Een vegetatiewaardering van het stroomdallandschap van het Merkske (N.-Br.), gebaseerd op een floristische inventarisatie

The vegetation of the valley of the rivulet Merkske; an evaluation based on a floristic investigation The valley of the meandering rivulet Merkske (in the province of Noord-Brabant, Netherlands, and the province of Antwerp, Belgium) is menaced with deterioration by the intended canalization of the main part of the rivulet (fig. 1 and 2). A new method is described to evaluate the vegetation of this landscape, based on a floristic inventory of all species of vascular plants in each square kilometre in the investigated area. This method is inspired by the Flora statistics proposed bij VAN DER MAAREL (1971). He classified the Dutch species of vascular plants in 19 sociological-ecological groups and in 9 frequency categories (from 1 = very rare, to 9 = very common). For each sociological-ecological group the floristic value (Wg) can be determined by using the formula given on p. 163. All floristic values taken together, determine the floristic value of a square kilometre (Wf). The meaning of the other symbols in the formula is as follows: a1 = number of species found in a square kilometre in frequency category i of a sociological-ecological group; fi=the frequency category (1-9); ni=number of species found in the Netherlands in frequency category i of a sociologicalecological group. The vegetation value of a sociological-ecological group can be determined by multiplication of the floristic value of the group with a certain factor. The list of multiplication factors (see p. 166) has been drawn up by the Nijmegian phytosociologist Van der Maarel. The multiplication factor of each sociological-ecological group depends on the vulnerability with regard to influences such as environmental disturbances, and to the rareness by the group in Europe. The vegetation value of a square kilometre is determined by adding the values for all groups together. This method can also been tested in othei landscape types in the Netherlands. Remarkably, none of the 180 square kilometres compared shows a higher combination of the floristic value and the vegetation value than 7 of the 24 square kilometres in the valley of the rivulet Merkske (fig.8 and 10): over 150 and over 650 respectively, demonstrating the great botanical significance of the most important parts of the vegetation of the Merkske valley, when considered nationally. Nature conservancy is therefore urged to conserve the best parts of the vegetation in this valley. In the eastern part, where the rivulet has already been canalized, the vegetation value is considerably lower (fig. 8,c), indicating how canalization of the western part of the rivulet will affect the vegetation in the western part. Finally the method introduced is discussed in general. Not all problems met with during the investigations have been solved finally. It is for instance rather difficult to evaluate properly the importance of a square kilometre with only heath or outer-dune vegetations, both poor in species. Therefore provisionally, floristic and vegetation values are marked with an asterisk in case the square kilometres fulfill the following requirements: 1) the number of sociological-ecological groups is smaller than 10; 2) the number of species is smaller than 100; 3) the floristic value is higher than the number of plant species recorded. Advantages of the method are: the possibility to gather data on flora and vegetation of a certain area in a short time, and to give an exact and objective numerical evaluation of their value, which can also be used by non-biologists because of the compatibility of the values on the national level. The method should also have its uses in other countries, e.g. Great-Britain and Scandinavia. In scientific respect this method of investigation must not be the final research in the area under study, but the results should be the starting point for plant ecological, landscape-ecological, and phytosociological research.