The response of European forests to the change in summer temperatures : a comparison between normal and warm years, from 1996 to 2006

Abstract: This study shows how six different European forests react during the summer to abnormally warm temperatures, from 1996 to 2006. With the forecasted increase of summer temperatures over Europe, the carbon balance of forests – photosynthesis (GPP), ecosystem respiration and Net Ecosystem Exchange - in likely to change. In this thesis, the photosynthesis and the respiration of six European forests were studied from 1996 to 2006, in order to see how they react to abnormally warm summer temperatures. The parameters used are temperature, vapor pressure deficit and soil water content, precipitation and net radiation. The eddy covariance technique was used to obtain the data. Linear regression was used to find trends and correlations between the various parameters. Common responses can be found between the six forests, depending on common features they share, such as phenology (evergreen or deciduous), foliage (needleaved or broadleaved) and location in Europe (the six forests were distributed in Sweden, Germany, France and Italy). This study shows that the most marked changes in GPP during very warm years are found in deciduous forests. Evergreen forests are characterized by their respiration, which is less sensitive to temperature than those of deciduous forests. Moreover, it was found that the NEE is more often negative (respiration is higher than GPP fluxes, and CO2 is released by the forests) in needleleaved forests than in broadleaved forests. The location also plays a role in the carbon balance of the forests, as respiration is higher in the northern sites. It is less obvious for GPP, for the highest value are found in the central Europe forests (France and Germany). These findings show that if temperatures keep increasing during the summer, both respiration and GPP will be impacted, and marked changes are likely to be seen in the carbon balance.