Observations of canopy light penetration and net ecosystem exchange of CO2 under different sky conditions in a mid-western mixed deciduous forest

Understanding long-term change in terrestrial carbon sequestration dynamics is an important goal for modeling future change for Earth's climate. One aspect receiving significant attention is the observed enhancement of carbon sequestration under skies with high concentrations of aerosols. Previous research has shown that for the Morgan-Monroe State Forest (MMSF) in south-central Indiana, net CO2 uptake is enhanced by up to 20% under certain cloud conditions compared with equivalent clear sky conditions, despite reduction in total solar radiation of up to 50%. Here we examine the relation between atmospheric scattering, light penetration within the canopy and net ecosystem exchange of CO2 for the MMSF AmeriFlux flux tower site. Data presented include observations and modeling of diffuse and direct components of photosynthetically active radiation (PAR), eddy covariance measurements of CO2 fluxes and a canopy tower instrumented with nine levels of quantum sensors within the canopy. The role of atmospheric scattering on NEE is also evaluated under different climatological and hydrological controls and phenological development from a seven-year dataset. The mechanism of enhanced canopy light use efficiency is investigated through analysis of the vertical profile of canopy PAR with respect to the vertical distribution of LAI under varying sky conditions. The goal is to produce parametric equations for light use efficiency of the forest that are dependent on sky conditions.