Because leaves represent a substantial fraction of total plant hydraulic resistance, their hydraulic architecture plays a major role in determining stomatal regulation of water loss. In trees, water limitation increases with height due to the effects of path-length resistance and gravity on xylem tension. We characterized changes in leaf hydraulic architecture and its consequences for gas exchange along a gradient of increasing height in Douglas-fir trees. Leaf hydraulic conductance (K_leaf), and leaf hydraulic vulnerability were analyzed using pressure-volume curves and a timed rehydration technique. A range of leaf xylem anatomical properties were analyzed with an image analysis system consisting of a compound microscope and video camera, and gas exchange was measured using a Li-Cor 6400 portable photosynthesis system.

Results/Conclusions

Determination of leaf hydraulic vulnerability curves indicated that values of leaf water potential (Ψ_l) corresponding to nearly complete loss of leaf hydraulic conductance (K_leaf) declined by 0.016 MPa per meter increase in height. Similarly, values of Ψ_l at which daily stomatal closure was initiated decreased by 0.020 MPa per meter increase in height. These parallel changes resulted in a 1:1 relationship between values of Ψ_l at initiation of stomatal closure and loss of K_leaf allowing stomata to remain open at more negative values of Ψ_l at the tops of tall trees. Adaptations to reduce leaf hydraulic vulnerability involved trade-offs that reduced shoot water transport capacity. Maximum K_leaf was reduced by 0.082 mmol m^-2 MPa^-1 s^-1 for every one-meter increase in height. To determine the basis for the observed trends of decreased hydraulic vulnerability and efficiency with height, we analyzed leaf tracheid anatomical properties of foliage from the tops of Douglas-fir trees along a height gradient from 5 to 55 m. Total tracheid lumen area per needle cross section, hydraulic mean diameter of leaf tracheid lumens, total number of tracheids per needle cross section, leaf tracheid length and pit number per tracheid decreased with height by 18.4 mm² m^-1, 0.029 mm m^-1, 0.42 m^-1, and 5.3 mm m^-1, and 0.07 m^-1respectively. Tracheid thickness-to-span ratio (t_w/b)² increased with height by 1.04 x 10^-3 m^-1. Coordinated adjustments in leaf hydraulic architecture with increasing height were associated with constraints on gas exchange that may dominate hydraulic constraints associated with path length and changes in leaf-specific conductance. Photosynthesis at ambient [CO₂] decreased exponentially with height and increased linearly with K_leaf-max. Vertical tension gradient may be the ultimate driver of these changes.