P1: Spatial above- and belowground complementarity
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.
- to analyse crown, branch and leaf traits in relation to tree-tree interactions (TSPs and local neighbourhood)
- to analyse trait variability related to canopy filling and light interception;
- to quantify how canopy space use complementarity translates into enhanced tree growth.
- to analyse species richness effects on fine-root production
- to analyse effects of species composition on fine-root turnover and distribution
- to identify the connection between genetic identity, functional traits and tree growth
P2: Complementarity through trait variation
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.
- to quantify to which degree leaf traits are adjusted to different light conditions along the interaction plane of two tree species
- to analyze trait variation as a response to species and functional diversity of the local neighborhood of the TSP andthe dependence of traits and trait variation on soil nutrient availability
- to relate trait variation and performance of the TSP
- to quantify different components of trait variation to complementarity effects of ecosystem functions
- to characterize tree species by their root morphological and anatomical root traits
- to assess the response of root and mycorrhizal traits to resource patches, different tree neighborhoods, and site-scale resource gradients
- to test whether complementary traits translate into enhanced tree productivity
P3: Growth and temporal stability
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.
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.
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.
- Assess how neighborhood diversity affects water use efficiency (~stomatal closure) and growth during drought years for tree species with contrasting stomatal control (water savers vs. spenders)
- Test how competitive asymmetry is controlled by diversity at the level of TSPs and their neighborhood based on biometric measurements
- Analyze whether diverse forests express complementarity in seasonal growth and whether this translates into higher productivity
- Analyze whether diversity advances the start of the growing season
- Relate seasonal growth complementarity to functional diversity of available proxy traits
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.
- Herbivory & community structure of herbivore feeding guilds across space & time
- Top-down vs. bottom-up control
- Consequences for tree growth & productivity
P5: Leaf fungal pathogens and endophytes
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.
- to estimate the leaf area infected by fungal pathogens
- to identify all fungal species found on the trees and shrubs using morphological and molecular approaches
- to monitor pathogen load and fungal species richness under different crown interaction points and to relate these patterns to microclimate
- to study the effect of pathogen load on tree and shrub productivity
- to study the temporal course of fungal species composition, in parallel with the leaf chemical composition
- to study leaf decomposition rates and to relate them to species leaf litter composition, pathogen load and local neighbourhood diversity
P6: Biodiversity effects on leaf and litter metabolomes
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.
- to assess the effects of plot and local diversity on the volatile and non-volatile metabolomes of leaves
- to analyse the metabolomic composition of leaf litter
- to assess the relationship between herbivory and volatile organic compouds (VOCs)
- Characterise N-acquisition strategies, incl. soil microbes
- Explore how trees and soil microbes acquire available N via chemical, temporal and spatial niche differentiation
- Examine if tree species benefit from shared mycorrhizal networks
- Analyse degree of complementarity N- uptake in local neighbourhoods over different soil depths and seasons
P7: Rhizosphere and root microbes
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.
- to investigate the bidirectional impact of plant-plant interactions in modifying rhizosphere microbiota
- to assess the role of shrub species in modulating the tree-tree and shrub-tree interaction zone rhizosphere microbiomes
- to assess the interplay between target plant species, neighbour trees, mycorrhizal type, and other biotic and abiotic environmental factors in shaping the overall microbial community composition
- to relate the rhizosphere microbial co-occurrence patterns and functional gene profiles to plant traits, root exudates and the overall TSQ functional dissimilarity
- Analyze how root associated bacterial and fungal community composition of mono- and hetero-specific TSPs change within and across diversity levels and environmental conditions
- Assess the link between root associated microbial communities with root traits, nitrogen acquisition and exudation profiles
P8: Soil multifunctionality
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.
- To analyse if dissimilarity of TSPs increases functional diversity of soil microorganisms and soil nematodes
- To analyse if more complex environments (diversity of the local neighbourhood) steepen this slope due to a higher diversity of organic inputs and microhabitats, which is expected to increase the functional diversity of soil microorganisms and soil nematodes
- To analyse if increased functional diversity of soil organisms results in increased biomass of microorganisms and nematodes (consumer biomass)
- To identify the functional diversity of soil microorganisms involved in nitrogen cycling processes
- To link microbial functional diversity with corresponding ecosystem functioning by measuring soil extracellular enzyme activities and ecosystem process rates
- To determine the response of microbial functional gene diversity and corresponding ecosystem functioning to the functional dissimilarity of the local tree neighbourhood
P9: Modelling generalised diversity interactions
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.
- To model the dynamics of herbivore, pathogen and rhizosphere populations on landscapes (grids) composed of tree individuals as habitat compartments (grid cells)
- To establish spatially-explicit models of tree individuals, their tree-tree interactions, and neighborhood effects
- To predict tree growth in novel models synthesizing population- and individual-level processes for animals and trees, respectively, which will address interaction processes such as enemy dilution and facilitation
Prof. Dr. Weiguo Sang, Assist. Prof. Dr. Shaopeng Wang
- To model the change between TSP partners and the whole patch change through time
- To simulate tree interactions for forest development
- To use the models to understand the dynamic interactions of trees in mature forests