Inter-specific competition in mixed forests of Douglas-fir (Pseudotsuga menziesii) and common beech (Fagus sylvatica) under climate change - a model-based analysis

Mixed forests feature competitive interactions of the contributing species which influence their response to environmental change. We analyzed climate change effects on the inter-specific competition in a managed Douglas-fir/beech mixed forest. Therefore, we initialised the process-based forest model 4C with published fine root biomass distributions of Douglas-fir/beech stands and a stand composition originating from yield tables to simulate forest growth under regional climate change scenarios for a Dutch and a German site. The number of days when the tree water demand exceeded the soil water supply was higher for Douglas-fir than for beech. After 45 simulation years the proportion of basal area covered by beech increased from one to seven percent. Beech's competitive strength is mainly explained by the fine root biomass distributions and is highest under the historic climate and the driest climate change scenarios. Higher net primary production (NPP) under warmer/wetter climate but decreased NPP under warmer/drier conditions confirms Douglas-fir's high sensitivity to limited water supply. Simulated climate change does not substantially alter the interaction of the two species but the drought-stressed trees are more susceptible to insects or pathogens. The concept of complementary water use highlights the importance of mixed forest for climate change adaptation.