Divergent Climate Responses Decouple Stem Growth from Canopy Activity Across Mongolian Pine Plantations in Northern China

Stem growth and canopy activity are two of the key processes in trees and are associated with one another. These two processes and their relationships may be altered under climate change, yet the underlying mechanisms by which climatic factors impact them remain unclear. This study examined stem growth and canopy activity and their responses to climate using tree-ring width data from 614 increment cores and satellite-based normalized difference vegetation index (NDVI) data from 17 Mongolian pine (Pinus sylvestris var. mongolica Litv.) plantation sites in northern China. Stem growth was not significantly (p > 0.05) correlated with canopy activity at both broad spatial and individual-site scales. Broadly, stem growth was primarily driven by temperature-induced water stress whereas canopy activity was constrained by growing season precipitation, causing a difference in the spatial patterns of these two processes across the Mongolian pine plantations. Locally, stem growth was more vulnerable to high-temperature-exacerbated drought stress during the growing season and more affected by pre-growing-season hydroclimatic variability than canopy activity, especially at more arid sites, which could decouple interannual correlations between the two processes. Our study reveals climate-driven decoupling between stem growth and canopy activity, complicating the estimations of overall carbon dynamics, and thereby underscores the necessity of comprehensively assessing forest dynamics and carbon sequestration.