Vegetation clumping modulates global photosynthesis through adjusting canopy light environment.

Area of Science:

• Ecology
• Remote Sensing
• Climate Modeling

Background:

• The spatial distribution of photoelements in vegetation, or clumping index (CI), influences canopy light absorption and photosynthesis.
• CI is not commonly integrated into terrestrial biosphere models for ecosystem carbon cycle estimations.
• Recent advancements in remote sensing have produced global CI products, enabling broader impact assessments.

Purpose of the Study:

• To integrate CI into the Community Land Model version 5 (CLM5) radiative transfer scheme.
• To quantitatively assess the impact of CI on canopy absorbed radiation and gross primary production (GPP).
• To analyze the uncertainty and seasonal variation of CI using multiple remote sensing products.

Main Methods:

• Revised CLM5 incorporating CI into its radiative transfer scheme.
• Utilized multiple global CI products derived from remote sensing data.
• Evaluated changes in direct and diffuse radiation absorption, sunlit/shaded canopy fractions, and Vcmax.

Main Results:

• Revised CLM5 showed reduced direct (9%-15%) and diffuse (23%-34%) radiation absorption in sunlit canopies.
• Shaded canopies absorbed more diffuse radiation (3%-18%) and exhibited enhanced photosynthesis.
• A net increase in GPP of 1.0 ± 0.12 PgC year⁻¹ was observed, driven by higher light use efficiency in shaded leaves.
• Uncertainty in GPP due to CI datasets was substantial, especially in tropical regions.

Conclusions:

• Considering CI in terrestrial biosphere models is crucial for accurate estimations of canopy radiation absorption and GPP.
• The uncertainty associated with CI datasets significantly impacts model outputs, necessitating careful data selection.
• Future research should focus on refining CI estimation and its integration into global carbon cycle models.