Restoring the fire-grazing interaction promotes tree-grass coexistence by controlling woody encroachment

Woody encroachment can convert grasslands and savannas to shrublands and woodlands, so understanding the processes which regulate woody encroachment is necessary to conserve or restore these ecosystems. We hypothesized that recreating the fire-grazing interaction would limit woody encroachment because focal grazing increases fuel accumulation on unburned areas and increases browsing on emergent woody plants in burned areas. This study was conducted in the Grand River Grasslands of Iowa and Missouri (USA) on 11 sites (15.4-35.0 ha). Each site was assigned to one treatment: patch-burn-graze (n = 4), with spatially discrete prescribed fires and free access by cattle (the fire-grazing interaction); graze-and-burn (n = 4), with free access by cattle and one burn of the entire site every 3 yr; or burn-only (n = 3), with one site-wide burn every 3-5 yr and no grazing. The burn-only treatment increased woody encroachment fourfold compared to the graze-and-burn and patch-burn-graze treatments (130.2, standard error [SE] = 16.0; 20.9, SE = 12.0; and 46.3, SE = 10.8; plants/200 m(2)). The patch-burn-graze treatment had 2-3 cm more accumulated fuel and woody plants which were 12% shorter, on average, than the other treatments (comparing eight common species). The movement of large herbivores also appeared to decrease the frequency of woody species which spread vegetatively. Our work illustrates how the fire-grazing interaction may control woody encroachment and shows that cattle substitute, at least partially, for endemic large herbivores.