This dataset provides information about the fine root biomass inventory conducted in April 2018 in plots of the interdisciplinary research training group 'RTG2300: Enrichment of European beech forests with conifers. The aim of this study was to examine how enrichment of beech forests with conifer species impacts fine root biomass and other root traits across variable site conditions.The study was carried out in Northern Germany (federal state of Lower Saxony, Germany) and comprised four stand clusters so-called 'quintets'. Each quintet contained five neighboring forest stands: three monospecific stands of European beech (Fagus sylvatica; Be), Norway spruce (Picea abies; Sp), or Douglas- fir (Pseudotsuga menziesii, Dg), and two mixed stands, one composed of European beech and Norway spruce, and one composed of European beech and Douglas-fir. In each of twenty stands, plots of 0.25 ha in size were established. The quintets were clustered according to their geographical location into two regions, "south" and "north", with two quintets in each region. The southern plots are located in higher altitudes with lower mean annual temperatures and a higher annual precipitation. Growing conditions on the northern plots are less favorable than on the southern sites, in particular with respect to precipitation.For fine root sampling, we used a systematic regular sampling grid design with a 10x10m grid established across each study plot. A total of ten grid cells were systematically selected from total 25 grid cells. At each selected cell, one root core was taken. Sampling was carried out from the end of March to mid-April 2018. We used a soil corer of 8 cm diameter to extract soil from the organic layer and mineral soil (0-60 cm). The organic layer which varied by depth was sampled as a whole, while the mineral soil was divided into twelve subsamples, each of 5 cm depth. In total, 200 soil cores with 2147 samples were collected and stored in plastic bags at 4 °C until further processing in the laboratory (maximum storage time four months).Living fine roots and dead roots (necromass) were expressed as fine root dry mass per soil volume (g m-3) and as dry matter per square meter of ground area (g m-2) per soil depth. Specific fine root surface area (SRA, cm2 g-1), and specific fine root length (SRL, m g-1) were calculated from fine root area and fine root length divided by dry fine root biomass. Root area index (RAI, root surface area per ground area, m2 m-2) was estimated by multiplying specific fine root surface area by the fine root biomass in a horizon

Soil cores that did not reach the maximum depth of 60 cm because of obstacles, the actual soil depths reached were considered as maximum depths. Only 20 % of the cores reached a maximum sampling depth of 60 cm and 66 % of the cores reached a sampling depth of at least 50 cm. In all of the plots there was at least one core that reached deeper soil layers of at least 50 cm. From the observed biomass in deeper soil layers, we estimate that approximately 94 % of the root biomass was sampled across all plots. Southern plots were slightly stonier compared to northern plots, resulting in a slightly higher proportion of sampled root biomass in the north (96 % in the north vs. 92 % in the south). Thus, we are confident that the effect of any such potential bias due to different sampling depths is largely negligible.Acknowledgements:The study was conducted as part of the Research Training Group 2300 funded by the German research funding organization (Deutsche Forschungsgemeinschaft – DFG). We gratefully acknowledge the administrative support by Serena Müller and the indispensable help of Andreas Parth, Michael Unger, Karl-Heinz Heine, Julian Meyer and Ulrike Westphal during soil core sampling. We thank Dirk Hölscher and Dietrich Hertel for the advice. We also thank the numerous student helpers fore carefully washing the soil samples and extracting fine roots. We are grateful to Kathleen Regan (USA) for linguistic corrections.