Effects of soil temperature on the nitrogen economy and growth of mountain birch seedlings near its presumed low temperature distribution limit

Direct and indirect effects of soil temperature on the nitrogen economy and growth characteristics of mountain birch, Betula pubescens ssp. tortuosa, seedlings were assessed in a subarctic environment close to the species' altitudinal distribution limit. One-year-old seedlings were grown for one season in silica sand at three soil temperatures (5, 10 and 15 degrees C) and at two nutrient levels in a factorial design. Another set of seedlings was grown at the same temperatures but in natural birch woodland soil and supplied with distilled water only. Nitrogen economy and growth were more strongly affected by soil temperature than by nutrient supply or substrate type. The mountain birch seedlings showed no net growth or nutrient uptake at 5 degrees C. Increasing soil temperature resulted in a series of plant responses. The must important responses comprised the following sequence of events: 1) increased root N uptake rate, 2) a rise in plant nitrogen concentrations and 3) increased leaf productivity which was the main proximate factor responsible for 4) an increased growth rate. The amount of dry matter produced per unit plant nitrogen increased with both the soil temperature and plant nitrogen concentration. The distribution of mountain birch in relation to soil temperature is discussed; low soil temperatures and their effect on nitrogen uptake may be a critical factor limiting mountain birch growth on subalpine heaths and at its altitudinal distribution limit.