Despite hollow trees being valuable for biodiversity, urban hollow trees have received less attention. Hollow lime trees in old manor parks in southern Sweden host as many specialist and red-listed saproxylic beetle species as similar trees in open wood pastures or overgrown former wood pastures (Jonsell 2012). However, man-made parks with mainly planted trees can also be important if they contain certain natural elements, if they are located close to potential source areas or if they are old enough (Siitonen 2012b). In Britain, saproxylic beetle fauna in landscape parks that include relic trees are strikingly richer than fauna in parks without such continuity (Harding and Alexander 1994 Alexander 1998). 2010) have shown that rare and threatened species frequently occur in such habitats. Surveys of saproxylic beetles and dipterans in old parks in Central Europe (Franc 1997), Britain (Denton and Chandler 2005) and Northern Europe (Andersson 1999 Jonsell 2004, 2012 Sverdrup-Thygeson et al. Some of these trees are sufficiently old to contain hollows. Even more importantly, most major cities maintain large tree populations (in the order of tens to hundreds of thousands of trees per city) by actively planting and tending trees in parks and along streets (Nowak et al. As urban areas continue to expand, valuable habitat patches containing old trees may become surrounded by built-up environments. Mature hollow trees can also be found in cities. three times greater than in the forests (Lõhmus and Liira 2013). ![]() For example, the basal area of large (> 40 cm) trees is approximately three times greater in old manor parks in Estonia compared to mature forest patches in the surrounding landscape, and the proportion of broadleaved trees ( Tilia spp., Quercus robur and Fraxinus excelsior) is also ca. In most regions of Europe, old hollow trees occur mainly in various semi-natural habitats created and maintained by human activity, including pasture woodlands, hedgerows, avenues and parks (Oleksa et al. Numerous cavity-dwelling saproxylic species have become threatened (Speight 1989 Nieto and Alexander 2010) because the number of large old trees has decreased and continues to decline, both in Europe and globally (Gibbons et al. Cavity formation and hollowing are normal life cycle stages for nearly all long-lived deciduous trees such as oak ( Quercus spp.) and lime ( Tilia spp.) (Schwarze et al. Decaying wood regularly occurs in mature living trees as heartwood decay caused by heart rot fungi. Many are strict habitat specialists living only in tree cavities (Speight 1989 Siitonen 2012a). Hollow trees are an important habitat for a large number of saproxylic invertebrates. We conclude that saproxylic beetle fauna living in hollow urban trees can be efficiently sampled with small trunk window traps or containers placed on the inner walls of hollows. ![]() Trunk windows were the most efficient trap type also in terms of saproxylic species and individuals per handling time. The time increased with increasing sample weight, i.e. Time spent separating insects from samples was the most laborious work stage. However, pitfall traps caught certain species more efficiently than the other two trap types. Trunk window and aluminium foil traps had similar assemblage and trapping efficiency, and were significantly more efficient than pitfall traps. The traps caught a total of 4004 saproxylic beetle individuals belonging to 131 species. ![]() A total of 30 traps of each trap type were set in 15 trees. We compared the species richness and species composition of saproxylic beetle assemblages between trunk window, aluminium foil tray and pitfall traps placed inside hollow trees in the Helsinki metropolitan area, Finland. The aim of our study was to find an efficient trap type for sampling saproxylic beetles in hollow urban trees. Sampling in urban areas includes the risk of trap failure due to human interference, which needs to be considered when designing sampling. Large old trees occur frequently in cities, but the saproxylic fauna inhabiting these trees has been poorly studied. Hollow trees are an important habitat for a large number of saproxylic invertebrates, many of which are rare or threatened.
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