Ecological assessment of habitat representativity
International and European environmental organizations are making serious efforts to stop the loss of biodiversity in various habitat types, including forests (European Commission, 1992; CBD, 2006; Oja, 2009). For that purpose, the Natura 2000 Network of important habitats in the EU has been established. The Natura 2000 Standard Data Form for habitat evaluation (European Commission, 1996a, 1996b, 2006) sets terminology and principles for the inventory and monitoring of habitats, including their representativity and conservation status. The major focus in interpretation manuals for fieldworkers is on the description of the habitat and its deductive comparison to a typical example (Airaksinen & Karttunen, 2001; Vicenikova & Polak, 2003; Guth & Kucera, 2005; Aunins, 2010). In the case of Estonia, the comparative description of an old-growth forest is used to determine the habitat's representativity in three- or four-level grade system (Ministry of Environment, 2002; Paal, 2002, 2007; Palo, 2004). This representativity grading system consists of four grades, from grade A (an old-growth forest, historically continuous and without management signs) to D (hardly suits under the definition of a habitat type, but has a potential to develop into one in the future).
The commonly used representativity grading for forest habitats is based on an expert opinion given in the field according to mapping manuals. In theory, the adequate evaluation and monitoring of habitat type forests require the description of various aspects of these habitats, which would provide information about their status. The majority of monitoring and forest inventory methods are based on the evaluation of the number of forest structural elements and the registration of indicator species (Noss, 1990, 1999; Lindenmayer et al., 2000; Korjus, 2002; Andersson et al., 2003; Liira & Kohv, 2004; Brang et al., 2008; Winter et al., 2008; Adermann, 2009; Lamb et al., 2009; Oja, 2009). Informative characteristics are called indicators, but they must be tested for causal correlations, universality of use, power of extrapolation, and robustness for errors in practice, and they must be easy and inexpensive to apply (Liira & Kohv, 2010). There is no single universal indicator trait that would work uniformly in all habitats, and therefore a complex of indicative traits should be considered (Jonsson & Jonsell, 1999; Buchs, 2003; Zenner, 2004; Ranius & Jonsson, 2007; Liira & Kohv, 2010; Lohmus & Kraut, 2010).
Only recently researchers in Estonia have become interested in more specific topics, such as the distinction of old near-natural forests rich in structural elements from managed ones and whether it varies along environmental gradients (Trass et al., 1999; Kohv & Liira, 2005; Liira et al., 2007; Liira & Sepp, 2009; Sepp & Liira, 2009; Liira & Kohv, 2010; Lohmus & Kraut, 2010). Increasingly more attention has been paid to responses of forest species to changes in stands (Vellak & Paal, 1999; Meier et al., 2005; Vellak & Ingerpuu, 2005; Lohmus & Lohmus, 2008; Juriado & Liira, 2009; Meier & Paal, 2009). It is also very important to consider direct and indirect anthropogenic effects (Pikk, 2003; Palo et al., 2004; Remm, 2005; Liira et al., 2007; Kaasik et al., 2008).
The first updates toward standardized methods have been made in the evaluation and monitoring of the Natura 2000 habitat type forests that focus on biological diversity in Estonia (Viilma & Palo, 2008; Adermann, 2009; Liira, 2009). However, in order to ensure methodological adequacy and suggest future improvements in the list of structural indicators, the methods need to be tested in the field for practicality and universality of use in a wide range of habitats.
The goal of our research was to test the effectiveness of widely used forest structural indicators for the characterization of the representativity grade of forests belonging to some most common Natura 2000 habitat types, to compare the conformity of the trait-based approach with the grading given by an expert, and to determine which stand characteristics are non-specific to habitat type. …