Comparing Stomatal Conductance of Street Tree Species in Traditional Tree Pits and Green Infrastructure Storm Water Trenches

Urban combined sewer systems (CSS) such as that of Philadelphia can lead to combined sewer overflows (CSO) during intense storm events. CSO generate serious environmental concern due to their introduction of untreated waste into local waters. Philadelphia’s Green City, Clean Waters initiative calls for significant implementations of green infrastructure in order to control the amount of storm water runoff entering the sewer system. One such structure is a storm water tree trench, which consists of soil-filled tree pits installed on top of a large gravel-filled catchment area beneath the sidewalk. Storm water, collected through curbside inlets, is distributed through the catchment area via a perforated pipe. The trenches are designed to retain large volumes of water, with a portion eventually percolating into the soil below, and some being accessed and transpired by the trees above. In order to assess the effectiveness of the storm water trenches in controlling CSO, it is necessary to evaluate the water relations of the various associated tree species. Stomatal conductance measures the concentration of water vapor released from tree leaves, and thus it serves as an excellent proxy for total evapotranspiration as well as water stress. The stomatal conductance of 25 trees of 13 different species/cultivars located on two blocks in the Mt. Airy section of Philadelphia was monitored from June-October 2015. In assessing 25 different trees, the mean ranks of conductance data by species were significantly different, suggesting some species may be better suited for use in urban GI trench trees than others. Two potential benefits of these species are 1) they have higher overall levels of conductance resulting in greater water movement out of the system via evapotranspiration following rain events, and 2) they may be less susceptible to water stress during periods of low precipitation. Furthermore, pairwise comparisons revealed that Koelreuteria paniculata Laxm. and Prunus sargentii Rehder trees conducted significantly more water on average within storm water trenches compared to traditional, isolated tree pits, whereas Quercus macrocarpa Michx. and Quercus robur L. showed the opposite tendency. This research provides useful insight for further development of storm water tree trenches by showing that tree species selection can influence the effectiveness of the system.