After a consideration of the policy framework for the development of monitoring and indicators of biodiversity in Europe, the major biogeographic influences on biodiversity and the major key factors (or drivers) of biodiversity, plants were identified as a potential indicator of biodiversity.
Data on the diversity of plants were collected in 1km{2} sites, or �land use units� in eight European countries - Portugal, Spain, France, Switzerland, Hungary, Ireland, Finland and the UK.
Six sites were established along land use intensity gradients. Data were obtained by a standardised protocol and quantified the impacts of land use intensity and biogeography.
After statistical evaluation with data on other components of biodiversity, plants were found to be a potentially useful indicator of biodiversity, a significant predictor of the richness of birds, butterflies, and lichens but not a good indicator of soil biodiversity. Plants have a number of other advantages and disadvantages as indicators of biodiversity.
Plant sampling for biodiversity monitoring is cost-efficient and accurate, although it has some seasonal constraints for certain plant groups and regions. The structural component of vegetation showed the more direct responses to land use gradients and is also related to plant diversity itself and to other components of biodiversity like e.g. birds.
Total vascular plant diversity was consistently associated to habitat heterogeneity, and the different components of the land-use gradient were complementary in their contributions to plant species richness. But different patterns were found for plant species according to their life forms. Herbaceous plants, that account for the main part of plant diversity, reached their maximum values of species richness and diversity in mixed landscapes. Woody plants, being a minor component of the European flora but playing a major role in structuring habitats, showed peaks of richness in forested landscapes, and their diversity was strongly related to the vertical complexity of the vegetation. Hence the potential of plants as indicators of biodiversity requires the consideration of their structural and compositional components.