Abstract
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 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 significantly wider crowns (p < 0.001) than in monocultures. On average, we found relatively small effects of species mixing on stand growth and structure. Scots pine benefiting 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.
Highlights
There are many reasons for systematically analysing as many different tree species mixtures as possible
Greater crown canopy packing in mixed stands due to species differences in crown morphology and light ecology has been identified as an important cause of overyielding (Pretzsch and Schütze 2016; Williams et al 2017)
The same was true for standing stock, which on average amounted to 436 m3 ha−1 in Scots pine and 360 m3 ha−1 in oak monocultures
Summary
There are many reasons for systematically analysing as many different tree species mixtures as possible. Karst)/European beech (Fagus sylvatica L.) or Scots pine (Pinus sylvestris L.)/European beech and other mixtures including European beech are very well analysed (Knoke et al 2008; Pretzsch and Schütze 2009); there is still a long way for forest science to establish a solid theory of mixing effects and general rules of species behaviour in mixture or even guidelines for combining and thinning tree species or functional groups of species (Forrester 2014). Combinations of species with more similar light ecology but complementary root space exploitation, such as Scots pine and oak, were analysed at selected sites, but so far not over a broader range of site conditions (Bello et al 2019)
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