Plantengeografische Elementen in de Nederlandsche Flora

The term “flora element” was introduced into phytogeography by H. Christ (1867) for classifying alpine plant species according to their habitat in other parts of the world as a means of facilitating historical investigation of the alpine flora. Later on (by Engler and others) the term flora element received a modified meaning including a genetical or historical indication of origin (region of origination, time or direction of immigration, etc.). M. Brockmann-Jerosch, C. Schroter, M. Rikli have sharply distinguished the different meanings in which the term flora clement had come to be used. In 1919 and 1923 J. Braun-Blanquet urged the necessity of restoring the term into its original regional-geographic signification and proposed a definition by which it would be possible to designate flora elements in an absolutely universal, objective and static way. Therefore he takes as an element such plant species and plant communities as are characteristic for a “regio” of the phytogeographic map after Flahault’s projections. As regards our country it seems impracticable for several reasons to follow Braun’s proposition: 1. The inclusion of sociological plant communities together with plant species in the same group is inappropriate. 2. The “regiones” as proposed and named by Flahault have not at all universally been accepted. 3. The regio eurosibirico-boreoamericana in the midst of which our country is situated is so enormously extensive and other regiones are so far away, that only very few of our native plants can be really characteristic for an other regio than ours. Therefore a classification of our flora on that basis can be of little use for historical research. 4. It is not to be ascertained quite objectively whether a plantspecies is characteristic for a certain region. It might even be held that a species cannot really be called characteristic for such a distant regio if it is spread unto our country here. Braun-Blanquet himself has not been able to stick constantly to his own definition. In a study of 1928: “Ueber die pflanzengeographischen Elemente Westdeutschlands” he feels compelled to avoid the term element for groups that he, as appears from the title, would like to name as such. Whilst we agree with Braun in confining the term flora element to its original geographic meaning, i.e. a group of species with the same or similar area over the world, we give up Braun’s postulation of universality to be obtained by connecting each element to a regio of a certain phytogeographic map system. Keeping in mind the purpose aimed at, viz. classification of the flora for historic geobotanical research, we come to the conviction that the elements of our native flora should be defined after the geographic position of the species area as situated in relation to our country. In this way I have discerned in the flora of the Netherlands a boreal, a mediterranean, an atlantic and a continental element. Following Wangerin’s suggestion we may find an indication for different elements in distribution peculiarities to be observed within our own region. As a definition I propose, e.g. for the boreal element: The boreal element of our flora contains the native plant-species the centre of whose area lies distinctly north of us, so that they reach or approach their southern limit here with us. As for the boreal and atlantic element, this conception will meet with little opposition because it agrees with the method usually adopted in neighbouring countries. The atlantic element may be subdivised into north atlantic, south atlantic, euatlantic, subatlantic, etc. These subdivisions must not be taken as elements, as it is preferable to have only a small number of relatively large elements. The term continental element is liable to more opposition, but can be fully accounted for. It is meant to enclose the species with an extensive distribution in the more eastern parts of the european continent and that are near to their west limit here. In this way it is rather vaguely delimited, but in no greater degree than are the other elements too. Nevertheless the west limit of continental species may naturally be less sharply drawn than the east limit of atlantic species is set by the severer climate. That the term continental element tends to include the ecological indication: adapted to continental conditions of climate and soil, is not against the accepted rules, provided that I don’t define it after the ecological adaptations, but after the geographic position of the area. Moreover the same objection applies to the atlantic and other elements. The geographic situation of our country at the west coast of Europe involves that many species find their west boundary here. These may be distinguished from really continental species by the fact that a plant in approaching the natural limit of its area will appear to be rare or at least local and bound to specially favourable localities. It is obvious that in the above way of discerning elements a great part of the native flora cannot be ascribed to either of these four flora elements because for these species our country lies near to the “middle” of their area. These might be taken as a fifth “home element” for which I propose the term “mesochorous element”,