Environmental controls on the carbon isotope composition of ecosystem-respired CO2 in contrasting forest ecosystems in Canada and the USA

Abstract

We compared the carbon isotope composition of ecosystem-respired CO 2 (δ13CR) from 11 forest ecosystems in Canada and the USA and examined differences among forest δ 13CR responses to seasonal variations in environmental conditions from May to October 2004. Our experimental approach was based on the assumption that variation in δ13CR is a good proxy for short-term changes in photosynthetic discrimination and associated shifts in the integrated ecosystem-level intercellular to ambient CO2 ratio (c1/ca). We compared δ13CR responses for three functional groups: deciduous, boreal and coastal forests. The δ13CR values were well predicted for each group and the highest R2 values determined for the coastal, deciduous and boreal groups were 0.81, 0.80 and 0.56, respectively. Consistent with previous studies, the highest correlations between δ13CR and changes in environmental conditions were achieved when the environmental variables were averaged for 2, 3 or 4 days before δ13C R sample collection. The relationships between δ 13CR and environmental conditions were consistent with leaf-level responses, and were most apparent within functional groups, providing support for our approach. However, there were differences among groups in the strength or significance, or both, of the relationships between δ13CR and some environmental factors. For example, vapor pressure deficit (VPD) and soil temperature were significant determinants of variation in δ13CR in the boreal group, whereas photosynthetic photon flux (PPF) was not; however, in the coastal group, variation in δ13CR was strongly correlated with changes in PPF, and there was no significant relationship with VPD. At a single site, comparisons between our δ13CR measurements in 2004 and published values suggested the potential application of δ13CR measurements to assess year-to-year variation in ecosystem physiological responses to changing environmental conditions, but showed that, in such an analysis, all environmental factors influencing carbon isotope discrimination during photosynthetic gas exchange must be considered. © 2007 Heron Publishing.