• drought;
  • pipe model;
  • sapwood;
  • water relations;
  • water vapour pressure deficit


Future increases in air temperature resulting from human activities may increase the water vapour pressure deficit (VPD) of the atmosphere. Understanding the responses of trees to spatial variation in VPD can strengthen our ability to predict how trees will respond to temporal changes in this important variable. Using published values, we tested the theoretical prediction that conifers decrease their investment in photosynthetic tissue (leaves) relative to water-conducting tissue in the stem (sapwood) as VPD increases. The ratio of leaf/sapwood area (AL/AS) decreased significantly with increasing VPD in Pinus species but not in Abies, Pseudotsuga, Tsuga and Picea, and the average AL/AS was significantly lower for pines than other conifers (pines: 0.17 m2 cm−2; nonpines: 0.44 m2 cm−2). Thus, pines adjusted to increasing aridity by altering above-ground morphology while nonpine conifers did not. The average water potential causing a 50% loss of hydraulic conductivity was −3.28 MPa for pines and −4.52 MPa for nonpine conifers, suggesting that pines are more vulnerable to xylem embolism than other conifers. For Pinus ponderosa the decrease in AL/AS with high VPD increases the capacity to provide water to foliage without escalating the risk of xylem embolism. Low AL/AS and plasticity in this variable may enhance drought tolerance in pines. However, lower AL/AS with increasing VPD and an associated shift in biomass allocation from foliage to stems suggests that pines may expend more photosynthate constructing and supporting structural mass and carry less leaf area as the climate warms.