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Background: Experimental manipulations of tree diversity have often found overyielding in mixed-species plantations. While most experiments are still in the early stages of stand development, the impacts of tree diversity are expected to accumulate over time. Here, I present findings from a 31-year-old tree diversity experiment (as of 2018) in Japan. Results: I find that the net diversity effect on stand biomass increased linearly through time. The species mixture achieved 64% greater biomass than the average monoculture biomass 31 years after planting. The complementarity effect was positive and increased exponentially with time. The selection effect was negative and decreased exponentially with time. In the early stages (≤ 3 years), the positive complementarity effect was explained by enhanced growths of early- and mid-successional species in the mixture. Later on (≥ 15 years), it was explained by their increased survival rates owing to vertical spatial partitioning — i.e. alleviation of self-thinning via canopy stratification. The negative selection effect resulted from suppressed growths of late-successional species in the bottom layer. Conclusions: The experiment provides pioneering evidence that the positive impacts of diversity-driven spatial partitioning on forest biomass can accumulate over multiple decades. The results indicate that forest biomass production and carbon sequestration can be enhanced by multispecies afforestation strategies. [ABSTRACT FROM AUTHOR]

While tree diversity has been proven to enhance above‐ground forest biomass in many instances, the effect on below‐ground biomass has often been neutral, or even negative. This raises the questions of whether above‐ground results are the product of a reduced allocation below‐ground in more diverse forests, which could imply a more efficient acquisition of below‐ground resources per unit root, and whether the effects of diversity have been correctly quantified and interpreted in the past.We addressed these issues using data from a tree‐based functional diversity experiment where 24 mixtures and their respective monocultures were analysed in a fully replicated design. We used species‐level measurements of above‐ and below‐ground biomass to assess how adding below‐ground biomass changes our interpretations regarding the roles of complementarity effects (niche partitioning or facilitation between species) and selection effects (dominance of high‐yielding species within mixtures).Mixtures showed overyielding of above‐ground biomass but not below‐ground. Although remaining significantly positive, this translated into diversity effects of a lesser magnitude when adding below‐ground biomass. However, species did not reduce their allocation to below‐ground biomass within mixtures compared to monocultures. Rather, this combination of above‐ground overyielding and below‐ground neutral effect was caused by the dominance in mixtures of species with a low below‐ground allocation strategy. Another consequence of these species dominance is that their actual contribution to total productivity within mixtures was overestimated when using only above‐ground biomass, due to their relatively small below‐ground biomass. This caused an overestimation of the selection or dominance effects, thereby causing an underestimation of the relative importance of complementarity effects.Synthesis. This study emphasizes the need to consider both above‐ and below‐ground biomass components when determining the origin and strength of diversity effects, or when attempting to link tree diversity to ecosystem services such as carbon sequestration. The statistical partitioning of net effects is a widely used method, but one which can lead to an overestimation of the role of individual dominant species when below‐ground biomass is not taken into account, hence diminishing the role of positive interactions between species. [ABSTRACT FROM AUTHOR]

Experimental forest plantations to study biodiversity-ecosystem functioning (BEF) relationships have recently been established in different regions of the world, but subtropical biomes have not been covered so far. Here, we report about the initial survivorship of 26 tree species in the first such experiment in subtropical China. In the context of the joint Sino-German-Swiss Research Unit 'BEF-China,' 271 experimental forest plots were established using 24 naturally occurring tree species and two native commercial conifers. Based on the survival inventories carried out in November 2009 and June 2010, the overall survival rate was 87 % after the first 14 months. Generalized mixed-effects models showed that survival rates of seedlings were significantly affected by species richness, the species' leaf habit (deciduous or evergreen), species identity, planting date, and altitude. In the first survey, seedling establishment success decreased with increasing richness levels, a tendency that disappeared in the second survey after replanting. Though evergreen species performed less well than deciduous species with establishment rates of 84 versus 93 % in the second survey, their planting success exceeded the general expectation for subtropical broad-leaved evergreen species. These results have important implications for establishing mixed-species plantations for diversity conservation and improvement of ecosystem functioning in the Chinese subtropics and elsewhere. Additional costs associated with mixed-species plantations as compared to conventional plantations also demonstrate the potential of upscaling BEF experiments to large-scale afforestation projects. [ABSTRACT FROM AUTHOR]