Impacts of persistent underground coal fires on radial growth of Picea schrenkiana on the northern slope of the Tianshan Mountains

Abstract

Using tree-ring width data from Picea schrenkiana samples collected from six cross-sections in the underground coal fire area on the northern slope of the Tianshan Mountains, we established a tree-ring width chronology and analyzed the mechanisms by which underground coal fires affect tree radial growth by integrating climatic factors, soil temperature, and NDVI data. The results indicate that tree mortality across the six cross-sections predominantly occurred around 2018–2019, with mortality rates higher in the top sections than in the lateral sections, and higher in cross-section A than in cross-section B within the top sections. In the northeastern part of cross-section A, the underground coal fire spread at an annual rate of 4.36 m from 2014 to 2021, with the fastest spread occurring in 2018, reaching approximately 17.2 m. Since the 1990s, the missing-ring rate has generally increased, peaking in 2020, and living trees showed higher missing-ring rates than dead trees, except in sections A and E. The climatic response of the BYH site was similar to that of the lower-altitude site (BYG2), showing significant negative correlations with May temperature and significant positive correlations with February and April–May precipitation; additionally, the BYH chronology exhibited significant negative correlations with June temperature and precipitation. The Mann-Whitney U test revealed that the responses to temperature and precipitation in most months differed significantly (p < 0.05) between the BYH and BYG2 chronologies. Analysis of NDVI data over the past 18 years indicated a sustained effect of underground coal fires on tree growth, with the recovery of tree growth in the top section showing a negative correlation with distance from the fire zone (p < 0.1), closely linked to the upward heat transfer that led to pronounced soil warming. Living trees exhibited negative correlations with soil temperature (stronger in the 2000s), while dead trees showed positive correlations (stronger in the 2010s); the latter reflects a disruption of normal growth-climate feedback under long-term thermal stress. In addition to thermal stress, insect infestations (in sections C and D) acted as compounding factors accelerating tree decline, while the spatial distribution of combustion channels, geological interlayer, and individual tree differences collectively contributed to the spatial heterogeneity of mortality patterns. This study reveals the spatial heterogeneity of forest tree growth under the influence of underground coal fires and its complex relationships with climatic and soil environmental factors, providing dendroecological empirical evidence for a deeper understanding of the ecological impacts of underground coal fires.