Rapid recent range-margin rise of tree and shrub species in the Swedish Scandes

Abstract

1. Recent elevational range-margin performance of tree and shrub species was studied at a site in the Swedish Scandes. The methods included comparisons of historical and present-day range-margin records (m a.s.l.) in conjunction with age-determination of newly established saplings. 2. Since the early 1950s, the range-margins of Betula pubescens ssp. tortuosa (mountain birch), Picea abies (Norway spruce), Pinus sylvestris (Scots pine), Sorbus aucuparia (rowan) and Salix spp. (willows) have advanced by 120-375 m to colonize moderate snow-bed communities. The non-native Acer platanoides (Norway maple) has become established just below the birch forest-limit. In concert with tree-limit rises by 100-150 m in the same region, the present results suggest a shift in reproduction and a significant break in the late-Holocene vegetation history. 3. Ring-counting (in 2000) of a subsample of the recovered saplings revealed that, with one exception, they were aged between 7 and 12 years, i.e. they germinated after 1987. Since 1988 there has been strong and consistent winter warming, with some very warm summers, and this may ultimately have forced the vegetational changes. 4. Reduced summer snow-retention has favoured seedling establishment and juvenile growth, and mild winters, with reduced risk of frost-desiccation, have enhanced survivorship and height increment. 5. Certain seed-regenerating tree and shrub species have tracked recent climate change quite rapidly and more sensitively than vegetatively propagating field-layer species. Such species-specific responses may give rise to novel high-elevation vegetational patterns in a hypothetically warmer future world.