03 April 2018

Model-based analysis of the impact of diffuse radiation on CO2 exchange in a temperate deciduous forest.

Diffuse fraction, the ratio of diffuse to direct incoming photosynthetically active radiation, explains a large proportion of variability in net carbon uptake.

Science

We used 10 years of eddy-covariance observations, combined with a model of carbon and water cycling in forest ecosystems, to examine the impact of changes in diffuse light on carbon cycling in a northeastern temperate deciduous forest, and found that changes in diffuse fraction explained a large proportion of the interannual variability in total forest carbon uptake. Large biases in different canopy radiative transfer models imply needed model improvements.

Impact

Temperate forests exert a large control on the global carbon cycle and provide multiple ecosystem services to society. They are highly sensitive to incoming radiation, and in particular the amount of radiation that is diffuse, rather than direct. We quantify the sensitivity to diffuse fraction and show that it is poorly represented by existing radiative transfer schemes.

Summary

Clouds and aerosols increase the fraction of global solar irradiance that is diffuse light. This phenomenon is known to increase the photosynthetic light use efficiency (LUE) of closed-canopy vegetation by redistributing photosynthetic photon flux density (400–700 nm) from saturated, sunlit leaves at the top of the canopy, to shaded leaves deeper in the canopy. We combined a process-based carbon cycle model with 10 years of eddy covariance carbon flux measurements and other ancillary data sets to assess 1) how this LUE enhancement influences interannual variation in carbon uptake, and 2) how errors in modeling diffuse fraction affect pre- dictions of carbon uptake. The sensitivity of GPP to increases in diffuse fraction was highest when the diffuse fraction was low to begin with, and lowest when the diffuse fraction was already high. Diffuse fraction also explained significantly more of the interannual variability of modeled net ecosystem exchange (NEE), than did total irradiance. Our findings highlight the importance of incorporating LUE enhancement under diffuse light into models of global primary production.

Contact
Trevor F. Keenan
Lawrence Berkeley National Laboratory (LBNL)
Publications
Lee, MS, DY Hollinger, TF Keenan, AP Ouimette, SV Ollinger, and AD Richardson.  2018.  "Model-Based Analysis of the Impact of Diffuse Radiation on CO2 Exchange in a Temperate Deciduous Forest."  Agricultural and Forest Meteorology 249: 377-389, doi:10.1016/j.agrformet.2017.11.016.