Land ownership affects diversity and abundance of tree microhabitats in deciduous temperate forests
Introduction
Forest management drives forest biodiversity (Putz et al., 2000). Negative consequences of forestry affect many species (e.g. Brunet et al., 2010, Lonsdale et al., 2008). This may in turn reduce quality of life because human well-being and biodiversity are closely related (Cardinale et al., 2012). On the other side, wood is needed as a regrowing resource for a wide range of human purposes. Against the backdrop of this conflict of interests, extended knowledge of the factors which influence forest biodiversity is required. One crucial link between forest management and forest biodiversity are tree microhabitats (TMHs). They are forest features which are strongly regulated by forest management and at the same time are a determining factor for forest biodiversity (Regnery et al., 2013a, Winter et al., 2005).
TMHs are referred to in the relevant literature as ‘special tree structures’ (Winter et al., 2005), ‘tree microhabitat structures’ (Michel and Winter, 2009), or ‘tree microhabitats’ (Larrieu and Cabanettes, 2012, Larrieu et al., 2012, Regnery et al., 2013a, Vuidot et al., 2011, Winter and Möller, 2008). TMHs such as tree holes, are determining factors of forest biodiversity (Michel and Winter, 2009). In France, 41% of forest-related birds are tree hole dwellers (Blondel, 2005). More than half of Germany’s bat species use tree roosts (Dietz, 2013). Decay holes create TMHs for rare epiphytic lichens and mosses (Fritz and Heilmann-Clausen, 2010). Many xylophilous insects depend on TMHs because of their specialization on dead wood, lignicolous fungi, mould or sap-runs. For other insect species, the tree nests of birds, bats or social insects are essential habitats (Köhler, 2000).
Diversity and density of TMHs are key attributes of biodiversity and are particularly present in old-growth forests. A higher number of species occur where TMHs are more abundant (Winter et al., 2005, Regnery et al., 2013a). TMHs have therefore been proposed as indicators for measuring forest ecosystem biodiversity (Michel and Winter, 2009). In Germany, forests are the second most widespread land cover type after agriculture (Bolte and Polley, 2006) and cover 31% of the land area (BMELV, 2005). Prior to human logging activities, 85–95% of the land surface had been covered by forest ecosystems (Korneck et al., 1998). Hence, forest structures and tree characteristics such as TMHs are of crucial significance for the conservation of large-scale biodiversity. National policy agendas have taken biodiversity requirements for forest management into account (e.g. BMUB, 2015).
The impact of land ownership on biodiversity has been revealed in previous studies, e.g. by Lovett-Doust et al. (2003), who revealed significantly greater rare-species richness on publicly owned land in comparison to private land in Canada. Schmithüsen and Hirsch (2010) summarised forest ownership data from 23 European countries and calculated that publicly owned forest lands represent 50.1% of all forests. Roughly 20% of German forests are state owned, 33% are owned by municipalities or other public bodies like churches and trusts and 47% are privately owned. Fifty-seven percent of the private forests in Germany are smaller than 20 ha (Depenheuer and Möhring, 2010). Private forest owners of large and medium-scale forests frequently have strong economic interests (Pickenpack, 2004). In contrast, small-scale forest owners vary more in their management objectives and are increasingly driven by recreation and nature conservation motives (Härdter, 2004). Small-scale private forests are frequently managed very extensively for firewood cutting or remain unexploited (Bieling, 2004).
In Germany and other European countries, public forests have been legally declared a public welfare good, which must be managed in a way that supports a multitude of goals, for example, maintaining recreation functions or serving as a model for biodiversity conservation (Schaich, 2013). However, public forest management has developed on the basis of a centuries-old tradition of timber production orientated management (Pistorius et al., 2012), which continues to shape forestry in the present (Schaich and Plieninger, 2013).
The influence of forest management on TMH density or diversity has rarely been studied (e.g. Larrieu et al., 2012, Vuidot et al., 2011, Winter et al., 2005). At the stand level, the time span since the last cutting has been suggested as a predictor of TMH density. 90 years old stands had almost 13 times higher TMH density than up to 30 years old stands (Regnery et al., 2013b). The relationship between forest ownership and TMH has, to our knowledge, not yet been studied. Information on the relationship between ownership, management and TMH provisioning is needed for the design of effective policies which aim to halt the loss of biodiversity or to promote more sustainable use of forest resources; especially if such policy frameworks are to be applicable to landscapes with different land ownership types.
We investigated the occurrence of tree microhabitats in adjacent broadleaf forest stands under three different forest ownership types – small-scale private forest, municipal forest and state forest – in the Swabian Alb Biosphere Reserve located in south-western Germany. Our main objective was to quantify to which extend forests under these distinct ownership types vary in terms of the density and composition of TMHs. Schaich and Plieninger (2013) found in the same region differing basal areas and differing diameter distribution when comparing forests of the three ownership types. Based on those findings we hypothesised: (1) The occurrence of TMHs on trees is influenced by the management approaches applied under different forms of forest ownership; (2) small-scale private forests in the Swabian Alb provide more TMHs than the publicly managed forests in the region; and (3) the diversity of TMHs is higher in small-scale private forests.
Section snippets
The study region Swabian Alb Biosphere Reserve
We collected data in the Swabian Alb Biosphere Reserve which is located in south-western Germany about 50 km south-east of Stuttgart (9°45′43–9°07′07 easting, 48°37′49–48°12′30 northing). It covers an area of about 85,300 ha and encompasses the lands of 29 municipalities which are subsumed within three administrative districts (Ministerium für Entwicklung und Ländlichen Raum, 2008). The region is predominantly rural and in 2005 the Biosphere Reserve had about 151,000 inhabitants (Ministerium für
TMHs on the tree scale
The survey recorded a total of 1569 trees and a total of 1800 TMHs. The average number of different TMHs per tree is 0.98 (SD = 1.19) in state-owned forests, 1.39 (SD = 1.45) in small-scale private forests and 0.92 (SD = 1.18) in municipal forests. An ANOVA revealed significant (p < 0.05) differences at p < 0.001 for the three different types of ownership. Subsequent Tukey post hoc tests found no significance in the comparison between state forests and municipal forests (p = 0.73), whereas comparing
Main findings
We analysed the indirect impact of forest ownership type on TMH occurrences and compared the importance of land ownership with that of biotic factors and abiotic landscape variables. Our main result is that forest ownership type has a strong effect on density and diversity of TMHs.
The results of the study support our three hypotheses. Small-scale private forests provide more TMHs per forest area, a higher TMH diversity and more TMHs per tree in comparison to municipal forests and state-owned
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