Tree colonisation of abandoned arable land after 27 years of horse-grazing: the role of bramble as a facilitator of oak wood regeneration

Abstract

The impact of horse-grazing on natural tree regeneration on abandoned arable fields was studied in ‘Baronie Cranendonck’, a 98 ha nature reserve near the Dutch–Belgian border. The study area comprised a vegetation mosaic of Corynephorus grassland and dry heath with juniper shrub on former drift sand, planted pine wood and oak wood. Several small-size former arable fields occurred, which were abandoned in 1972. From 1973 onwards the whole reserve was put under grazing management to integrate the abandoned fields with the adjacent natural vegetation and to maintain an open habitat. For this purpose Iceland ponies ( Equus caballus) were introduced at a density of 8–10 animals per km 2. In 2000 an inventory was made of the vegetation and of woody regeneration. Also the habitat use of the ponies was assessed during the growing season from March to October. The former arable fields were covered by mesotrophic grassland with patches of bramble shrub ( Rubus fruticosus) and soft-rush tussocks ( Juncus effusus). At a few places these grasslands had evolved to natural grassland communities with heather ( Calluna vulgaris). The ponies preferentially grazed the former fields. During the growing season the ponies spent 76% of their grazing-time in these areas. Although grazing by rabbits ( Oryctolagus cuniculus) had a large impact on vegetation development during the first few years after abandonment, their current impact on vegetation development is insignificant since they are now at very low densities (<1 animal per ha). Woody regeneration was encountered at several places on the former arable fields. Highest densities were found within patches of bramble, heather or soft-rush. Bramble especially was found to facilitate the regeneration of browse-sensitive species, i.e. pedunculate oak ( Quercus robur) and alder buckthorn ( Frangula alnus). All saplings taller than 100 cm (oak) or 200 cm (buckthorn) were associated with bramble shrub. The establishment of bramble on the former arable fields seemed essential for oak wood regeneration. Facilitation of the development of wood-pasture under horse-grazing on abandoned land is outlined.