Permafrost degradation driven by climate warming is reshaping boreal forest ecosystems, but the mechanisms linking thaw-induced soil changes to tree growth remain unclear. Here, we analysed a century-long (1901–2020) tree-ring record from Larix gmelinii in north-eastern China, integrating basal area increment (BAI), stable nitrogen (δ15N) and carbon (δ13C) isotopes, and C/N ratios with active layer thickness (ALT) dynamics. During the recent warming period (1987–2020), ALT increased markedly, accompanied by accelerated declines in δ15N, reductions in C/N ratios, limited increases in BAI, and weakened correlations between BAI and growing-season temperature. Despite substantial releases of soil organic carbon and nitrogen from permafrost thaw, tree-ring δ15N and C/N continued to decline, indicating persistent nutrient limitation. Since the mid-twentieth century, tree growth has exhibited strengthened associations with δ15N and C/N, with δ15N emerging as the dominant correlate with BAI. These results demonstrate that rapid permafrost thaw modifies both soil and climatic conditions, thereby reducing nitrogen availability, constraining tree productivity, and weakening the climate–growth relationship. Our findings demonstrate the utility of tree-ring geochemical proxies for assessing long-term ecosystem responses and carbon–nitrogen interactions in boreal forests under climate warming.
