Hydraulic constraints modify optimal photosynthetic profiles in giant sequoia trees
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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 (δ13C) 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.
KeywordsSequoiadendron giganteum Sap flow Hydraulic conductance Tree size Hydraulic limitation Xylem conduit widening
We are grateful to the Save the Redwoods League, which supported this work through the Redwoods and Climate Change Initiative. We thank Rob York with the UC Berkeley Center for Forestry for permission to work at Whitaker’s Forest. We also thank the US National Park Service, California State Parks, California Department of Forestry and Fire Protection, and US Forest Service for research permissions and support. We thank Stephen Sillett for valuable help with field measurements, data analysis, and reviewing earlier versions of the manuscript, and Bob Van Pelt, Marie Antoine, Jim Campbell-Spickler, Russell Kramer, Tobe Sherrill, and Bryan Kotwica for field assistance. We also thank two anonymous reviewers for comments and suggestions that substantially improved the manuscript.
Author contribution statement
ARA, WLB, CSW and TED conceived and designed the study. ARA, SSOB, WLB, CSW, CBW, RRN, GWK and TED conducted the fieldwork. ARA, and SSOB analyzed the data. ARA wrote the manuscript; all others provided editorial advice and feedback.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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