RESPONSES TO NATURAL CLIMATIC VARIATION AND EXPERIMENTAL WARMING IN 2TUNDRA PLANT-SPECIES WITH CONTRASTING LIFE FORMS - CASSIOPE-TETRAGONAAND RANUNCULUS-NIVALIS

Two circumpolar tundra plant species, the evergreen dwarfshrub Cassiop e tetragona and the perennial herb Ranunculus nivalis, were studied at Latnjajaure in northern Swedish Lapland during three consecutive grow ing seasons (1993-95) as a contribution to the ITEX programme. Open-to p chambers (OTCs) were used in a passive heating experiment, and the p erformance of the plants in unmanipulated controls was correlated with climatic fluctuations among the years. Phenological, vegetative, and reproductive variables were measured. In both species phenological res ponses were controlled mainly by ambient air temperature. In the everg reen C. tetragona vegetative growth was controlled mainly by the influ x of global solar radiation and was not temperature-dependent, whereas the opposite applied in the herbaceous R. nivalis. Vegetative growth in C. tetragona was rather stable among years as well as between treat ments, whereas it was strongly influenced by annual climate in R. niva lis. Both species increased their reproductive success with increasing temperature, but R. nivalis was also radiation-dependent in this case , probably because of its green, photosynthetic nutlets. Ovule number in R. nivalis increased steadily in the experimentally heated plots du ring the study in response to the constant temperature amelioration ab ove the ambient. At the community level, evergreen C. tetragona seems to have low competitive ability under warmer conditions. The situation for vernal low-growing herbs like X. nivalis is more complex; despite a strong positive response to increased temperature, they may exhibit decreased reproductive success if overgrown by a vigorous graminoid c anopy.