Vertical distribution, abundance, and biology of oribatid mites (Acari) developing inside decomposing spruce needles in a podsol soil profile

Three species of oribatid mites, Atropacarus cf. striculus, Rhysotritia ardua, and Adoristes ovatus, whose juveniles develop inside decomposing needles of spruce (Picea abies), were studied in a podsol soil in two different microhabitats: 1) Patches underneath large spruce trees, with no ground vegetation and a thick mat of fallen needles, and 2) mosscovered patches between trees. Externally intact needles from different depths in the organic layers, down to 2 cm, were dissected, and the species and development stage of the mites encountered inside each needle were registered. The percentage of needles colonized by mites, and the percentage of late development stages, increased markedly with increasing depth level. At 1-2 cm below the lower limit of the green moss mat about a third of the examined needles were, or had been, colonized. In vegetation-free patches the corresponding figure was about 20%. The dominant species within needles was A. cf. striculus, although A. ovatus represented 19% of the total mite number in moss sites and 24% in vegetation-free sites. R. ardua was very rare, and was encountered only in the moss sites. A. ovatus colonized the needles earlier than A. cf. striculus, resulting in an increased dominance of the latter species with increasing depth. There was no sign that the colonization affected the needle decomposition rate. It was estimated that the average density of juvenile mites developing inside needles at any one time may be as high as 40 000-60 000 per m super(2).