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Field manipulations of light, temperature, nutrients, and length of growing season in directions simulating global environmental change altered biomass of the four most abundant vascular plant species in tussock tundra of northern Alaska. These species are Betula nana, Ledum palustre, Vaccinium vitis-idaea, and Eriophorum vaginatum. Biomass response to our treatments reflected changes in both growth and mortality, with growth being stimulated by treatments that enhanced biomass, and mortality being enhanced by all treatments (except in Vaccinium). Those species with highest leaf and stem turnover (the graminoid and deciduous shrub) initially showed large positive responses to nutrient addition. By contrast, slow-turnover evergreen species showed little initial change in production in response to our manipulations, and their long-term biomass responses were in the opposite direction to those of the responsive species. Short-term measurements of leaf expansion, photosynthesis, and phosphate uptake showed little correlation with net production or biomass change in response to manipulations because of compensatory mechanisms at levels of growth and allocation. Changes in nutrient distribution among species accounted for many of the long-term changes in biomass and productivity. Processes that are readily integrated at annual time steps (e.g., shoot growth, shoot mortality, allocation) were more useful than instantaneous physiological measurements in predicting decadal vegetation changes because (1) compensating responses among physiological processes buffer plant responses at progressively longer time scales, (2) species interactions in the community buffer ecosystem processes such as productivity and nutrient cycling from changes in growth of individual species, and (3) different time lags between physiological, demographic, and ecosystem processes complicate modeling of long-term responses from short-term mechanisms. Key words: arctic; climatic change; light; nutrients; phenology; photosynthesis; productivity; temperature; tundra., INTRODUCTION Physiological ecologists confronted with the challenge of predicting the role of terrestrial ecosystems in global processes (Houghton et al. 1990) have sought to provide a mechanistic understanding of the [...]