Abstract ID: 3.13300 | Accepted as Talk | Talk | TBA | TBA

Current uppermost tree positions represent cold boundaries of realized niche of trees which may or may not overlap with the boundaries of fundamental niche. Climatic characterization of uppermost tree locations allows an evaluation whether current treelines are in equilibrium with climate or they lag behind the pace of warming. Furthermore, climatic characteristics of treelines may indicate potential treelines where trees are absent due to various disturbances or long-term land-use pressures. Surprisingly, we lack comprehensive climatological characterization of European treelines. In this study, we systematically screened European treeline regions, identified uppermost tree positions and calculated treeline bioclimatic metrics derived from CHELSA dataset including growing season length, growing season temperature, degree days and growing season daily precipitation. We identified 5100 locations of uppermost trees in more than thousand 20 km gridcells that overlaid treeline areas. For each point we recorded treeline form, ground cover and topographic features. We hypothesized that treelines face varying climatic conditions with trade-offs between growing season heat sums and growing season moisture stress. Our results show that according to climatic metrics, European treelines can be divided into five distinct groups: (1) very warm and dry located predominantly in southern Europe, (2) moderately cold and dry located mainly in eastern Europe and in inland parts of Scandes and in very northern part of Europe, (3) cold found mainly in the Alps and Pyrenees, (4) moderately cold and wet with very short growing season located mainly in maritime parts of Scandes, and (5) moderately cold with very long growing season found in Scotland and Ireland. Treeline climatic groups differ mainly in their location, and to lesser extent also in terrain morphology and treeline form. Accordingly, the most influential predictors of treeline temperature metrics were geographical position, slope steepness, distance to summits and presence of broadleaves/conifers. Warmer treelines were drier suggesting trade-off between temperature and moisture-limited tree growth. Identified differences in temperature metrics between treeline groups indicate that there is a complex gradient of meteorologic conditions that influence occurrence and intensity of tree growth allowing woody plants to achieve tree stature. Our study brings a climatic context of European treelines.