Divergent Temporal Trends in Carbon Assimilation-Growth Coupling Across Global Forests

Understanding how trees allocate assimilated carbon to growth is essential for projecting forest carbon sequestration. However, the temporal dynamics of the relative roles of carbon assimilation (source) versus cambial activity (sink) in regulating tree growth remain poorly understood. Using multiple gross and net primary productivity (GPP and NPP) data and tree-ring width records from 2126 sites worldwide, we quantified the temporal trend of the correlation between carbon assimilation and tree growth (R trend) over the past three decades. We found that the R trend varied substantially across sites, with 57.8% of the sites exhibiting negative trends and 42.2% showing positive trends, reflecting a temporal increase in decoupling between carbon assimilation and tree growth in many forests, but a strengthening of coupling in others. Precipitation strongly affected the spatial variation in the R trend, with its importance only slightly lower than that of temperature. Sites with lower baseline precipitation tended to exhibit more positive R trends, whereas those with higher baseline precipitation more often exhibited negative R trends, with these patterns potentially modulated by soil properties and plant traits. These empirical negative effects of precipitation on the R trend were further reproduced by 13 current Dynamic Global Vegetation Models (DGVMs). In addition, model outputs under different future climate scenarios indicated that the coupling between carbon assimilation and tree growth is likely to strengthen across broader regions by the end of the 21st century due to the ongoing climate change. Overall, our findings reveal the importance of incorporating precipitation influences for improving the understanding of the temporal dynamics of the coupling between carbon assimilation and tree growth under future climate change.