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Axial variation of xylem conduits in the Earth’s tallest trees


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In the Earth’s tallest tree species, axial variation in conduit size matches theoretical predictions minimizing the accumulation of hydraulic resistance with height growth, within a constraint of maximum conduit diameter.


Hydraulic limitations to tree height can be mitigated by widening the conducting elements toward a tree’s base. However, size limits of tracheid and vessel dimensions may constrain this compensation mechanism as the water transport pathway elongates. Moreover, variation in conduit size is poorly described in tall trees even though their long transport paths have high potential for hydraulic resistance. Here, we evaluated whether axial variation in conduit diameter was uniquely structured, or matched theoretical predictions in Sequoia sempervirens, Sequoiadendron giganteum, and Eucalyptus regnans that were 86–105 m tall and exceeded 85% of the maximum height for each species. Across Sequoia and Sequoiadendron, tree top tracheids maintained constant width, whereas tree base tracheids in the outermost ring were 20% wider in taller trees, suggesting maintenance of basipetal conduit widening with height growth. In all trees, the observed widening decreased at a rate per unit path length that fitted well to a power function with an exponent consistent with hydraulic compensation. However, below about 60 m from the tree tops, conduit diameters approached an asymptote beneath the power function, indicating a limit to maximum conduit size. Quantifying the distribution of base-to-top hydraulic resistance suggested that the minimal hydraulic benefit gained with increasingly wider conduits near the tree base may trade off with other factors such as maintaining mechanical strength or reducing fluid volume. We summarize these results into an anatomical model of height growth that includes limits to axial variation in conduit diameter and is supported by many physiological and anatomical observations.

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This research was supported by a grant from the National Science Foundation (IOS-1010769). T.A. was supported by the project UNIFORALL (University of Padova, Progetti di Ricerca di Ateneo CPDA110234). Anthony Ambrose, Wendy Baxter, Tom Greenwood, Joe Harris, Rikke Reese Næsborg, and Giacomo Renzullo helped collect critical samples. Anthony Ambrose, John Battles, Cindy Looy, and Stefania Mambelli provided constructive comments on the manuscript. We are grateful to Ion Maher and Tony Fitzgerald of Kinglake National Park, Jay Harris of California State Parks, Koren Nydick (then, with Sequoia and Kings Canyon National Parks), and Rob York of Whitaker’s Forest Research Station for research permissions.

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Correspondence to Cameron B. Williams.

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Communicated by Nardini.

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Williams, C.B., Anfodillo, T., Crivellaro, A. et al. Axial variation of xylem conduits in the Earth’s tallest trees. Trees 33, 1299–1311 (2019).

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  • Coast redwood
  • Giant sequoia
  • Hydraulic resistance
  • Metabolic scaling theory
  • Mountain ash
  • Tall trees
  • Tree height growth
  • Wood anatomy