title: Hydraulic constraints modify optimal photosynthetic profiles in giant sequoia trees.
abstract: Optimality theory states that whole-tree carbon gain is maximized when leaf N and photosynthetic capacity profiles are distributed along vertical light gradients such that the marginal gain of nitrogen investment is identical among leaves. However, observed photosynthetic N gradients in trees do not follow this prediction, and the causes for this apparent discrepancy remain uncertain. Our objective was to evaluate how hydraulic limitations potentially modify crown-level optimization in Sequoiadendron giganteum (giant sequoia) trees up to 90 m tall. Leaf water potential (Ψ l ) and branch sap flow closely followed diurnal patterns of solar radiation throughout each tree crown. Minimum leaf water potential correlated negatively with height above ground, while leaf mass per area (LMA), shoot mass per area (SMA), leaf nitrogen content (%N), and bulk leaf stable carbon isotope ratios (δ(13)C) correlated positively with height. We found no significant vertical trends in maximum leaf photosynthesis (A), stomatal conductance (g s), and intrinsic water-use efficiency (A/g s), nor in branch-averaged transpiration (E L), stomatal conductance (G S), and hydraulic conductance (K L). Adjustments in hydraulic architecture appear to partially compensate for increasing hydraulic limitations with height in giant sequoia, allowing them to sustain global maximum summer water use rates exceeding 2000 kg day(-1). However, we found that leaf N and photosynthetic capacity do not follow the vertical light gradient, supporting the hypothesis that increasing limitations on water transport capacity with height modify photosynthetic optimization in tall trees.
authors.affiliation: Department of Integrative Biology, University of California, Berkeley, CA, 94720, USA. ambrose@berkeley.edu.
authors.forename: Anthony R
authors.identifier: 0000-0003-1706-7431
authors.initials: AR
authors.lastname: Ambrose
authors.affiliation: Department of Integrative Biology, University of California, Berkeley, CA, 94720, USA.
authors.forename: Wendy L
authors.initials: WL
authors.lastname: Baxter
authors.affiliation: Department of Integrative Biology, University of California, Berkeley, CA, 94720, USA.
authors.forename: Christopher S
authors.initials: CS
authors.lastname: Wong
authors.affiliation: School of Plant Biology, University of Western Australia, Perth, 6009, Australia.
authors.forename: Stephen S O
authors.initials: SS
authors.lastname: Burgess
authors.affiliation: Department of Integrative Biology, University of California, Berkeley, CA, 94720, USA.
authors.forename: Cameron B
authors.initials: CB
authors.lastname: Williams
authors.affiliation: Department of Integrative Biology, University of California, Berkeley, CA, 94720, USA.
authors.forename: Rikke R
authors.initials: RR
authors.lastname: Næsborg
authors.affiliation: Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ, 86011, USA.
authors.forename: George W
authors.initials: GW
authors.lastname: Koch
authors.affiliation: Department of Integrative Biology, University of California, Berkeley, CA, 94720, USA.
authors.forename: Todd E
authors.initials: TE
authors.lastname: Dawson
mesh_terms: D010788:Photosynthesis; D018515:Plant Leaves; D018526:Plant Transpiration; D032488:Sequoiadendron; D014197:Trees; D014867:Water
chemical_list: D014867:Water
keywords: Hydraulic conductance; Hydraulic limitation; Sap flow; Sequoiadendron giganteum; Tree size; Xylem conduit widening