Effects of shading, nutrient application and warming on leaf growth and shoot densities of dwarf shrubs in two 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 enhancement with and without NPK fertilizer addition to a heath and a fellfield 450 m above sea level and 1150 m above sea level, respectively, in northern Sweden. Amongst the treatments shading generally had the greatest effects on the growth of individual shoots. It decreased the specific leaf weight of all species, but did not affect the activation of new meristems. Fertilizer addition stimulated both the growth of individual shoots and meristem activation at the fellfield, but stimulated only activation of new meristems at the heath, which led to increased leaf area index and leaf biomass per ground area. Temperature enhancement at the fellfield generally increased shoot growth and meristem activation and explained mole of the total experimental variance than the fertilizer addition. At the heath, shoot growth was largely unaffected by temperature changes, but two of five species responded by activation of new meristems. Evergreen and deciduous species responded similarly to the treatments. However, predominantly boreal species extending 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 their range. This suggests that predicted future increase of air temperature will enhance the growth of dwarf shrubs most strongly in climatically harsh environments, but the growth of boreal species may increase also in climatically mole benign areas. Increased nutrient availability will, on the contrary, probably increase growth more than changes in temperature in climatically benign sites.