Tree layer composition is a crucial factor for the stability and diversity of forest ecosystems, and many factors such as nutrient availability or the light regime are influenced by trees and their species-specific properties. The occurrence and abundance of herbs as indicators for site characteristics are likely to be related to the variability in tree layer diversity and changes in environmental variables. To test this hypothesis, the effect of tree layer composition on herb layer diversity was studied in mixed deciduous beech (Fagus sylvatica) forest reserves on the Swiss Plateau. Applying a transect design we recorded tree and herbaceous species composition and measured key top soil properties and light availability in seven forest reserves. Tree layer diversity did not affect herb layer diversity at the study sites. However, a species-specific influence of trees on the herb layer was evident: a high proportion of beech had a negative effect on herb layer diversity, and the presence of ash (Fraxinus excelsior) was correlated with a thick Ah horizon and high soil pH, which enhanced herb layer diversity. Spruce (Picea abies) and pine (Pinus sylvestris) were related to a thick forest floor (litter) layer, which negatively affected herb layer diversity.