Botanical evaluation of the Drenthian district

Abstract

The Biological Station of Wijster is situated approximately in the centre of the so-called Drenthian district, one of the 11 phytogeographical districts of the Netherlands. This district also extends into NW Germany and consists mainly of base-poor fluvioglacial and drift-sands with underlying boulder clay. The climate is boreo-atlantic, with fairly high rain-fall (about 31 inches), little sunshine, the highest summer P/S quotient of the Netherlands (June to August) and the coldest winters and coldest nights throughout the year. The flora is rather poor, but comparatively rich in boreal and atlantic species: 138 boreal species of vascular plants, bryophytes and lichens, 49 of which have, within the Netherlands, their main distribution in this district (cf. list), and 124 (sub) atlantic species, 18 of which occur mainly in Dr, as far as the Netherlands are concerned (cf. list). Some of the Northern species are even subarctic and widely separated from their main (Scandinavian) area, e.g. Carex aquatilis, Cornus suecica, and Sphagnum majus (= S. dusenii). It is argued at length, that for several reasons the theory of “glacial relics” is highly improbable, even for the bryophytes and lichens typical of erratic boulders (Megalithic tombs), which are so frequently found in Dr. Many non-boreal and non-atlantic species show a boreal resp. atlantic ecological behaviour in Dr. Originally the district was almost entirely covered with deciduous woods (mainly oak) and ombrotrophic bogs. The latter included the largest on the continent of NW Europe. Very little has been preserved of either. As far back as 2500 B.C. man started to convert the drier woods into heathland, which, until large-scale reclamation was initiated around 1900 A.D., completely dominated this region. The most salient features of present-day natural vegetation are the following. On moderately rich, shallow sand overlying boulder clay a special type of oak-wood is found with a dense understorey of holly, up to 8 m high, containing some interesting species like Trientalis europaea, Polygonatum verticillatum, Plagiothecium latebricola and, on still richer soil, Stellaria nemorum ssp. glochidisperma. Ilex is a very common tree in Dr, and so is Salix pentandra. The latter is restricted to brook valleys; these valleys are particularly rich in species of woody plants. The poor Querco-Betuletum is well developed in Dr. Betula verrucosa is typically absent as a native tree. The abundance of Empetrum nigrum in the heath is another characteristic feature of Dr. Dr is also the world centre of the wet-heath association Ericetum tetralicis. Both dry and wet heath show a variety of communities from very poor to rich. Juniper scrub is found in Dr more than anywhere else in the Netherlands. It belongs to a yet undescribed association, viz. the Dicrano-Juniperetum, and more specifically, to a geographical vicariant, restricted to Dr, the Senecio sylvaticus-vicariant. So far no less than 128 vascular plants, 96 bryophytes and lichens and more than 300 macrofungi have been found in this juniper scrub. The ombrotrophic bogs, some of which had a peat layer of up to 12 m thick, have now been reclaimed, but for a few relic stands, protected by State and private Nature Conservancy. They belong to the “flat bog” type (“Flachhochmoor”) and still contain such species as Sphagnum pulchrum, S. balticum, S. fuscum, Dicranum bergeri, Drosera anglica and D. obovata. Salt spray indicators are absent, except from a few coastal bogs. The succession in bog pools and heath pools is discussed at length. The moderately poor heath pools or fens and their margins contain such interesting species as Sparganium angustifolium, Carex limosa, Scheuchzeria palustris, Utricularia neglecta, Sphagnum majus, S. angermanicum, S. girgensohnii and S. russowii. With regard to its epiphytic vegetation, the Drenthian district is a.o. characterized by two communities, each of which consists of a mixture of species not normally found growing together else-where: (1) the community of Parmelia furfuracea and P. acetabulum and (2) the community of Aulacomnium androgynum and Tetraphis pellucida. Finally, the communities of the stream valleys in the Drenthian district are discussed here (about 55 streams, belonging to 9 river catchments). In Medieval times their fringing woodlands (Alnion and Alno-Padion) were transformed into meadow and poor pasture. During the past 50 years, drainage operations and the increasing use of fertilisers have largely destroyed these centuries-old semi-natural landscape types. A few relics have, however, been preserved as Nature Reserves and are maintained by being managed according to ancient agricultural practices. The trophic status of the water of a natural or semi-natural stream increases as one passes downstream. Corresponding to this gradient the following synecosystems may be recognised in the stream valleys: