Unravelling the influence of climate change on tree growth patterns and phenology in European forests

Rising temperatures and droughts increasingly affect tree growth in European forests. However, there is a limited understanding of how the changing climate alters intra-annual growth dynamics and phenology, which are key drivers of tree productivity and carbon uptake. To address this knowledge gap, we investigated how changes in temperature, precipitation and vapour pressure deficit (VPD) affect tree growth dynamics as well as the timing and duration of the growing season using a network of automatic dendrometers installed on more than 2500 trees of various functional types (broadleaf, coniferous, deciduous, evergreen) in major European forest ecosystems - boreal, temperate, and Mediterranean. In this ongoing project, the dendrometers have been measuring tree growth with high precision (1 µm) and frequency (15 minutes) for over 4 years, covering both climatically average and unusually hot years 2022 and 2023. The growth and growth phenology variables derived from dendrometer data were modelled as a function of the climate variables obtained from the E-OBS database. Significant variations in intra-annual growth dynamics were observed across all forest ecosystems over the study period. The growing period was substantially shorter in hot and dry years compared to hot and wet years or years with average conditions. This reduction was primarily due to a significantly earlier growth cessation (by over a month in some years), which offset a slightly earlier growth onset after winter dormancy (by up to 7 days). The large shifts in growth cessation to earlier dates were strongly associated with lower precipitation and higher VPD during the month with maximum growth (May-July), while the earlier growth onset was related to elevated early spring temperatures. Also, higher VPD and lower precipitation were the main causes of reduced growth rates in the hot and dry years. Although the magnitude of the effect varied, the pattern of precipitation and VPD strongly influencing growth and phenology, with temperature playing a lesser role, was consistent across ecosystems and species. Because lower growth rates and the shorter growing season were strongly linked to a decline in total yearly growth in all the ecosystems, it is evident that changes in atmospheric dryness and water availability are likely to be the main drivers of climate-induced shifts in tree growth phenology, productivity and carbon uptake in European forests under a warming climate.