Stand growth and structure of mixed-species and monospecific stands of Scots pine (Pinus sylvestris L.) and oak (Q. robur L., Quercus petraea (Matt.) Liebl.) analysed along a productivity gradient through Europe

Past failures of monocultures, caused by wind-throw or insect damages, and ongoing climate change currently strongly stimulate research into mixed-species stands. So far, the focus has mainly been on combinations of species with obvious complementary functional traits. However, for any generalization, a broad overview of the mixing reactions of functionally different tree species in different mixing proportions, patterns and under different site conditions is needed, including assemblages of species with rather similar demands on resources such as light. Here, we studied the growth of Scots pine and oak in mixed versus monospecific stands on 36 triplets located along a productivity gradient across Europe, reaching from Sweden to Spain and from France to Georgia. The set-up represents a wide variation in precipitation (456–1250 mm year −1 ), mean annual temperature (6.7–11.5 °C) and drought index by de Martonne (21–63 mm °C −1 ). Stand inventories and increment cores of trees stemming from 40- to 132-year-old, fully stocked stands on 0.04–0.94-ha-sized plots provided insight into how species mixing modifies stand growth and structure compared with neighbouring monospecific stands. On average, the standing stem volume was 436 and 360 m 3 ha −1 in the monocultures of Scots pine and oak, respectively, and 418 m 3 ha −1 in the mixed stands. The corresponding periodical annual volume increment amounted to 10.5 and 9.1 m 3 ha −1 year −1 in the monocultures and 10.5 m 3 ha −1 year −1 in the mixed stands. Scots pine showed a 10% larger quadratic mean diameter ( p