The role of climatic variability in controlling carbon and water budgets in a boreal Scots pine forest during ten growing seasons

This study reports eddy-covariance measurements of carbon and water fluxes in a boreal Scots pine (Pinus sylvestris) forest during the 1999-2008 growing seasons. We found that gross primary productivity (GPP) was limited by low rainfall and low summer temperatures, whereas evapotranspiration (ETP) was limited by temperature and precipitation. GPP was less sensitive to increased ETP in the years with little rainfall than in the years with abundant rainfall. During low-rainfall years with long-lasting drought periods and few rainy days (2000 and 2003), GPP was reduced, whereas during the low-rainfall year (2006) with evenly distributed seasonal precipitation it remained at the same level as that during the abundant-rainfall years. Ecosystem water-use efficiency (WUE) was found to be sensitive to increased ETP and vapour-pressure deficits. However, there was no difference between abundant- and low-rainfall years in the responses of WUE to climatic and edaphic variables, most likely due to the coupled feedback of the carbon and water cycles to drought. This study suggested that combined frequency (also seasonality) of precipitation and total amount of rainfall are needed to better evaluate the feedback of carbon and water budgets in boreal forests to drought, especially under expected climate change.