The yew (Taxus) communities of the region (England) lie chiefly in valleys opening to the south and east. Yew establishes itself with difficulty in grassland and suffers from grazing by rabbits; it develops therefore primarily in a scrub relatively immune to rabbits which may be either of Juniperus or Crataegus. The former is rapidly suppressed by shading, leading to the early appearance of pure yew wood. In the latter the succession is much slower and frequently passes through intermediate stages in which yew and Fraxinus are mixed. In both cases yew ultimately becomes dominant because of its longevity. Development of the yew wood begins normally in the most sheltered part of the valleys and progresses to leeward, where growth of the pioneer scrub is favored through protection from wind, or at the head of valleys in the juniper sere. Degeneration of the yew wood is caused by the death of the oldest yews; a protective scrub is not formed in the gaps, young yews fail to colonize, and the whole community ultimately disappears. Later re-colonization of such abandoned areas may take place, following the reappearance of scrub. The usual succession is from scrub through ash-oak to beech wood. The frequent mixture of Fraxinus and Taxus shows that these occupy essentially coordinate positions in the sere, while the normal succession is interrupted by the longevity and dense shade of the yew. Since the yew wood is normally unable to perpetuate itself, it is regarded as a temporary phase of the scrub.