Relationships between daytime carbon dioxide uptake and absorbed photosynthetically active radiation for three different mountain/plains ecosystems

[1] Mean midday values of eddy covariance CO2 flux and absorbed photosynthetically active radiation (APAR), derived from solar irradiance and normalized difference vegetation index (NDVI), were measured from May to September 1999 with an aircraft at 60-90 m above ground level over three different ecosystems dominated by native plant species in southeastern Wyoming (mixed conifer forest, mixed short-grass prairie, and sagebrush shrubland). The midday net CO2 uptake at each site followed seasonal trends, with summer values occurring later over the forest than over the other sites. At the landscape scale, linear relationships were observed between CO2 uptake and APAR for the grassland and shrubland, with increasing APAR leading to increasing CO2 uptake. Over the forest, however, the CO2 uptake was only weakly related to APAR, but an additional linear relationship with infrared surface temperature (TIR) implied that respiration may have been more important than APAR in determining net CO2 uptake by the forest. The regression slope for net CO2 uptake versus APAR for the grassland data agreed with those from other observations.