Biomass of forest stands under climatic change: a German case study with the Frankfurt biosphere model (FBM)

In this contribution, we perform a case study of the German forests. We couple the Frankfurt biosphere model (FBM) with a model of the age class development (AGEDYN). The coupled model is applied to simulate the temporal development of carbon pools in German forests under the influence of climate change taking into account changes in the age class structure. In the base case, the growth of forest stands is simulated using a temporally averaged climate dataset, being representative for the contemporary climate conditions. To assess the sensitivity of forest growth to changes in environmental conditions, the FBM is run in several scenarios. In these simulations the effects both of climate change and of the direct effect of increased levels of atmospheric CO2 on photosynthesis (CO2 fertilization) on forest growth are assessed. In another simulation run with the FBM both effects - climate change and CO2 fertilization are combined. In simulations under present day's climate conditions a good agreement is gained between simulation results and statistical data of the present standing stock carbon density of Germany's forests. A pure climate change leads to a decrease of the annual increments as well as to the climax standing stocks. The negative effect of climate change alone is overcompensated by enhanced photosynthesis in the simulations with combined climate change and CO2 fertilization. In the transient case, the coupled model is used in two scenarios describing first a continuation of present day's climate conditions and second a transient climate change from present conditions (1990) to 2 x CO2 conditions in 2090. Here, the simulations indicate that changes in the forest's age class structure can have a stronger influence on the future carbon balance of the forests in the considered region than the combined effect of climate change and CO2 fertilization.