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.

Effect of Leaf Water Potential on Internal Humidity and CO2 Dissolution: Reverse Transpiration and Improved Water Use Efficiency under Negative Pressure

“…This means that the saturation vapor concentration over a surface of water potential zero (w i, 0 ) is 0.92 mol m −3 (Haynes, 2010 ). The realistic leaf water potential values can be assumed to vary from −0.5 to −2.2 MPa (Ambrose et al, 2016 ). It is difficult to assess what the actual value for stomatal conductance would be during such conditions due to the difficulty in measuring the actual stomatal conductance (see Figure 5A ) and also due to technical limitations of measuring small transpiration rates at high RH, when the transpiration may be also masked by condensation on the surfaces (e.g., Altimir et al, 2006 ).…”

Tip‐to‐base xylem conduit widening as an adaptation: causes, consequences, and empirical priorities

“…Congruent with this idea, in an experiment in the tropical tree Moringa oleifera, Echeverría et al (2019) found that vessel diameter and number change with height growth in a way that maintained hydraulic conductance constant with leaf area. Some researchers, however, have documented drops in photosynthetic productivity and stomatal conductance with height growth (Koch et al, 2004;Ambrose et al, 2016). Maintaining constant (or increasing) conductance with height growth is likely especially important for forest tree species, which grow up through a dark and humid understory into much higher radiation environments with higher evaporative demand (Gleason et al, 2018b).…”

Optimal carbon partitioning helps reconcile the apparent divergence between optimal and observed canopy profiles of photosynthetic capacity

“…It has long been known that stomatal conductance is often systematically suppressed in upper‐canopy leaves (Ryan & Yoder, 1997; Delzon et al ., 2004; Koch et al ., 2004). Such suppression would cause optimal photosynthetic capacity to be lower than expected in the upper canopy (Peltoniemi et al ., 2012; Buckley et al ., 2013), and could arise from low leaf water potentials, caused either by the greater hydraulic resistance encountered in transporting water to more distal sites in the canopy, or by elevated evaporative demand in more sunlit locations (Ambrose et al ., 2016; Bachofen et al ., 2020). Indeed, stomatal conductance responds negatively both to reduced water potential and increased evaporative demand (Buckley, 2019).…”