Palynologisch en stratigrafisch onderzoek van de Peelvenen

As an introduction to a number of researches of his own the author wishes to give the following data: „Veen” has two meanings in Dutch: 1. in a petrographic sense (peat) Von Büllow’s definition was accepted: „Torf” ist zu deflnieren als ein meist dunkles, kohlenstoffreiches und ± saures Gemenge unvollständig spezifisch-zersetzter Pflanzenteile, das erdgeschichtlich jüngste Glied der Verwantschaftsreihe der Kohlen, dessen Bildung noch heute andauert.” 2. in a plant-sociological-geographic sense (bog) the following definition has been suggested: a bog is a plot, the surface of which consists of a layer of peat, either covered or not with vegetation, with which that layer is genetically connected. The classification of bogs according to their position with regard to the water-level of the surroundings (Staring) and that of the geological chart were rejected on account of their ambiguous character. The classification suggested by Van Baren according to the environment in which the bogs have been formed, was likewise thought insufficient. Preference was given to the classification according to the plants which gave rise to the peat (eutrophic, mesotrophic and oligotrophic bogs) and according to the origin of the water needed for peat formation (topogenous, ombrogenous and soligenous bogs). The conditions of peat-formation are of a botanical (presence of a vegetation and micro-organisms), climatologic (presence of a certain temperature and moisture) and geological nature (presence of a basin, valley or dead river-branch, certain level of ground water, a possible impervious layer). With reference to a number of authors (Picardt; Van Lier; Grisebach, Venema and Staring; Weber) the alteration in conception as to peatformation from the 17th via the 18th and 19th to the 20th century has been given. The word „Peel” cannot be derived from „palus”. Nothing is certain about its origin. It may mean the low land, bog or marsh. The bogs of the Peel lie on the Brabant-Limburg border-plateau (fig. 2). Lorié and Pannekoek van Rheden have shown that the peatformation of the Peel is likely to have occurred in channels, which have been formed by the Meuse, in co-operation with wind and rain (fig. 4). The bogs were therefore in the first instance topogenous formations, which afterwards developed into ombrogenous bogs. For his own research the author collected peat in three ways: 1. by cutting lumps of peat from open profiles; 2. by boring with a simple peat-bore (photograph 1); 3. by boring with the Utrecht peat-bore, an improvement on Dachnowski’s (fig. 5). To assist in the pollen-analytic examination the samples were treated according to Erdtman’s method. The latter has the following advantages compared with the usual treatment with a 10% KOH-solution: 1. the surface-structures of the pollen-grains are more distinct and as a result the grains themselves can be recognized better; 2. the pollen is more concentrated, so that in spite of the method taking up much time, a saving of time is possible. How the method is applied may be found in the chapter concerned (p. 38 and following). For the stratigraphic examination the samples were broken apart in a glass-bowl of water and viewed with a binocular microscope. Dry sandy samples were broken in water, when seeds and other vegetative parts came floating to the top; next they were put with a brush on thick blotting paper and studied through the binocular microscope. The designations for the sediments and species of peat have been derived from Fægri & Gams. For Scheuchzeria peat a new designation has been added. A plea was made for replacing the word pollen-analysis by „palynology”. A survey of the observations and examinations up to abt. 1935 closes the introduction (see the diagrams of Weber, Erdtman and Duyfjes in the figs. 6, 7, 8 and 9). The author’s own research refers to the Southern and Astense Peel, as in the remaining grounds of the geological chart indicated I 4v (= raised bog) no samples could be taken owing to the digging off having progressed too far. 10 profiles were examined. The situation of the bore-sites has been given in the geological chart of the grounds (fig. 3). The result of the examination (figs. 10—27) and the discussion on it may be summerized as follows: Zoning of pollen-diagrams The sub-zoning of the late- and post-glacial periods according to Blytt & Sernander has proved useful as a zoning of pollen-diagrams, provided atlantic and sub-boreal are joined. It is desirable to replace Blytt & Sernander’s terminology by a different one, because the authors gave a climatologic connotation to their names of periods. The limit between pleistocene and holocene was drawn between preboreal and boreal as Florschütz did. As phases of the holocene the following names were suggested: young post-glacial = sub-atlantic mid post-glacial = sub-boreal and atlantic old post-glacial = boreal. Neither in the Peel nor elsewhere in Holland have Allerød-deposits been found. They are not likely to be found either, as on account of the long distance from the land-ice-margin the flora will have been hardly or not at all influenced by the Allerød interstadial period. For Holland therefore the zoning of the late-glacial according to Firbas (1935) may be considered sufficient. The names of the periods do not bear a climatologic connotation as those of the post-glacial phases do. For the sake of a unity the following names have been suggested: young late-glacial = pre-boreal mid late-glacial = sub-arctic period old late-glacial = arctic period. Forest-history In a table (p. 98), in which likewise the Peel diagrams of Weber, Erdtman and Duyfjes have been inserted, the examined profiles have been arranged from North to South. From each profile it has been stated whether it originated in a certain period (+) or not (—). The sub-arctic phase was characterized by forests of Betula and Pinus and was followed by the pre-boreal phase, in which Corylus and Alnus occurred. Also from the other Dutch diagrams (see list on p. 99) it appeared that in the Netherlands the Alnus pollen occurs with an equal frequency before, during and after that of the Quercetum mixtum. The old post-glacial zone of the diagrams shows a peak in the Pinusline. In contrast with the from Mid-Europe there is not always a maximum in the Corylus-curve after the Pinus-peak. In other Dutch diagrams this phenomenon is likewise found. Only in 28% of all Dutch profiles with a boreal zone does a hazel-maximum succeed a Pinus one. They often co-incide (16%), while in the remaining cases no hazelpeak has been established. There is no fixed order of sequence in the occurrence of the components of the Quercetum mixtum, either in the Peel or elsewhere in Holland. The mid post-glacial is the phase of culmination of warmth-loving forest elements: Alnus pollen shows the highest percentage in this zone. Quercus pollen also occurs in great quantities, while Ulmus and Tilia take up an important place up to the „Grenzhorizont”. The absolute and empiric Fagus pollen limits are found at different heights in the mid post-glacial zone of the diagrams, the rational limit lies somewhere near the „Grenzhorizont”. In the young post-glacial phase the Fagus pollen attains fairly high percentages (up to 30%). The maxima in the East and South-east of the Netherlands are between 20% and 38%; they decrease towards the coast and increase towards the South-east (Hautes Fagnes, Belgium) and East (Germany). It seems incorrect to class the Netherlands almost entirely among the oak-alderterritory poor in beeches, as Firbas did. An attempt has been made to fit the Peel-diagrams into Overbeck & Schneider’s zonation system. For the territory for which it has been made there are already difficulties (p. 104), for use in the Peel and other Dutch diagrams there are even more objections (p. 68, 104). Godwin’s zonation system appeared to be a little less forced, but not quite useful on account of too many details. From his horizons that of Ulmus proved useless for the continent. Neither for the Peel nor for the Netherlands and its surrounding territory can a detailed zonation system be designed. It has proved difficult to proceed any farther than Rudolph’s „Grundsukzession”: birch, pine-hazel-mixed oak-forest-beech, in which the alder generally joins the mixed oak-forest and the hornbeam the beech. Before drawing far-reaching conclusions from the course of the curves (as has been done by some authors) more palynological researches are needed in accordance with the actuality principle, known from geology. Pollen-grains from warmth-loving trees in seemingly sub-arctic spectra In profile 4 (Deurnse Peel II) pollen-grains of Abies, Alnus, Picea, Tilia, Ulmus and Corylus were found in the „late-glacial” zone (figs. 14, 15). Investigations were made as to which of the following possibilities would be the cause of their appearance: 1. in taking and preparing the samples pollution occurred; 2. pollen-transport over long distances has taken place; 3. the pollen-grains found have got secondarily into the deposit; 4. warmth-loving trees have occurred in favourable circumstances in the late-glacial phase or 5. in an interstadial period or in an interglacial phase. The said pollen-grains probably hail from a Würm interstadial or interglacial phase. Interglacial peat On the site of the bore-point 7 it was possible to collect samples from the layers under the peat. The upper 40 cm of the diagram Griendtsveen IX (fig. 27) of this profile proved a repetition of the lower 40 cm of the Griendtsveen I profile (fig. 18). The diagram shows that pollen of Carpinus, Picea and Abies occurs showing the deposit to be of interglacial age. The pollen-curves, however, pass unnoticed from an interglacial into a post-glacial portion. The limit is likely to be found between the two, about 30 cm below the mowing field. There is therefore a great stratigraphic hiatus. Pollen-analytically it could not be decided from which interglacial period the profile hails; on account of its situation on the middle terrace, it was deemed likely that it was an Eem sea deposit. The examined profile probably corresponds to Jessen & Milthers’ zone g; showing it to have been formed at the end of the Eem sea period. The Meuse therefore cannot have flowed through this part of the Astense Peel after the mid Eemean phase. Stratigraphy This is difficult to summarize. Compare various profiles. Individual mention may be made here of: 1. peat on a podsol layer; this was found in two places (Deurnse Peel I Kraaienhut and Griendtsveen VIII). Peat-formation may be thought to have occurred in the following way: heather started growing on drift-sand giving rise to a podsol layer. As the latter is impervious the vegetation surface became marshy. The heath was replaced by a Caricetum from which peat arose. Gradually more Eriophorum occurred, from which almost pure vaginatum peat arose. The bog-surface grew moister and moister, Sphagnum cuspidatum and Scheuchzeria could grow on it and formed a „Vorlaufstorf”. Only then could non-extremehydrophile Sphagna join in peat-formation. 2. the occurrence of Scheuchzeria-peat after the „Grenzhorizont” period. This species of peat, which is often found at the basis of the old Sphagnum-peat as a mesotrophic transition vegetation, has for the Netherlands only been found in the young post-glacial phase in the Peel (Deurnse Peel I Kraaienhut, Griendtsveen V and VIII and Nederweerd). At present the plant is very rare. The severe decline of this plant was also observed elsewhere. Probably it is caused by the gradual drying up or reclaiming of the raised bogs. Of the present station of Scheuchzeria near Ommen a short description has been given (p. 59 and photographs 2, 3, 4). 3. the „Grenzhorizont”. Where the young Sphagnum-peat has not been dug for the preparation of moss-litter, the Peel bogs show a clear „Grenzhorizont” (photograph 8). The conceptions about its origin have been discussed. The distinct separation between the old and the young Sphagnum-peat was not considered sufficiently explained. Though on the whole the „Grenzhorizont” is synchronous in the North-west European profiles, the point of transition from old to young Sphagnumpeat was fairly unstable and easily changeable as to time. Generally the date of the „Grenzhorizont” is fixed at about 500 A.D., though there are differences in opinion. There is a lack of archeological correlation which renders a correct dating impossible. Interference of man in the Peel Three ways of interference were stated: 1. peat has been dug off for the greater part in the territory of the Peel: young Sphagnum-peat for the preparation of moss-litter, old Sphagnum-peat for fuel. The trees which appeared when the bog was dug up in the „Veenderij der Maatschappij Griendtsveen” are sometimes in so good a condition, that they are used for building sheds. The 1 st, 2nd and 4th beam in the foreground of the shed in photo 5 has been sawn from a 30 m long subfossil pine. 2. in a native peat-digging it was possible to collect recent young Sphagnum-peat. 40 to 50 years ago the peasants living there had dug peat in holes, which were afterwards left to themselves. Sphagnum started growing again and the holes were filled in again. The diagram (fig. Griendtsveen VII) represents the surrounding heath with scattered pines and birches, sown by the wind, and a pine-plantation close by. 3. in the profiles Nieuwe Peel, Griendtsveen VI and VII it has been fixed by the indications given by Firbas, that only in the surface layers of the bog has corn-pollen occurred. So in these parts cultivation of cereals will be of recent date. This also appeared from the history of the reclamation of the said territory.