Effect of smoke on subcanopy shaded light, canopy temperature, and carbon dioxide uptake in an Amazon rainforest

Daytime Net Ecosystem CO2 uptake (NEE) in an Amazon forest has been shown to increase significantly during smoky periods associated with biomass burning. We investigated whether the increase in CO2 uptake is caused by increased irradiance in the lower canopy, which results from increased above-canopy diffuse light, or by decreased canopy temperature, which results from decreased above-canopy net radiation. We used Sun photometers measuring aerosol optical depth to find nonsmoky (Aerosol Optical Depth (AOT) < 0.35), smoky (AOT > 0.5) and very smoky (AOT > 0.7) periods for the Tapajos region in the Amazon. Using a network of subcanopy photosynthetic photon flux density (PPFD) sensors, we detected a similar to 4 mu mol m(-2) s(-1) increase in subcanopy diffuse light during smoky periods relative to nonsmoky periods. Using a pyrgeometer to measure upwelling longwave radiation and, hence, canopy surface temperature, we found a similar to 0.5 degrees C cooling relative to air temperature during smoky periods. We modeled subcanopy irradiance based on the subcanopy PPFD sensors and combined this with subcanopy leaf photosynthesis measurements to determine how the increased lower canopy light affected NEE. We used the relationship between temperature and NEE measured by eddy covariance to determine the effect of decreased canopy temperature on canopy CO2 uptake. We found that the increase in CO2 uptake at high aerosol optical depths is primarily a result of increased shaded light in the subcanopy (accounting for similar to 80%) and to a lesser extent the effect of decreased canopy temperature (accounting for similar to 20%).