Pasturing has a dramatic effect on a forest.  Grazing eliminates almost every bit of forest understory with the exception of the thorny bushes that you see here, such as barberry (Berberis spp.), multiflora rose (Rosa multiflora), raspberries (Rubus spp.), and gooseberries (Ribes spp.).  To make matters worse, compaction from grazing animals slows the growth of trees.