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 (Ψl) 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 (gs), and intrinsic water-use efficiency (A/gs), nor in branch-averaged transpiration (EL), stomatal conductance (GS), and hydraulic conductance (KL). 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
- Field CB, Mooney HA (1986) The photosynthesis-nitrogen relationship in wild plants. In: Givnish TJ (ed) On the economy of plant form and function. Cambridge University Press, Cambridge, pp 25–55Google Scholar
- Hirose T, Ackerly DD, Traw MB, Ramseier D, Bazzaz FA (1997) CO2, elevation, canopy photosynthesis, and optimal leaf area index. Ecology 78:2339–2350Google Scholar
- McDowell NG, Bond BJ, Dickman LT, Ryan MG, Whitehead D (2011) Relationship between tree height and carbon isotope discrimination. In: Lachenbruch B, Dawson TE (eds) Meinzer FC. Size- and age-related changes in tree structure and function. Spring, Berlin, pp 255–286Google Scholar
- Zeppel M (2013) Convergence of tree water use and hydraulic architecture in water-limited regions: a review and synthesis. Ecohydrology 6:889–900Google Scholar