In this project, we curated and provisioned long-term data acquired over almost 50 years at the Stillberg ecological treeline research site in the Swiss Alps that was established in 1975 by planting 92,000 tree seedlings of Larix decidua, Pinus cembra and Pinus mugo spp. uncinata. The long-term data comprise 6.5 million ecological and environmental records from the 40-year afforestation experiment, as well as from a 9-year free-air CO2 enrichment experiment crossed with a 6-year soil warming experiment, a 12-year nutrient addition experiment, and an 8-year multifactorial seed addition and vegetation removal experiment. Our data further include 38 million records of 25 meteorological parameters measured at an hourly resolution from 1975 to 1996 and at a 10-minute resolution since 1997. All datasets and the corresponding metadata are provided as open research data (ORD) on the environmental data portal EnviDat ( www.envidat.ch). The enhanced availability and quality of these ecological and environmental long-term data in a harmonized, well documented, and publicly available form is expected to foster whole-system approaches and inter- and transdisciplinary research syntheses on ecological processes in changing mountain ecosystems, e.g., within the eLTER framework. Continued monitoring of the Stillberg research site and open data sharing will help to further the mechanistic understanding of ecological processes in treeline ecotones under global change supporting the development of adequate climate change adaptation strategies.