Greater Coupling Between Tree Radial Growth and Canopy Activity Is Associated With Weaker Drought Resistance in Drier Closed Forests.

Understanding tree radial growth and its resistance to extreme droughts is crucial for predicting forest carbon sequestration in response to climate change. Photosynthates derived from canopy activity are fundamental to tree radial growth and drought resistance. However, the extent to which tree radial growth is coupled with canopy activity and whether such coupling is associated with growth resistance to extreme droughts remain unclear. Here, we compiled a pan-continental paired dataset of tree radial growth (represented by tree-ring chronologies) and canopy activity (estimated by a vegetation index with 30-m spatial resolution) for 1985-2022 across 2222 closed forests. We quantified the spatial patterns and underlying drivers of the coupling between tree radial growth and canopy activity (Cgc) and evaluated the associations between Cgc and growth resistance to extreme droughts. Tree radial growth in 10.6% of the studied forests was closely coupled with canopy activity, as indicated by the significantly positive Cgc (p < 0.05). Drier forests exhibited greater Cgc values than dry sub-humid and humid forests. Machine learning models revealed that the positive Cgc values were driven primarily by more homogeneous climate sensitivities of tree radial growth and canopy activity, together with a much drier soil and a greater growing-season diurnal temperature range. Linear mixed-effects models consolidated the independent negative associations of Cgc with growth resistance to extreme droughts. Forests with significantly positive Cgc values presented 13%-21% greater radial growth reduction during extreme drought events compared to those with insignificantly positive or negative Cgc. Our findings highlight that although the coupling between tree radial growth and canopy activity is not widespread across the Northern Hemisphere, it exacerbates forest vulnerability to extreme drought events, particularly in arid closed forests.