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

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

Aim of study: We aim at (i) developing a reference definition of mixed forests in order to harmonize comparative research in mixed forests and (ii) review the research perspectives in mixed forests.Area of study: The definition is developed in Europe but can be tested worldwide.Material and Methods: Review of existent definitions of mixed forests based and literature review encompassing dynamics, management and economic valuation of mixed forests.Main results: A mixed forest is defined as a forest unit, excluding linear formations, where at least two tree species coexist at any developmental stage, sharing common resources (light, water, and/or soil nutrients). The presence of each of the component species is normally quantified as a proportion of the number of stems or of basal area, although volume, biomass or canopy cover as well as proportions by occupied stand area may be used for specific objectives. A variety of structures and patterns of mixtures can occur, and the interactions between the component species and their relative proportions may change over time.The research perspectives identified are (i) species interactions and responses to hazards, (ii) the concept of maximum density in mixed forests, (iii) conversion of monocultures to mixed-species forest and (iv) economic valuation of ecosystem services provided by mixed forests.Research highlights: The definition is considered a high-level one which encompasses previous attempts to define mixed forests. Current fields of research indicate that gradient studies, experimental design approaches, and model simulations are key topics providing new research opportunities.Keywords: COST Action; EuMIXFOR; mixed-species forests; admixtures of species.

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.

Mixed mountain forests of European beech (Fagus sylvatica L.), Norway spruce (Picea abies (L.) Karst), and silver fir (Abies alba Mill.) cover a total area of more than 10 million hectares in Europe. Due to altitudinal zoning, these forests are particularly vulnerable to climate change. However, as little is known about the long-term development of the productivity and the adaptation and mitigation potential of these forest systems in Europe, reliable information on productivity is required for sustainable forest management. Using generalized additive mixed models this study investigated 60 long-term experimental plots and provides information about the productivity of mixed mountain forests across a variety of European mountain areas in a standardized way for the first time. The average periodic annual volume increment (PAI) of these forests amounts to 9.3 m3ha−1y−1. Despite a significant increase in annual mean temperature the PAI has not changed significantly over the last 30 years. However, at the species level, we found significant changes in the growth dynamics. While beech had a PAI of 8.2 m3ha−1y−1 over the entire period (1980–2010), the PAI of spruce dropped significantly from 14.2 to 10.8 m3ha−1y−1, and the PAI of fir rose significantly from 7.2 to 11.3 m3ha−1y−1. Consequently, we observed stable stand volume increments in relation to climate change.