Ectomycorrhizal (ECM) symbioses are omnipresent in the Arctic, and shifts in ECM communities may be expected in a future warmer climate. As large functional differences exist among ECM fungal genera, species and isolates, this could affect several important ecosystem processes, such as plant C fixation and allocation belowground and plant uptake of different nutrient pools. The objectives of this project are 1) to characterize responses of ECM associations of a circumpolar plant species (Betula nana L.) to manipulations of air temperature and soil nutrient availability and 2) to relate functional characteristics of ECM communities to ecosystem C and N cycling.The project consists of three elements: 1. Quantitative analyses of responses in ectomycorrhizal morphotype communities associated with B. nana to manipulations of air temperature and soil nutrient availability at two sites (Abisko, Sweden, and Toolik Lake, Alaska). Fieldwork 2002. 2. Field and growth chamber studies of functional differences among ectomycorrhizal fungi and communities from Abisko and Toolik Lake. Fieldwork 2002. 3. Along a natural gradient, relationships between ECM communities and availability and uptake of different N forms will be examined under natural conditions. Fieldwork 2003-04.