EFFECTS OF SHADING, NUTRIENT APPLICATION AND WARMING ON LEAF GROWTH AND SHOOT DENSITIES OF DWARF SHRUBS IN 2 ARCTIC-ALPINE PLANT-COMMUNITIES

Responses in shoot growth and activation of new meristems of dominant dwarf shrubs were measured after six years of shading or temperature e nhancement with and without NPK fertilizer addition to a heath and a f ellfield 450 m above sea level and 1150 m above sea level, respectivel y, in northern Sweden. Amongst the treatments shading generally had th e greatest effects on the growth of individual shoots. It decreased th e specific leaf weight of all species, but did not affect the activati on of new meristems. Fertilizer addition stimulated both the growth of individual shoots and meristem activation at the fellfield, but stimu lated only activation of new meristems at the heath, which led to incr eased leaf area index and leaf biomass per ground area. Temperature en hancement at the fellfield generally increased shoot growth and merist em activation and explained mole of the total experimental variance th an the fertilizer addition. At the heath, shoot growth was largely una ffected by temperature changes, but two of five species responded by a ctivation of new meristems. Evergreen and deciduous species responded similarly to the treatments. However, predominantly boreal species ext ending above the tree-line responded more strongly to the temperature enhancement than species with a main arctic-alpine distribution. Hence , it appears that the species responded more at the coldest part of th eir range. This suggests that predicted future increase of air tempera ture will enhance the growth of dwarf shrubs most strongly in climatic ally harsh environments, but the growth of boreal species may increase also in climatically mole benign areas. Increased nutrient availabili ty will, on the contrary, probably increase growth more than changes i n temperature in climatically benign sites.