When exposed to shifts in the environment, plants can adjust their traits to maintain optimal performance under more restrictive conditions. This capacity of plants is known as phenotypic plasticity and can occur over short and/or long periods of time. However, although trait plasticity could be one of the most important factors driving tolerance to climatic stress, to date very little is known about its magnitude and almost no data is available in global trait databases. Our objectives are to determine the seasonal phenotypic plasticity in a large array of aboveground (e.g. photosynthetic properties) and belowground (e.g. root properties) functional traits across a broad range of tree species in European forests.