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

Abstract

Mean midday values of eddy covariance CO 2 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 CO 2 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 CO 2 uptake and APAR for the grassland and shrubland, with increasing APAR leading to increasing CO 2 uptake. Over the forest, however, the CO 2 uptake was only weakly related to APAR, but an additional linear relationship with infrared surface temperature (T IR ) implied that respiration may have been more important than APAR in determining net CO 2 uptake by the forest. The regression slope for net CO 2 uptake versus APAR for the grassland data agreed with those from other observations.