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

Abstract

Accurate 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. © 2007 The Authors.