Permafrost degradation alters the environmental signals recorded in tree-ring lignin methoxy group δ²H in...

Climate warming has profoundly altered the status of permafrost and has caused extensive permafrost degradation in the Northern Hemisphere. However, long-term observations investigating the hydrological dynamics of permafrost and its ecological effects on plant growth are lacking. Previous studies have reported tree-ring stable hydrogen isotope ratios of lignin methoxy groups (δ²H_LM) as an archive of hydrological signals. This study sampled tree-ring cores from a Larix gmelinii forest in Nanwenghe Forest Park, Northeastern China, and separately measured the tree-ring δ²H_LM for earlywood and latewood from 1900 to 2020. Earlywood and latewood δ²H_LM values, as well as the difference between them, showed no significant long-term trend from 1900 to 1987; however, they both exhibited significant increasing trends since 1988 at rates of 2.6 ‰ and 4.9 ‰ per decade, respectively. This variance changes the magnitude of the difference between the two chronologies and can be explained by the shift in source water δ²H values during tree growth. Based on a structural equation model analysis, when the influence of permafrost melting weakened due to permafrost degradation, the growing season temperature was better recorded in latewood δ²H_LM through the effects of precipitation δ²H from July to September. Based on the environmental response of tree-ring δ²H_LM in the permafrost region, permafrost degradation influences the source water δ²H values of trees, thereby affecting the expression of temperature signals in tree-ring δ²H_LM. The novel results in this study provide a new perspective on permafrost degradation based on the dynamic responses of tree-ring δ²H_LM to source water δ²H during permafrost degradation.