INTRA-SPECIFIC VARIATION IN NITROGEN ECONOMY AMONG 3 MOUNTAIN BIRCH PROVENANCES

Intra-specific relationships between growth traits and nitrogen econom y were studied for seedlings of mountain birch (Betula pubescens ssp. totuosa) originating from three climatically different regions within the northern forest margin in subarctic Fennoscandia. The experiment w as carried out using potted seedlings grown in Abisko, northern Sweden , through two complete growing seasons. The seedlings were grown in pe at and assigned to two different temperature and fertilization treatme nts according to a factorial design. The treatments were ambient and c a. 2.5 degrees C elevated temperature (passive greenhouse), and soil n utrient fertilization corresponding to 1 and 10 g nitrogen m(-2) seaso n(-1). Significant differences among provenances were found In almost all growth and nitrogen economy traits studied The differences in grow th rate, which reached 47%, were explained primarily by leaf weight ra tio (biomass allocation to leaves) and secondly by leaf area productiv ity (biomass production per unit leaf area). Both the nitrogen product ivity and residence time of nitrogen (with respect to aboveground nitr ogen losses) varied between provenances: no significant relationship w as found between these two traits. Differences in residence time of ni trogen were more related to biomass loss ratio (leaf biomass losses pe r unit total plant biomass production) than to nitrogen concentration of abscised leaves. The intra-specific variation in nitrogen economy b etween mountain birch provenances is discussed with respect to the pos sibility of significant genotype x environment interaction.