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| Registros recuperados: 16 | |
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| Janssen, C.R.. |
| Single vegetation plots in forests and bogs in the deciduous and coniferoushardwood formation in northwestern Minnesota were studied according to the field methods of the Zürich-Montpellier system. Plots and species were arranged so that the greatest coincidence of occurrences was obtained. Species that show approximately similar amplitudes were united in sociological groups. Plant communities typified by these groups were placed in a hierarchy. The plant communities appear to intergrade but are not completely continuous. Definite concentrations of occurrences remain present in the ordination. Fourteen main types of vegetation are recognized. Their relations to the environment are discussed, and their relations to vegetation in other regions are indicated.... |
| Tipo: Article / Letter to the editor |
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| Ano: 1967 |
URL: http://www.repository.naturalis.nl/record/534710 |
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| Janssen, C.R.. |
| This year Prof. Dr. F.P. Jonker, Frits as he is known among his friends, will retire from the formal academic life at the State University of Utrecht: a long and busy life of 49 years, devoted to teaching, administration, and scientific research. Looking back on all these years, one realises the important contributions that Jonker has made to botanical science in general and to palaeobotany in particular, both in The Netherlands and abroad, as well as the impact he has exerted on his surroundings, culminating in the vigorous activities of the Laboratory of Palaeobotany and Palynology at Utrecht. To describe Jonker’s life history is indeed to describe the history of his laboratory. To understand the significance of Jonker and the character of the “lab”, we... |
| Tipo: Article / Letter to the editor |
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| Ano: 1978 |
URL: http://www.repository.naturalis.nl/record/534719 |
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| Janssen, C.R.. |
| It is commonly accepted that percentages of pollen in a pollen diagram do not express the exact composition of forests in earlier times. This inaccuracy is due to several factors, for instance the different quantities of pollen produced by plants, the distance of transport etc. A pollen diagram tells us only the change in pollen rain on the locality where we collected soil samples. In studying a pollen diagram we find a close relation between the variations in the percentages of a certain species and the area occupied by this species in the vegetation. When the percentage of pollen of a species increases, we conclude generally that the relative area occupied by this species in the vegetation increases too. However, such a connection might be doubted. The... |
| Tipo: Article / Letter to the editor |
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| Ano: 1959 |
URL: http://www.repository.naturalis.nl/record/535044 |
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| Janssen, C.R.. |
| A pollen diagram from a lake in the former bed of the eastern arm of Lake Agassiz in northern Minnesota records a vegetation of spruce forest followed by immigration successively of Pinus banksiana and (or) P. resinosa at 10 000 B.P., then Abies and Pteridium, and still later Alnus. Between 8000 and 7000 B.P. prairie and (or) Quercus savanna prevailed on the uplands, followed by deciduous forests of mainly Quercus, Ostrya virginiana, Fraxinus pennsylvanica, and possibly Populus sp. Slightly later, Pinus strobus migrated into the area, resulting in a gradual decline of pollen of deciduous forest types. Betula pollen, however, rises, and there is an indication of a return to prairie conditions prior to 3000 B.P. During the 8000-7000 B.P. dry interval the... |
| Tipo: Article / Letter to the editor |
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| Ano: 1969 |
URL: http://www.repository.naturalis.nl/record/535048 |
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| Janssen, C.R.. |
| The present study deals with the late-glacial and post-glacial development of the vegetation in the loess region of South Limburg (Netherlands). In the lateglacial time the continental element in the vegetation (Artemisia) is very pronounced, probably due to the particular soil conditions in South Limburg. The zonation of FIRBAS (1949) has been applied to the South Limburg diagrams. Zone I: Tundra, a temporary amelioration of the climate (Bölling time) can be observed. Zone II: Closed pine forests present (Alleröd time). Zone III: In the lower part of the zone Pinus dominant, afterwards Belula. Zone IV: Decrease of the percentages of the herbs; Pinus dominant in the vegetation; first appearance of Corylus. Zone V: Pinus and Corylus dominant; in the lower... |
| Tipo: Article / Letter to the editor |
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| Ano: 1960 |
URL: http://www.repository.naturalis.nl/record/534940 |
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| Janssen, C.R.. |
| In recent years pollen analysis has turned increasingly from historical plant geography towards paleoecology. More and more the main interest lies in a reconstruction of the past vegetation instead of simply floristics of a region. Vegetation as a rule is made up of communities¹) that can be described quantitatively in terms of species composition or qualitatively in terms of structure. |
| Tipo: Article / Letter to the editor |
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| Ano: 1970 |
URL: http://www.repository.naturalis.nl/record/535170 |
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| Janssen, C.R.. |
| Pollen samples were taken along nine transects across local vegetational belts bordering bogs or ponds in overall deciduous and coniferous-deciduous forest regions. Three types of pollen rain are distinguished: local, extralocal, and regional. Local pollen rain is derived from plants that grow at or very close to the sampling point. High local values are common in all sorts of vegetation types: lakes, bogs, marshes, and forests. Extralocal pollen rain is derived largely from trees that grow on the slopes and upland adjacent to the sampling site, but not extensively over larger areas. When excluded from the pollen sum along with the local types, its curves increase slightly, but noticeably, along the transect as the source of dispersal is approached. The... |
| Tipo: Article / Letter to the editor |
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| Ano: 1966 |
URL: http://www.repository.naturalis.nl/record/535204 |
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| Janssen, C.R.; Hove, H.A. ten. |
| Three pollen diagrams from the Peel peatlands, a raised bog area in the southern part of The Netherlands reveal the post-Boreal vegetation history of that region. There are two or three land-occupation phases in the Neolithic and Bronze ages, that show but low values of terrestrial herbs. They are negatively correlated with Ulmus. The Fagus curve starts slightly below the first occupation phase at around 2500 B.C. In the Subatlantic there is an Iron age and a Medieval occupation phase with higher values of terrestrial herbs. Fagus shows two maxima, one at the beginning of our era and another at around 700 A.D. The trend of the pollen curves for bog species is discussed in relation to bog development. |
| Tipo: Article / Letter to the editor |
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| Ano: 1971 |
URL: http://www.repository.naturalis.nl/record/534844 |
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| Janssen, C.R.. |
| The pollen assemblages of a core in the coniferhardwood formation in northwestern Minnesota are compared with the floristics of the recent vegetation in the region. Percentage levels of the main tree components have been compared first with those from recent surface samples taken at the same short distance from various types of upland forests and second with the regional values of the pollen rain in this area (McAndrews 1966). To that end all the data were recalculated on the basis of special pollen sums. The regional diagram of Stevens Pond shows basically the same assemblage zones as established by McAndrews but without the late-glacial Picea-Populus assemblage zone. The pollen in the following Pinus-Pteridium assemblage zone has been interpreted as... |
| Tipo: Article / Letter to the editor |
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| Ano: 1967 |
URL: http://www.repository.naturalis.nl/record/534791 |
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| Registros recuperados: 16 | |
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