TRANSFORMATIONS OF EARTHWORM COMMUNITIES DURING POST-LOGGING SUCCESSIONS IN THE FORESTS OF THE NORTHWEST CAUCASUS

The paper presents study findings of the earthworm population at three phases of postlogging succession of coniferous–broad-leaved forests in the Northwest Caucasus (heads of the Pshekha and Belaya rivers). Three types of forest communities corresponding to the stages of forest vegetation succession were examined: early stage – aspen–hornbeam–honeysuckle smallgrass forests, intermediate stage – fir–hornbeam small-grass forests, late stage – beech–fir deadcover forests. Type of soil: brown forest soils. It has been shown that during the post-logging succession significant changes in the earthworm community occur only at the late stage, when the biomass of anecic worms increases significantly. Only at the late stage there are four stable morpho-ecological groups of Lumbricidae, despite the fact that at this stage the structure of ecological-cenotic plant groups becomes simpler and oligodominant dead-cover communities with a predominance of boreal species are formed. At all stages, species with the CrimeanCaucasian type of habitat (D. schmidti, D. mariupolensis) are prevalent. Demographic structure of the earthworm community in all types of forests is stable, represented by different ontogenetic states with a significant predominance of juvenile worms (65–78%).

information about any studies focusing on the earthworm population dynamics in forests at the different succession stages formed after logging in the Northwest Caucasus.
Study objective: to assess the dynamics of earthworm communities in coniferous-broadleaved forests at different stages of post-logging succession in the Northwest Caucasus.

MATERIALS AND METHODS
Studies were carried out in summer 2016 at three stages of post-logging succession of the forests in the Northwest Caucasus (heads of the Pshekha and heads of the Belaya river) at an altitude of 650-700 m above sea level were analyzed. Based on the population-ontogenetic approach to the state of plant communities (Smirnova et al., 1988;2006;Evstigneev et al., 1992;Smirnova, 2004) three stages of succession were identified: earlyaspen-hornbeamhoneysuckle small-grass forests, intermediatefir-hornbeam small-grass forests, latebeechfir dead-cover forests with the oldest trees of over 450 years old . At each stage three experimental plots 50x50 m 2 in size were allotted. Soil and zoological studies were carried out according to the standard method: 16 soil samples 25x25 cm in size were taken up to the depth of species occurrence from the experimental plots with different stages of succession (Gilyarov, 1975). Earthworms (Lumbricidae) in the soil and fauna in deadwood were counted. Data on the quantity and biomass are given only based on the results of soil sample analysis. Earthworms were fixed in 95% alcohol. Earthworm species were identified using the guide of T. S. Vsevolodova-Perel (1997). Morpho-ecological groups of Lumbricidae are given according to the classification of T. S. Perel (1979). The following ontogenetic states were included into the earthworm demographic structure: juvenile, subadults and adults (Shashkov, 2016). Cocoons of earthworms were not assessed separately.

RESULTS AND DISCUSSION
In three forest types 8 species of earthworms were identified belonging to 4 morphoecological groups and 4 types of habitats (Table 1).
Species composition of earthworms and composition of morpho-ecological groups in general is in line with the earthworm population in similar forest types in the medium-altitude mountain forests of the Northwest Caucasus (Rapoport, 2014;Rapoport, Tsepkova, 2015;Geraskina, 2016).

epi-endogeiс
The total number of earthworms does not differ significantly in the three forest types with the different succession status and ranges from 36 to 42 spec./m 2 unlike biomass which at the late stage is twice as high compared to the previous stages ( Fig. 1).  D. attemsi and E. fetida. In soil, predominant species (80%) in terms of both quantity and biomass are soil species D. schmidti and A. jassyensis (Fig. 2, 3). Demographic structure of the most numerous group of endogeic species D. shmidti, A. jassyensis, D. tellermanica is dominated by juvenile earthworms. At the same time 90% of specimens at the period of counting in the group of epigeic, epi-endogeic and anecic worms were adults and subadults (Fig. 4).
In the present study, the properties of tree plant debris and ground cover are most favorable for saprophages at the early and intermediate stagesin aspen-hornbeamhoneysuckle small-grass and fir-hornbeam small-grass forests with a high share of nemoral, nitrophilous and meadow-forest-edge species , where mixed litter from leaf debris of different tree and undergrowth species is formed. Most studies have shown the benefit of mixed deciduous and coniferous debris for the activity of the soil saprophage community (Sariyildiz et al., 2004;Sariyildiz, Küçük, 2008), since hard-to-decompose coniferous litter provides a favorable habitat for epigeniс mesofauna, and easily decomposable deciduous debris provides an accessible source of carbon and nitrogen (Sayad et al., 2012).
Studies in the forests of Central Germany have shown that the more diverse the composition of the forest stand is, the greater the density and higher the functional diversity of earthworms are.
Monodominant beech forests are inhabited with only one group of epigeiс earthworms (Cesarz et al., 2007). At a late stage, beech-fir dead-cover forests with a high proportion of boreal species, despite the unfavourable (in terms of trophic conditions) fir and beech debris, a full-fledged community of earthworms is formed. It has a high proportion of endogeic species and anecic D.
mariupolensis which is evidence of favourable soil conditions of forest brown soils for earthworms and is indicative of the late succession stage in the forest development. The diversity of morpho-ecological groups of earthworms in forests with hard-to-decompose debris is also exemplified by beech forests of the Central Caucasus, which is associated with a favourable combination of soil conditions and the presence of a thick layer of slowlydecomposable litter (Rapoport et al., 2017).