NUTRIENT RELATIONS OF MOUNTAIN BIRCH GROWTH AT AND BELOW THE ELEVATIONAL TREE-LINE IN SWEDISH LAPLAND

The effect of nutrient enrichment on growth of mountain birch at different elevations in Swedish Lapland is reported. Fertilizer experiments using nitrogen (N), phosphorus (P), and a complete fertilizer containing nitrogen, phosphorus, potassium and micronutrients (NPK) were conducted on established trees at tree-line, forest limit and valley sites and their growth and nutrient characteristics were compared to those of non-fertilized control trees. Leaf area and weight in NPK- and N-fertilized trees were about 30% greater than in control and P-fertilized trees at all elevations but specific leaf area showed no significant change. No noticeable elevational effect on specific leaf weight was found. Total non-structural carbohydrate concentration levels did not vary with elevation nor were they correlated with concentrations of foliar nitrogen, phosphorus, or potassium in control or treatment groups. Leaf concentration of nitrogen, phosphorus, and potassium generally increased with elevation in control trees. In spite of this, application of nitrogen-containing fertilizer (N and NPK) increased height growth most at the tree-line and least at the lowest elevation. Phosphorus application caused increased above-ground growth only at the tree-line in 1982 but not at other elevations or at the tree-line during the 2 following years. Number of long shoots was increased by all treatments, but most by N and NPK. Leaf nutrient concentrations of nitrogen, phosphorus and potassium in control trees did not correlate strongly with growth when trees at all elevations were analysed together. However, when analyses were performed within elevations, the correlation between nitrogen and growth increased and was statistically significant at the tree-line. Correlation between growth and nutrient concentration was weakened by fertilizer application. Thus, nutrient application affected foliar nutrient concentration differentially and reduced the within-zone relationships between growth and nutrient concentration. The growth response to nutrient application was evident for 2 subsequent years.