Latitudinal Gradients in Position and Climatology of Upper Tree Limits Across Europe

ABSTRACT Aim The upper tree limit (UTL) marks the upper boundary of the realized niche of trees. It is commonly located below the treeline, which represents the cold boundary of the fundamental niche of trees. We hypothesized that the elevational offset between the UTL and the treeline is partly systematic and can be predicted using topographic and bioclimatic variables. These variables were further used to develop a climatic classification of European UTLs. Location Europe except Ural. Taxon Tree species from the families Pinaceae , Fagaceae , and Betulaceae . Methods We identified current UTLs and calculated their bioclimatic metrics, based on which UTLs reaching treeline isotherm were subset. UTL elevation and its offset from the treeline were modelled using latitude, diurnal temperature range, climatic water balance, distance to summit, and warming trends. Results Along with latitude, distance to summit and diurnal temperature range explained the elevational gradient of European UTLs (root mean square error = 196 m) and the offset between UTLs and treelines. European UTLs were classified into five distinct bioclimatic groups: (1) warm and dry UTLs in southern Europe, (2) UTLs in the highest European mountains, (3) Scotland's UTLs with a very long growing season, (4) UTLs in the western Scandes with the lowest degree days and the shortest growing season, and (5) UTLs from the eastern Scandes and Kola with a relatively dry and short growing season and with a low diurnal temperature range. Only UTLs in groups 2,4, and 5 reach the treeline. Main Conclusions Our study shows that, beyond random and land‐use driven factors, deviations of UTLs from the treeline are partly systematic and reflect latitudinal and regional variation in available space above the UTL and diurnal temperature range. We provide a bioclimatic framework for the distribution and regional dynamics of European UTLs.