Plant hydraulic traits have been conjectured to be coordinated, thereby providing plants with
a balanced hydraulic system that protects them from cavitation while allowing an efficient transport of
water necessary for photosynthesis. In particular, observations suggest correlations between the water
potentials at which xylem cavitation impairs water movement and the one at stomatal closure, and
between maximum xylem and stomatal conductances, begging the question as to whether such coordination
emerges as an optimal water-use strategy under unpredictable rainfall. Here mean transpiration is
used as a proxy for long-term plant fitness and its variations as a function of the water potentials at 50%
loss of stem conductivity due to cavitation and at 90% stomatal closure are explored. It is shown that coordination
between these hydraulic traits is necessary to maximize , with rainfall patterns altering the
optimal range of trait values. In contrast, coordination between ecosystem-level conductances appears not
necessary to maximize . The optimal trait ranges are wider under drier than under mesic conditions,
suggesting that in semiarid systems different water use strategies may be equally successful. Comparison
with observations across species from a range of ecosystems confirms model predictions, indicating that
the coordinated functioning of plant organs might indeed emerge from an optimal response to rainfall
variability.