The causes of upper tree limits in the mountain ranges of Central Europe north of the Alps – A stem growth perspective

Abstract

AbstractQuestionsGlobally, mean growing season temperature at the climatic treeline was found to converge to a common isotherm, presumably related to similar growth limitations by low temperature. However, temperatures for high‐elevation tree limits are often higher than the corresponding thermal limit for tree growth, suggesting that trees are regionally absent from the potential treeline. We explore whether the regional uppermost tree stands in Central Europe are limited by low growing season temperature, reflect the action of a “summit syndrome” (harsh climatic conditions near summits of insufficiently high mountains), or have resulted from historical land use or other disturbances.LocationMountains of Central Europe, north and east of the Alps (48–51°N, 10–20°E).MethodsIn each region, the highest elevation, 3‐m tall individuals of Picea abies were sampled to measure radial and height growth and to record symptoms of wind‐induced damage (abrasion, bending, breakage). Trees were grouped based on their growth performance and growing season temperature. Differences in variables indicative of a summit syndrome (wind‐induced damage, elevation distance to nearest summit, height increment above 2 m stem height) were tested among groups.ResultsWe identified four particular groups of trees. The first contains trees characterized by low growth rates at cold tree limits located distant from summit areas. These trees are considered to represent the climatic treeline. The second group consists of trees growing well at relatively “warm” tree limits located near summits. These upper tree limits are probably a consequence of disturbances, human interventions or tree establishment lagging behind the warming over the past century. Trees from the remaining two groups are characterized by intermediate temperatures and growth rates. They are usually located near summits and exhibit various ranges of symptoms of mechanical impact due to summit conditions. Their current range limits reflect a combination of the repeated loss of biomass, thermally‐limited growth and restricted space for further upslope advance.ConclusionsOur study shows that comparing tree growth, growing season temperature and symptoms of biomass loss is helpful for explaining the formation of regional upper tree limits.