Thesium linophyllon parasitizes and suppresses expansive Calamagrostis epigejos

Root-hemiparasitic interaction between the dominant grass Calamagrostis epigejos and the hemiparasite Thesium linophyllon was studied to assess the potential of the parasite to regulate dominance of the grass, which is expanding into species-rich steppe grasslands. First, we aimed to identify physiological links between the two species as a principal indicator of the parasitic relationship. Second, we analysed the dynamics of the two species in the vegetation of a steppe grassland at the foot of the Bükk Mountains, Hungary, where their joint presence is recorded in a long-term permanent plot monitoring dataset to detect patterns associated with the parasitic ecological interaction. Numerous well-developed functional haustoria of Th. linophyllon were identified on the root systems of C. epigejos. The joint dynamics of C. epigejos and Th. linophyllon displayed clear signs of the parasitic interaction: (1) the dynamics of Th. linophyllon frequency was positively associated with the initial cover of C. epigejos; (2) maximum recorded cover values of the two species were strongly positively correlated; and (3) the extent of C. epigejos decrease in the vegetation was significantly positively associated with maximum Th. linophyllon cover recorded throughout the monitoring period. We demonstrate that C. epigejos can be parasitized by Th. linophyllon, which restricts abundance of the grass. Th. linophyllon thus has potential to act as a native biological control of C. epigejos in steppe grasslands.