Water relations of street trees in green infrastructure tree trench systems

Abstract

Urban trees provide a number of ecosystem services through the process of transpiration, including evaporative cooling and returning stormwater to the atmosphere. Collocating street trees and engineered stormwater catchments in ‘tree trenches’ is a space-efficient strategy for providing these services and managing stormwater simultaneously; such systems are therefore being installed in a growing number of cities worldwide. However, tree trenches are designed to infiltrate rapidly, which could limit water availability in trenches and soil pits, thereby restricting transpiration between storms and eventually impacting tree health and survival. Focusing on tree trenches in Philadelphia, USA, we sought to determine how the water relations of young trees varied with atmospheric and soil moisture conditions, if water limitation affected photosynthesis and basal growth, and whether interspecific differences in water use were related to leaf economics. Measurements of stomatal conductance (gs) from 13 species and cultivars indicated that water use varied widely across taxa; mean gs ranged from 161 to 507 mmol m−2 s-1, as did the responsiveness of gs to increasing vapor pressure deficit. Mean gs was correlated with photosynthesis across all taxa, with gas exchange appearing to influence growth rates in the majority of species. We found no evidence that the sorting of taxa was related to leaf economics, but it was modestly consistent with prior assessments of tolerance to xylem cavitation risk. Strong differences in leaf water potential between the two taxa for which it was measured (Acer × freemanii and Platanus × acerifolia, both of which are isohydric) suggested that some taxa may have avoided water limitation by accessing supplemental water sources such as the soil beneath trenches. These results demonstrate that young individuals of common street tree taxa vary considerably in their ability to maintain favorable water relations through hot and dry periods in tree trenches as they are typically designed.