Florastatistieken als bijdrage tot de evaluatie van natuurgebieden

Flora statistics as a contribution to the evaluation of nature areas Flora statistics of an area are defined as a quantitatively elaborated species list of that area. Important aspects are: the total number of species as compared with the average number on an area of equal size, a rarity index and a plant sociological spectrum of the flora. Quantitative evaluation of nature areas, i.e. areas comprising natural or/and semi-natural biotic communities, based on diversity studies by PRESTON (1962) and WILLIAMS (1964) as well as plant geographical and sociological studies by WESTHOFF (1956, 1958), is now being further developed at the Nijmegen Geobotany Section. Detailed data on the distribution of vascular plant species from 1900 onwards are available GOETHART & JONGMANS, 1900—1910, I.V.O.N. = Institute for Vegetation Research, 1910—1950 and now Rijksherbarium, Leiden). A ‘1900’ and a ‘1970’ picture have been composed from those data. For general use of evaluation techniques a standard-list for the Dutch flora will be necessary. It is proposed to include most of the subspecies, minor species and neophytes in this list. All colleagues are invited to give their comments on a stencilled draft-list, which will be sent on request. Species frequency, measured on the basis of a grid of over 1600 ‘hour-squares’, measuring 5 X 25/6sq km, is classified according to a logarithmic frequency class division in which both the number of hour-squares and the average number of plant units per square are taken into account. Table 1 gives the interval boundaries, and corresponding verbal descriptions ranging from extremely rare to extremely common. In this way a lognormal frequency distribution is reached for the 1900 situation, see fig. 1. The 1970 situation shows a considerable shift towards the left. With the help of these data the average number of species per hour-square could be estimated: 250 species in 1900, 180 species in 1970. The species-area relation for the Netherlands given by ADRIANI & VAN DER MAAREL (1968) only needs a small correction. The decline in the number of hour-squares of all rare species (categories 1, 2, 3) from 1900 to 1970 by 80 % as calculated earlier could be confirmed. To refine conclusions on the biological value of an area a plant-sociological spectrum is necessary. Table 2 gives a division, based on earlier attempts, into 19 sociological-ecological species groups, each of which being defined by one or more units from the survey of plant communities of the Netherlands by WESTHOFF & DEN HELD (1969). In the stencilled draft-list socio-ecological group and frequency category in 1900 and 1970 are presented for each species. Table 3 gives the flora-statistics concerning the above mentioned data.