Canopy-scale d 13 C of photosynthetic and respiratory CO 2 fluxes: observations in forest biomes across the United States

The d 13 C 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 13 C discrimination by terrestrial vegetation (DA), and depends on d 13 C values of respired CO2 fluxes (d 13 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 d 13 CR from dominant forest ecosystems in the United States between 2001 and 2003. Our observations indicate considerable summer-time variation in the weekly value of d 13 CR 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 d 13 CR showed a smaller range (2‐3%), which appeared to significantly correlate with soil water availability. Values of d 13 CR were less variable during the growing season at the deciduous forest (Harvard Forest). We suggest that the negative correlation between d 13 CR 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 d 13 CR and DA could be of sufficient magnitude globally to impact partitioning calculations of CO2 sinks between oceanic and terrestrial compartments.