Biophysical controls on plant water status exist at the leaf, stem, and root levels. Therefore, we pose that hydraulic strategy is a combination of traits governing water use at each of these three levels. We studied sap flux, stem water storage, stomatal conductance, photosynthesis, and growth of red oaks (Quercus rubra) and red maples (Acer rubrum). These species differ in stomatal hydraulic strategy and xylem architecture and may root at different depths. Stable isotope analysis of xylem water was used to identify root water uptake depth. Oaks were shown to access a deeper water source than maples. During non‐limiting soil moisture conditions, transpiration was greater in maples than in oaks. However, during a soil dry down, transpiration and stem water storage decreased by more than 80% and 28% in maples but only by 31% and 1% in oaks. We suggest that the preferential use of deep water by red oaks allows the species to continue transpiration and growth during soil water limitations. In this case, deeper roots may provide a buffer against drought‐induced mortality. Using 14 years of growth data, we show that maple growth correlates with mean annual soil moisture at 30 cm but oak growth does not. The observed responses of oak and maple to drought were not able to be explained by leaf and xylem physiology alone. We employed the Finite‐difference Ecosystem‐scale Tree Crown Hydrodynamics model version 2 plant hydrodynamics model to demonstrate the influence of root, stem, and leaf controls on tree‐level transpiration. We conclude that all three levels of hydraulic traits are required to define hydraulic strategy.

Stable isotope ratios of hydrogen and oxygen have been applied to water cycle research for over 60 years. Over the past two decades, however, new data, data compilations, and quantitative methods have supported the application of isotopic data to address large-scale water cycle problems. Recent results have demonstrated the impact of climate variation on atmospheric water cycling, provided constraints on continental- to global-scale land-atmosphere water vapor fluxes, revealed biases in the sources of runoff in hydrological models, and illustrated regional patterns of water use and management by people. In the past decade, global isotopic observations have spurred new debate over the role of soils in the water cycle, with potential to impact both ecological and hydrological theory. Many components of the water cycle remain underrepresented in isotopic databases. Increasing accessibility of analyses and improved platforms for data sharing will refine and grow the breadth of these contributions in the future. ▪ Isotope ratios in water integrate information on hydrological processes over scales from cities to the globe. ▪ Tracing water with isotopes helps reveal the processes that govern variability in the water cycle and may govern future global changes. ▪ Improvements in instrumentation, data sharing, and quantitative analysis have advanced isotopic water cycle science over the past 20 years.

Local meteoric water lines (LMWLs) represent the site‐specific long‐term covariation of hydrogen and oxygen stable isotope ratios. LMWLs have practical utility as a hydrologic framework and as benchmarks for evaluating hydroclimatic processes in isotope‐enabled climate models. In this manuscript, we characterize the global distribution of LMWLs and compare them to LMWLs from model data. To evaluate the sensitivity of the covariance of stable isotope ratios to data set length, we paired time series rarifaction with Bayesian ellipse estimation. We then applied a threshold of 48 months and estimated LMWLs at 398 sites in 25 Köppen climate classes using orthogonal distance regression. Slopes ranged from 4.8 to 10.9, with an average of 7.64 ± 0.64. Intercepts ranged from −24‰ to 27‰, with an average of 6.85 ± 6.2‰. We identified three processes: (1) subcloud evaporation of rain, (2) atmospheric remoistening by rainfall evaporation, and (3) conditions of snow formation as important controls on slopes and intercepts in arid, humid, and seasonally snowy regions, respectively. We compared observational LMWLs with those from a suite of isotope‐enabled climate models. At arid and snowy sites, model data produced higher slopes and intercepts than observational data. At humid sites, model data exhibited dampened variability in slopes and intercepts relative to observational data. These results indicate potential for improvement in the precipitation and/or isotope parameterizations of raindrop evaporation, advection of reevaporated water, evapotranspiration fractionation, and supersaturation in mixed‐phase clouds. This meta‐analysis demonstrates LMWLs utility for identifying specific hydroclimatic and isotopic processes in observations and models.

Continued anthropogenic CO2 emissions are expected to drive widespread changes in precipitation characteristics. Nonetheless, projections of precipitation change vary considerably at the regional scale between climate models. Here, it is shown that the response of plant physiology to elevated CO2, or CO2 physiological forcing drives widespread hydrologic changes distinct from those associated with CO2 radiative forcing and has a role in shaping regional-scale differences in projected daily-scale precipitation changes. In a suite of simulations with the Community Climate System Model, version 4 (CCSM4), reduced stomatal conductance from projected physiological forcing drives large decreases in transpiration and changes the distribution of daily-scale precipitation within and adjacent to regions of dense vegetation and climatologically high transpiration. When atmospheric conditions are marginally favorable for precipitation, reduced transpiration dries the boundary layer and increases the likelihood of dry day occurrence. In CCSM4, the annual number of dry days increases by upward of 15 days yr−1 over tropical land and the continental midlatitudes. Decreases in transpiration from physiological forcing also increase the number of heavy precipitation events by up to 8 days yr−1 in many tropical forest regions. Despite reductions in the land surface contribution to atmospheric moisture, diminished surface latent heat fluxes warm the forest boundary layer and increase moisture convergence from nearby oceans, enhancing instability. The results suggest that consideration of the radiative impacts of CO2 alone cannot account for projected regional-scale differences in daily precipitation changes, and that CO2 physiological forcing may contribute to differences in projected precipitation characteristics among climate models.