Last-century vegetational changes in northern Europe. Characterisation, causes, and consequences

Abstract

In the face of recent changes in environmental conditions and climate, longterm studies provide important insights into patterns and processes of vegetational change. In northern Europe, however, long-term studies are rare for many ecosystems. This thesis uses a new approach that uses historical phytosociological data-sets to study changes in the vegetation of alpine, mire, and arctic habitats and regions across northern Europe over the past decades. Because plot relocation due to the use of non-permanent plots may bias the detection of change, the thesis investigates if observed changes are larger than what is expected by chance. Furthermore, to find out if observed changes in vegetation are consistent between different habitats and regions in northern Europe, a meta-analysis of 15 data sets from arctic, alpine, and mire sites is presented. The results of the resurvey conducted in alpine Sikkilsdal, Central Norway, show that most species have shifted their distributional range upwards along the elevational gradient since the first sampling in the 1920s. These upward shifts were found to be independent of whether upper, lower, or optimum elevation were considered. As the largest shifts were found for species growing in snow-bed habitats, the results suggest climate warming and alterations in snow-cover duration to be important drivers of the observed range shifts. In the Åkhult mire (South Sweden), changes over a period of 54 years were found predominantly for species of dwarf-shrubs and trees, whereas several typical mire species have decreased or disappeared from the study site. Drier mire surface and higher nutrient availability due to a warmer climate are identified as the most plausible drivers explaining the observed turnover in species composition. On Jan Mayen Island, similar changes in vegetation were found during time periods of 19 and 80 years. Over both time-scales, graminoid and woody species were found to have increased, whereas several snow-bed related species have decreased. However, whereas the main trend is similar over both time-scales considered, discrepancies in the trends of some species suggest that long-term changes are only partly predictable from short-term studies. The results of the meta-analysis show that the arrangement between species has changed more than is expected by chance, indicating that nonrandom changes have occurred in the studied arctic, alpine, and mire habitats during the past century. Vegetation stability was found to vary sitespecifically. Observed patterns could not be explained by different factors, such as time-scale, plot number, species diversity, or productivity. The thesis shows that non-random vegetational changes have occurred independent of which habitat or site is considered. As the observed changes in vegetation are in line with several other studies on vegetational dynamics focusing at different temporal and spatial scales and using permanent plots, this thesis demonstrates that historical phytosociological data-sets may successfully be used in the way presented here. These results unlock a valuable archive to identify recent vegetational changes in relation to environmental change. Moreover, observations of increased growth of woody plants and graminoids, upward shifts in species ranges, and decreases in species mostly associated with wetter habitats indicate trends in vegetation towards more competitive and nutrient-demanding species. With regard to predicted changes in climate, further changes may be assumed, the effects of which are likely to be most pronounced in areas where species are adapted to low temperatures and low nutrient availability, such as in high mountain areas, raised bogs, and in the Arctic.