The effects of temperature, precipitation and nutrient perturbations, and their interactions, are being assessed on two contrasting arctic ecosystems to simulate impacts of climate change. One, a high arctic polar semi-desert community, is characterised by a sparse, low and aggregated vegetation cover where plant proliferation is by seedlings, whereas the other, a sub-arctic dwarf shrub heath, is characterised by a complete vegetation cover of erect, clonal dwarf shrubs which spread vegetatively. The developmental processes of seed production were shown to be highly sensitive, even within one growing season, to specific environmental perturbations which differed between sites. At the polar semi-desert site, there was a striking effect of the temperature enhancement treatments on phenology and seed-setting of Dryas octopetala ssp. octopetala, with almost no seed-setting occurring in plots experiencing ambient temperatures. By contrast, there were no significant effects of temperature enhancement alone on fruit production of Empetrum hermaphroditum at the sub-Arctic dwarf shrub heath site, although fruit production was significantly influenced by the application of nutrients and/or water. The response of a dominant high arctic dwarf shrub to increased temperature suggests that any climate warming may stimulate seed-set. This could be particularly important in the high Arctic where colonisation can proceed in areas dominated by bare ground and where genetic recombination may be needed to generate tolerance to predicted changes of great magnitude. In the sub-Arctic, however, the closed vegetation is dominated by clonally-proliferating species and recruitment from seedlings is rare. Plant fitness will increase here in response to any increased vegetative growth resulting from higher nutrient availability in warmer organic soils.