Indicators of plant species and community diversity in grasslands

Abstract

Parameters which are directly related to both land use change and biodiversity may be useful tools to indicate biodiversity in marginal landscapes. In these landscapes for about five decades abandonment of cultivation, especially in favour of extensive grassland use and succession on abandoned fields, has led to considerable changes in the landscape structure. Regions such as the Lahn-Dill Highlands (Hesse, Germany), formerly characterized by small-parcelled crop and grassland rotation, increasingly feature old grassland communities over large areas. Impacts of changes in the landscape structure on the floristic–phytocoenotic diversity have been studied in two landscape tracts of this region that today are mainly used as grassland. Reconstruction of land use dynamics based on multitemporal aerial photograph interpretations of the period from 1945 to 1997 confirm the predominance of cultivation until ca. 1960 in both areas. On the basis of phytosociological surveys in one stand abandoned 3 years before the survey and in each three grassland stands of different age classes (11–27, 28–38, 39–46 and over 46 years), the floristic–phytocoenotic diversity of these stands is characterized as follows: (i) Flora and vegetation are clearly differentiated in relation to stand age. (ii) The vegetation of the older (>38 years) stands is more comparable among one another than is the vegetation of younger stands. (iii) 19- to 33-year-old stands have the highest number of exclusive species. (iv) Old stands (>46 years) have the highest α-species richness. (v) The stands can be classified into different vegetation types in relation to age. The floristic–phytocoenotic diversity is associated with site differences of the above age classes. Older stands are more frequent at upper slopes and the pH values of their soils are lower. With a small methodological outlay, grassland stands of different age and species diversity can be differentiated by red–green–blue (RGB) colour tonal values from false-colour infrared (FCIR) aerial photographs. The results open up possibilities for the qualitative and quantitative indication of floristic–phytocoenotic diversity in grasslands on the basis of stand age, site factors and also green and blue tonal values from the respective FCIR aerial photographs. Furthermore the results indicate that it is necessary to retain old grassland stands, as well as a mosaic of extensively used grassland stands of different ages to retain plant species and community diversity in the study region.