Throughout the southern US, past forest management practices have replaced large areas of native
forests with loblolly pine plantations and have resulted in changes in forest response to extreme
weather conditions. However, uncertainty remains about the response of planted versus natural
species to drought across the geographical range of these forests. Taking advantage of a cluster of
unmanaged stands (85–130 year-old hardwoods) and managed plantations (17–20 year-old loblolly
pine) in coastal and Piedmont areas of North Carolina, tree water use, cavitation resistance,
whole-tree hydraulic (Ktree) and stomatal (Gs) conductances were measured in four sites covering
representative forests growing in the region. We also used a hydraulic model to predict the resilience
of those sites to extreme soil drying. Our objectives were to determine: (1) if Ktree and stomatal
regulation in response to atmospheric and soil droughts differ between species and sites; (2) how
ecosystem type, through tree water use, resistance to cavitation and rooting profiles, affects the water
uptake limit that can be reached under drought; and (3) the influence of stand species composition on
critical transpiration that sets a functional water uptake limit under drought conditions. The results
show that across sites, water stress affected the coordination between Ktree and Gs. As soil water
content dropped below 20% relative extractable water, Ktree declined faster and thus explained the
decrease in Gs and in its sensitivity to vapor pressure deficit. Compared to branches, the capability
of roots to resist high xylem tension has a great impact on tree-level water use and ultimately had
important implications for pine plantations resistance to future summer droughts. Model simulations
revealed that the decline in Ktree due to xylem cavitation aggravated the effects of soil drying on tree
transpiration. The critical transpiration rate (Ecrit), which corresponds to the maximum rate at which
transpiration begins to level off to prevent irreversible hydraulic failure, was higher in managed forest
plantations than in their unmanaged counterparts. However, even with this higher Ecrit, the pine
plantations operated very close to their critical leaf water potentials (i.e. to their permissible water
potentials without total hydraulic failure), suggesting that intensively managed plantations are more
drought-sensitive and can withstand less severe drought than natural forests.
