Canopy-scale dC of photosynthetic and respiratory CO2 fluxes: observations in forest biomes across the United States

The dC values of atmospheric carbon dioxide (CO2) can be used to partition global patterns of CO2 source/sink relationships among terrestrial and oceanic ecosystems using the inversion technique. This approach is very sensitive to estimates of photosynthetic C discrimination by terrestrial vegetation (DA), and depends on d C values of respired CO2 fluxes (d CR). Here we show that by combining two independent data streams – the stable isotope ratios of atmospheric CO2 and eddy-covariance CO2 flux measurements – canopy scale estimates of DA can be successfully derived in terrestrial ecosystems. We also present the first weekly dataset of seasonal variations in dCR from dominant forest ecosystems in the United States between 2001 and 2003. Our observations indicate considerable summer-time variation in the weekly value of dCR within coniferous forests (4.0% and 5.4% at Wind River Canopy Crane Research Facility and Howland Forest, respectively, between May and September). The monthly mean values of dCR showed a smaller range (2–3%), which appeared to significantly correlate with soil water availability. Values of dCR were less variable during the growing season at the deciduous forest (Harvard Forest). We suggest that the negative correlation between dCR and soil moisture content observed in the two coniferous forests should represent a general ecosystem response to the changes in the distribution of water resources because of climate change. Shifts in dCR and DA could be of sufficient magnitude globally to impact partitioning calculations of CO2 sinks between oceanic and terrestrial compartments.