Many of the accessible sessile oak woods were virtually clear-felled for timber during World Wars I and II, resulting in predominantly single-aged stands less than 100 years old. This does not, however, diminish their importance as sources of biodiversity, as some of these single-aged stands in humid situations support diverse and internationally important bryophyte and lichen populations. However, the problems associated with single-aged stands are often compounded by sufficiently high levels of sheep grazing to prevent successful regeneration.