Plot data of three primeval European beech forests in eastern Slovakia - Glatthorn, Jonas; Feldmann, Eike; Pichler, Viliam; Hauck, Markus; Leuschner, Christoph (In supplement to: Glatthorn, J et al. (2018): Biomass Stock and Productivity of Primeval and Production Beech Forests: Greater Canopy Structural Diversity Promotes Productivity. Ecosystems, 21(4), 704-722, https://doi.org/10.1007/s10021-017-0179-z, 2017)