Vegetation response to climate warming across the forest-tundra ecotone: species-dependent upward movement

Questions Rising temperatures are predicted to cause upward shifts and reorganisation of mountain vegetation. This study analyses how field layer vegetation across the forest-tundra ecotone has responded over a 22-year period. Main questions are: (a) have vegetation composition, richness and diversity changed; (b) have abundance of functional plant groups and individual species changed; and (c) which environmental factors regulate vegetation distribution and composition? Location Central Norway. Methods The study uses vascular plant species recordings and environmental data from permanent 1 m x 1 m quadrats (n = 266), established in 1994 and revisited in 2016, along transects from forest to high alpine areas (750-1,500 m a.s.l.). Changes in vegetation composition (species and functional group levels) and influence of environmental factors are analysed using ordination and mixed-effect models. Results Ordination shows an overall upward vegetation movement corresponding to 0.5 +/- 0.1 m/y, and compositional homogenisation across the ecotone over time. Changes at species and functional group levels vary across the ecotone. Species richness and diversity increase over time due mainly to an increase of herbs and graminoids in the forested part of the ecotone. Evergreen woody species increase in abundance across the entire ecotone and most strongly above the forest. Deciduous woody species abundance is stable at group level but shows large variation at species level. Species-level responses deviate from group-level responses in all functional groups. Vegetation distribution and composition are environmentally explained by altitudinal distance to the treeline and microtopography. Conclusions Our results show how increased temperature impacts vegetation movements and reorganisation through mainly species-specific responses with low within-functional-group coherency. The apparent upward shift is moderate compared to the increase in temperature over the study period, but larger than in similar studies, although grazing pressure might co-control change rate. Species-specific responses and response rates highlight the need for detailed empirical data to predict and understand vegetation responses in a warming climate.