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As major terrestrial carbon sinks, forests play an important role in mitigating climate change. The relationship between the seasonal uptake of carbon and its allocation to woody biomass remains poorly understood, leaving a significant gap in our capacity to predict carbon sequestration by forests. Here, we compare the intra-annual dynamics of carbon fluxes and wood formation across the Northern hemisphere, from carbon assimilation and the formation of non-structural carbon compounds to their incorporation in woody tissues. We show temporally coupled seasonal peaks of carbon assimilation (GPP) and wood cell differentiation, while the two processes are substantially decoupled during off-peak periods. Peaks of cambial activity occur substantially earlier compared to GPP, suggesting the buffer role of non-structural carbohydrates between the processes of carbon assimilation and allocation to wood. Our findings suggest that high-resolution seasonal data of ecosystem carbon fluxes, wood formation and the associated physiological processes may reduce uncertainties in carbon source-sink relationships at different spatial scales, from stand to ecosystem levels., (© 2024. The Author(s).)

The establishment of mixed-species forests with complex structures and high biodiversity has emerged as a key approach for enhancing productivity and maintaining the stability of plantation forests. However, how the mixing effect changes with stand development and climate variation is still unclear. Additionally, little is known about how the mixing effects vary between even- and uneven-aged mixed planting strategies based on the same species combinations and site conditions. In this study, we used dendrochronological methods to reconstruct 37 years of annual growth series for even-aged mixed plantations (EMs) and uneven-aged mixed plantations (UEMs) of Pinus massoniana and Castanopsis hystrix , as well as their respective monocultures at the individual tree (286 trees in total) level. We found that in EMs, the growth of P. massoniana and C. hystrix was 20.8 % lower and 26.2 % higher than those of their monocultures, respectively. However, the growth of P. massoniana remained unchanged during the first 17 years of mixing, after which it was significantly suppressed by C. hystrix. The growth of P. massoniana and C. hystrix in the UEMs was 20.8 % and 26.2 % lower than that in their monocultures, respectively. Remarkably, the mixing effect on the growth of P. massoniana shifted from suppression to promotion after 30 years of mixing. Furthermore, the relative growth of P. massoniana in the UEMs increased with dry season precipitation. These results indicate that transforming pure plantations into uneven-aged mixed plantations may be a crucial forest management strategy for enhancing tree growth and to improving adaptation to future precipitation pattern changes. However, in EMs, thinning should be conducted before fast-growing, shade-intolerant species experience intense light competition. • Even-aged mixture (EM) decreased the growth of both P. massoniana and C. hystrix. • Uneven-aged mixture (UEM) decreased the growth of P. massoniana but increased that of C. hystrix. • The growth of P. massoniana in the EM became suppressed after 17 years of mixing. • The growth of P. massoniana in the UEM became promoted after 30 years of mixing. [ABSTRACT FROM AUTHOR]