Mixed-species forests have often been shown to enhance above-ground ecosystem properties and functions compared to their mono-specific counterparts. For example, they are often more productive than pure stands. However, the underlying mechanisms of positive diversity-ecosystem functioning relationships have been analysed mainly for above-ground processes, with less attention paid to the role of below-ground interactions. Consequently, our understanding of the functioning of mixed forests is still largely incomplete. To promote diverse, productive, and resilient forests capable of adapting to the impacts of climate change, a comprehensive understanding of the functioning of mixed-species forest is indispensable. Fine roots generally play a fundamental role for plant growth and fitness, but also in carbon and nutrient cycling. Nevertheless, as to how species diversity affects below-ground functions driven by fine roots, including soil resource exploitation, remains largely unknown. Methodological constraints related to root research and inconsistent root classification bear major challenges for analysing the role of the below-ground ecosystem component. Consequently, contradictory results of previous studies do not allow broad conclusions to be drawn about the role of fine roots for positive biodiversity-ecosystem functioning relationships.
The overarching goal of this thesis was to assess the effect of tree diversity on fine-root soil exploitation and decomposition in four wide-spread European forest types.
The main research objectives were:
(1) To assess the soil space occupation by tree fine roots in response to tree species mixing
(2) To examine soil exploitation strategies by tree fine roots and mycorrhizal partners in response to tree species mixing
(3) To investigate tree fine-root litter decomposition rates in response to tree species mixing
In total, 63 mostly mature forest plots distributed across four sites across Europe were selected from an existing exploratory plot network (FunDivEUROPE) in semi-natural forests. The sites were located in four countries and representative of boreal (Finland), hemiboreal (Poland), mountainous beech (Romania), and thermophilous deciduous forests (Italy). The plots either represented tree species mixtures with three target species or mono-specific stands. Within each plot, five tree neighbourhoods (triplets) were selected for soil sampling and subsequent incubation of root litter samples. In the centre of each of these neighbourhoods, soil cores at three depth increments (0-10, 10-20, 20-30 cm) were taken in spring 2017. The following year, in spring 2018, 1,330 litter bags with fine-root material were incubated near the soil sampling spots for one year.
In total, 928 soil samples were processed in the laboratory, and morphological, chemical, and microbial fine-root traits were measured. The vertical distribution of fine roots across soil depths was examined. Roots were sorted by species, and the functional classification approach was applied to distinguish absorptive, i.e., the first three most distal root orders, from transport fine roots, i.e., fourth or fifth-order roots with a diameter ≤2 mm. Moreover, ectomycorrhizal diversity and abundance data from nearby soil samples were integrated into subsequent analyses.
Fine-root decomposition rates were determined via mass loss after one year of incubation. Initial fine-root traits of tree species that were incubated were measured to determine initial litter quality.
Across all sites, tree species mixing significantly affected tree fine-root traits and decomposition rates. Tree species mixtures supported on average less biomass of absorptive fine roots than corresponding mono-specific stands. This underyielding was mainly reflected in negative complementarity effects, and to a lesser extent, in negative selection effects. The species-specific and overall rooting patterns across the three soil depth layers did not provide evidence for vertical root stratification in mixtures. Nevertheless, as total length density of absorptive fine roots (i.e., across the entire soil profile) did not significantly differ between mixtures and mono-specific stands, overall soil space occupation by tree fine roots and thereby the trees’ resource uptake capacity did not change in response to mixing. Instead, an increased root length density in mixtures in the most nutrient-rich soil depth (0-10 cm) indicates an enhanced soil resource uptake capacity compared to pure stands.
The second analysis suggested that the observed underyielding of biomass of absorptive roots in response to tree species mixing was related to changes in fine-root traits. Fine roots in mixtures were characterised by higher specific root lengths, lower diameters, lower root tissue densities, and higher root nitrogen concentrations than trees in pure stands. Overall, these changes at the community level suggest a shift in soil resource acquisition strategies by trees in mixtures compared to mono-specific stands towards a faster resource foraging. A higher ectomycorrhizal colonisation intensity of roots and, at the same time, higher diversity and abundance of ectomycorrhizae in soil samples in mixtures compared to mono-specific stands suggest positive biotic feedbacks from mycorrhizae likely enhancing soil resource capture by trees in mixtures. An important finding was that thin-rooted broadleaved tree species showed stronger responses to mixing than thick-rooted conifer tree species, particularly in terms of root morphology and ectomycorrhizal colonisation.
The decomposition study suggested that decomposition rates of mixed-species fine-root litter in mixed tree neighbourhoods can differ from component single-species litter in mono-specific neighbourhoods. As such, mixed-species litter decomposed faster than single-species litter across the four study sites. Differences in micro-environmental conditions between mixed and mono-specific tree neighbourhoods rather than interactions among litter species in mixed-species litter likely caused these non-additive effects. Nevertheless, the analyses further showed that initial chemical traits explained a greater proportion of the variability in the data than tree diversity. The additional incubation of standard root litter species across the plot network further suggests that macro-climate and regional-scale differences, as well as litter species identity, may be more important predictors of fine-root litter decomposition than tree diversity.
This thesis enhances our understanding of overall tree diversity effects on ecosystem functioning by shedding more light on the role of the hidden half, i.e., the below-ground component of forest ecosystems. The obtained results provide evidence for positive below-ground species interactions in mixtures, possibly enhancing soil resource acquisition by trees. Hence, these findings contribute to a better mechanistic understanding of positive diversity-productivity relationships in forest ecosystems. Overall relatively consistent tree species mixing effects on fine-root soil exploitation and decomposition across a broad range of environmental conditions and different species compositions in four wide-spread European forest types demonstrate the generality of the results.