The role of vegetation canopy structure in the variability of the terrestrial carbon sink

The terrestrial carbon sink currently accounts for the removal of about one quarter of the total atmospheric carbon dioxide emitted to the atmosphere by human activity. Inter-annual variability (IAV) in atmospheric CO₂ is in large part driven by in the IAV in terrestrial carbon fluxes, namely gross primary productivity (GPP) and respiration. A mechanistic understanding of the contributions of vegetation to IAV in terrestrial carbon fluxes would help constrain the the relationship between GPP and climate drivers, and therefore improve our ability to predict long-term changes to the growth rate of atmospheric CO₂ due to terrestrial processes. Here, we use a version of the Community Land Model with a multilayer vegetation canopy (CLM-ml) to investigate the role of canopy structure on terrestrial carbon uptake and its IAV. We conduct multiannual single-point CLM-ml simulations at several FLUXNET sites, using meteorological observations from towers as boundary conditions, to study the varying effects of radiation, temperature and water vapor vertical structure within the canopy airspace on canopy photosynthesis. Specifically, we examine (1) the representation of plant hydraulic responses to soil water to understand the role of soil moisture on coupled photosynthesis-transpiration mechanisms, and (2) the effects of structural factors such as leaf clumping and leaf angle on the distribution and absorption of radiation vertically within the canopy and consequently photosynthesis. Finally, we describe how canopy structure factors into IAV of GPP, both by comparing sensitivity runs in the multilayer model with simulations using a traditional big-leaf canopy model (CLM4.5). We evaluate the model results with flux tower GPP and other in-situ data to understand the role of canopy structure in carbon uptake.