How treatment, storm events and changed climate affect productivity of temperate forests in SW Germany

The impact of heavy storm events, changed climate and treatment on the productivity of temperate forests was investigated for three representative stands in Southwest Germany. The simulation experiment was conducted on a Norway spruce, a mixed and a Douglas-fir stand. We applied a business-as-usual scheme (BAU), further a storm adapted management (SAM) and an early reduction in the tree density (BFB6). Further, we investigated the effects of two different climates, a current and a climate change scenario based on the A1B emissions with a HeadCM3 model chain. Simulations were run by the core functions of a single-tree growth simulator, with an empirical storm risk model, enabling to predict single-tree probabilities of being damaged. Further growing conditions were changed by modelling site index as a function of climate. The simulations were run over a period from 2010 to 2500, and we investigated standing and harvested volume and net present value (NPV). Results show that storm frequency has a major impact on all output variables, followed by treatment. For the Douglas-fir stand, treatment is even more important for mean harvested volume, while it does not play a major role in the mixed stand. Compared with storm frequency and treatment, change in precipitation and temperature is less influential. There is a clear negative climate change effect on harvest levels for the spruce and mixed stand, while Douglas fir shows a distinct positive reaction. BFB6 shows the highest harvested and standing volume for both coniferous stands, but lower NPVs due to cutting of premature timber. BAU displays the best performance for NPV, while SAM yields the worst results for harvested and mean standing timber as well as for NPV. Assumptions and limitations of the study are discussed mainly referring to the long simulation period, and it is concluded that before implementing extreme strategies—like SAM—with effects similar to those of the disturbances, a forward-looking adaptive management should be considered.