Increase in plant productivity in the tundra in Sweden is not related to increasing presence of warm-adapted vascular plant species

Abstract

Over the past decades, Arctic and alpine biomes have warmed faster than the global average. Substantial vegetation changes have been reported as a response to this warming, including increased productivity and the spread of warm-adapted (thermophilic) species that are less resistant to grazing pressure into higher latitudes and altitudes. At a local and regional level, however, reported vegetation changes are much more variable. Here, we investigate community changes of vascular plant species in the mountain tundra in Sweden during the period 2003 to 2020 by combining remote sensing and vegetation monitoring at 419 permanent plots. We first investigated changes in vegetation productivity by analysing the trend in the normalized difference vegetation index (NDVI). We then used plot averages of species-specific environmental indicators weighted by species occurrences to test whether the vegetation changed towards a more thermophilic state with a decreased resistance to grazing by mammalian herbivores during that time. We found an increase in vegetation productivity in ca 23% of the sites, and no trend in ca 55% of the sites. There was no general trend towards an increased thermophilization of vascular plant communities. Furthermore, at the plot scale, the change in NDVI during the study period was not explained by thermophilization of the plant community. However, warm-adapted forbs increased more than cold-adapted species. We conclude that, although part of the vascular vegetation is getting more thermophilic, the observed increase in vegetation productivity during the last two decades is not driven by an increase in warm-adapted species. We also suggest that reindeer grazing, which occurs in almost the entire study region, contributes to the preservation of cold-adapted Arctic–alpine biodiversity by selectively feeding on warm-adapted vascular species.