Variations in the Frequency and Biomass of Corticolous Earthworms in the Eastern Hyrcanian Forests

Background: Earthworms make up a significant portion of the soil's invertebrate biomass and play a vital role in enhancing the physical, chemical, and biological characteristics of the soil.Earthworms enhance ecosystem performance and boost primary production by altering the soil structure and decomposing organic matter.Earthworms are classified into different categories based on their behavior and feeding habits.The most important ones include epigeic, anecic, and endogeic earthworms.Additionally, some earthworms, such as arboreal and corticolous species, inhabit unusual environments.This study aims to investigate the effect of dead wood on the biodiversity and abundance of corticolous earthworms in the forests of the eastern Hyrcanian region.In this regard, earthworm species found beneath the bark of deadwood (four species) were identified and classified in this research.Additionally, an attempt was made to analyze the effect of different deadwood species and their degrees of decay on various indices of earthworm biodiversity and evenness.To evaluate the impact of different locations within the deciduous temperate forests of eastern Hyrcania, the abundance of earthworms was examined and compared under the forest canopy and in forest gaps.Methods: The study area is located in District-1 of Dr. Bahramnia Forestry Project, the eastern part of Hyrcanian forests.The research was conducted in parcels 1, 4, 7, 8, 31, and 32, which respectively include tree cover types of oak-hornbeam, ironwood-hornbeam, hornbeamironwood, beech-hornbeam, and beech-hornbeam.In this study, 119 samples of fallen deadwood from four species (beech, hornbeam, oak, and ironwood) were randomly selected from beneath the forest canopy and forest gaps at two elevations, 150 and 950 meters.These samples were classified into four degrees of decay based on their physical characteristics.After separating the bark with a sharp knife, the earthworms underneath were collected and initially stored in 15% ethanol, then transferred to 75% ethanol after one day, and finally to a 4% formalin solution.The earthworms were identified to the genus level using available identification keys.Their mass was measured after drying in an oven for 48 h, with an accuracy of 0.0001 g.Data analysis included the calculation of diversity indices (Shannon and Simpson), evenness (Pielou), and richness (Margalef and Menhinick) indices to investigate the diversity of earthworms.Diversity indices were analyzed using PAST (version 3.04) software.Statistical tests were conducted using SPSS to assess the diversity, abundance, and biomass of earthworms across variables.Results: In a study of four deciduous tree species (hornbeam, beech, oak, and ironwood), four genera of corticolous earthworms were identified: Eisenia, Dendrobaena, Dendrodrillus, and Apporecoeda, with respective abundance percentages of 32%, 20.46%, 45.94%, and 1.6%.Notably, Eisenia showed the highest abundance and biomass among corticolous earthworms, primarily in hornbeam trees.The impact of tree species was significant on corticolous earthworm biodiversity, though differences in earthworm abundance and biomass were not statistically significant.The highest Shannon and Simpson diversity indices were recorded for oak (0.16 and