Litter quality and microtopography as key drivers to topsoil properties and understorey plant diversity in ancient broadleaved forests on decalcified marl

In forest ecosystems, litter quality is a major driver for soil and understorey characteristics, but elevation, microtopography and subsoil properties may also be important. We tested the importance of each factor in two ancient mixed forests on decalcified marl, dominated by trees with different litter quality such as European hornbeam, with high-palatable litter, and beech, with low-palatable litter. We mapped elevation, differences in local height (microtopography), tree distribution and understorey cover on slopes ranging from crest to bottom, and sampled 200 7 × 7 m grid cells for characteristics of litter input, understorey, topsoil and subsoil. In both forests, elevation decreased gradually, but microtopography showed irregular patterns of depressions and mounds of a few cm below or above average local height. Tree distribution was not affected by elevation or subsoil properties, but clearly by microtopography. Adult beech was abundant on local mounds, while hornbeam was more common in local depressions. Topsoil and understorey characteristics were mainly affected by litter quality (tree species dominance) and microtopography. Litter quality had separate effects from microtopography, but could reinforce this. High litter quality (hornbeam) and low local height both led to high earthworm activity, low litter mass, high erosion, impermeable clay layers close to the surface, high pH, high soil moisture and high diversity of the understorey. Low litter quality (beech) and high local height both led to low earthworm activity, high litter mass, low erosion, low pH, low soil moisture and low plant diversity. Beech and hornbeam may act as ecosystem engineers, which change habitat conditions and local hydrology, and make habitats more suitable to themselves, and/or unsuitable to the other. However, they also increased spatial complexity of the forest and length of the habitat gradient. This may increase forest biodiversity as a whole, but also resilience to prolonged wet or dry periods.