Impact of extreme inter-annual climatic events on the net ecosystem carbon dioxide exchange of a Sitka spruce forest

Temperature and precipitation are key climatic drivers of the global carbon cycle and play an important role in the greenhouse gas mitigation potential of terrestrial ecosystems. The impacts of extreme climatic variability, which in this study were defined by differences in rainfall and temperature of >5% (IPCC, 2012) relative to the long-term site mean (1978-2007), were assessed using eddy covariance-based measurements of net ecosystem carbon dioxide exchange (NEE). Precipitation in 2009 and 2010 was 1156 mm and 741 mm, respectively and approximately 35% higher and 16% lower than the 30 year mean for this region (1978-2007). The differences in precipitation in 2009 were not uniformly distributed throughout the year and occurred largely during the growing season (April-August). The mean annual air temperature in 2010 was ~17% lower than the 30 year mean, and characterized by a number of extended sub-zero temperature events during the winter months. These climatic differences resulted in a 1.07 t C ha yr-1 difference between the annual estimates of NEE in 2009 (8.14 × 1.94 t C ha yr-1) and 2010 (9.21 × 0.99 t C ha yr-1) respectively. The measured NEE in 2009 and 2010 represented a 5.6% decrease and a 6.9% increase relative to the long-term mean measured at this site (2002-2008; 8.62 × 1.39 t C ha yr-1). The components of NEE, gross primary productivity (GPP) and ecosystem respiration (Reco), also showed differences between years. In 2009, GPP was ~19% lower when compared to 2010, most likely due to a reduction in stand photosynthesis at higher irradiances during the growing season that was correlated with higher soil water availability. The extended sub-zero temperatures experienced during the winter of 2010 had a greater impact on GPP, relative to Reco, resulting in a net loss of carbon during these periods. Variations in GPP were, however, positively correlated with Reco in both years. NEE was correlated with temperature in all years, with a slope (negative) of 0.2 to 0.3 g C m-2 d-1 per degree increase in air temperature. At temperatures below ~0oC the forest was a carbon source, whilst there was a progressive increase in sink capacity as temperatures increased up to ~20 degrees C. The observed decrease in NEE was dependent on both the duration of exposure and the extent to which the temperature was reduced below zero. This information indicates that while temperature remains the key driver of NEE in this forest ecosystem, the observed reduction in NEE during periods of high water availability in 2009 suggests that under future climatic scenarios the magnitude and timing of extreme precipitation events may also have a significant impact on annual carbon gain.