Solving the puzzle of leaf-to-canopy scaling: accounting for the carbon costs of water transport and light capture decouples optimal canopy profiles of photosynthetic capacity and light.
Abstract
Leaf-level models of photosynthesis and transpiration are usually scaled to the canopy level based on a prediction from optimization theory: namely, that leaf photosynthetic capacity should be roughly proportional to daily irradiance in each canopy location. Unfortunately, that prediction is consistently disproven by empirical data showing that, although photosynthetic capacity does increase with daily irradiance within a canopy, the relationship is saturating, so that capacity per unit irradiance is lower in upper/sunlit locations than in lower/shaded locations. Predictions converge with observations when ad hoc constraints on stomatal or mesophyll conductance in upper canopy locations are included in simulations. Yet the economic rationale for such ad hoc constraints, within the framework of optimization theory, remains elusive.
Here I show that these constraints emerge from optimal partitioning of C and N between canopy locations ("modules") and of C between carbon pools (stem vs leaf) to maximize net growth (photosynthesis minus the amortized carbon costs of light capture and water transport), subject to negative feedbacks of low water potential on water transport. I also show that it is not generally optimal for the marginal carbon revenue of water (often denoted dA/dE) to be spatially invariant in the canopy. Thus, leaf-to-canopy scaling is consistent with optimization theory, but only when the theory is expanded to include the (previously implicit) carbon costs of photosynthetic inputs (water, N and light). This has implications for scaling in models, and for interpreting the ecological and agronomic implications of observed canopy profiles of photosynthetic capacity and gas exchange.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B51A..04B
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0439 Ecosystems;
- structure and dynamics;
- BIOGEOSCIENCES;
- 0466 Modeling;
- BIOGEOSCIENCES;
- 0476 Plant ecology;
- BIOGEOSCIENCES