Net primary production and light use efficiency in a mixed coniferous forest in Sweden

ABSTRACT Simple light use efficiency ( ɛ ) models of net primary production (NPP) have recently been given great attention (NPP = ɛ × absorbed photosynthetically active radiation). The underlying relationships have, however, not been much studied on a time step less than a month. In this study daily NPP was estimated as the sum of net ecosystem exchange (NEE) and heterotrophic respiration ( R h ) of a mixed pine and spruce forest in Sweden. NEE was measured by eddy correlation technique and R h was estimated from measurements of forest floor respiration ( R f ) and the root share of R f . The total yearly NPP was on average 810 g C m −2 year −1 for 3 years and yearly ɛ was between 0.58 and 0.71 g C MJ −1 , which is high in comparison with other studies. There was a seasonal trend in ɛ with a relatively constant level of approximately 0.90 g C MJ −1 from April to September Daily NPP did not increase for daily intercepted radiation above 6 MJ m −2 d −1 , indicating that between‐years variation in NPP is not directly dependent on total Q i . The light was most efficiently used at an average daytime temperature of around 15 °C. At daytime vapour pressure deficit above 1400 Pa ɛ was reduced by approximately 50%.