Biodiversity-ecosystem function in a willow biomass production system

The relationship between biodiversity and ecosystem function is an important issue in ecology. Stands of Salix (willow) are suitable model systems to study this relationship. Salix and other crops are mainly grown in monoculture in agricultural systems. Species or genotypes grown in monoculture share the same functional traits and can therefore be expected to compete strongly for resources. In contrast, different species or genotypes grown in mixed culture vary in their functional traits and may use different niches, leading to reduced competition. Thus, higher diversity in functional traits can increase ecosystem functions such as productivity and litter decomposition. This thesis examined how individual Salix genotypes affect community shoot biomass, litter decomposition and fungal diversity when grown in pure and mixed cultures of different genotypes. Three field sites were established in Central and Northern Europe (Freiburg and Rostock in Germany, Uppsala in Sweden). Within each location, plots were planted with pure and mixed communities of four Salix genotypes (‘Björn’, ‘Jorr’, ‘Loden’ and ‘Tora’) that differ in their morphological and functional traits. In addition to the field study, the two taxonomically and physiologically most distinct genotypes (‘Loden’ and ‘Tora’) were grown under two different nutrient treatments in pure and mixed communities in a pot study. Genotypes exerted different influences on the Salix community in which they were grown, although there was no general increase in productivity, decomposition rate or fungal diversity with increasing genetic richness. Among other findings, one genotype (‘Jorr’) enhanced productivity and litter decomposition when grown in mixed communities. Another genotype (‘Tora’) had a negative effect on productivity, litter decomposition and fungal diversity when added to a community. ‘Tora’ benefited from the presence of other genotypes, but under nutrient poor conditions it performed worse in the presence of a competitor. Litter chemistry differed between genotypes, and decomposition showed a distinct pattern between sites. Fungal communities were affected by different drivers including leaf chemistry, soil properties and genotype identity, but not by genetic diversity. The results suggest that the specific functional trait combinations of individual genotypes affect their response to mixture as compared to monoculture and that the attributes of individual genotypes are more important than genetic richness for the ecosystem functions studied here