NITROGEN NUTRITION AND ISOTOPE DIFFERENCES AMONG LIFE FORMS AT THE NORTHERN TREELINE OF ALASKA

Natural abundances of nitrogen isotopes, delta(15)N, indicate that, in the same habitat, Alaskan Picea glauca and P. mariana use a different soil nitrogen compartment from the evergreen shrub Vaccinium vitis-id aea or the deciduous grass Calamagrostis canadensis. The very low delt a(15)N values (-7.7 %) suggest that (1) Picea mainly uses inorganic ni trogen (probably mainly ammonium) or organic N in fresh litter, (2) Va ccinium (-4.3 %) with its ericoid mycorrhizae uses more stable organic matter, and (3) Calamagrostis (+0.9 %) exploits deeper soil horizons with higher delta(15)N values of soil N. We conclude that species limi ted by the same nutrient may coexist by drawing on different pools of soil N in a nutrient-deficient environment. The differences among life -forms decrease with increasing N availability. The different levels o f delta(15)N are associated with different nitrogen concentrations in leaves, Picea having a lower N concentration (0.62 mmol g(-1)) than Va ccinium (0.98 mmol g(-1)) or Calamagrostis (1.33 mmol g(-1)). An exten ded vector analysis by Timmer and Armstrong (1987) suggests that N is the most limiting element for Picea in this habitat, causing needle ye llowing at N concentrations below 0.5 mmol g(-1) or N contents below 2 mmol needle(-1). Increasing N supply had an exponential effect on twi g and needle growth. Phosphorus, potassium and magnesium are at margin al supply, but not interaction between ammonium supply and needle Mg c oncentration could be detected. Calcium is in adequate supply on both calcareous and acidic soils. The results are compared with European co nditions of excessive N supply from anthropogenic N depositions.