Whither Diffuse? Synthesizing the Influence of Diffuse Light on the Biosphere

The role of diffuse radiation on the biosphere has been discussed in detail over the past twenty years. Because complex forest canopies may more effectively utilize diffuse radiation, the so-called diffuse fertilization effect has been postulated to increase photosynthesis and concomitant transpiration rates. Early publications suggested that the diffuse fraction of radiation increases under aerosol loadings or clouds of moderate optical thickness. This increase in diffuse has the potential to increase carbon uptake by forested ecosystems. However, modeling and observational studies show mixed results on the overall impact of diffuse light on total net ecosystem exchange. Atmospheric radiative transfer models calculate the split of total incoming solar radiation into direct and diffuse components, and canopy radiative transfer models simulate the distribution of radiation throughout the canopy. Observations of diffuse light are limited and typically only available at the top of the canopy, leading to uncertainties in the overall canopy radiative response. The influence of diffuse radiation on total net ecosystem exchange, and its partner, transpiration remains uncertain. I will discuss methodologies to address this question and provide closure on the global impacts of diffuse light in the forest canopy.