Urban trees contribute strongly to the water balance of cities rendering essential a quantitative understanding of their water use and its regulation. Tree evapotranspiration and sapflow and their sensitivity to atmospheric drought (vapor pressure deficit; VPD) have been described by several empirical, mathematical functions representing the plant water transport system. Leaves represent a major bottleneck in the control of plant water transport yet their contribution to the regulation of sapflow and its response to VPD has remained unclear. For seven common urban trees of Los Angeles, we tested hypotheses for the role of leaf hydraulic vulnerability and functional traits in determining the response of sapflow to VPD. We measured the response of leaf hydraulic conductance (Kleaf) to declining water status (leaf bulk water potential; Ψ) and quantified the sensitivity to hydraulic decline as the leaf water potential at 50% loss of Kleaf (P50). We hypothesized that (1) Kmax would strongly predict whole tree transpiration rates and sensitivity to VPD and that (2) P50would drive whole tree sensitivity to VPD. To expand this framework, we also compared the strength of relationships between these leaf and whole plant water use performance traits and associated hypothesized functional traits measured in the field.
Across the 7 species of urban trees, we found substantial variation (>27-fold) in leaf hydraulic conductance for well-hydrated leaves (Kmax), and in P50 (from –0.3 to –2.3 MPa), as well as in the sensitivity of sapflow to VPD as estimated using the transpiration sensitivity model described by Litvak et al. in 2012 (>2-fold). We found novel relationships between the control of sapflow and leaf physiological and functional traits. The sensitivity of sapflow to VPD was positively related to Kmax and P50 across species, indicating that species with leaves that transport water more efficiently when hydrated, yet reduce their conductance during dehydration, also reduce their transpiration and sapflow most strongly at high VPD. Whole tree water use and its sensitivity to VPD showed weak coordination or independence from other functional traits, such as leaf mass per area, leaf density, or the water potential at which leaf turgor is lost (Ψtlp). This study provides novel insights into the hydraulic basis for tree water use and an improved ability to predict whole tree behaviour from leaf physiology across urban tree species.