When Has an Abandoned Field Become a Semi-Natural Grassland or Heathland? Rasmus Ejrnæs Æ Jaan Liira Æ Roar S. Poulsen Æ Bettina Nygaard Received: 8 October 2007 / Accepted: 30 June 2008 / Published online: 23 August 2008 Ó Springer Science+Business Media, LLC 2008 Abstract This study presents a meta-analysis of a col- lective dataset describing the succession from abandoned fields to semi-natural grassland and heathland vegetation over the past century. The study objectives were to develop a method for statistical discrimination between abandoned fields and semi-natural habitats and to analyze the proba- bility that an abandoned field had developed into a semi- natural habitat. A statistical classification model was developed, based on lists of vascular plants from 2059 plots from Danish semi-natural grasslands and heathlands, and abandoned fields of varying age. This model was shown to discriminate effectively between abandoned fields and semi-natural habitats, and it was found to be potentially useful for the detection of abandoned fields approaching semi-natural vegetation. We suggest that the model may help clarify restoration targets and assess biological con- dition in formerly cultivated areas. Statistical modeling revealed that succession age, period of abandonment and succession trajectory had significant effects on the proba- bility that abandoned fields reached the semi-natural phase. Our study indicates that restoration projects targeting grassland and heathland should take local species pools and soil fertility into account. Keywords Artificial neural networks Á Habitat quality Á Old-field Á Restoration Á Secondary succession Á Supervised classification Introduction There is increasing focus on habitat-based conservation as a supplementary strategy to species conservation (e.g., EC Habitats Directive 1992; Danish Nature Protection Legis- lation 1997). In the European Union, the Habitats Directive combines these two approaches. The protection of habitats after the directive implies their monitoring, assessment of their conservation status, and efforts to achieve a favorable conservation status (EC Habitats Directive 1992). Past habitat loss may be compensated by, e.g., reclamation of arable land or improved pastures. A proper definition and discrimination of target habitats is therefore indispensable for the implementation of habitat protection programs (e.g., Ejrnæs and others 2004). The intention of the Habitats Directive (EC Habitats Directive 1992) is to protect natural habitats of Community interest, but although the Annex I of protected habitats builds on an extensive classification of habitats (Anon 1991), it does not specify how to discriminate between protected and nonprotected habitats along the continuum from natural to cultural. And there is not much help from the scientific literature here: although terms, such as natural or semi-natural vegetation are used frequently in the lit- erature, commonly agreed definitions are hard to find. According to Westhoff (1983), the term semi-natural is Nomenclature: Species names follow Flora Europaea (Tutin and others 1964–1993). R. Ejrnæs (&) Á B. Nygaard Department of Wildlife Ecology and Biodiversity, National Environmental Research Institute, University of Aarhus, Grenaavej 12, DK-8410 Roende, Denmark e-mail: rej@dmu.dk J. Liira Institute of Botany and Ecology, University of Tartu, Lai 40, Tartu EE-51005, Estonia R. S. Poulsen Technical and Environmental Department, Municipality of Aalborg, Boulevarden 13, DK-9100 Aalborg, Denmark 123 Environmental Management (2008) 42:707–716 DOI 10.1007/s00267-008-9183-6