Velev, N., I. Apostolova, Z. Rozbrojová, and P. Hájková. “The Alliances Arrhenatherion, Cynosurion and Trifolion Medii in Western Bulgaria – Environmental Gradients and Ecological Limitations”. Hacquetia, vol. 9, no. 2, Aug. 2015, doi:10.2478/v10028-010-0012-0.