Although the boreal forest has been proposed to accommodate increasing amounts of CO2, the large-scale response of photosynthesis to widespread noxious gases is largely unknown. Here we present carbon isotope ratios for ring series from trees subjected to different levels of SO2 emitted from a copper smelter. Our results indicate that noxious gases drastically lower the ability of trees to capture CO2 in the region of the smelter, and that this ability may also be reduced in an extensive region of the Canadian boreal forest undergoing effects from diffuse pollution. This raises concerns about the proposed increased capacity of boreal forests to sequester excess anthropogenic CO2.

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