Herb layer response to broadleaf tree species with different leaf litter quality and canopy structure in temperate forests

How do broadleaf tree species affect humus characteristics, herb layer composition and species diversity through their leaf litter quality and canopy structure? Mixed broadleaf forests in Brandenburg, NE Germany. We studied the herb and tree layer composition in 129 undisturbed stands using a 10-degree cover-abundance and percentage scale, respectively. The main floristic gradients were extracted by non-metric multidimensional scaling. Effects of tree species on the herb layer were analysed with partial Spearman rank correlation. We assessed affinities for specific tree species using indicator species analysis. Both beech and oak influenced herb layer composition mainly through their litter quality, which resulted in deep Ol and Of horizons, respectively. The less dense canopy of oak, in contrast to the dense beech canopy, enhanced species diversity in favour of indifferent herb species (species not closely tied to forests). Lime was correlated with a distinct floristic gradient, but a direct effect on the herb layer cannot be proven with the available data. Effects of hornbeam were less pronounced. The relationship between the tree and herb layer must be partly attributed to pH differences. However, tree species effects on humus characteristics and on light flux to the ground were largely responsible as well. The results suggest that tree species can influence herb layer composition and diversity, but the missing correlation with lime and hornbeam raise questions requiring further detailed investigation.