Herb layer characteristics, fly communities and trophic interactions along a gradient of tree and herb diversity in a temperate deciduous forest

Abstract

The human-induced loss of biodiversity from many ecosystems has led to increased interest in the relationship between biodiversity and ecosystem functioning. Research in this field is mostly focused on the producer level, i.e. the diversity of plants. Plant diversity can positively affect the diversity and abundance of higher trophic level organisms, for example insects. In addition, it has been shown that plant diversity can impact plant-insect interactions such as pollination and herbivory. Most studies on plant diversity effects have been conducted in grasslands, whereas forest ecosystems have received less attention so far. In particular, herbaceous plant communities of the forest understorey and their associated insect communities have rarely been the focus of biodiversity studies. In this thesis we investigated plant diversity effects in the herb layer of a near-natural temperate deciduous forest. We explored possible linkages between the plant diversity of the tree and the herb layer. In addition, we investigated how the diversity of tree and herb layer influence diversity, abundance and community composition of fly communities in the herb layer. Finally, we studied trophic interactions (herbivory, pollination-dependent seed production, seed predation) in relation to tree and herb diversity. The study was conducted on 100 plots along a gradient of tree diversity (2 9 species) and herb diversity (2 28 species) in the Hainich National Park (Central Germany). Vegetation surveys of the herb layer were performed in two consecutive years. Flies were sampled with pan traps and suction sampling. To study trophic interactions, potted individuals of the common understorey herbs Lathyrus vernus and Primula elatior were set out on 40 plots for three months in 2009. On these phytometer plants, we assessed invertebrate herbivore damage as well as seed production as a proxy for pollination success. Half of the L. vernus individuals had been treated with plant parasitic nematodes to investigate if links between belowground and aboveground herbivory are affected by the diversity of the surrounding plant community. Also in 2009, we performed a seed removal experiment using different predator exclusion treatments to study seed predation in the herb layer. Species richness and cover of the herb layer increased with increasing tree diversity. In addition, the environmental variables canopy cover and humus layer mass had a negative effect both on herb layer species richness and cover. Herb layer species richness was positively related to increasing soil pH and the distance to the forest edge. The proportion of forbs increased with increasing tree diversity, whereas the proportion of tree saplings decreased. The proportion of graminoids was not affected. Both tree and herb diversity influenced fly abundance, while abundances of the locally common fly taxa Empidoidea and Phoridae responded to herb diversity only. Relationships between fly abundance and tree and herb diversity were positive in the study location dominated by lime (Lindig), but absent or weakly negative in the beech-dominated location (Thiemsburg). Fly family richness and the species richness of Empidoidea and Phoridae were not affected by tree or herb diversity. In contrast, herb cover showed a strong and consistently positive association both with fly richness and abundance. Plots which were similar in their herb and tree layer composition also had more similar fly communities. In our phytometer experiment, foliar herbivore damage decreased with increasing tree diversity in P. elatior. In L. vernus, foliar herbivore damage only responded to belowground herbivory, with higher aboveground herbivory in nematode treated plants. Seed production per plant increased with increasing tree diversity in L. vernus and decreased with increasing herbivore damage in both species. The seed predation experiment showed that the seed removal rate was positively related to herb diversity for seeds of L. vernus, but only weakly so in the case of P. elatior. Predator exclusion treatments revealed that both vertebrates and invertebrates act as seed predators of L. vernus, while seed removal of P. elatior is mainly performed by invertebrates. Slugs appeared to be the dominant invertebrate seed predators in our study system. In conclusion, we found that the diversity of tree and herb layer affects plant and fly communities in the herb layer of Germany s largest connected deciduous forest. In addition, we demonstrated that several types of trophic interactions, namely herbivory, pollination-dependent seed production and seed predation, are related to tree or herb layer diversity. As a consequence, overall forest biodiversity and the functioning of trophic interactions are expected to benefit from forestry practises that promote high tree diversity, which goes along with a diverse and well-developed herb layer.