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

Abstract

The δ 13 C values of atmospheric carbon dioxide (CO 2 ) can be used to partition global patterns of CO 2 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 (Δ A ), and depends on δ 13 C values of respired CO 2 fluxes ( δ 13 C R ). Here we show that by combining two independent data streams – the stable isotope ratios of atmospheric CO 2 and eddy‐covariance CO 2 flux measurements – canopy scale estimates of Δ A can be successfully derived in terrestrial ecosystems. We also present the first weekly dataset of seasonal variations in δ 13 C R from dominant forest ecosystems in the United States between 2001 and 2003. Our observations indicate considerable summer‐time variation in the weekly value of δ 13 C R 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 δ 13 C R showed a smaller range (2–3‰), which appeared to significantly correlate with soil water availability. Values of δ 13 C R were less variable during the growing season at the deciduous forest (Harvard Forest). We suggest that the negative correlation between δ 13 C R 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 δ 13 C R and Δ A could be of sufficient magnitude globally to impact partitioning calculations of CO 2 sinks between oceanic and terrestrial compartments.

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