The increasing disturbances in monocultures around the world are 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.

Here, we 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. Structural equation modelling was employed to further explore the direct and indirect influence of climate, overyielding, species asynchrony and additive effect (i.e. temporal stability expected from the species growth in monospecific stands) on temporal stability in mixed forests.

We showed that by adding only one tree species to monocultures, the level (overyielding: +6%) and stability (temporal stability: +12%) of stand growth increased significantly. We identified the key effect of temperature on destabilizing stand growth, which may be mitigated by mixing species. We further confirmed asynchrony as the 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.

Synthesis and applications. 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. 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. 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.

The research unit is the forest stand. We used data from a total of 261 forest stands belonging to three triplet-transects across Europe. Each triplet consists of a plot established in a two species mixed stands, and two plots on the respective monospecific stands; the three stands are located close to each other under similar environmental conditions. The species composition of the mixtures changes in the three triplet-transects. The first transect covers monospecific and mixed stands of Fagus sylvatica and Pinus sylvestris (32 sites, 96 stands), the second of Quercus petraea and Pinus sylvestris (35 sites, 105 stands), and the third of Picea abies and Pinus sylvestris (20 sites, 60 stands). Plot sizes varies from 0-02 to 0.15 ha depending on stand density a local site characteristics.

In each plot the diameter of all trees was measured, and two increment cores per tree were taken at a 1.3 m stem height in a sample of approximately 20 trees per species and plot. Annual ring widths were measured and cross-dated using standardized dendrochronological techniques. The studied period was 2000-2013 for the beech-pine transect and 2004-2017 for the oak-pine and spruce-pine transects (except in five triplets where the period was 2000-2013), the last year corresponding to triplet establishment. Using data from cored trees, tree diameter increment-diameter models were fitted by year, species and plot to estimate diameter increments of noncored trees for the studied period. Dead trees during the last 14 years were estimated using stumps, standing and lying dead trees, and their decomposition status. Based on measured tree diameters and annual diameter increments we estimated species and stand annual basal area (BA) and basal area growth (BAI), which conforms the dataset.

Annual climate data were obtained from meteorological weather stations located in the proximity of each triplet (50 triplets). When local station data were not available, national digital climatic atlas data (24 triplets) or more general gridded data (13 triplets) were used (mostly CRU gridded database). For each triplet mean and standard deviation of annual precipitation (P) and mean annual temperature (T) for the studied period were calculated.

The are presented in an excel-table which can be open freely in different ways (open office, google docs, etc.)