INTRODUCTION

Ecological restoration of natural forests on former agricultural land or on sites formerly planted with non-indigenous tree species has become a central component of sustainable forest management in many parts of the world (Angelstam et al. 2004; Hérault et al. 2005). To become operational at the stand or the landscape level, the principles of sustainable forest management need to be broken down into objectives and in indicators to monitor.

To define the ecological objectives of restoration efforts, a reference or target system is essential. This information generally consists of contemporary ecological data from a reference site (Bakker et al. 2000). In the case of forest restoration, the reference may consist of ancient forests (Honnay et al. 2002). Ancient forest sites are commonly defined as sites that have been continuously wooded since a reference date in the past, and it is generally accepted that this continuity has not been broken by forest management practices such as coppicing but only by an alternative land use such as cultivation or pasture (Peterken 1996). The reference date varies between regions and countries and, very pragmatically, reflects the availability of the first detailed land-use maps (e.g. 1600, Peterken (1974); 1700, Rackham (1980); 1789, Lawesson et al. (1998); 1850, Grashof-Bokdam (1997)).

After setting the objectives, indicators can be measured repeatedly to examine whether they directionally change towards the objectives and to evaluate to what extent the objectives have been reached.