Climate-Induced Permafrost Degradation Exerts Species-Specific Impacts on Pine and Larch Growth in the Da Xing’Anling (Hinggan) Mountains, Northeast China

Abstract: In permafrost regions, forests exhibit a heightened sensitivity to shifts in hydroclimate. To examine the responses of trees to climatic and hydrological changes in hemi-boreal forests in northeast China, we collected a total of 405 tree-ring cores from key conifer tree species, including Larix gmelinii (Xing'an larch) and Pinus sylvestris var. mongolica Litv. (Scots/Mongolian pine), across nine sampling sites in the Da Xing'anling Mountains in the western part of northeast China. Basal area increment (BAI) was calculated as a metric for reflecting tree-ring growth. The analysis of stable hydrogen isotope ratios in lignin methoxy groups (δ2HLM) was conducted to probe alterations in water supply for trees under a persistently warming climate. Employing the Bayesian stable isotope mixing model (MixSIAR), our findings unveiled that ground surface temperature (encompassing dormancy and growing-season temperatures) and permafrost type, based on thermal stability, controlled significantly over the contributions of the current year precipitation (CYP) and soil stored moisture (SSM) to trees. Moreover, Scots/Mongolian pine and Xing'an larch responded differently to climate warming and permafrost degradation. In the Da Xing'anling Mountains, the discernible fluctuations in plant water supply are more pronounced in the shallow-rooted Xing'an larch, while Scots/Mongolian pine with deeper taper roots gain greater advantages from permafrost degradation. Over prolonged periods, the growth of Xing'an larch may encounter water constraints due to alterations in soil water infiltration prompted by rising soil temperatures and permafrost degradation. In contrast, Scots and Mongolian pine species may flourish even before the complete thaw of permafrost, courtesy of moisture supplementation from the degrading permafrost. Subsequently, the Scots and Mongolian pines may employ this mechanism to confront the challenges akin to those faced by the Xing'an larch. Our findings hold significance in assessing ecological risk in the hemi-boreal forests of northeast China and analogous northern regions, guiding judicious management decisions under a warming climate.