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
H. Pretzsch , M. Steckel , M. Heym , P. Biber , C. Ammer , M. Ehbrecht , K. Bielak , F. Bravo , C. Ordóñez , C. Collet , F. Vast , L. Drössler , G. Brazaitis , K. Godvod , A. Jansons , J. de‑Dios‑García , M. Löf , J. Aldea , N. Korboulewsky , D. O. J. Reventlow , A. Nothdurft , M. Engel , Maciej Pach , Jerzy Skrzyszewski , M. Pardos , Q. Ponette , R. Sitko , M. Fabrika , M. Svoboda , J. Černý , B. Wolff , R. Ruiz‑Peinado , M. del Río
AbstractPast 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 diferent tree species in diferent mixing proportions, patterns and under diferent 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 monospecifc 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 modifes stand growth and structure compared with neighbouring monospecifc stands. On average, the standing stem volume was 436 and 360 m3 ha−1 in the monocultures of Scots pine and oak, respectively, and 418 m3 ha−1 in the mixed stands. The corresponding periodical annual volume increment amounted to 10.5 and 9.1 m3 ha−1 year−1 in the monocultures and 10.5 m3 ha−1 year−1 in the mixed stands. Scots pine showed a 10% larger quadratic mean diameter (p<0.05), a 7% larger dominant diameter (p<0.01) and a 9% higher growth of basal area and volume in mixed stands compared with neighbouring monocultures. For Scots pine, the productivity advantages of growing in mixture increased with site index (p<0.01) and water supply (p<0.01), while for oak they decreased with site index (p<0.01). In total, the superior productivity of mixed stands compared to monocultures increased with water supply (p<0.10). Based on 7843 measured crowns, we found that in mixture both species, but especially oak, had signifcantly wider crowns (p<0.001) than in monocultures. On average, we found relatively small efects of species mixing on stand growth and structure. Scots pine benefting on rich, and oak on poor sites, allows for a mixture that is productive and most likely climate resistant all along a wide ecological gradient. We discuss the potential of this mixture in view of climate change.
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