The increasing loss of biodiversity prompts the need for a greater understanding of ecosystem functioning, particularly in forest ecosystems. The present work investigates if increased proportions of different deciduous tree species in beech forest ecosystems affect the microbial community due to an increase in litter quality (leaf and root litter) during different seasons. Furthermore, this study focused on whether the increase of complementarity effects with increasing plant diversity observed in long-term grassland experiments can also be seen in species-rich forest ecosystems. Using microbial phospholipid fatty acids (PLFAs), the microbial communities from topsoils in different forest sites in the Hainich National Park, Germany, were analysed. In addition, we used the hydrogen isotopic signature (D, deuterium) of throughfall, soil water and n-alkanes from sun and shade leaf lipids to investigate the water use strategy of Fagus sylvatica during a period of drought. The results provide evidence that competition in the rooting zone by other tree species promotes strategies of drought avoidance in F. sylvatica, and also show complementary water use. Microbial communities are strongly influenced by abiotic controls, however species-specific tree characteristics affected the microbial community, especially in the photosynthetically active period when decomposition rates and root activity are highest. Our results suggest that a dense root network in association with arbuscular mycorrhizal fungi strongly supported microbial growth in the more diverse forest stands. It can be concluded that in species-rich forest ecosystems complementary resource use plays a crucial role for biotic interactions and ecosystem functioning, in particular maintaining the coexistence of different trees species.