Mixing of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) enhances structural heterogeneity, and the effect increases with water availability

The mixing of tree species with complementary ecological traits may modify forest functioning regardingproductivity, stability, or resilience against disturbances. This may be achieved by a higher heterogeneityin stand structure which is often addressed but rarely quantified. Here, we use 32 triplets of mature andfully stocked monocultures and mixed stands of Scots pine (Pinus sylvestris L.) and European beech (Fagussylvatica L.) located along a productivity and water availability gradient through Europe to examine howmixing modifies the stand structure in terms of stand density, horizontal tree distribution pattern, verticalstand structure, size distribution pattern, and variation in tree morphology. We further analyzehow site conditions modify these aspects of stand structure. For this typical mixture of a light demandingand shade tolerant species we show that (i) mixing significantly increases many aspects of structuralheterogeneity compared with monocultures, (ii) mixing effects such as an increase of stand density