Impacts of clouds on ecosystem carbon dioxide exchange over a spectrum of climates

The presence of clouds in the sky creates spatial heterogeneity in the solar radiative forcing over the landscape and changes the relative proportions of diffuse and direct components in surface solar radiation.It can also affect or be associated with changes in temperature, moisture, latent heating and precipitation. All these factors influence the biophysical and ecophysiological processes that control carbon dioxide exchanges of terrestrial ecosystems. Many field observations have shown that the highest net ecosystem exchanges (NEE) of carbon dioxide tend to occur under partly cloudy conditions. Our recent studies in a boreal forest and a temperate forest have shown that the maximal NEE occurred under sky conditions with surface irradiance equivalent to 70-80% of the clear-sky irradiance and forests can tolerate about 50% reduction in surface irradiance caused by clouds without lowering their carbon absorption capacity. The present study extends this analysis to a spectrum of vegetation and climate types by utilizing Fluxnet Dataset.