<b>Abstract</b><br/><p>The interplay of forest stand and environmental factors shape soil organic C (SOC) storage in forest ecosystems but little is known about their relative impacts in different soil layers. Moreover, how environmental factors modulate the impact of stand factors, particularly species mixing, on SOC storage, is largely unexplored.  In this study conducted in 21 forest triplets (two-species mixed stand and respective monocultures nearby) distributed in Europe, we tested the hypothesis that stand factors (functional identity and diversity) have stronger effects on topsoil (FF+0-10 cm) C storage than environmental factors (climatic water availability, clay+silt content, oxalate-extractable Al - Al_ox) but that the opposite occurs in the subsoil (10-40 cm). We also tested the hypothesis that functional diversity improves SOC storage under high climatic water availability, clay+silt contents, Al_ox. We characterized functional identity as the proportion of broadleaved species (beech and/or oak), and functional diversity as the product of broadleaved and conifer (pine) proportions. The results show that functional identity was the main driver of topsoil C storage while climatic water availability had the largest control on subsoil C storage. Contrary to expectations, functional diversity decreased topsoil C storage under increasing climatic water availability but the opposite was observed in the subsoil. Functional diversity effects on topsoil C increased with increasing clay+silt content, while its effects on subsoil C was negative at increasing Al_ox content. This suggests that functional diversity effect on SOC storage along environmental gradients depends on the specific environmental factor and the soil depth under consideration.</p>

<b>Abstract</b><br/><p><span lang="EN-US">The increasing disturbances in monocultures around the world </span><span lang="EN-US">are</span><span lang="EN-US"> testimony to their instability under global change. Many studies have claimed that temporal stability of productivity increase with species richness, although the ecological fundaments have mainly been investigated through diversity experiments. To adequately manage forest ecosystems, it is necessary to have a comprehensive understanding of the effect of mixing species on the temporal stability of productivity and the way in which this it is influenced by climate conditions across large geographical areas. </span></p> <p><span lang="EN-US">Here, we </span><span lang="EN-GB">used a unique dataset of 261 stands combining pure and two-species mixtures of four relevant tree species over a wide range of climate conditions in Europe to examine the effect of species mixing on the level and temporal stability of productivity. </span><span lang="EN-US">Structural equation modelling was employed to further explore the direct and</span> <span lang="EN-US">indirect influence of</span><span lang="EN-US"> climate</span><span lang="EN-US">, overyielding, species asynchrony and additive effect (i.e. temporal stability expected from the species growth in monospecific stands) on temporal stability in mixed forests. </span></p> <p><span lang="EN-US">We showed that by adding only one tree species to monocultures</span><span lang="EN-US">,</span><span lang="EN-US"> the level (overyielding: +6%) </span><span lang="EN-US">and </span><span lang="EN-US">stability (temporal stability: +12%) of stand growth increased significantly. We </span><span lang="EN-US">identified </span><span lang="EN-US">the key effect of temperature on destabilizing stand growth, which may be mitigated by mixing species. We further </span><span lang="EN-US">confirmed</span><span lang="EN-US"> asynchrony as </span><span lang="EN-US">the </span><span lang="EN-US">main driver of temporal stability in mixed stands, through both the additive effect and species interactions, which modify between-species asynchrony in mixtures in comparison to monocultures. </span></p> <p><span lang="EN-GB">Synthesis and applications</span><span lang="EN-US">.</span> <span lang="EN-GB">This study highlights the emergent properties associated with mixing two-species, which result in resource efficient and temporally stable production systems. We reveal the negative impact of mean temperature on temporal stability of forest productivity and how the stabilizing effect of mixing two species can counterbalance this impact. </span><span lang="EN-US">The overyielding and temporal stability of growth addressed in this paper are essential for ecosystem services closely linked with the level and rhythm of forest growth. </span><span lang="EN-US">Our results underline that mixing two species can be a realistic and effective nature-based climate solution, which could contribute towards meeting EU climate target policies. </span></p>

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