Trees handled half to about three-fourths of the stormwater output at Morton Arboretum’s main parking lot, 5 acres of green infrastructure. Permeable pavers cover about half the area. The lot’s medians consist of 3 m wide bioswales graded at a 3:1 slope and amended with sand, clay, and organic matter. Underneath are perforated polyethylene storm sewers designed for a 10-year storm event.
The study, conducted by Morton Arboretum’s Center for Tree Science found that three factors most influence tree performance in a bioswales: 1) Most importantly how well leaf stomata conduct water vapor out of the plant, 2) Total leaf area/mature size of the tree and 3) The health/condition of the tree. Bioswale trees were paired with like trees installed at the same time in non-bioswale areas of the Arboretum.
In brief, the work shows that trees in bioswales account for between 46% to 72% of total water outputs from bioswales. As you would expect, not all trees perform equally well. Larger trees with more leaf area transpire more. For example, Quercus macrophylla had the highest conductance rates whether the trees were inside bioswales or the companion comparisons in the Arboretum.
Various aspects of the research were conducted at differing times. Stomatal conductance was studied for one growing season, from July to September 2014. Other measured aspects, such as the volume of stormwater inputs/outputs were monitored for 3 years.
The list of trees ranked by transpiration performance
Syringa pekinensis Morton
Acer miyabei Morton
Acer x freemanii Jeffersred
Carpinus caroliniana Walter
A separate study taking a look at the growth rates of bioswale trees showed that Ulmus, Cercis, Fraxinus and Gymnocladus all grew faster in the bioswales compared to companions growing in the Arboretum.
Tree Species Suitability to Bioswales and Impact on the Urban Water Budget, Scharenbroch, Bryant C.; Morgenroth, Justin; Maule, Brian, Journal of Environmental Quality by the The American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Inc., 10.2134/jeq2015.01.0060.
Doubly Green Trees, Soil Science Society of America blog post by Susan Fisk: http://www.soils.org/discover-soils/story/doubly-green-trees