Urban trees provide several ecosystem services through the process of transpiration, including stormwater overflow mitigation and transpirational cooling. Although planting trees in engineered green infrastructure (GI) systems can potentially increase their ability to provide such services, water availability can oscillate between extremely high and low levels in GI systems, and transpiration rates can be reduced as a result. To determine the extent to which common street tree cultivars experience limitations to transpiration in a typical GI tree trench, we tracked temporal variation in plant water relations and associated environmental controls in a set of replicate trees planted in a highly instrumented trench in Philadelphia, USA through the 2015 growing season. Specifically, we measured stomatal conductance (gs) and leaf water potential (Ψmd) at mid-day, meteorological conditions, volumetric soil water content (θv), and water available at depth in Platanus × acerifolia 'Bloodgood' (London plane tree) and Acer × freemanii 'Armstrong' (a red maple-silver maple hybrid). We then used linear modeling to determine the extent to which environmental factors contributed to temporal variation in gs and Ψmdthrough the growing season.
P. × acerifolia and A. × freemanii had similar gs rates (mean ± SE: 286 ± 6 mmol m-2 s‑1) when soil water was readily available (θv > 18%). However, when soil moisture was less available, P. × acerifolia had a more favorable water status, with Ψmd rarely exceeding -1.5 MPa and gs averaging 341 ± 11 mmol m-2 s-1. In A. × freemanii, Ψmd and gs declined sharply with decreasing θv; Ψmd frequently exceeded -1.5 MPa and mean gs declined to 64% of that measured in P. × acerifolia. In addition, gs increased with increasing atmospheric vapor pressure deficit (VPD) in P. × acerifolia, a pattern that was exacerbated under greater soil moisture levels. In contrast, gs remained constant across moderate VPD conditions in A. × freemanii, but declined when it exceeded approximately 2.0 kPa. Despite all trees having similar root system sizes and bole diameters when planted, our results indicate that P. × acerifolia was able to avoid physiological stress even when soil water became scarce during rain-free periods, whereas A. × freemanii was not. Although both cultivars can contribute substantially to transpiration-derived ecosystem services, P. × acerifolia may be better able to maximize these services in GI settings that become periodically dry.