LAUSR

Dead trees, and particularly downed logs, play an important role in the dynamics of forest ecosystems. The contribution of decaying wood to the carbon and nutrient pool of forest soils depends on tree species and the degree of wood decay but the extent to which downed logs affect forest properties in temperate forests has rarely been quantified. In order to investigate the impact of decaying wood on soil properties, we carried out a study in a mixed beech forest of the Liresar district (Mazandaran province, northern Iran). We first investigated if and how the presence of downed logs affects soil quality and function by comparing soils underneath degraded logs and nearby soils of the two dominant forest species, beech and hornbeam. Then, we explored how these effects occur as downed logs decompose by comparing wood with four degrees of decomposition of both tree species. The decay degree of downed logs was classified in four classes (DC1 to DC4). Eight dead trees of each tree species were selected as the center of each sample plot and three composite soil samples under each decaying log and 100 cm near a decaying log (soil around logs) were collected at two soil depths (i.e., 0–15 and 15–30 cm) and then analyzed for main physico-chemical characteristics and microbial activity. Our study showed that downed logs affected physical (5% wetter than control soils), chemical (lower by 2% the pH, increased 100% organic C, and total N in the case of hornbeam and P by 2%) and biological (i.e., soil microbial respiration was enhanced by 10% and , microbial biomass of C 620 mg kg−1, 351.5 mg kg−1 and N 66.47 mg kg−1, 32.18 mg kg−1 by, respectively) characteristics of soils, thus resulting in downed tree soil microsites that strongly differ from those without downed logs. The presence of down logs increased soil microbial activity and soil fertility as wood decayed. Thus, the presence of downed logs should be considered as an important factor influencing forest soils and should be taking into account in forest management practices.