Modelling the short-term effects of climate change on the productivity of selected tree species in Nordic countries

A boreal version of the process-based simulation model, BIOMASS, was used to quantify the effect of increased temperature and CO ₂-concentrations on net primary production (NPP). Simulations were performed for both coniferous (Pinus sylvestris, Picea abies) and deciduous broad-leaves stands (Fagus sylvatica, Populus trichocarpa), growing in different Nordic countries (Denmark, Finland, Iceland, Norway and Sweden), representing a climatic gradient from a continental climate in Finland and Sweden to a maritime in Denmark, Norway and Iceland. Simulations with elevated temperature increased NPP by ca. 5-27% for the coniferous stands, being less for a Scots pine stand growing in a maritime climate (Norway) compared with a continental (central Sweden, eastern Finland). The increase in NPP could largely be ascribed to the earlier start of the growing season and more rapid recovery of the winter-damaged photosynthetic apparatus, but temperature-driven increases in respiration reduced carbon gain. The effect of elevated temperature on NPP was similar in the P. trichocarpa stand on Iceland, mainly caused by an earlier budbreak and a more rapid leaf development in spring. Increased temperature reduced, however, NPP for the F. sylvatica stand in Denmark, since elevated temperature had no effect on budbreak but increased the water deficit and water demand during the summer and lowered photosynthesis. Increased CO₂-concentrations had an additional effect on NPP by 25-40% for the conifers and beech, which originated from increased photosynthesis, through enhanced carboxylation efficiency in summer and improved water use efficiency (beech). The effect of elevated CO₂ on NPP was somewhat less for the P. trichocarpa by 13%.