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

• Stomatal conductance controls δ 13 C and δ 18 O fractionation in the cambial region. • Post-carboxylation processes have minimal impact on δ 13 C in cambium and xylem. • P ex effect alters δ 18 O signals in forming xylem cellulose of black spruce. • Cambium and xylem use different water sources during the growing season. The stable isotopic composition of carbon (δ 13 C) and oxygen (δ 18 O) 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 δ 13 C and δ 18 O 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 (δ 13 C cam and δ 18 O cam ) and developing xylem cellulose (δ 13 C xc and δ 18 O xc ). Strong positive correlations were observed between δ 13 C cam and δ 18 O cam , with an increasing trend along the growing season. Conversely, negative relationships were observed between δ 13 C xc and δ 18 O xc , characterized by an increasing trend in δ 13 C xc and a decreasing trend in δ 18 O xc . The results illustrated that stomatal conductance is the dominant physiological factor controlling seasonal fractionation of δ 13 C cam and δ 18 O cam . Increasing proportional exchanges between xylem water and sugars at the sites of cellulose synthesis (i.e., P ex effect) are thought to be strong enough to completely blur the observed trends in δ 18 O cam during the growing season. This suggests that δ 18 O xc 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.