Abstract: Understanding how endangered tree species respond to rapid warming is essential for assessing their future survival. Here, we build up a 282 years tree-ring width chronology of Rhododendron protistum var. giganteum in southeastern Tibetan Plateau to test whether rising temperatures have reshaped the climatic controls on radial growth. Structural equation modeling shows that pre-growing-season warming enhances atmospheric dryness, intensifying the negative effect of vapor pressure deficit (VPD) on growth. Segmented analyses around a 1988–1989 breakpoint reveal a clear regime shift: before the late 1980s, tree growth was primarily constrained by growing-season climate; after warming accelerated, growth became dominated by pre-growing-season temperature, VPD, and moisture availability. The strengthened negative impacts of early-season temperature and VPD, together with the reduced influence of growing-season climate, demonstrate that warming has fundamentally altered the climate-growth sensitivity of this species. Because R. giganteum already occupies a narrow ecological niche and exhibits slow growth, increasing pre-growing-season warming and atmospheric drying pose a substantial threat to its persistence under future climate scenarios. Our findings highlight the urgent need to incorporate early-season climatic stress into conservation strategies for this critically endangered R. giganteum.

Key words: Southeastern Tibetan Plateau, Rhododendron protistum var. giganteum, Tree rings, Winter-spring warming