Floristic and structural data of oak dune forests in western Jutland (Kaergard and Blabjerg) were collected in 1998 and compared with previous studies of Danish oligotrophic oak forests. A hierarchical divisive clustering resulted in four main clusters, containing two coastal oak vegetation types. Quercus robur-Carex arenaria and Q. robur-Pleurozium schreberi communities, both restricted to the oak dune forest areas in western Jutland, and two more widespread oak forest types. Q. robur-Vaccinium myrtillus and Q. robur-Holcus mollis communities. The low-growing Q. robur-C. arenaria shrub and forest vegetation occurs primarily in the outer dunes, with the tallest trees on the leeward sides of the dunes, while a Q. robur-P schreberi community is found in the inner parts of the dune system. The similarity to a considerable number of the acidophilous vascular plant species closely relates the Danish oak dune forest communities to the Pinus sylvestris or P. sylvestris-Q. robur vegetation described in neighbouring countries, suggesting that R sylvestris probably had a prominent role in the Danish oak dune forests before human exploitation caused its extinction in the 18th century. The low-growing oak shrub in the outer dunes. aged between 52 and 85 years, generally seems to have remained unchanged in floristic composition and structure since 1964 when the previous study on the shrub was performed. In the dune valleys, oak stand-level dieback is widespread and was latest recorded in 1985. Such retrogressive processes are probably due to a combination of low winter temperatures and other adverse climatic conditions. In the central parts of the oak dune forest, the oak trees, aged between 55 and 73 years, grow to a higher stature, but also here stand-level dieback is widespread leading to retrogression. Only in the innermost parts, a regular forest cover seems to prevail with trees of 68-104 years of age and the development of the field vegetation cover progresses into a typical forest community. This is exemplified by the decrease of light-demanding species and the establishment of true forest species like Oxalis acetosella and Dryopteris dilatata. The structure and poor-species diversity make the oak dune forest unique both in Denmark and on an international scale. We recommend further studies of the remnants of the original. and in the past probably more widespread dune forests in Denmark. and drawing up of a detailed management and conservation plan. The possibility of permitting the native and now reintroduced P. sylvestris to re-colonise the oak dune forest area and be part of the natural vegetation succession processes should be seriously considered. (C) 2002 Elsevier Science B.V. All rights reserved.
