The UK is the European stronghold for the Natura 2000 ‘Atlantic sessile oakwood’ (Quercus petraea) habitat, which is well represented in Wales – and where the more oceanic Welsh examples are of particular importance for their lower plant and lichen communities. These sessile oakwoods are climax communities invariably found on thin acid/base-poor soils. All of the more accessible examples are relatively young, i.e. less than 100 years old, and single-aged as a consequence of felling for timber during World Wars I or II. There are three reference states, determined by climate and management history.