Research Shows Urban Trees Offer Pollution Solution

Urban environments in Australia are not that different than those in the U.S. They also struggle with contaminated water running off and causing pollution. In response, cities often use natural landscapes of soil, grasses and trees. These biofiltration systems capture and filter the runoff.

How well do they filter runoff? Research on how soils and woody plants like trees filter water is lacking. In response, researchers at the University of Melbourne designed an experiment.

Researchers planted four different trees commonly used as street trees in Australia in three different soil types (leaving some unplanted to serve as controls). To these, they applied either a solution similar to storm water or regular tap water. The storm water contained high levels of nutrients such as nitrogen and phosphorus. High levels of these nutrients are typically present in storm water in part because of fertilizer runoff.

Researcher Liz Denman, now a member of the Landscape and Urban Design group at VicRoads, explained that high levels of nitrogen and phosphorus can quickly go from nutrient to contaminate. The levels stimulate algal blooms—or higher than normal algae content in water bodies. Algae consume oxygen in the water, especially as they die and decompose. They also block the sun from reaching areas below them, upsetting the natural state of the water body. The lower oxygen levels can result in fish deaths, a loss of biodiversity in the waters and dead zones.

Some Australia states use mandates as a response. “The planning provisions in the Australian state where I live have a clause that relates to water management in residential subdivisions,” she said. “This mandates targets for pollutant load reductions. In most cases, these guidelines require the incorporation of water quality treatment systems.”

Through the 13-month experiment, the group measured how well the different tree species grew when watered with storm water. They also looked at how well the trees took extra nutrients out of the storm water.

The trees studied were from four species: red box, brush box, white bottlebrush and oriental plane. All four species receiving storm water grew significantly larger than those receiving tap water, demonstrating they all have the potential to thrive under urban conditions.

Additionally, the results showed the trees are good at unpacking nutrients from storm water. Scientists collected the water that drained from the bottom of the experimental pots for a period of two hours after watering. This water contained fewer undesired nutrients when it came from the planted soils. The unplanted soils actually leached nutrients. The different tree species did not show much difference in how many nutrients they removed from the storm water. They all performed well in the study.

“Species selection was not essential to maximize nutrient removal performance of biofiltration systems,” Denman said, although overall it was more difficult for the trees and soil to reduce nitrogen than phosphorus.

Based on these findings, Denman suggested using a variety of trees. “Biodiversity of vegetation within our cities is important and street tree selection should not be based on a single criterion,” Denman said.

Additionally, one species will not fit everywhere. “The selection of storm water treatment measures suitable for highly urbanized streets is restricted by the limited available space combined with the need for functional and aesthetically pleasing landscapes,” she said.

Biofiltration systems are already in place in many large Australian cities. Denman added that further evaluations are needed to test the effectiveness of the trees as they mature or go through extremely wet or dry periods.

Source: American Society of Agronomy, Crop Science Society of America, Soil Science Society of America