| dc.description.abstracteng | During the past two decades, much research has focused on understanding the role of biodiversity for ecosystem functioning and the provision of ecosystem goods and services for humanity. While several experiments with artificial grasslands and herbaceous communities provided clear evidence for a positive relationship between plant species diversity and aboveground productivity in species-rich communities less is known as to whether these results also apply to natural or near-natural communities. Only few data exist on the biodiversity-productivity relation in natural and semi-natural forests and confirmation for a positive diversity effect on aboveground productivity of such ecosystems is rather weak. Moreover, only little information on the diversity-productivity relationship is available so far with regard to the belowground compartment. Some recent belowground studies found a higher standing fine root biomass and productivity in species-rich compared to species-poor stands or monocultures of temperate trees, pointing forward to complementarity in soil space exploration and resource use of the root systems of coexisting tree species.
By using a replicated tree cluster approach with 100 small mature tree groups with variable tree species composition (all possible monospecific, 2-species and 3-species combinations of the five tree species Acer pseudoplatanus, Carpinus betulus, Fagus sylvatica, Fraxinus excelsior, and Tilia cordata) in Hainich National Park (Thuringia, Germany), this study attempts for the first time to separate possible tree species diversity and tree species identity effects on fine root dynamics with a focus on aspects of spatial distribution, root morphology and nitrogen uptake in a mixed old-growth broad-leaved forest. The tested main hypotheses were that (i) tree species identity has a larger effect on standing fine root biomass than tree species diversity, (ii) identity effects on tree fine root productivity are more important than a diversity effect, and (iii) the coexisting five tree species differ in their preference for specific nitrogen (N) forms.
First, I conducted a fine root inventory that included tree roots ≤2 mm in diameter and analysed the fine root bio- and necromass in all cluster plots differing in tree species diversity and tree species composition. I used a key to distinguish between the fine roots on species level by their morphological attributes. In order to proof the evidence of belowground overyielding in terms of fine root productivity, an ingrowth core study was carried out in the 100 cluster plots. In additional monospecific study plots of the five tree species, I investigated species-specific differences in the preference for NH4+, NO3- and glycine uptake using a 15N tracer experiment.
The results revealed no evidence of a positive diversity effect on standing fine root biomass and thus of overyielding in terms of fine root biomass. Fine root necromass decreased from 136 g m-2 in the monospecific cluster plots to 118 g m-2 in the 3-species plots. Instead, there was evidence for a significant species identity effect on fine root biomass. An up to 10-20% higher fine root biomass was recorded in 2-species cluster plots with the presence of A. pseudoplatanus and F. sylvatica than in cluster plots with presence of C. betulus. A 100% higher fine root biomass was found for monospecific cluster plots of F. sylvatica and F. excelsior in comparison to plots of C. betulus. Fine roots of F. excelsior generally tended to be over-represented in the 2- and 3-species mixed cluster plots compared to the respective monospecific plots pointing at apparent belowground competitive superiority of F. excelsior in this mixed forest.
Fine root productivity on plot level was not significantly different between monospecific, 2- and 3-species cluster plots and ranged from 97 to 139 g m-2 yr-1 while fine root turnover increased from 0.39 yr-1 in the 1-species plots to 0.64 and 0.56 yr-1 in the 2- and 3-species plots (difference signifficant at P <0.1). On the species level, large differences in the mean fine root growth rate were found among the five tree species in the monospecific cluster plots. Hence, T. cordata showed an up to five times higher fine root growth rate than C. betulus and about two times higher rates than F. sylvatica in the respective monospecific plots. Comparing the species-specific root growth rate in monospecific with the mixed-species cluster plots revealed a higher productivity in mixtures for F. excelsior, A. pseudoplatanus and T. cordata, but lower values for C. betulus. Fine root turnover was similar for the five species in the monospecific plots and tended to be higher for F. excelsior, F. sylvatica and A. pseudoplatanus in the mixed-species than monospecific plots. The presence of F. excelsior significantly influenced fine root productivity and turnover with accelerated root turnover in species-richer plots.
Apparent root nitrogen uptake rates of the five tree species were in the range of 5-46 µg N g-1 root h-1 for NH4+, 6-86 µg N g-1 root h-1 for NO3- and 4-29 µg N g-1 root h-1 for glycine during the first hour after tracer application. C. betulus, T. cordata and A. pseudoplatanus seemed to prefer NH4+ over NO3-, while F. excelsior showed equal preference for both inorganic N forms and F. sylvatica apparently preferred NO3-.
This study found no evidence for spatial root system complementarity and belowground overyielding in the mixed stands of Hainich forest. Tree species identity effects on root productivity and turnover were much more important with a key role apparently played by F. excelsior. | de |