Weekly carbon and oxygen isotope dynamics in black spruce: A case study in the northeastern boreal forest of Quebec, Canada

The stable isotopic composition of carbon (δ13C) and oxygen (δ18O) in tree rings is widely used to explore tree eco-physiological dynamics across various time scales. However, interpreting these isotopic signals is challenging due to multiple interacting factors, including gas exchange at the leaf level, stored carbohydrate reserves, and xylem water, whose timing and interactions during the growing season remain poorly understood. In this study, weekly δ13C and δ18O signals were tracked within the cambial region and forming xylem of black spruce (Picea mariana (Mill.) BSP.) in boreal forests of Quebec, Canada. The study covered three consecutive growing seasons (2019–2021) at two forest sites with differing temperature and soil water content. Weekly isotopic profiles were developed for the cambial region (δ13Ccam and δ18Ocam) and developing xylem cellulose (δ13Cxc and δ18Oxc). Strong positive correlations were observed between δ13Ccam and δ18Ocam, with an increasing trend along the growing season. Conversely, negative relationships were observed between δ13Cxc and δ18Oxc, characterized by an increasing trend in δ13Cxc and a decreasing trend in δ18Oxc. The results illustrated that stomatal conductance is the dominant physiological factor controlling seasonal fractionation of δ13Ccam and δ18Ocam. Increasing proportional exchanges between xylem water and sugars at the sites of cellulose synthesis (i.e., Pex effect) are thought to be strong enough to completely blur the observed trends in δ18Ocam during the growing season. This suggests that δ18Oxc signals differ from those originating in the earlier cambium sink. These findings highlight the need to carefully consider the processes influencing isotopic signals to avoid misinterpretations in dendroclimatological studies.