunities for hydrologists, biogeoscientists, forest ecologists and others interested in the hydrology and biogeochemistry of wooded ecosystems. 

“If we are going to develop better biogeochemistry models, we need to know where the hotspots and hot moments are — the times at which more things are being cycled in and through our forest ecosystem. These hot moments could be periods, seasons or even times within events where certain areas of the forest floor have more biogeochemical reactivity than at other times,” Levia says. 

One burgeoning question is whether, and to what extent, trees in urban forest fragments of varying sizes and shapes differentially intercept and transport atmospheric pollutants. Dowtin’s research aims to answer this precise question, among others.

Back in Wilmington, there is also the economic and safety benefit to consider, says Dowtin. “I can’t say that one tree with rough bark is going to stop city flooding, but if we are strategic in planting certain trees that can capture water on leaf surfaces or slow water down on bark surfaces, I think we can help provide some solutions to some urban water issues.”

Article by Karen B. Roberts

Video by Leah Dodd

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