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Seedling establishment is important to understanding the formation and dynamics of alpine treeline, which would be vulnerable to the unpredictable freezing events in the early growing season.However, characteristics of the growing-season freezing event and its effects on the survival and growth of seedlings at and above treeline remain poorly understood because continuous and detailed measurements of microclimate at trecline ccotoncs are rare.In this study, we conducted a 4-yr reciprocal transplant experiment of fir and juniper treeline seedlings on opposite slopes of a valley in the Sergyemla Mountains where annual mean air-temperature differs by 0.5-1.6 K and annual precipitation is similar.The microclimate data were obtained from four automatic weather stations at and above both treelines.The frequency, intensity and duration of the growing-season freezing events across different habitats generally increased from north-facing to south-facing slopes.The freezing days in the early (AprilJune) and late (September-October) growing season were mainly determined by daily minimum net radiation and relative humidity.In the north-facing fir forest, the freezing days were only found in the late growing season, compared to the frequent occurrences in the early and late growing season across the south-facing juniper forest and the two non-forested sites above both treelines.The survival rates of transplanted fir seedlings on the south-facing slope greatly decreased by <60% in forested habitat and by <40% in non-forested habitat, while their survival rates on the north-facing slope did not change in forested habitat and slowly decreased by 85% in non-forested habitat.The survival rates of transplanted juniper seedlings decreased by 68%-84% with unclear change trends among different slopes and habitats.In pooled data across slopes and habitats, annual top-shoot growth rates decreased in fir seedlings but varied little in juniper seedlings with increasing freezing days in the early growing season.The data provided evidence that the seed-based fir seedlings were especially vulnerable to the wanning-induced increase of freezing events in the early growing season, which can be helpful for understanding the reason why there is little change in the fir treeline position after 200 years of warming.