How does photosynthetic response to elevated CO2 compare at the leaf and canopy scales? A case study with Eucalyptus saligna saplings grown in whole-tree chambers

Abstract:

Accurate representation of canopy photosynthesis (A) and its sensitivity to rising atmospheric CO₂ (C_a) in terrestrial biosphere models is essential for the robust projection of the carbon-climate feedback under climate change. Theory indicates that the sensitivity of A to rising C_a depends on the most limiting process. At low C_a, A is mostly limited by Rubisco carboxylation and increases near-linearly with rising C_a. Above an inflection point, A becomes mainly limited by RuBP regeneration and is less sensitive to further increases in C_a. Hence, the relative contribution of Rubisco- and RuBP regeneration-limitation to A is a critical modeling feature that constrains the sensitivity of terrestrial carbon cycle responses to rising C_a. Whilst leaf-level A-C_a measurements are abundant and different leaf-to-canopy scaling schemes are available for canopy A estimates, the extent to which canopy A-C_a responses differ from those of the leaf is largely untested. Here we describe C_a drawdown experiments using Eucalyptus saligna saplings grown in whole-tree chambers. The whole-canopy A-C_a responses were quantified for different canopy layers (i.e. whole canopy, top + middle, and top layers). We incorporated leaf-level measurements into the multi-assumption architecture and testbed (MATT) to estimate canopy A-C_a sensitivity based on different leaf-to-canopy scaling schemes. Our assessment identifies crucial disagreement in canopy A-C_a estimates between the data and modeling results. This data-model intercomparison therefore has direct implications to reduce model-based variability in predicting the terrestrial A-C_a responses under future climate change.