What In the Arctic, climate change is warming tundra landscapes at twice the rate as the rest of the world. Predicting how plant communities change with climate warming, and quantifying the associated effects on ecosystems, is one of the principal goals in ecology. Plant functional traits are measurable characteristics, such as plant height or leaf size, which reflect different strategies for competing with other plants or coping with climate change. For example, diversity in leaf thickness or nitrogen content strongly determines ecosystem carbon balance through direct effects on photosynthetic carbon uptake and microbial litter decomposition rates. Consequently, plant traits provide a quantifiable link between vegetation change and ecosystem processes that feedback to the climate. Why As the tundra warms, plants take up CO2 through photosynthesis, while microbial decomposition of soil carbon increasingly releases greenhouse gases into the atmosphere. Permafrost soils store twice as much carbon as currently found in the atmosphere, making it a great concern whether tundra environments will act as net sources or sinks of atmospheric carbon, and thus exacerbate or mitigate climate change in the future. Ultimately, this depends on the balance between plant carbon uptake and microbial decomposition losses. However, plants are not just responsible for carbon sequestration; they also regulate microbial decomposition rates through changes in their leaf and root trait characteristics. Nevertheless, we know little about plant functional responses to long-term tundra warming. How In WarmTraits, I want to quantify how climate warming duration affects a very comprehensive suite of the most important above- and belowground plant functional traits. I will use a rare opportunity to destructively sample in a long-term field experiment where small greenhouses have warmed arctic tundra plots for one, 11, and 21 years, respectively. Moreover, I will investigate how plant responses to warming affect important ecosystem functions, such as carbon and nitrogen cycling rates, using stable isotopic labelling. I will plan and host a field workshop, teaching students in trait-based climate change ecology while providing them with hands-on experience on how to measure traits. Thereby, the students gain invaluable research experience while efficiently collecting WarmTraits data. SSR This project improves our mechanistic knowledge of tundra ecosystem responses to increasing climate warming duration, with direct implications for feedbacks to our global climate. The results will aid researchers and modellers improve predictions of a changing climate, which - ultimately - benefits us all.