Impacts of summer water limitation on the carbon balance of a Scots pine forest in the southern upper Rhine plain

Regional climate models for Central Europe predict more frequent and longer-lasting periods of low precipitation and high air temperature in summer for the second half of the 21st century. Such periods of low water availability will be of major importance for photosynthetic activity, plant growth and the carbon budget of ecosystems. From 2004 to 2007, we investigated the effects of water limitation on a 45-year-old Scots pine forest in the upper Rhine plain. This region with its relatively high air temperature and low precipitation during the vegetation period already now represents the expected future precipitation and temperature conditions for large areas of Central Europe. Using the eddy covariance technique to measure net ecosystem CO2 exchange, we found that below the permanent wilting point (i.e., 12 vol.%), net carbon uptake was controlled rather by temperature than by photosynthetically active radiation. In 2005, relatively moderate air temperature and comparatively high precipitation amounts in summer lead to optimal growth conditions and to an annual carbon sink of 600 g C m−2 year−1 for this pine forest. In contrast, dry and hot conditions in July 2006 resulted in soil moisture values below the permanent wilting point, subsequent stomatal closure and a 40% reduction of the net carbon uptake, leading to an annual C sink of only 380 g C m−2 year−1 for the pine stand. We thus conclude that regional-scale impacts of limited water availability on ecosystem sink capacity are no single events, but already now a re-occurring phenomenon in some regions of Central Europe.