Above- and belowground complementarity of resource use is considered an important mechanism for increased biomass production in diverse tree communities. Complementary space occupation and spatial niche differentiation in tree crowns and the rhizosphere may contribute to competitive reduction and may facilitate individual tree and community productivity. Plasticity in crown and root architecture, in turn, are mediated by tree-tree interactions at the local neighbourhood level, improving three-dimensional (3D) space filling and enhancing biomass density in diverse neighbourhoods.

In-depth analyses of 3D space filling and above-/belowground biomass density along diversity gradients, however, received little attention. In P1 we will analyse (i) effects of local neighbours on complementarity in crown and root architecture of the TSPs by quantifying the spatio-temporal dynamics of above- and belowground growth and biomass allocation patterns, and (ii) how these processes will translate into enhanced tree growth.

Chemical and morphological leaf and root traits of trees have been shown to vary (1) among species, (2) within species and among individuals, and (3) within individuals. So far it is unknown how these three components of trait variation contribute to complementarity effects of ecosystem functions. In the TreeDì design with comparing mono- and heterospecific TSPs, complementarity effects are easily visible in over-yielding.

Increased complementarity can be achieved through increasing trait variance between the two trees of a TSP or by increasing trait variance within the individual TSP trees. In addition, the contribution of the three components of trait variation to complementarity can vary with the abiotic environment, such as differences in soil nutrient supply, and the biotic environment, such as differences in tree species identity and diversity of the local neighborhood.

Water use
Theory predicts that diverse communities use resources more efficiently than monocultures. Under water shortage resource complementarity of water use may buffer growth decline. Carbon isotopes (d¹³C) in tree rings are used to infer diversity effects during drought.

Temporal complementarity
Variation in growth phenology – expected to occur in diverse mixtures – is predicted to exploit the resource ‘time’ more efficiently – as first results suggest. Using dendrometers we will test the hypothesis that diverse mixtures have an extended growth period.

Asymmetry
Size dependency of relative growth rates of two interacting individuals can be used to infer competitive asymmetry. Low asymmetry promotes coexistence and complementarity. We hypothesize that asymmetry is minimal in a diverse neighborhood and at intermediate levels of trait dissimilarity.

Local tree interactions might explain community-level associations of herbivores, their enemies, and tree diversity, but are likely modified by the wider tree neighborhood. The relative importance of potential bottom-up and top-down mechanisms underlying such associational effects, and their scale-dependence, remain unclear. Analyses of functional consumer traits, aided by molecular methods, may facilitate a mechanistic understanding.

Foliar fungal pathogens negatively affect plant performance, directly or indirectly by changing the outcome of competition. In contrast, it is not known how leaf fungal pathogen infestation affects other ecosystem functions such as leaf litter decomposition. As leaf litter decomposition is dependent on the chemical composition of the leaf tissue and fungi feed on organic compounds in the leaf, fungal infection can be expected to have an effect on leaf decomposition rates. There is also evidence that leaf inhabiting fungi play a direct role in leaf litter decomposition.

Leaf fungal pathogens are mostly highly host-specialized, which also applies to the BEF-China experiment. In the BEF-China experiment, both fungal richness and fungal infestation increase with increasing proportion of conspecifics in the local neighbourhood. Such neighbourhood effects are thought to be mediated by the modification of sun exposure, local microclimate or soil conditions.

Root exudates play a critical role in tree growth and survival. They mobilize nutrients and govern interactions with soil microorganisms and neighbors. The chemical profiles of root exudates are species-specific but also depend on the biotic and abiotic environment of the tree. Consequently, differences in tree community composition and (functional) diversity may lead to belowground niche differentiation with regards to nitrogen acquisition and allelopathy. In this sub-project, we test the hypothesis that differences in tree diversity and local neighborhood composition leads to complementarity in root chemical traits, which are reflected in exudate metabolome profiles and spatial-temporal nitrogen acquisition strategies.

Tree species identity, plant successional stage, plant diversity level, and mycorrhizal type affect the soil microbial community composition. Plant genus diversity was also found to be an important predictor of ectomycorrhizal fungal community composition in subtropical forests. Both arbuscular and ectomycorrhizal fungal diversity and identity are known to influence tree species diversity and composition. Plant diversity level influences fungal community composition and the plant beta-diversity predicts the beta-diversity of fungal functional groups including mycorrhizal fungi.

Despite the number of recent advances in the rhizosphere and endosphere microbiome research, there is a limited knowledge on the inter- and intra-kingdom variation in community composition, co-occurrence network patterns, the functional gene profiles, and the drivers of these network patterns and functional gene profiles in the interaction zones of mono- and hetero-specific tree species pairs.

The functional dissimilarity of species is hypothesised to determine the positive relationship between biodiversity and ecosystem functioning.However, functional dissimilarity effects might be context-dependentand strongest in complex environments (e.g. high resource complexity), where dissimilar species are able to occupy different niches.Thus, studying  the effects of functional  dissimilarity of interacting  trees in neighbourhoods of different diversity on the functional diversity and biomass of soil organisms may provide a generalisable framework to link biodiversity to ecosystem functions across above-belowground ecosystem compartments.

Individual tree-based modeling approaches rest on the assumption that resource supply and tree performance are spatially explicit processes at a local neighborhood scale. Models built on pairwise interactions suffer from the fact that local neighborhoods beyond the interaction partners are not taken into account.

The focus of TreeDì, on the one hand, on binary interactions between tree species pairs (TSPs), and on the other hand, on the wider neighborhood surrounding the TSPs allows to model interaction coefficients between tree pairs as a function of neighborhood diversity. This neighborhood diversity can also take functional or phylogenetic diversity into account. This project will develop a novel modeling approach addressing the interplay between (i) tree individuals, (ii) the local abiotic environment and (iii) the populations of herbivores, pathogens and rhizosphere species composing the above- and belowground food webs.