Defoliation and site differences influence vegetative spread in grassland

Plants spread vegetatively at rates that depend on both their own and their neighbours' traits. We tested hypotheses that such rates also interact with defoliation intensity and differ between sites. Well-established monoculture patches (20 x 20 cm) of five grass species were arranged in plots so that every species patch had all the remaining species as neighbours. Sites were in central Scotland, UK, and at a drier location in eastern Scotland. Plots were cut at 3 cm ('short') or 6 cm ('tall'), either uniformly or in a chessboard pattern. Invasiveness and resistance to invasion followed the transitive hierarchy, Agrostis capillaris > Festuca rubra > Lolium perenne = Holcus lanatus > Poa trivialis, except that P. trivialis strongly invaded H. lanatus at the damper, more fertile site. 'Tall' patches spread and intermixed most, independent of species. The effects of cutting neighbouring patches depended on both invading and invaded species' traits. Thus, defoliation altered the relationships between species, as did differences between sites, influencing both the speed and direction of species replacement.