Diversity did not influence soil water use of tree clusters in a temperate mixed forest

Compared to monocultures, diverse ecosystems are often expected to show more comprehensive resource use. However, with respect to diversity–soil-water-use relationships in forests, very little information is available. We analysed soil water uptake in 100 tree clusters differing in tree species diversity and species composition in the Hainich forest in central Germany. The clusters contained all possible combinations of five broadleaved tree species in one-, two- and three-species clusters (three diversity levels), replicated fourfold (20 one-species, 40 two-species and 40 three-species clusters). We estimated soil water uptake during a summer dry period in 0–0.3 m soil depth, based on throughfall and soil moisture measurements with a simple budgeting approach. Throughout the whole vegetation period in 2009, soil water uptake was additionally determined at a higher temporal resolution and also for a greater part of the soil profile (0–0.7 m) on a subset of 16 intensive clusters. During the dry spell, mean soil water uptake was 1.9 0.1 mm day−1 in 0–0.3 m (100 clusters) and 3.0 0.5 mm day−1 in 0–0.7 m soil depth (16 clusters), respectively. Besides a slightly higher water use of Fraxinus clusters, we could not detect any effects of species identity or diversity on cluster water use. We discuss that water use may indeed be a conservative process, that differences in tree-species-specific traits may be compensated for by other factors such as herb layer coverage and tree spatial arrangement, and that diversity-driven differences in water use may arise only at a larger scale. We further conclude that with respect to stand water use "tree diversity'' alone is not an appropriate simplification of the complex network of interactions between species traits, stand properties and environmental conditions that have varying influence on stand water use, both in space and time.