Evaluating the influence of design strategies and meteorological factors on tree transpiration in bioretention suspended pavement practices

Impervious surfaces, such as roads, parking areas, and buildings, found in cities throughout the world have significant impacts on urban hydrology due to increased volumes and peak flow rates of run‐off delivered to receiving waterbodies. Bioretention practices are a common stormwater control measure used to mitigate the impacts of urban run‐off. When coupled with suspended pavement systems, which provide tree roots with an uncompacted soil matrix that enhances root access to oxygen and water, engineers can design subsurface alternatives to manage urban stormwater. Two suspended pavement systems designed to function as subsurface bioretention practices were installed on the campus of the University of Tennessee, Knoxville, Tennessee, USA. Sap flow sensors using the heat ratio method were installed in two bald cypress ( Taxodium distichum ) trees to characterize the role of transpiration in the suspended pavement systems. Mean transpiration rates were greater when water availability was higher in the bioretention media. Regression models indicated that atmospheric vapour pressure deficit (kPa) was the most influential environmental parameter on tree transpiration and that stomatal regulation of water losses was evident when water was limiting. Findings from this study illustrate how tree transpiration rates can vary, even between individual trees of the same species, on the basis of conditions within the practice, and provide insight to practitioners on how design parameters influence fine‐scale tree–water relations in bioretention systems to maximize the contributions of transpiration on system hydrology.