Open-grown trees as key habitats for arthropods in temperate woodlands: The diversity, composition, and conservation value of associated communities
Introduction
Open temperate woodlands host rich communities of plants and animals, including many endangered organisms (Bengtsson et al., 2000, Benes et al., 2006, Spitzer et al., 2008, Bergmeier et al., 2010, Hédl et al., 2010, Bugalho et al., 2011, Horak et al., 2014, Ramírez-Hernández et al., 2014). In Europe, they support a high biodiversity of tree-associated organisms. Evidence that a large proportion of European forests naturally occurred with open canopies for most of the Holocene has substantially increased in recent years (Vera, 2000, Whitehouse and Smith, 2004, Alexander, 2005, Birks, 2005). The open structure of these forests was formerly maintained by disturbances caused by fires or windthrows (Niklasson et al., 2010, Adámek et al., 2015, Hultberg et al., 2015), and by the grazing of large herbivores (Bengtsson et al., 2000, Vera, 2000). Since the mid-Mesolithic, these natural processes have been supplemented and later substituted by various human activities with a strong impact on woodland habitats. Slash-and-burn practices and later various silvopastoral management practices such as wood-pasturing and coppicing (Rackham, 1998, Szabó, 2009) have sustained the open structure of many European woodlands.
The situation changed in the 18th century with the industrial and agricultural revolutions. Intensification of forestry management together with abandonment of traditional silvicultural practices led to a substantial increase in canopy closure, and consequently to a transition from open woodlands to closed-canopy mesic forests (Bürgi, 1999, Hédl et al., 2010, Kopecký et al., 2013). Today, open woodlands are scarce, fragmented, and still declining (Miklín and Čížek, 2014, Varga et al., 2015). Yet these open woodlands still host a rich, specialised community of arboricolous invertebrates (Dolek et al., 2009, Horak et al., 2014, Ramírez-Hernández et al., 2014).
A typical characteristic of open woodlands is the presence of scattered, open-grown, often large and old trees. Such trees are considered keystone ecological features for biodiversity in various temperate and boreal regions (Read, 2000, Manning et al., 2006, Fischer et al., 2010, Hall and Bunce, 2011, Lindenmayer et al., 2012, Lindenmayer et al., 2014, Siitonen and Ranius, 2015). Although these trees are referred to by various synonyms, such as isolated trees, dispersed trees, pasture trees, paddock trees, and remnant trees (see Manning et al., 2006), here, we call them collectively “solitary trees”, meaning trees with well-developed and separated tree crowns, growing in isolation from closed-canopy forests. Wide-crowned trees can only develop in open conditions, which were formerly common in wood-pastures (Plieninger et al., 2015a, Hartel et al., 2013), traditional fruit or chestnut orchards (Horak et al., 2013, Plieninger et al., 2015b), noblemen’s hunting parks (Fletcher, 2015), and in coppice with standards woods (Altman et al., 2013). Today, solitary trees occur in remnants of these habitats (Varga et al., 2015, Plieninger et al., 2015a), as well as in game reserves, parks, and tree alleys (Horak et al., 2014, Jonsell, 2011).
Solitary trees in wood-pastures are important breeding sites for birds because they often develop hollows (Hartel et al., 2014). Horak et al. (2014) found solitary trees were particularly attractive for saproxylic beetles since deadwood exposed to the sun is warmer, which enhances larval development. On the other hand, some groups of organisms, such as fungi or lichens, were found to be similarly rich at the edges of closed canopy forests (Horak et al., 2014). Moreover, in temperate forests the richness of arthropods often depends not only on the horizontal openness of the stand, but also on vertical stratification (Floren and Schmidl, 2008, Ulyshen, 2011), which can result in significant differences in communities between canopy and understorey strata. Therefore, regarding the conservation of woodland biodiversity, the question is to what extent forest edges or forest canopies can substitute for the role of fully open-grown trees. There is a need to explore communities from these habitats separately and then to compare them with solitary trees.
The aim of this study is to examine the ecological role of open-grown, solitary trees in maintaining temperate woodland biodiversity in comparison with closed-canopy forests. We compare the species density, composition, and conservation value of arthropod communities found on solitary trees with the communities found in the canopy and understorey at the forest edge and in the forest interior. We focused on four arthropod groups with a wide range of life-histories: beetles, bees and wasps, ants, and spiders.
Section snippets
Study area
This study was conducted in alluvial woodlands in South Moravia, Czech Republic (48°45′-48°50′N, 16°45′-16°55′E, alt. 160–170 m a.s.l.), within the floodplain of the lower Dyje (Thaya) river. The flat landscape is composed of managed hardwood forests and meadows with old solitary trees. The prevailing trees are pedunculate oak (Quercus robur), narrowleaf ash (Fraxinus angustifolia), hornbeam (Carpinus betulus), field maple (Acer campestre), limes (Tilia cordata, T. platyphyllos), European white
Results
In total, we sampled and identified 9492 arthropods. We recorded 349 species of beetles (7515 individuals), 116 species of bees and wasps (805 individuals), 18 species of ants (839 individuals), and 53 species of spiders (333 individuals); see Table 1 for details on threatened species. Of the beetles, 89% of species were obligate or facultative saproxylics, while the remaining 11% were species that occasionally occur on trees. Among the bees and wasps, the ratio was 74% saproxylics to 26%
Discussion
Solitary trees, followed by forest edges, were the most important habitats for maintaining a high diversity of tree-associated species in all four focal arthropod groups. Not only did they support the richest assemblages, but also many threatened species. Below, we discuss the relative importance of each of the studied habitats to our focal taxa, and the likely causes of the observed patterns, limitations of our study, and the conservation implications of our findings.
Flight interception traps
Acknowledgements
This study and its authors were supported by the Czech Science Foundation (P504/12/1952), and the Grant Agency of the University of South Bohemia (168/2013/P and 044/2013/P). We would like to thank B. Büche, J. Háva, D. Hauck, Z. Kejval, J. Mertlík, P. Průdek, L. Spitzer, R. Stejskal, J. Straka, P. Tyrner and J. Vávra for identification and/or revision of the arthropod material. Carl Wardhaugh for reviewing the manuscript and English revision. Židlochovice Forest Enterprise, a division of the
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