The influence of changes in climate and land-use on regeneration dynamics of Norway spruce at the treeline in the Swiss Alps.
Recent changes of climate and land use are often regarded to affect the European Alpine region substantially and to trigger an increase in the elevation of the upper treeline. The patterns of tree invasion on a subalpine meadow at 1900 m asl in Sedrun, Canton Grisons, Switzerland, were studied in order (1) to reconstruct the process of tree establishment and tree-growth dynamics in space and time, and (2) to evaluate the influence of site properties, land use change and climate on these processes. Dendroecological analysis of 105 Norway spruce (Picea abies) combined with an assessment of 48 vegetation plots and 17 soil profiles revealed that the trees were established in one main period (1965-1980s), starting 15 years after the abandonment of the agricultural use of the meadow, and that there is a pronounced environmental gradient along the forest-meadow ecotone. Tree establishment and height growth were favoured close to the former forest edge, but all saplings irrespective of their distance to the forest edge and their age showed increased radial growth since 1990, coinciding with a period of higher summer temperatures in the region. Therefore, we conclude that the observed treeline dynamics in Sedrun are the result of both land use and climate change: Tree establishment was triggered by the abandonment of the agricultural use of the meadow, and strongly favoured by particularly good growing conditions in a warm decade, which illustrates the sensitivity of conifers near the alpine treeline to temperature fluctuations.