Abstract
Mixed forests are suggested as a strategic adaptation of forest management to climate change. Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) are tree species of high economic and ecological value for European forestry. Both species coexist naturally in a large part of their distributions but there is a lack of knowledge on the ecological functioning of mixtures of these species and how to manage such stands. This paper analyses these species’ intra- and inter-specific competition, including size-symmetric vs. size-asymmetric competition, and explore the effect of weather conditions on tree growth and competition. We studied basal area growth at tree level for Scots pine and Norway spruce in mixed versus pure stands in 22 triplets of fully-stocked plots along a broad range of ecological conditions across Europe. Stand inventory and increment cores provided insights into how species mixing modifies tree growth compared with neighbouring pure stands. Five different competition indices, weather variables and their interactions were included and checked in basal area growth models using a linear mixed model approach. Interspecific size-asymmetric competition strongly influenced growth for both tree species, and was modulated by weather conditions. However, species height stratification in mixed stands resulted in a greater tree basal area growth of Scots pine (10.5 cm2 year−1) than in pure stands (9.3 cm2 year−1), as this species occupies the upper canopy layer. Scots pine growth depended on temperature and drought, whereas Norway spruce growth was influenced only by drought. Interspecific site-asymmetric competition increased in cold winters for Scots pine, and decreased after a drought year for Norway spruce. Although mixtures of these species may reduce tree size for Norway spruce, our results suggest that this could be offset by faster growth in Scots pine. How inter-specific competition and weather conditions alter tree growth may have strong implications for the management of Scots pine-Norway spruce mixtures along the rotation period into the ongoing climate change scenario.
Highlights
Mixed stand management has been proposed as preferable to monocultures due to their potential to provide a reasonable combi nation of timber production, ecological functions and forest ecosystem services (Forrester, 2017; Jonsson et al, 2019)
Basal area increment was on average higher in mixed stands than in pure for Scots pine trees, but no differences were found for the in crements of Norway spruce growth (Fig. 2 and Table S2)
There were significant differences in tree height and diameter between mixed and pure stands for both species, with Scots pine bigger and Norway spruce smaller in mixed stands
Summary
Mixed stand management has been proposed as preferable to monocultures due to their potential to provide a reasonable combi nation of timber production, ecological functions and forest ecosystem services (Forrester, 2017; Jonsson et al, 2019). Mixed forests may be more resilient and resistant to biotic and abiotic disturbances and stresses resulting from climate change (Guyot et al, 2016; Pretzsch et al, 2013). Mixed forest stands may improve resilience to extreme and increasingly frequent drought events in Europe (Jactel et al, 2017), contrasting results were found depending on species composition (Grossiord, 2019; Steckel et al, 2020). Trees may exhibit different climate–growth responses depending on the species of neighbouring trees. Such differences are crucial for understanding the effects of climate change on the growth and vulner ability of forests
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