Release from competition and protection determine the outcome of plant interactions along a grazing gradient

Plant interactions a re suggested [0 shift from competition to facilitation and co llapse with increasi ng grazing pressure. 1he existence of this fu ll range of plant interactions and the role of underlying mechanisms (i .e. release from competition and protecting effect) in response to herbivory remains poo rly documented and mainly described in terrestrial systems. We use a large grazing disturbance gradient (fivc levels of grazing) [0 test its efFect on the outcome of plant interact ions and underlying mechanisms in freshwarer ecosystems. In a mesocosm experiment, we manipulated the presence of neighbouring plants to test their negarive (comperition) or protective (faci li tation) cHeers on targer plants along the grazing pressure grad ient. We predicted that plant interactions 1) shift from competition to indirect facilitation with increased grazing pressure, 2) indirect faci litatio n collapses at high levels of grazing, 3) release from compet it ion mainly drives the outcome in lowly grazed conditions and, 4) decreased protect ion occurs in highl y grazed condi tioll s respollsib le For the co ll apse of faciliLarioll. lltis stud y shows rhe occurrence of the full range of outcomes in plant interactions under a wide spectrum of grazing pressure and indicates how the complex combination of underlying mechanisms shapes variations in plant interactions. We show that both, the release from competition and the increased protection by neighbouring plants drove the shift from competition to indirect facilitation . Declined protection by neighbouring plants resulted in a co llapse of indirect faci litation for survival under intense hetbivory. Our study provides the first experimental evidence of indirect faci litation structuring freshwater ecosys tems therebyva lidating important ecological concepts mainly developed for terrestrial ecosystems.