Vegetation water content (VWC) plays a key role in transpiration, plant mortality, and wildfire risk. Although land surface models now often contain plant hydraulics schemes, there are few direct VWC measurements to constrain these models at global scale. One proposed solution to this data gap is passive microwave remote sensing, which is sensitive to temporal changes in VWC. Here, we test that approach by using synthetic microwave observations to constrain VWC and surface soil moisture within the CliMA Land model. We further investigate the possible utility of sub-daily observations of VWC, which could be obtained through a satellite in geostationary orbit or combinations of multiple satellites. These high-temporal-resolution observations could allow for improved determination of ecosystem parameters, carbon and water fluxes, and subsurface hydraulics, relative to the currently available twice-daily sun-synchronous observational patterns. We find that incorporating observations at four different times in the diurnal cycle (such as could be available from two sun-synchronous satellites) provides a significantly better constraint on water and carbon fluxes than twice-daily observations do. For example, the root mean square errors (RMSE) of projected evapotranspiration and gross primary productivity during drought periods was reduced by approximately 40%, when using four-times-daily relative to twice-daily observations. Adding hourly observations of the entire diurnal cycle did not further improve the inferred parameters and fluxes. Our comparison of observational strategies may be informative in the design of future satellite missions to study plant hydraulics, as well as when using existing remotely sensed data to study vegetation water stress response.

Water returned to the atmosphere as evapotranspiration (ET) is approximately 1.6x greater than global river discharge and has wide-reaching impacts on groundwater and streamflow. In the U.S. Midwest, widespread land conversion from prairie to cropland has altered spatiotemporal patterns of ET, yet there is no consensus on the direction of change in ET or the mechanisms controlling changes. We aimed to harmonize findings about how land use change affects ET in the Midwest. We measured ET at three locations within the Long-Term Agroecosystem Research (LTAR) network along a latitudinal gradient with paired rainfed cropland and prairie sites at each location. At the northern locations, the Upper Mississippi River Basin (UMRB) and Kellogg Biological Station (KBS), the cropland has annual ET that is 84 and 29 mm/year higher, respectively, caused primarily by higher ET, likely from soil evaporation during springtime when agricultural fields are fallow. At the southern location, the Central Mississippi River Basin (CMRB), the prairie has 69 mm/year higher ET, primarily due to a longer growing season. To attribute differences in springtime ET to specific mechanisms, we examine the energy balance using the Two-Resistance Method (TRM). Results from the TRM demonstrate that higher surface conductance in croplands is the primary factor leading to higher springtime ET from croplands, relative to prairies. Results from this study provide critical insight into the impact of land use change on the hydrology of the U.S. Corn Belt by providing a mechanistic understanding of how land use change affects the water budget.

The relationships between water potential and water content in plants and soil have long been of interest, and there is increasing focus on understanding how these fundamental measures are linked at larger spatial and temporal scales. In this Perspective, we explore how the theory of pressure-volume (PV) relationships can be applied at ecosystem scale. We define and evaluate the concept and limitations of the ecosystem and vegetation pressure-volume curves and discuss its application using existing data. As a proof of concept, plot-scale aboveground vegetation PV curves were generated from equilibrium (e.g. predawn) water potentials and water content of the above ground biomass of nine plots including tropical rainforest, savanna, temperate forest, and a long-term Amazonian rainforest drought experiment. Initial findings suggest high consistency among sites where the steady-state water:biomass ratio is approximately 1:3, while the relative values of ecosystem hydraulic capacitance and accessible water storage (the water volume between saturation and a threshold) do not vary systematically with biomass. The ecosystem-scale PV relationship provides a thermodynamically consistent steady-state view of ecosystem form and function and a biophysically robust basis for the interpretation of remote sensing data of vegetation and soil water content, with promise for revealing useful trends across ecosystems.

The coordination of plant leaf water potential (ΨL) regulation and xylem vulnerability to embolism is fundamental for understanding the tradeoffs between carbon uptake and risk of hydraulic damage. There is a general consensus that trees with vulnerable xylem regulate ΨL more conservatively than plants with resistant xylem. We evaluated if this paradigm applied to three important eastern US temperate tree species, Quercus alba L., Acer saccharum Marsh., and Liriodendron tulipifera L., by synthesizing 1600 ΨL observations, 122 xylem embolism curves, and xylem anatomical measurements across ten forests spanning pronounced hydroclimatological gradients and ages. We found that, unexpectedly, the species with the most vulnerable xylem (Q. alba) regulated ΨL less strictly than the other species. This relationship was found across all sites, such that coordination among traits was largely unaffected by climate and stand age. Quercus species are perceived to be among the most drought tolerant temperate US forest species; however, our results suggest their relatively loose ΨL regulation in response to hydrologic stress occurs with a substantial hydraulic cost that may expose them to novel risks in a more drought-prone future. We end by discussing mechanisms that allow these species to tolerate and/or recover from hydraulic damage.

The coordination of plant leaf water potential (ΨL) regulation and xylem vulnerability to embolism is fundamental for understanding the tradeoffs between carbon uptake and risk of hydraulic damage. A legacy of observations in drylands suggests plants with vulnerable xylem more carefully regulate ΨL than plants with resistant xylem. We synthesized over 1600 ΨL observations, 122 xylem embolism curves, and xylem anatomical measurements of Quercus alba L., Liriodendron tulipifera L., and Acer saccharum Marsh. across ten contrasting forests to evaluate if the paradigm linking conservative ΨL regulation to vulnerable xylem applies to temperate deciduous trees. Additionally, we explored generalizable patterns of hydraulic trait acclimation in relation to forest age and climate. Contrary to the dryland paradigm, we found that the tree species with the most vulnerable xylem (e.g., Q. alba) regulated ΨL less strictly (anisohydric behavior) than the species with xylem more resistant to embolism (e.g., A. saccharum and L. tulipifera). This relationship was found across all sites, suggesting coordination among traits was largely unaffected spatio-temporal factors. Our findings indicate drought-response traits of temperate deciduous forest species are coordinated in fundamentally different ways than vegetation in arid climates.