Diurnal, seasonal and interannual variability of carbon isotope discrimination at the canopy level in response to environmental factors in a boreal forest ecosystem

ABSTRACTAccurate estimation of temporal and spatial variations in photosynthetic discrimination of 13C is critical to carbon cycle research. In this study, a combined ecosystem–boundary layer isotope model, which was satisfactorily validated against intensive campaign data, was used to explore the temporal variability of carbon discrimination in response to environmental driving factors in a boreal ecosystem in the vicinity of Fraserdale Tower, Ontario, Canada (49°52′30″N, 81°34′12″W). A 14 year (1990–1996 and 1998–2004) hourly CO2 concentration and meteorological record measured on this tower was used for this purpose. The 14 year mean yearly diurnal amplitude of canopy‐level discrimination Δcanopy was computed to be 2.8 ± 0.5‰, and the overall diurnal cycle showed that the greatest Δcanopy values occurred at dawn and dusk, while the minima generally appeared in mid‐afternoon. The average annual Δcanopy varied from 18.3 to 19.7‰ with the 14 year average of 19 ± 0.4‰. The overall seasonality of Δcanopy showed a gradually increasing trend from leaf emergence in May–September and with a slight decrease at the end of the growing season in October. Δcanopy was negatively correlated to vapour pressure deficit and air temperature across hourly to decadal timescales. A strong climatic control on stomatal regulation of ecosystem isotope discrimination was found in this study.

Diurnal, seasonal and interannual variability of carbon isotope discrimination at the canopy level in response to environmental factors in a boreal forest ecosystem Journal Articles