The nitrogen economy of mountain birch : As related to environmental conditions and genotype

Sammanfattning: Responses of mountain birch growth to changed environmental conditions arestrongly affected by birch genotype, and nitrogen economy is a key factor influencingthe winter survival of first-year seedlings. These are some of the findings of a series ofstudies, in which growth, nitrogen economy and winter survival were analysed usingpot-grown seedlings of mountain birch (Betula pubescens ssp. czerepanovii) exposedto various environmental conditions in northern Sweden. Differences between birchgenotypes were studied at the individual, ecotype and provenance levels. Theexperimental factors included temperature, nutrient availability and ultravioletradiation. In addition, the influence of neighbouring vegetation on birch nitrogeneconomy was evaluated. Nutrient availability and temperature were recorded in situduring one year in three mountain birch woodland soils. Environmental effects on nitrogen economy varied between years, during agrowing season and between seedling age groups. In many cases differences in growthbetween birch genotypes were only apparent under certain types of environmentalconditions. An interaction between genotype and environment was apparent in thecase of elevational ecotypes. Thus, an increase in leaf nitrogen content with altitudewas attributed to both a phenotypic adjustment and a genetic adaptation to decreasingtemperature. The rate of nitrogen accumulation in mountain birch during the growingseason determined the survival rate of first-year seedlings the following winter. Theenvironmental conditions, neighbouring vegetation, and genotype all affected thenitrogen accumulation rate of young seedlings and could influence their survival. Soiltemperature was concluded to have a major influence on the survival of young birchseedlings owing to its strong impact on the root nitrogen uptake rate.