Maximum height in a conifer is associated with conflicting requirements for xylem design

Copyright © 2008 National Academy of Sciences. All rights reserved. The copy here is the final accepted version before it was copyedited by the journal. The final version is available at http://www.pnas.org/.Despite renewed interest in the nature of limitations on maximum tree height, the mechanisms governing ultimate and species-specific height limits 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 used morphological measurements to estimate the hydraulic properties of the bordered pits between tracheids in Douglas-fir trees along a height gradient of 85 m. With increasing height, the xylem structural modifications that satisfied hydraulic requirements for avoidance of runaway embolism imposed increasing constraints on water transport efficiency. In the branches and trunks, the pit aperture diameter of tracheids decreases steadily with height, whereas torus diameter remains relatively constant. The resulting increase in the ratio of torus to pit aperture diameter allows the pits to withstand higher tensions before air-seeding, but at the cost of reduced 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 this species. Likewise, the twig water potential corresponding to the threshold for runaway embolism would be attained at a height of about 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