Impact of changing sky conditions on light regimes and carbon uptake by terrestrial ecosystems

This paper presents research on the role of diffuse light on terrestrial ecosystem productivity utilizing gross ecosystem production derived from eddy covariance measurements of ecosystem exchanges of CO2. Examples are provided from a mid-latitude deciduous broadleaf forest site, as well as from initial results of a FLUXNET synthesis study. Results are presented first on the testing and validation of four models to derive the fraction of diffuse PAR from global PAR or solar radiation measurements above the canopy. These models are needed to assess the role of diffuse light at sites where this measurement is not available. Most plant functional types show a significant increase in light use efficiency (LUE) under higher fractions of diffuse light. However, the trade-off between the increase in LUE of diffuse light and the corresponding decrease in the quantity of available light can produce different results (including a reversal in sign) under different levels/types of atmospheric turbidity. Since gross ecosystem productivity is the largest single carbon flux associated with the terrestrial biosphere, the diffuse light effect produces an important bio-climate feedback to scenarios of changing light environments.