A new species of Arctoseius Thor, 1930 (Acari: Ascidae) from taiga regions of the Palaearctic, with a key to Arctoseius species of Fennoscandia, NW Europe.

A new mesostigmatid mite species belonging to the genus Arctoseius Thor, 1930, is described based on material from subarctic coniferous forests, Finland, and Altai highlands, Russia. Arctoseius ambiguus sp. nov. is similar to Arctoseius venustulus (Berlese, 1916), but can be distinguished from the latter by its larger size, wider dorsal shield, weakly sclerotized presternal platelets, pronounced differences in the length of opisthonotal setae, and distinct punctation of the anal shield. A key for 15 Arctoseius species recorded in continental North-Western Europe (Fennoscandia) is given. The Fennoscandian fauna of Arctoseius species is rather specialised. It includes four European temperate species and five cryobiontic (arctic, arcto-montane, arcto-boreo-montane) ones.

Evaluation of different similarity indices as measures of succession in arthropod communities of the forest floor after clear-cutting.

Communities of spiders (Araneae) and beetles (Coleoptera) living in the soil and litter of clear-cut areas were compared with those of intact forest stands. Sixteen different indices of similarity were tested on three sets of material: spiders and beetles examined during one year in three clear-cut areas felled 3, 6 and 9 years earlier, and spiders in one clear-cut area examined during 7 successive years after felling. Other sources of evidence showed that succession in the spider community was divergent for at least 7 years after felling.The indices that seemed to express the changes best were: (1) Kendall's rank correlation test, (2) the Bray-Curtis measure, (3) Renkonen's percentage similarity, (4) the correlation coefficient r (2 to 4 after logarithmic transformation of data), (5) the Canberra metric, and (6) the diversity overlap (R 0). The properties of the indices are discussed.

Habitat patchiness affects decomposition and faunal diversity: a microcosm experiment on forest floor.

Environmental heterogeneity has been intensively studied, but little is known about relationships between habitat patchiness and soil processes. The aim of this study was to investigate (1) the impact of patchiness of the litter layer on the decomposer community and litter decomposition rate, and (2) whether the impact of soil fauna on the rates of processes differs in relation to patchiness. An experiment was carried out in microcosms with coniferous forest humus and four kinds of litter with different C:N ratios or stages of decomposition, either separately (i.e. in patches) or mixed with each other. Microarthropod species diversity was better maintained in the patchy systems. In the absence of soil fauna, community respiration was higher in the patchy microcosms, but in the presence of fauna the opposite pattern was observed. The contribution of soil fauna to the rate of decomposition was clearly greater in the mixed litter systems. Based on the results, a hypothesis is presented that in the patchy litter layer the soil fungi can create connections between different materials located some centimeters apart, thus enhancing decomposition, while in the mixed litter the scale of millimeters is more appropriate for the soil fauna, known to accelerate the process rates.

Influence of soil fauna and habitat patchiness on plant (Betula pendula) growth and carbon dynamics in a microcosm experiment.

We tested (1) how the presence of a diverse soil faunal community affects ecosystem carbon balance and (2) whether habitat patchiness modifies the influence of soil fauna on plant growth and carbon dynamics. We constructed cylindrical microcosms that contained coniferous forest humus and different litter materials either mixed or in separate patches, and in the presence or absence of diverse soil mesofauna. A birch seedling was planted in the centre of each microcosm. The experiment continued for two growing periods during which net carbon assimilation was measured continuously. At the end of the experiment, the microcosms were destructively sampled for plant biomass, soil fauna, and soil physical and chemical properties. All systems, independently of treatment, were net CO2 producers in the beginning. In the presence of a diverse fauna, the plant growth was drastically increased, and the mixed-litter systems respired more than the patchy ones. During the second season, the patch effect disappeared, while the birch seedlings and mosses continued to grow better in the microcosms with diverse fauna. In the long term, patchiness did not modify the effect of fauna on plant growth or carbon balance. By the end of the experiment, the carbon balance approached zero in the refaunated microcosms, while it remained negative in the "simple" systems. The weak impact of patchiness in comparison to the faunal effect may be due to a homogenising role of plant roots and progressive decay of the substrates.