Disentangling leaf area and environmental effects on the response of the net ecosystem CO2 exchange to diffuse radiation.

There is an ongoing discussion about why the net ecosystem CO2 exchange (NEE) of some ecosystems is less sensitive to diffuse radiation than others and about the role other environmental factors play in determining the response of NEE to diffuse radiation. Using a six-year data set from a temperate mountain grassland in Austria we show that differences between ecosystems may be reconciled based on their green area index (GAI; square meter green plant area per square meter ground area) - the sensitivity to diffuse radiation increasing with GAI. Our data suggest diffuse radiation to have a negligible influence on NEE below a GAI of 2 m2 m-2. Changes in air/soil temperature and air humidity concurrent with the fraction of diffuse radiation were found to amplify the sensitivity of the investigated temperate mountain grassland ecosystem to diffuse radiation.