Shifting mosaics in grazed woodlands driven by the alternation of plant facilitation and competition

Free-ranging large grazers, such as cattle and horses, are increasingly rei ntroduced to former agricultural areas in Western Europe in order to restor e natural and diverse habitats. In this review we outline mechanisms by whi ch large grazers induce and maintain structural diversity in the vegetation (mosaics of grasslands, shrub thickets and trees). This variation in veget ation structure is considered to be important for the conservation of biodi versity of various plant and animal groups. The process of spatial associat ion with unpalatable plants (associational resistance) enables palatable pl ants to establish in grasslands maintained by large grazers. In this way, s hort unattractive (thorny, low quality or toxic) species facilitate taller unattractive shrubs, which facilitate palatable trees, which in turn outsha de the species that facilitated their recruitment. Established trees can, t herefore, not regenerate under their own canopy, leading to cyclic patch dy namics. Since this cyclic dynamic occurs on a local scale, this contributes to shifting mosaics. The mechanisms involved in creating and maintaining t he resulting shifting mosaics are described for temperate floodplain and he athland ecosystems, including the effects on nutrient transport within graz ed landscapes. How grazing leads to shifting mosaics is described in terms of plant functional types, allowing potential generalisation to other ecosy stems. The resulting interaction web of grasses, unpalatable forbs and shru bs, palatable light-demanding trees and shade-tolerant trees is discussed, and was found to contain various interesting direct and indirect effects. T he key process contributing to spatial diversity in vegetation structure is the alternation of positive (facilitation) interactions between plant spec ies at one life cycle stage, and competitive displacement at another stage. Crazing thus causes directional successional sequences to change to shifti ng mosaics. The implications of this theory for nature conservation are dis cussed, including the relevant management problems, possible choices and pr actical solutions. We conclude that the theoretical framework outlined in t his review provides helpful insights when coping with nature conservation i ssues in temperate woodland habitats.