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 cm³, were collected from diverse microhabitats (moss on ground, lichens on tree twigs lying on ground, moss on tree trunk at ground level and 1.5 m above ground, moss on decaying wood, log and stump, and decaying wood) 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 tree stumps and in lichens were distinct from the other communities, while mesostigmatid communities on tree trunks (both at ground level and 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 Zerconopsis are reported from Norway for the first time. Six records are also new to Fennoscandia, including (Oribatida) Quadroppia monstruosa, Eueremaeus valkanovi, Ramusella furcata, and (Mesostigmata) Dendrolaelaps rectus, D. multidentatus, and D. tenuipilus. In addition, several rare species were detected, e.g., Achipteria magna, Oribotritia berlesei, and Subiasella quadrimaculata, and two were found in their northernmost locality (O. berlesei, E. valkanovi). These results confirm the unique character and high conservation value of the studied forest in Norway, Fennoscandia and at a European scale.

2.1. Study Site

Samples were collected in a plant-rich broadleaf forest located in Kjeøya (59.093° N 11.222° E, 120 m a.s.l.), a peninsula in Viken province, Halden municipality, in Eastern Norway (Figure 1). The study site was characterized by an oceanic climate, with mean annual temperature 6.4 °C and annual precipitation 820 mm [38]. Summer is relatively mild with average temperatures between 16.0–16.7 °C in July and August. In the coldest months (January and February) the average temperatures are between −2.9 °C and −3.8 °C. The vegetation zone is Boreonemoral and slightly oceanic sensu [39]. The bedrock is composed mainly of different gneiss and granite rocks. 

The forest (Figure 2) has an area of 2.32 ha and is considered an important habitat, since many rare species, mainly fungi, have been detected there [40]. Forest was characterized by old and large oak (mostly common oak, Quercus robur L.), hollow oaks of 100 years and older (>50 cm dbh) [41], and small-leaved lime (Tilia cordata Mill.) trees. Other tree species were hazel (Corylus avellana L.), common ash (Fraxinus excelsior L.), Norway maple (Acer platanoides L.), and some additional Norway spruce [Picea abies (L.) H.Karst] and Scots pine (Pinus sylvestris L.). The herb vegetation was partly sparse.

2.2. Sampling and Identification

In total, 18 samples, each of a volume of 500 cm³, were collected on 12 June 2017 from several microhabitats: (1) mosses on ground (four samples), (2) lichens on tree twigs lying on ground (three samples), (3) mosses on tree trunks on ground level (three samples), (4) mosses on tree trunks 1.5 m above ground (two samples), (5) mosses on stump (one sample), (6) mosses on decaying wood (three samples), and (7) decaying wood (two samples).

Arthropods were extracted using modified Tullgren funnels for 14 days into 90% ethanol and sorted out from the samples under stereomicroscope. Oribatida were mounted on temporary slides with cavity in lactic acid and adult specimens were identified using the keys of [42–45], while juveniles were identified based on [46–61]. The nomenclature of oribatid species follows [62, 63] and partly [45,57,58]. Mesostigmata were mounted on permanent slides in PVA mounting medium (Lactic Acid, Poly Vinyl Acetate and Phenol Solution, BioQuip Products, Inc., Compton, CA, USA) and identified following [64–84]. Full names of species are given in Table 1 while in other tables and figures abbreviations are used.  Specimens representing all species are deposited at the University Museum of Bergen, Norway. Information on other mite groups that were sorted out from the samples will be published later.

Habitat preferences (Table 1) of oribatid mites are based on [45,85–87], and those of Mesostigmata on [67,69,70,72,73,75–78,80–82,88–91]. These habitats include the following types: aquatic (reproduction and all stages of life cycle in water or at its margins), hygrophilous (living in wet places), mesohygrophilous (preferring high moisture but not wet places), xerophilous (living in dry places), arboricolous (living on trees), lichenicolous (living on lichens), merocenophilous (living in bark beetle galleries and anthills), muscicolous (living in mosses), praticolous (meadow species), silvicolous (forest species), tyrphophilous (bog species) and eurytopic (occurring in more than three habitat types).

The new records of Oribatida for Norway are based on the checklist [92] and later publications [15,37,93]. Those new to Fennoscandia are based on [94,95] and later publications [31,96–107]. The new records of Mesostigmata for Norway are based on [91,108–110] and those new to Fennoscandia are based on [90,111].

2.3. Statistical Analyses

Oribatid and mesostigmatid mite populations were quantified as abundance (individuals in 500 cm³), dominance (D, percentage of specimens of a particular species in the average abundance of Oribatida or Mesostigmata), and frequency (F, percentage of the samples where the species was present), and by the number of species (mean per sample in habitat and total species richness per habitat), and the Shannon (H') diversity index [112]. Categories summarizing the status of occurrence of species (Table 2) follow [113].

The basic statistical descriptors included the mean values and standard deviation and were calculated in MS Excel. The species similarities of the Oribatida and Mesostigmata between microhabitats were analyzed using an unweighted pair group method with arithmetic mean (UPGMA) with Bray-Curtis coefficient [114] using MVSP 3.2 [115]. Chao −1 indices and individual-based rarefaction curves were computed using 100 randomizations in EstimateS for Windows (version 9) [116].