The coordination of whole-plant and branch-level transpiration provides unique insight in of canopy-atmosphere decoupling and hydraulic capacitance

Rising global temperatures increase evaporative demand and transpiration (E), straining plant hydraulic pathways and limiting vegetation productivity. The theoretical basis for how water supply (i.e., soil moisture) and demand (i.e., vapor pressure deficit (D)) affect plant water potential gradients () is well developed; yet, fully mechanistic predictions of organ- and whole-plant remain uncertain due an abundance of low-throughput measurements (i.e., leaf pressure chamber) collected at weekly and seasonal timescales. While weekly timescales reflect soil drying, it is too coarse to capture faster-acting hydrodynamic processes, including stomatal responses to D and phenotypic variation (e.g., degree of isohydricity or hydraulic capacitance (C)) in the presence of rapidly shifting environmental conditions. Here, we harness the high-temporal resolution of stem water potential (Stem) measurements from psychrometers installed at a height of ~ 20 m, together with E from sapflux (JS) paired with pressure chamber measurements to characterize the diel and seasonal as they correspond to hydraulic adjustments of forests in southern Indiana. Specifically, we ask, how does canopy E and C adjust to seasonal D gradients and soil moisture limitations? We focus on broadleaf deciduous species spanning a gradient of iso- to anisohydric water-use phenotypes. Most of the 2021 growing season was characterized by high and non-limiting soil moisture levels. However, variability in D nonetheless drove substantial variation in , C, and E, which we were able to quantify with this high-resolution dataset. As predicted, isohydric tulip poplars maintained a higher Stem relative to the anisohydric oaks for both pressure chamber and psychrometer measurements. Whole-plant C in oaks was 35% higher than tulip poplars, though expressed greater diel variation. Branch JS lagged whole-plant JS by 7 and 100 minutes for oaks and tulip poplars, respectively. The elevated C along with lower canopy-atmosphere decoupling allows the anisohydric oaks to maintain E and during periods of high D. Coupling high-temporal resolution measures of C, Stem, and E at the branch and whole-plant scales provides a novel first principle approach for constraining uncertainty for predicting and the importance of isohydric regulation.