PhD Defence: Seid Muhie Dawud

Title: Functional significance of tree species diversity and species identity on soil organic carbon, C/N ratio and pH in major European forest types

Principal supervisor
Professor Lars Vesterdal (chair)
Department of Geosciences and Natural Resource Management, Faculty of Science, University of Copenhagen, DK

Co-supervisor
Professor Karsten Raulund-Rasmussen
Department of Geosciences and Natural Resource Management, Faculty of Science, University of Copenhagen, DK

Assessment Committee
Professor Inger Kappel Schmidt (chair)
Department of Geosciences and Natural Resource Management, Faculty of Science, University of Copenhagen, DK

Professor Kris Verheyen
Forest and Nature Lab, Ghent University, Belgium

Director of Research Laurent Augusto
Centre INRA Bordeaux-Aquitaine, France

Summary
Forests provide different ecosystem functions and services including soil carbon sequestration and nutrient supply to maintain growth and productivity. This PhD thesis explored tree species diversity and tree species identity (conifer proportion of basal area) effects on soil C stock and nutrient supply (C/N ratio and pH). The studies were carried out in (1) forest floor and mineral soil to 20 cm depth across six different sites of major European forest types based on samples from one to five tree species mixtures along a latitudinal gradient from Spain to Finland, (2) soil profiles down to 40 cm within comparable environmental conditions in a Polish forest area, and (3) a trans-boundary approach in adjacent monoculture stands of Douglas-fir and beech at two common garden sites in Denmark. The thesis also included tree species diversity effects on fine root biomass, production and turnover under 8 and 12 years old common garden stands established in two contrasting bioclimatic regions.

In all the studied contexts, tree species identity (confers versus broadleaves) was stronger than diversity in consistently driving variability of the examined soil properties and root characteristics, particularly in topsoil layers. Diversity did not affect fine root characteristics of the young forests and effects on soil properties were different under the investigated contexts. Across the different European sites, diversity had no effect on C/N ratio and pH but under comparable environmental conditions in Poland, diversity led to higher topsoil pH. However, there was a negative effect on N status as indicated by the higher C/N ratios in the deeper soil layers (20-40 cm). Further investigations are needed to unravel whether the increasing effect on soil C/N ratio results from more N-poor organic matter inputs or a more efficient uptake of N from the organic matter in diverse stands. Diversity positively influenced C stock particularly in mineral soils as concurrently observed across the six different sites, under the comparable environmental conditions and in two species mixtures as represented by the neighborhood approach. Root-related processes and inter-specific interactions could be the major driving factors in mature forests though there was no effect of diversity on fine root dynamics in young stands. The positive but generally additive diversity effects on soil C stock corresponded with previously reported positive effects on aboveground biomass production but did not mirror reported net diversity effects on above-ground biomass which were synergistic under more extreme climatic conditions (i.e., Spain and Finland) and additive otherwise (i.e. in Germany, Poland, Italy and Romania).

After the PhD defense there will be a reception at Rolighedsvej 23, in the canteen (common area) on ground floor in the new building.