Oecologia (1994) 100:406-412 9 Springer Verlag 1994 E.-D. Schulze 9 ES. Chapin, III - G. Gebauer Nitrogen nutrition and isotope differences among life forms at the northern treeline of Alaska Received: 14 June 1994 ! Accepted: 12 September 1994 Abstract Natural abundances of nitrogen isotopes, ~15N, indicate that, in the same habitat, Alaskan Picea glauca and P. mariana use a different soil nitrogen com- partment from the evergreen shrub Vaccinium vitis-idaea or the deciduous grass Calamagrostis canadensis. The very low 615N values (-7.7 %0) suggest that (1) Picea mainly uses inorganic nitrogen (probably mainly ammo- nium) or organic N in fresh litter, (2) Vaccinium (-4.3 %0) with its ericoid mycorrhizae uses more stable organic matter, and (3) Calamagrostis (+0.9 %0) exploits deeper soil horizons with higher 615N values of soil N. We con- clude that species limited by the same nutrient may coexist by drawing on different pools of soil N in a nutri- ent-deficient environment. The differences among life- forms decrease with increasing N availability. The differ- ent levels of 615N are associated with different nitrogen concentrations in leaves, Picea having a lower N concen- tration (0.62 mmol g-I) than Vaccinium (0.98 mmol g-l) or Calamagrostis (1.33 mmol g-l). An extended vector analysis by Timmer and Armstrong (1987) suggests that N is the most limiting element for Picea in this habitat, causing needle yellowing at N concentrations below 0.5 mmol g-1 or N contents below 2 mmol needle-1. Increas- ing N supply had an exponential effect on twig and nee- dle growth. Phosphorus, potassium and magnesium are at marginal supply, but no interaction between ammoni- um supply and needle Mg concentration could be detect- ed. Calcium is in adequate supply on both calcareous and acidic soils. The results are compared with European conditions of excessive N supply from anthropogenic N depositions. Dedicated to Prof. Dr. Drs. h.c.H. Ziegler on the occasion of his 70th birthday E.-D. Schulze ( ~ ) 9 G. Gebauer Lehrstuhl Pflanzenrkologie, Universit~it Bayreuth, D-95440 Bayreuth,Germany F.S. Chapin III Department of IntegrativeBiology,Universityof California, Berkeley, CA 94720, USA Key words Boreal forest 9 Nitrogen, phosphorus, and cation nutrition 9 Stable isotopes 9 Picea glauca Calamagrostis Vaccinium Introduction Nutrient availability is generally low in Alaskan spruce forests because nitrogen (N) and phosphorus (P) become bound in dead soil organic matter due to temperature- limited decomposition (Van Cleve et al. 1991). Tree growth responds to added N but responds to other ele- ments only in the presence of added N (Van Cleve and Zasada 1976; Chapin et al. 1986). Leaf N concentrations in white spruce are typically low (Van Cleve and Zasada 1976; Chapin and Tyron 1983) and are associated with low rates of photosynthesis (Hom and Oechel 1983) and respiration (Chapin and Tryon 1983). In order to analyze how different species and plant life forms coexist in such a nutrient-limited environment we investigated N and cation nutrition of Picea glauca and P. mariana and their associated ground flora on acid and alkaline soils near the northern limit of the Alaskan boreal forest belt. We hypothesize that under severe nu- trient limitation coexisting species may tap different sources for nutrient supply, and this may lead to signifi- cant differences in plant nutrition even in the same habi- tat. In order to test this hypothesis we compare natural stable isotope ratios of N (~15N-values) and N concentra- tions of spruce needles with leaves of other major com- ponents of the vegetation, especially the ericoid dwarf shrub Vaccinium vitis-idaea and the grass Calamagrostis canadensis. Nutrient limitation in the northern forest belt is severe enough to cause discoloration of Picea glauca needles. Trees of yellow appearance are found in patches, mainly in bogs, and in a mosaic with trees of green appearance. Generally, all needle age classes show yellow tips or whole-needle discoloration. While needle yellowing has received considerable attention in the context of Europe- an forest decline of Norway spruce (Schulze 1989), it is