Dynamics and driving forces of treeline fluctuation and regeneration in Central Norway during the past decades

Abstract

The aim of this study was to analyse recent treeline fluctuations in central Norway to estimate the influence of land use change and climatic change during the past decades. Four study areas were chosen along steep oceanic-continental gradients in central Norway, representing different climatic regions. The analysis was performed using a combination of landscape-ecological, dendroecological, remote sensing and human-geographical methods. Treeline alterations were analysed by correlating climate change signals and spatial land use changes. All study sites showed a spatial expansion of forest fragments within the alpine treeline ecotone but little regeneration above the recent treeline. We found that intensive historical land use by logging and grazing had decreased the treeline at the beginning of the 20th century. Recent regeneration was found, mainly due to land use extensification. Climate analysis indicated warmer and moister conditions over the past decades. We found that these changes were restricted to the winter months, only. Furthermore, regeneration occurred on plots formerly disturbed by heavy grazing pressure or cryoturbation. Climatic change during the past decades had no effect on treeline fluctuations. The most decisive driving forces for treeline fluctuation and regeneration were changes of land use intensity. On the one hand, grazing pressure was the initial trigger for the expansion of forest fragments within the treeline ecotone. On the other, high grazing intensity combined with other land use practises caused stable stages of depressed treelines.