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

Abstract

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 (δ¹³C_cam and δ¹⁸O_cam) and developing xylem cellulose (δ¹³C_xc and δ¹⁸O_xc). Strong positive correlations were observed between δ¹³C_cam and δ¹⁸O_cam, with an increasing trend along the growing season. Conversely, negative relationships were observed between δ¹³C_xc and δ¹⁸O_xc, characterized by an increasing trend in δ¹³C_xc and a decreasing trend in δ¹⁸O_xc. The results illustrated that stomatal conductance is the dominant physiological factor controlling seasonal fractionation of δ¹³C_cam and δ¹⁸O_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 δ¹⁸O_cam during the growing season. This suggests that δ¹⁸O_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.