Assessing climate-driven treeline dynamics via the habitat suitability index

Alpine treelines are climate-sensitive boundaries that serve as indicators of forest responses to global warming. However, predicting their long-term spatial dynamics under future climate conditions remains challenging. Focusing on Larix chinensis, the dominant species in the Taibai Mountain treeline ecotone (Qinling Mountains), this study constructed an ensemble species distribution modeling framework (integrating Generalized Linear Model, Random Forest, Maximum Entropy Model, eXtreme Gradient Boosting) to forecast spatiotemporal shifts using the habitat suitability index (HSI). The ensemble model accurately reproduced current distribution patterns. Results indicated that growing-season temperature-related variables were the primary limiting factors governing both the establishment and upward migration of tree species within the treeline ecotone, while the dominant contribution of elevation largely reflected the "temperature signal embedded in the elevation gradient". Future projections under multi-scenario climate conditions indicated a general expansion of suitable habitats, increasing by 4.65-18.96 km² (approximately 3.97-16.19% of the total study area) by the 2100s. Crucially, treeline shifts exhibited a pronounced aspect-dependent pattern: stable increasing zones were concentrated on southern slopes, while decreases occurred on northern slopes. Under all scenarios, increase dominated, with stable increasing zones covering up to 73.8% of the area under SSP585 by 2100. These findings clarify the distinct response mechanisms of high-altitude ecosystems and offer essential insights for formulating adaptive management strategies for mountain forests.