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.
