Opposite effects of litter and hemiparasites on a dominant grass under different water regimes and competition levels

Direct and indirect biotic interactions may affect plant growth and development, but the magnitude of these effects may vary depending on environmental conditions. In grassland ecosystems, competition is a strong structuring force. Nonetheless, if hemiparasitic plant species are introduced the competition intensity caused by the dominant species may be affected. However, the outcome of these interactions may change between wet or dry periods. In order to study this, we performed a potexperiment with different densities of the dominant species Schedonorus arundinaceus(1, 2 or 4 individuals) under constantly moist or intermittently dry conditions. The different Schenodorus densities were crossed with presence or absence of hemiparasites (either Rhinanthus minoror R. alectorolophus). Additionally, potsremained with bare ground or received a grass litterlayer (400 g m−2). We expected that indirect littereffects on vegetation (here Schedonorus or Rhinanthus) vary depending on soil moisture. We measured Schedonorus and Rhinanthusaboveground biomass and C stable isotope signature( δ13C) as response variables. Overall, Schedonorus attained similar biomass under moist conditions with Rhinanthusas in potsunder dry conditions without Rhinanthus. Presence of Rhinanthusalso increased δ13Cin moist pots, indicating hemiparasite-induced water stress. Litterpresence increased Schedonorus biomass and reduced δ13C, indicating improved water availability. Plants under dry conditions with littershowed similar biomass as under wet conditions without litter. Hemiparasites and litterhad opposite effects: hemiparasites reduced Schedonorus biomass while litterpresence facilitated grass growth. Contrary to our expectations, litterdid not compensate Schedonorus biomass when Rhinanthuswas present.