Abstract ID: 3.11308 | Accepted as Talk | Talk/Oral | TBA | TBA

Alpine treelines define the upper boundary of tree growth and serve as critical indicators of climate constraints on forest distribution. While treeline elevation is theoretically governed by temperature—following a mean growing-season temperature isotherm—empirical data reveal substantial deviations from this model. The mechanisms driving these deviations remain debated, partly due to research scale limitations and insufficient attention to taxon-specific patterns. To address these gaps, this study integrates two comprehensive global treeline datasets. The first dataset includes over 53,000 recorded treeline points from 70+ major mountain regions, forming a sub-meter resolution global database that captures tree species composition and canopy structure using biodiversity databases and machine learning algorithms. The second dataset, compiled from research spanning the past half-century, consists of over 2,000 treeline species records from 39 mountain regions in 43 countries, allowing for an in-depth taxon-specific analysis of treeline patterns. Our findings confirm that potential treelines align with a growing-season mean temperature isotherm of 6.6 ± 0.3°C, yet approximately two-thirds of actual treelines deviate from this threshold. The primary driver of these deviations is drought stress (51%), followed by the mountain elevation effect (MEE), which modulates surface heat conditions (27%). Additionally, treeline species richness peaks in northern mid-latitude mountains, corresponding with the highest observed treeline elevations. Taxon-specific analyses reveal that moisture availability and climatic variability further differentiate treeline distributions across genera. Despite these variations, a universal pattern emerges: treeline positions occur 30–40% below the thermal optimum of each genus’ niche range, reflecting a physiological constraint on tree survival at high elevations. By integrating elevational and taxonomic perspectives, this study provides a global synthesis of treeline distribution mechanisms. Our findings refine the understanding of species-specific responses to climate, contribute to biodiversity conservation in alpine environments, and improve predictions of forest dynamics under climate change.