Diversity and trophic structure of the soil fauna and its influence on litter decomposition in deciduous forests with increasing tree species diversity

Abstract

The relationship between species diversity and ecosystem function is an important topic of modern ecological research. Plant diversity has often been found to affect structural and biotic properties of ecosystems. Although the basic dependence of the soil food web on the resources provided by primary production is well studied, the knowledge of tree species diversity interactions with belowground processes and soil animal communities is still imperfect. This is especially true for forest ecosystems with mixtures of more than two tree species. In this study, the effect of increasing diversity of deciduous tree species on the composition of soil macrofauna communities and on leaf litter decomposition was studied. Nine research sites with comparable edaphic and climatic conditions were established in the Hainich National Park (Thuringia, Germany). They formed a diversity gradient ranging from pure beech (Fagus sylvatica), to medium-diversity stands built by beech, ash (Fraxinus excelsior), and lime (Tilia spp.), and to high-diversity stands dominated by beech, ash, lime, maple (Acer pseudoplatanus and A. platanoides), and hornbeam (Carpinus betulus) (Diversity Levels 1, 2 and 3).Canopy litter diversity had a marked effect on the composition of soil macrofauna communities. Though, the relationship between increasing tree species diversity and diversity or abundance of the animal taxa studied (gastropods, lumbricids, isopods, carabids and araneids) was not generally linear. Trophic groups responded differently to tree species diversity: the abundance of key groups of primary decomposers such as isopods and earthworms was promoted by a high diversity of litter resources with an increasing amount of high quality litter, whereas sapro- and phytophagous snails were most diverse and abundant in the medium-diversity forest sites. The activity density of predators in the litter layer such as spiders and carabid beetles decreased in the highly diverse plots due to the fast decomposition of the litter and the concomitant reduction of the litter habitat. On the other hand, predator diversity was positively correlated with the more diverse herb and litter layer in mixed sites. Overall, tree species diversity was an important driving force for soil macrofauna, but it was complemented by structuring forces exerted by single keystone species (e.g. beech).Litter decomposition was studied for the six common tree species on the Hainich sites: beech, lime, ash, maple (A. pseudoplatanus and A. platanoides) and hornbeam, as well as for site specific litter mixtures. The annual decomposition rates (k) were typical for these temperate tree species, with k = 0.5 for beech and k = 1.5 2 for the other tree species. The decomposition rate of Fagus increased with increasing tree species diversity and was further correlated to soil pH, moisture of the litter layer and isopod abundance. Litter decomposition of the other tree species was also faster in the mixed species sites (except for ash and hornbeam) and was positively correlated with earthworm abundance. Amounts and release rates of nutrients differed between tree species but were not consistently different between the diversity levels. Patterns of nutrient release were correlated to initial litter nutrient ratios and even more so with earthworm abundance. Me! re tree species diversity may not be the driver of decomposition processes and nutrient dynamics. Characteristic traits of individual tree species, e.g. of beech in contrast to lime and ash, influence soil and habitat properties as well as soil fauna communities. Thus, presence and abundance of certain tree species and their associated soil fauna (e.g. earthworms) play a key role through their direct and indirect effects on the whole forest ecosystem.