Background/Question/Methods
The mechanisms governing ultimate and species-specific limits on maximum tree height are not yet understood, but likely involve water transport dynamics. Tall trees experience increased risk of xylem embolism from air-seeding because tension in their water column increases with height due to path-length resistance and gravity. We hypothesized that as an adaptive response to the higher xylem tension in the tops of tall Douglas-fir trees, their xylem would show structural modifications that decrease the risk of embolism but also decrease the efficiency of water transport. We used morphological measurements to estimate the hydraulic conductance and vulnerability of the bordered pit connections between tracheids in trees ranging from 6 to 85.5 meters in height. Because water ascending within conifer xylem must pass through the bordered pits of overlapping tracheids, pit characteristics are a major determinant of tracheid and whole xylem hydraulic conductance. As a check on xylem hydraulic properties estimated from pit characteristics, we also measured daily minimum water potential on twigs excised from different heights, and constructed xylem vulnerability curves using the air-injection method on branches collected from a subset of heights.
Results/Conclusions
With increasing height, the xylem structural modifications that satisfied requirements for avoidance of runaway embolism imposed increasing constraints on water transport efficiency. The tracheid pit aperture diameter decreased steadily with height, whereas torus diameter remained relatively constant. The resulting increase in the ratio of torus to pit aperture diameter allowed the pits to withstand higher tensions before air-seeding, but at the cost of reduced pit aperture conductance. In branches, a 4.5 MPa increase in the air-seeding tension between 5 and 80 m height was attained at the expense of an 80% reduction in pit aperture conductance. Extrapolations of vertical trends for trunks and branches show that water transport across pits will approach zero at a height of 109 m and 138 m, respectively, which is consistent with historic height records of 100 - 127 m for Douglas-fir. Likewise, the twig water potential at which pit aperture conductance approaches zero (about -3.2 MPa) would be attained at a height of 107 m. Our results suggest that the ultimate height of Douglas-fir trees may be limited in part by the conflicting requirements for water transport and water column safety. This trade-off may impose structural limits on tree height concomitant with reduced capacity of processes such as photosynthesis and growth to adjust to vertical gradients of increasing xylem tension.