Temperature sensitivity of nitrogen productivity for Scots pine and Norway spruce

Environmental conditions control physiological processes in plants and thus their growth. The predicted global warming is expected to accelerate tree growth. However, the growth response is a complex function of several processes with both direct and indirect effects. To analyse this problem we have used needle nitrogen productivity, which is an aggregate parameter for production of new foliage. Data on needle dry matter, production, and nitrogen content in needles of Scots pine ( Pinus sylvestris) and Norway spruce ( Picea abies) from a wide range of climatic conditions were collected and needle nitrogen productivities, defined as dry matter production of needles per unit of nitrogen in the needle biomass, were calculated. Our results show that the nitrogen productivity for spruce is insensitive to temperature. However, for pine, temperature affects both the magnitude of nitrogen productivity at low needle biomass and the response to self-shading but the temperature response is small at the high end of needle biomass. For practical applications it may be sufficient to use a species-specific nitrogen productivity that is independent of temperature. Because temperature affects tree growth indirectly as well as through soil processes, the effects of temperature change on tree growth and ecosystem carbon storage should mainly be derived from effects on nitrogen availability through changes in nitrogen mineralization. In addition, this paper summarises data on dry matter, production and nitrogen content of needles of conifers along a temperature gradient.