Many of these woods were virtually clear-felled for timber during World Wars I and II, resulting in single aged stands of sessile oak 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.