Abstract: The growth responses of trees to hydrothermal conditions may vary across different tree species and change along altitudinal gradients. However, the patterns of these changes remain unclear, limiting the accuracy of forest growth predictions under future climate change. Using dendrochronological methods, we established five standardized tree-ring width chronologies for Picea likiangensis var. rubescens (north-facing slope) and Juniperus tibetica (south-facing slope) in eastern Tibet, and investigated the response characteristics of radial growth to hydrothermal factors through Pearson correlation analysis and other analytical approaches. The results showed that the growth of both species was primarily influenced by temperature and precipitation in the growing season (April to June). The correlation between tree-ring width and temperature for P. likiangensis changed from negative to positive with increasing altitude, with a significant positive relationship between the correlation coefficients and altitude (R²=0.86). Conversely, the correlation between treering width and precipitation changed from positive to negative with increasing altitude, and such change had a significant negative correlation with altitude (R²=0.93). The altitudinal threshold in which these correlations reversed was identified between 4360 m and 4410 m. In contrast, no such reversal was observed for J. tibetica on sunny slopes, and its radial growth remained negatively correlated with temperature and positively correlated with precipitation across the entire altitudinal gradient. These results suggest that the growth of P. likiangensis at higher altitudes is limited by low temperatures and by water deficit at lower altitudes. The growth of J. tibetica is consistently limited by water availability across all altitudes. The altitudinal threshold effect of tree growth in response to hydrothermal factors reveals the pronounced altitude dependence and ecological complexity of forest growth dynamics. Therefore, systematically examining and precisely defining the altitudinal thresholds of hydrothermal transitions is of significant scientific importance for improving the accuracy and reliability of forest dynamic prediction models.

Key words: Picea likiangensis var. rubescens, Juniperus tibetica, tree growth, climate response, altitude threshold