Woodlands formed by yew (Taxus baccata) are primarily found on the thin, dry, calcareous chalk soils in south-east England (Fig. 9.18) that are otherwise dominated by grazed grassland. Despite being poisonous, yew seedlings are prone to being grazed, so yew only usually develops beneath the spiny cover of hawthorn, Crataegus monogyna, and especially juniper, Juniperus communis bushes (Thomas and Polwart, 2003), where it is protected from herbivores.
Owing to its extreme tolerance of shade and the dense shade it creates, yew eventually outcompetes its nurse shrubs and any other trees that establish in the same way, and develops progressively into pure yew woodland, often with the dead woody remains of the former scrub beneath. The occasional holly Ilex aquifolium, or whitebeam Sorbus aria, may initially survive by outgrowing the yew in earlier years but as they die they are not replaced and the canopy of the yews fuses into a virtually continuous layer.
Though yew is extremely shade tolerant it cannot regenerate under its own canopy so its woodlands persist in a cycle of growth and destruction. As old trees die, a new growth of juniper or hawthorn in the gap creates the protected conditions for new yew seedlings to establish - this is cyclic succession as described above. However, since these gaps are often still too shaded for good shrub growth, most yew regeneration happens around the edges of existing woods, and the gaps left by dying trees revert back to grassland. So, most yew woods are single generation stands 'moving' across the landscape by regeneration at the edge, leaving individual relict trees behind until eventually even they die. As shrubs invade the now open grassland, the whole cycle can start again.
This helps to explain why Tittensor (1980) and others point out that most yew woods in England are quite young, largely resulting from the abandonment of land during the last two centuries as a result of the Napoleonic wars, agricultural neglect in the 1920s and myxomatosis killing rabbits.
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