Picture: Muir Woods, San Francisco. Credit: Billy Huynh (Unsplash)

Modelling photosynthesis is highly dependent on accurately capturing the light absorption by leaves within a tree canopy. The “architecture” of tree canopies in the real world is complex – leaf shape, size and orientation vary, tree crowns shade one another – all of these components together affect the light absorption of canopies.

By contrast to this real-world complexity, land surface models (LSMs) typically represent canopy architecture simplistically.  For example, many LSMs assume that light absorption can be accurately captured by assuming the canopy looks like one big leaf in full sunlight and one big leaf in the shade.

In this study, CLEX researchers and colleagues investigated to what extent it was possible to retrieve key structural parameters that would allow land surface models to still employ simple general calculations but improve simulated accuracy and minimise biases.

To explore this question, the researchers used data from digital hemispherical photography and 3D modelling of radiative transfer (the quantitative process to describe the absorption and partitioning of solar energy within the canopy) for two study sites with very different canopy architectures.

They found that if radiative transfer calculations ignored the 3D vegetation canopy structure it led to significant errors in shortwave radiation partitioning, or how solar energy was divided and used in different processes. When they accounted for 3D vegetation structure, canopy calculations included up to 3.5 times more direct transmittance of energy reaching the land surface.

The researchers then used specific structural calculations designed to broadly represent the fact that leaves are not randomly distributed in tree canopy, but instead clump together on branches: whole canopy ‘clumping indices’. These indices were then combined with existing calculations of the angle of sun (zenith angular variations) to evaluate the impact of clumping index on shortwave radiation transfer, or the amount of sunlight that reached the surface.

The impacts of this on photosynthesis were then evaluated at site level with the UKESM land surface model, JULES. A direct comparison was then made between real-world observations and the simulated photosynthesis in the land surface model.

The results indicated that considering zenith angular variations with broad vegetation canopy architecture parameters in the radiative transfer schemes of LSMs significantly improved photosynthesis prediction in light-limited ecosystems (RMSE 30% smaller), finding enhanced photosynthesis in the bottom canopy layers.

  • Paper: Braghiere, R., Quaife, T., Black, E., Ryu, Y. Chen, Q., De Kauwe, M. G. and Baldocchi, D. (2020) Influence of sun zenith angle on canopy clumping and the resulting impacts on photosynthesis. Agricultural and Forest Meteorology, in press.