Conifers decrease the amount of biomass apportioned to leaves relative to sapwood in response to increasing atmospheric evaporative demand. We determined how these climate-driven shifts in allocation affect the aboveground water relations of ponderosa pine growing in contrasting arid (desert) and humid (montane) climates. To support higher transpiration rates, a low leaf:sapwood area ratio (AL/As) in desert versus montane trees could increase leaf-specific hydraulic conductance (KL). Alternatively, a high sapwood volume:leaf area ratio in the desert environment may increase the contribution of stored water to transpiration. Transpiration and hydraulic conductance were determined by measuring sap flow (JS) and shoot water potential during the summer (June-July) and fall (August-September). The daily contribution of stored water to transpiration was determined using the lag between the beginning of transpiration from the crown at sunrise and JS. In the summer, mean maximum JS was 31.80±5.74 and 24.34±3.05 g m-2s-1 for desert and montane trees (a 30.6% difference), respectively. In the fall, JS was 25.33±8.52 and 16.36±4.64 g m-2-1 in desert and montane trees (a 54.8% difference), respectively. JS was significantly higher in desert relative to montane trees during summer and fall (P<0.05). Predawn and midday shoot water potential and sapwood relative water content did not differ between environments. Desert trees had a 129% higher KL than montane trees in the summer (2.41×10-5 versus 1.05x10-5 kg m-2 s-1 MPa-1, P<0.001) and a 162% higher KL in the fall (1.97×10-5 versus 0.75x10-5 kg m-2 s-1MPa-1 P<0.001). Canopy conductance decreased with D in all trees at all measurement periods (P<0.05). Maximum gC was 3.91 times higher in desert relative to montane trees averaged over the summer and fall. Water storage capacity accounted for 11 kg (11%) and 10.6 kg (17%) of daily transpiration in the summer and fall, respectively, and did not differ between desert and montane trees. By preventing xylem tensions from reaching levels that cause xylem cavitation, high KL in desert ponderosa pine may facilitate its avoidance. Thus, the primary benefit of low leaf:sapwood allocation in progressively arid environments is to increase KL and not to increase the contribution of stored water to transpiration.
- Biomass allocation
- Leaf-specific hydraulic conductance
- Water storage
- Xylem cavitation
ASJC Scopus subject areas
- Ecology, Evolution, Behavior and Systematics