Mixing of complementary tree species may increase stand productivity, mitigate the effects of drought and other risks, and pave the way to forest production systems which may be more resource-use efficient and stable in the face of climate change. However, systematic empirical studies on mixing effects are still missing for many commercially important and widespread species combinations. Here we studied the growth of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) in mixed versus pure stands on 32 triplets located along a productivity gradient through Europe, reaching from Sweden to Bulgaria and from Spain to the Ukraine. Stand inventory and taking increment cores on the mainly 60-80 year-old trees and 0.02-1.55 ha sized, fully stocked plots provided insight how species mixing modifies the structure, dynamics and productivity compared with neighbouring pure stands. In mixture standing volume (?12 %), stand density (?20 %), basal area growth (?12 %), and stand volume growth (?8 %) were higher Communicated by Peter Biber. than the weighted mean of the neighbouring pure stands. Scots pine and European beech contributed rather equally to the overyielding and overdensity. In mixed stands mean diameter (?20 %) and height (?6 %) of Scots pine was ahead, while both diameter and height growth of European beech were behind (-8 %). The overyielding and overdensity were independent of the site index, the stand growth and yield, and climatic variables despite the wide variation in precipitation (520-1175 mm year -1 ), mean annual temperature (6-10.5°C), and the drought index by de Martonne (28-61 mm°C -1 ) on the sites. Therefore, this species combination is potentially useful for increasing productivity across a wide range of site and climatic conditions. Given the significant overyielding of stand basal area growth but the absence of any relationship with site index and climatic variables, we hypothesize that the overyielding and overdensity results from several different types of interactions (light-, water-, and nutrient-related) that are all important in different circumstances. We discuss the relevance of the results for ecological theory and for the ongoing silvicultural transition from pure to mixed stands and their adaptation to climate change. Electronic supplementary material

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611. There is increasing evidence that species diversity enhances the temporal stability of 62 community productivity in different ecosystems, although its effect at population and tree 63 levels seems to be negative or neutral. Asynchrony between species was found to be one of 64 the main drivers of this stabilizing process. However, scarce research in this area has been 65 undertaken in forest communities, so determining the effect of species mixing on the stability 66 of forest productivity as well as the identity of the main drivers involved still poses a 67 challenging task. 3. Mixed stands showed a higher temporal stability of basal area growth than monospecific 76 stands at community level, but not at population or individual tree levels. Asynchrony 77 between species growth in mixtures was related to temporal stability, but neither overyielding 78 nor asynchrony between species growth in monospecific stands were linked to temporal 79 stability. Therefore, species interactions modify between-species asynchrony in mixed stands. 80Accordingly, temporal shifts in species interactions were related to asynchrony and to the 81 mixing effect on temporal stability. 4. Synthesis. Our findings confirm that species mixing can stabilize productivity at 83 community level whereas there is a neutral or negative effect on stability at population and 84 individual tree level. The contrasting findings as regards the relationships between temporal 85 stability and species asynchrony in mixed and monospecific stands suggest that the main 86 driver in the stabilizing process is the temporal niche complementarity between species rather 87 than differences in species specific responses to environmental conditions. 89 Keywords 91Temporal variability; mixed-species forests; plant-plant interactions; overyielding; 92 asynchrony; niche complementarity; organizational levels; 93 94

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Kamil Bielak 5 | Andrés Bravo-Oviedo 6,7 | Lluis Coll 8 | Miren del Río 6,7 | Lars Drössler 9 | Michael Heym 10 | Václav Hurt 11 | Magnus Löf 9 | Jan den Ouden 12 | Maciej Pach 13 | Abstract 1. When tree-species mixtures are more productive than monocultures, higher light absorption is often suggested as a cause. However, few studies have quantified this effect and even fewer have examined which light-related interactions are most important, such as the effects of species interactions on tree allometric relationships and crown architecture, differences in vertical or horizontal canopy structure, phenology of deciduous species or the mixing effects on tree size and stand density. Paper previously published as Standard Paper | 747 Journal of Ecology FORRESTER ET al.

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a b s t r a c tThe mixing of tree species with complementary ecological traits may modify forest functioning regarding productivity, stability, or resilience against disturbances. This may be achieved by a higher heterogeneity in stand structure which is often addressed but rarely quantified. Here, we use 32 triplets of mature and fully stocked monocultures and mixed stands of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) located along a productivity and water availability gradient through Europe to examine how mixing modifies the stand structure in terms of stand density, horizontal tree distribution pattern, vertical stand structure, size distribution pattern, and variation in tree morphology. We further analyze how site conditions modify these aspects of stand structure. For this typical mixture of a light demanding and shade tolerant species we show that (i) mixing significantly increases many aspects of structural heterogeneity compared with monocultures, (ii) mixing effects such as an increase of stand density and diversification of vertical structure and tree morphology are caused by species identity (additive effects) but also by species interactions (multiplicative effects), and (iii) superior heterogeneity of mixed stands over monocultures can increase from dry to moist sites. We discuss the implications for analyzing the productivity, for modelling and for the management of mixed species stands.