three lakes are very acid sensitive and therefore vulnerable to afforestation within their catchment areas. Woodlands Killarney possesses the most extensive area (approximately ) of semi-natural native woodland (woodland dominated by indigenous species) remaining in Ireland. Most of this woodland is encompassed by the national park. There are three main types of woodland in the park: acidophilous oak woodland (Quercus petraea-Ilex aquifolium) on Devonian sandstone; moss-rich yew woodland (Taxus baccata) on Carboniferous limestone outcrops; and wet woodland (also called carr) dominated by alder on low-lying swampy limestone soils on the lake edges. The woods in the park fall naturally into two sectors, along the geologic divide. The oak and yew woodlands are of international importance. Mixed woodland and conifer plantations also occur in the park. The mixed woodland on Ross Island has one of the richest herb layers in the park's woods. Grazing and rhododendron invasion threaten the park's woodlands. Rhododendrons affect approximately two-thirds of the oak woodlands. A rhododendron removal programme is under way in the park. The yew woodlands have been negatively affected by heavy grazing for many years. Oak woodlands The park is perhaps most famous for its oak woodlands, which are about in size. They form the largest area of native woodland remaining in Ireland and are a remnant of the woodland that once covered much of Ireland. Derrycunihy Wood is perhaps the most natural sessile oak (Quercus petraea) wood in Ireland. Most of the oak woodlands are located on the lower slopes of the Shehy and Tomy mountains, adjacent to Lough Leane. They are typically dominated by sessile oak, which favours the acidic soils of the sandstone mountains. The woods have Annex I status in the EU Habitats Directive because of their diverse and rich flora, most notably their bryophytes (mosses and liverworts). The oak woodlands typically have an understory of holly (Ilex aquifolium). Strawberry trees (Arbutus unedo) are a notable part of these woods. There are also scattered yews. The field layer includes bilberry and woodrush. The herb layer is not rich in species. Bryophytes, lichens and filmy ferns (Hymenophyllaceae), thrive in the humid oceanic climate. Species with restricted Atlantic distributions grow in the woods. The bryophytes in these woods are perhaps the best-developed Atlantic bryophyte community in Europe. The remote Glaism na Marbh valley has a particularly rich flora of bryophytes, some of which are scarce or absent in other parts of the