Understorey vegetation data related to Lammi LTER Site (LTER_EU_FI_004) from Valkea-Kotinen (ICP forests Level II monitoring plot Evo, nbr 19; SYKE monitoring plot FI01/VG3, 0.4 ha) during 1998 - 2014. Percentage covers on the ground level of all vascular, bryophyte and lichen species were determined on 16 permanently marked 2 m² sample quadrats on the subplot nro 1 (ICP forest II monitoring). In 1998 vegetation survey was carried out by the Finnish Environment Institute (SYKE) in the same points, but using 1 m² sampling quadrats.