Growth response of Mountain birch to air and soil temperature: is increasing leaf-nitrogen content an acclimation to lower air temperature?

Growth and nitrogen (N) economy of mountain birch are reported here in resp onse to temperature change. Mechanisms of temperature effects on plant grow th in temperate - arctic regions are discussed in the light of decreasing g rowth rates and increasing leaf-N contents along altitudinal and latitudina l temperature gradients. Mountain birch (Betula pubescens ssp. czerepanovii) seedlings were grown at two soil temperatures, air temperatures and nutrient concentrations in a f ull-factorial experiment during one growing season in northern Sweden. Changes in air and soil temperature affected aboveground growth more than b elowground growth. An increase in air temperature increased leaf area ratio and plant-N productivity while decreasing plant-N concentration and leaf-N content. A change in soil temperature affected root-N uptake rate and plan t-N concentration, similar to the effect of a change in nutrient supply. Ai r and soil temperature had interactive effects on growth rate, N productivi ty and leaf-N content. The results indicate that increasing leaf-N content with increasing altitud e and latitude is not only a passive consequence of weaker N dilution by re duced growth, but also a physiological acclimation to lower air temperature .