| Our knowledge about coordination of water transport at the whole-tree scale has been limited in its consideration of water storage and release from different tissues (i.e., sapwood and heartwood) along the xylem pathway. This constrains our ability to accurately model ecosystem water balance and plant species responses to current and projected climate changes, which is imperative considering that forests worldwide, including those in the Western US, are experiencing extensive mortality events due to human-induced drought and heat stress. Here, I utilized various novel methodologies to improve our understanding regarding whole-tree water transport and storage. This involved quantifying the coordination of sap flow and volumetric water content (VWC) at different tree trunk heights and xylem tissue depths in four tree species of contrasting hydraulic strategies (loblolly pine, southern red oak, Douglas-fir, and trembling aspen) over summer seasons in two environmental systems (mesic/semi-arid). Further, I utilized a novel in-situ borehole method to quantify the natural isotopic variation among different tree heights and depths at sub-daily timescales in a tulip poplar and loblolly pine tree. Lastly, I used an optimization model and the record-low precipitation conditions in the recent 2025-2026 winter in Idaho to predict the physiological performance of Douglas-fir and trembling aspen under projected climate scenarios. I found that water transport and storage were not well coordinated in the semi-arid species, and that angiosperm species displayed reliance on heartwood and sapwood stored water. Despite issues encountered with the novel in-situ borehole method, I found evidence for spatial variation of xylem water isotopes between different tree heights and tissue depths. I also found that relative humidity should be measured consistently in borehole in-situ approaches. Further, modelling efforts revealed that Douglas-fir and aspen may experience lower carbon uptake and increased vulnerability to mortality under low snowpack and hot, dry summers, conditions expected to be more prevalent in the future for the Intermountain West. My findings emphasize the importance of conducting spatially and temporally intensive studies across species and ecosystems to better understand water transport and storage, and they highlight the need for accurate modelling of plant species responses to current and projected climate changes. Keywords: water transport, water storage, hydraulic coordination, xylem water isotopes, plant physiological performance, climate change |