Investigating the hydrologic and water quality performance of trees in bioretention mesocosms

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Cities across the world are increasingly utilizing green infrastructure practices as part of their stormwater management programs. Bioretention areas have become a popular green infrastructure practice due to their widespread success in improving water quality and reducing runoff generated from impervious surfaces. Several studies have demonstrated that pollutant removal performance can be improved when plants are included in bioretention design; however, while numerous benefits of trees in urban areas have been identified, little knowledge of their contributions to stormwater management in green infrastructure currently exists. To address this need, a controlled mesocosm experiment was conducted to characterize the degree of stormwater treatment provided by bioretention columns planted with one of three native tree species commonly found across the eastern United States (Acer rubrum – red maple, Pinus taeda – loblolly pine, and Quercus palustris – pin oak). Tree pollutant removal performance was compared to nonvegetated mesocosms using a semi-synthetic stormwater mixture applied to the mesocosms over a period of 17 weeks. The hydrologic benefits of each species were characterized using data-logging scales placed below the mesocosms to compare evapotranspiration (ET) rates and drainage in each configuration. Differences in pollutant removal between tree species were largely not significant, indicating the dominant role of the bioretention media in mitigating dissolved and particle-bound constituents. Mesocosms planted with red maple (Acer rubrum) had significantly greater average ET rates (3.2 mm d−1) than all other configurations, attributable to plant development and increased growth and canopy size. All mesocosms planted with trees had significantly higher ET rates than the nonvegetated mesocosms, illustrating the role of transpiration in bioretention hydrology which, depending on species, accounted for 8.2–37.5% of average daily water losses from the mesocosms during testing. These results suggest that trees contribute to bioretention hydrology through evapotranspiration, and that significant differences between species exist and are likely related to growth rate.