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Net ecosystem productivity and its environmental controls in a mature Scots pine stand in north-western Poland

Although there have been many studies of the net ecosystem productivity (NEP) of different types of forests around the world, the CO2 dynamics in afforested pine stands of Central Europe are poorly understood. To fill this gap, continuous eddy-covariance (EC) measurements of net ecosystem exchange (NEE) were made from January 2008 to December 2013 in a 62-year-old temperate afforested Scots pine stand near Tuczno. The site is located in north-western Poland, where forests account for almost 30% of the land area and are dominated by Scots pine. Weather conditions during this 5-year period were mostly warm and wet. In all 5 years, air temperature (Ta) was higher than the 30-year (1983–2013) mean and by 3.3 ◦C during winter 2008, while precipitation (P) was noticeably higher only in summer months. The high productivity of the forest, which sequestered 118 Mg of CO2 per ha over the 5-year period, is likely because it was planted on fertile meadowland. Annual net ecosystem productivity (NEP = −NEE) ranged from 494 g C m−2 in 2012 to 765 g C m−2 in 2009, with an average of 645 g C m−2. The interannual variation in NEP was attributed more to the interannual variation in gross ecosystem photosynthesis (GEP) than to ecosystem respiration (R). Moreover, both annual NEP and GEP significantly decreased over the 5 years. This was the result of increasingly drier springs and wetter summers as time progressed during the 5-year period, as compared to the 30-year averages, which resulted in a gradual reduction in the growing season NEP and consequently the annual values. Seasonal values of NEP were highly correlated with Ta, photosynthetic photon flux density and vapor pressure deficit. The sensitivity of NEP to Ta was largely due to the much higher sensitivity of GEP to Ta compared to that of R. Although the interannual variability in NEP for separate seasons could not be explained using seasonal values of individual meteorological variables, a hygrothermal index, defined as P/10Ta, explained a large proportion of the interannual variability in NEP in spring and summer.