Radiative transfer and carbon assimilation in relation to canopy architecture, foliage area distribution and clumping in a mature temperate rainforest canopy in New Zealand

Simulations with the canopy model over 1 year using mean clumping factors revealed that clumping reduced total canopy radiation interception by 5%. Shoot-level irradiance was reduced at the top of the canopy but increased in lower canopy layers resulting in a more even canopy irradiance distribution. This enhanced annual canopy photosynthesis by 8% over a canopy with randomly distributed foliage. When the simulations were repeated to include the observed variation of γE with height, total canopy radiation absorption was 11% lower than for a random canopy and canopy photosynthesis was 12% greater than a random canopy. The combination of reduced canopy radiation absorption and increased photosynthesis due to foliage clumping resulted in a considerable enhancement of canopy light-use efficiency. These analyses reveal a significant advantage of clumped foliage over randomly distributed foliage in terms of carbon gain, even for a forest canopy with moderate leaf area index.