【英文摘要】We aim to understand mechanisms of alpine timberline formation in southeast Tibet. This study was conducted on the opposite slopes of a U-shaped valley at the peak of the Sergyemla Mountains where Abies georgei var. smithii and Juniperus saltuaria are the dominant tree species of timberlines on north-facing and south-facing slopes, respectively. During 2005-2009, we simultaneously monitored soil temperature and moisture and related meteorological factors for 12 plots with altitudinal vegetation changes from forests to shrublands and grasslands. We also examined altitudinal variations in within-species leaf water potential, leaf carbon isotope ratio (an integrated indicator of of carbon gain and loss of the entire history of a plant's leaf), maximum photosynthetic rate (Pmax) and photosynthetic nitrogen use efficiency (PNUE) etc. Our data indicated that: 1) The growing season mean soil temperature at -20cm was 0.8-1.0 K higher at the south-facing juniper timberline (7.0℃) than at the north-facing fir timberline (6.0-6.2 ℃), both fall in with the -10cm threshold temperature of 6.7±0.8℃ found in global climatic treelines. 2) The -20cm mean soil temperatures for shrublands and grasslands above both timberlines were generally higher than those of nearby forest sites (at lower altitudes), in which slope exposure, snow cover and vegetation variables rather than altitude were the major determining factors for spatial variability of seasonal mean soil temperatures across both timberline ecotones. 3) Slope exposure and canopy and snow covers had remarkable effects on the daily soil temperature amplitude that was associated with the contrasting distribution patterns of seed-based fir seedlings and root-sprouting juniper seedlings, suggesting an explanation for the slope-opposite distribution of both species timberlines in the Sergyemla Mountains.4) The low-temperature-enhanced water and nitrogen stress was the major cause for altitudinal decreases in leaf water potential, Pmax and PNUE that were associated with reduced plant growth rates. The high canopy stature and accompanied low air- and soil-temperature at the timberline limited the water uptake by roots and its transportation rate in the soil and plant, leading to a higher sensitivity to water stress in trees than in shrubs and grasses as indicated by the highest altitudinal variations in carbon isotope ratio of tree leaves. Given a threshold of low soil temperature at the timberline, tree species with high canopy stature would be replaced by low shrubs and/or grasses, which not only improves soil temperature and enhances the uptake of water and nitrogen by roots, but also decreases the pathway of water transportation in the plant.5) Altitudinal transect data indicated that canopy-mean foliar carbon isotope ratio was negatively correlated with plant-level Pmax and relative plant growth rate as well as ecosystem-level leaf area index and net primary productivity, which is generally confirmed by global literature data. The finding is useful for interpretation of carbon isotope signals in tree rings and soil profiles, or even in lake-core alkanes that may be stable over geological times.