Zur Struktur und Diversität der Bodenvegetation in Laubwäldern mit unterschiedlicher Baumartenvielfalt

Abstract

In this thesis, I investigate whether there are correlations between tree-layer diversity and herb-layer diversity as well as herb-layer productivity, and which environmental factors form the underlying mechanisms. Answers to these questions are very important, because many silvicultural management schemes allow for the development of mixed broadleaved stands instead of pure beech stands. The investigation area was in Hainich National Park (Thuringia, Central Germany). Main hypotheses of the project and related results: I. Herb-layer diversity is positively affected by increasing tree-layer diversity. It was found that herb-layer vegetation of deciduous forest stands rich in canopy species appeared to be more diverse than herb-layer vegetation of beech-dominated stands. It was surmised that herb-layer diversity was indirectly influenced by canopy tree species through the medium of the altered environmental factors soil pH and litter layer thickness. Apparently, lower beech proportion had a more profound effect than the number of secondary tree species. There were no correlations between herb-layer diversity and light transmissibility of the canopy layer, indicating that the light factor was not crucial for herb-layer diversity. At least for the Hainich research sites, the results indicated that small-scale light and soil heterogeneity is insignificant for herb-layer diversity. Several herb-layer species were found whose occurrence was particularly correlated with tree-layer diversity and environmental factors. Remarkably, all species positively correlated with soil pH were important for the phytosociological classification of the research sites. II. Herb-layer productivity is positively correlated with increasing herb-layer and tree-layer diversity. Herb-layer biomass was positively correlated with tree-layer diversity. Herb-layer productivity might be promoted in more diverse research sites by increased nutrient supply (particularly phosphorus) and base saturation as well as rising soil clay contents. It is also possible that greater beech proportion interfered with herb-layer productivity. Herb-layer biomass was also positively correlated with herb-layer diversity. Hence, this study hints that positive diversity-functioning relationships might occur in the herb-layer of the deciduous forest under investigation. Especially in slow-growing forest ecosystems it is difficult to identify underlying mechanisms and to recognise sampling effects and complementary effects, for instance. III. Beech (Fagus sylvatica) as an “ecosystem engineer” has an outstanding influence on the underlying ecosystem processes. Ecosystem engineers modify environmental factors to a great extent and have thereby an impact on biocoenosis composition and ecosystem functioning. Firstly, among the tree species occurring in the Hainich, beech (Fagus sylvatica) made itself conspicuous by its close correlations with herb-layer vegetation. Secondly, no other tree species on our research sites featured all characteristics that account for Fagus sylvatica’s competitiveness, such as shade tolerance, crown plasticity, and efficiency in taking up nutrients and water. In the Hainich, we found a strong impact of beech on environmental factors (litter-layer thickness, soil acidity) also in those stands, where beech was not predominant. The proportion of no other tree species was as closely correlated with those environmental factors influencing herb-layer vegetation as the proportion of beech.