| Summary: | Broadleaf forests are critical habitats for biodiversity and this biodiversity is in turn essential for their proper functioning. Mites (Acari) are a numerous and functionally essential component of these forests. We report the diversity of two important groups, Oribatida and Mesostigmata, in a broadleaf forest in Eastern Norway which is considered to be a biodiversity hotspot. Eighteen samples, each 500 cm<sup>3</sup>, were collected from diverse microhabitats (moss on ground, lichens on tree twigs lying on ground, moss on tree trunks at ground level, moss on tree trunks 1.5 m above ground, moss on decaying stump, moss on decaying log, and decaying wood from trees) from which 10,843 specimens and 95 species from 32 families of Oribatida, and 655 specimens of 34 species from 14 families of Mesostigmata were found. Only 30% of the species were previously recorded in broadleaf forests in Western Norway. Oribatid communities on decaying stump and in lichens were distinct from the other communities, while mesostigmatid communities on tree trunks (both at ground level and 1.5 m above ground) and in lichens differed most from other communities. Over 30% of the species were found in only a single microhabitat. Twenty-three species and the genus <i>Zerconopsis</i> are reported from Norway for the first time. Six records are also new to Fennoscandia, including (Oribatida) <i>Coronoquadroppia monstruosa</i>, <i>Eueremaeus valkanovi</i>, <i>Ramusella furcata</i>, and (Mesostigmata) <i>D</i><i>endrolaelaps rectus</i>, <i>D</i>. <i>multidentatus</i>, and <i>D</i>. <i>tenuipilus</i>. In addition, several rare species were detected, e.g., <i>Achipteria magna</i>, <i>Oribotritia berlesei</i>, and <i>Subiasella quadrimaculata</i>, and two were found in their northernmost locality (<i>O</i>. <i>berlesei</i>, <i>E</i>. <i>valkanovi</i>). These results confirm the unique character and high conservation value of the studied forest in Norway, Fennoscandia and at a European scale.
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