Mixture reduces climate sensitivity of Douglas-fir stem growth

Due to possible positive and compensatory interaction between species, mixed stands are a commonly accepted silvicultural response to reduce risks arising from climate change. Nonetheless, only a few species combinations have been studied more detailed so far revealing variable mixing effects. Here, we analyze the effect of the mixture of Douglas fir and European beech with regard to the species-specific climate sensitivity of growth. We focus on three hypotheses: (i) Species-specific long term growing performance and climate sensitivity do not differ between monocultures and mixed stands, (ii) species-specific growth reactions to severe drought events do not differ between monocultures and mixed species stands and (iii) species-specific growth reactions on severe drought events are not influenced by differing ecological growing conditions.To scrutinize the hypothesis we analyzed tree cores from both species taken from pure and mixed stands covering different site conditions and age classes. Tree ring characteristics were used to analyze the differences in climate related long-term growth responses in pure and mixed stands. Short-term responses were investigated by growth reaction indices on individual tree and stand level involving drought events during the years 1950-2010. Linear mixed models were applied to detect effects of ecological co-variables on the indices.Results reveal that Douglas-fir in mixed stands exhibit a significant improved growing performance compared to pure stands. European beech seems to react indifferently concerning its performance in mixture compared to pure stands.Differences in drought stress resistance and growth recovery time mainly arose between the species. Douglas-fir showed a significantly lower resistance and required more time to reach again its initial growth level compared to European beech. In mixture we found a trend that Douglas-fir growth recovery time is shortened and extended for European beech.The analysis along the ecological gradients showed that base-limited soils systems are more drought-tolerant during drought events. Lower basal area as a proxy for reduced stand competition decreased the relative growth loss by drought.We hypothesize that mainly spatial differentiation in height trigger enhanced diameter growth of Douglas-fir in mixture. Temporal differentiation expressed by deferred phenology attenuates climate sensitivity of this conifer. We conclude that in mixed Douglas-fir and European beech stands the former species is stabilized against climatic impacts. On the contrary, climate sensitivity of European beech is increased.