i i Forest Forest Biogeosciences and Forestry Biogeosciences and Forestry Dynamics of humus forms and soil characteristics along a forest altitudinal gradient in Hyrcanian forest Mohammad Bayranvand (1) , Moslem Akbarinia (1) , Gholamreza Salehi Jouzani (2) , Javad Gharechahi (3) , Giorgio Alberti (4) Humus forms are good indicators of environmental conditions and thus impor- tant in forest ecological processes. Altitudinal gradients are considered as nat- ural laboratory for evaluating soil ecological processes and humus form distri- bution. The objective of this study was to evaluate the macromorphology of humus forms along an altitudinal gradient (0-2000 m a.s.l.) covered with plain forest, mixed and pure forests and forest-grassland ecotone, in Alborz Moun- tains in northern Iran. In total, 225 humus profiles were evaluated. Forest stand variables including tree density, basal area, crown density, and height, forest floor and soil physico-chemical properties along with biological features were measured. We found that altitudinal gradients influence both humus forms distribution and soil properties but with different mechanisms. While soil properties (i.e., temperature, pH, CaCO3, soil N content, soil C/N and mi- crobial biomass N) were significantly correlated with altitude, the forest floor properties were more influenced by tree species composition. Particularly, the abundance of Mull was decreased in plain mixed forests compared to mountain pure forests, whereas the frequency of Amphi was increased. Moreover, Oligo- mull and Leptoamphi were abundant in mixed beech forests, while Eumacro- amphi, Eumesoamphi and Pachyamphi were only observed in pure beech for- ests. Such a distribution influenced soil fertility where higher values of nitro- gen (N), microbial biomass nitrogen (MBN) and pH were observed at lower alti- tudes under mixed forests compared to pure forests at higher altitudes. Keywords: Altitude Gradient, Plant-humus-soil Relationships, Humus Systems, Soil Microbial Biomass Introduction Forest humus is an indicator of the exist- ing environmental conditions (Ponge 2013), because it is the result of complex interac- tions between stand species composition (Labaz et al. 2014), soil properties (Ponge et al. 2011), soil micro- and macro-organ- isms’ activities and environmental factors (Badía-Villas & Girona-García 2018). Since humus forms show specifc morphological patterns (layering and structure – Jabiol et al. 2013), they are useful tool for assessing the health status of forests and the overall soil fertility (Salmon 2018). The current classifcation systems en- abled to distinguish fve humus systems and sixteen humus forms in terrestrial eco- systems (Jabiol et al. 2013, Zanella et al. 2018). Humus forms can be directly identi- fed in feld without the need for expensive laboratory tools (Zanella et al. 2018). According to Zanella et al. (2011), temper- ature, precipitation and vegetation compo- sition are the three most important factors afecting biological degradation of organic residues and contributing in the formation of diferent humus forms. On the other hand, altitude through changes in tempera- ture and precipitation, afects the distribu- tion of forest species, forest foor quality and quantity (Bayranvand et al. 2017b), soil characteristics (Ponge et al. 2011), micro- organism types and activities (Zhang et al. 2013, Xu et al. 2015), thus contributing in humus forms (Ascher et al. 2012, Salmon 2018). Altitudinal gradients are considered as natural laboratories for evaluating soil ecological processes (Labaz et al. 2014, Bo- jko & Kabala 2017). Understanding the complex interactions between soil and plant communities along altitude gradients can be used for the prediction of soil micro- bial activity and forest foor decomposition (Bojko & Kabala 2017, Xu et al. 2015). The natural broadleaf forests in northern Iran are similar to those in central Europe, northern Turkey and the Caucasus. In these forest ecosystems, composition of tree species changes with elevation (Bayran- vand et al. 2017a). Due to their unique topographical conditions compared to the oldest forest in Asia, Alborz mountains of- fers the potential to assess changes in for- est types and humus forms with altitude (Naqinezhad et al. 2013). So far, few stud- ies investigated the pattern of humus forms, forest foor features and soil micro- bial biomass along altitudinal gradients (Bayranvand et al. 2017b, Waez-Mousavi © SISEF https://iforest.sisef.org/ 26 iForest 14: 26-33 (1) Faculty of Natural Resources and Marine Sciences, Tarbiat Modares University, 46417- 76489, Noor, Mazandaran (Iran); (2) Microbial Biotechnology Department, Agricultural Biotechnology Research Institute of Iran (ABRII), Agricultural Research, Education and Exten- sion Organization (AREEO), Fahmideh Blvd., P.O. Box:31535-1897, Karaj (Iran); (3) Human Genetics Research Center, Baqiyatallah University of Medical Sciences, Tehran (Iran); (4) De- partment of Agri-food, Environmental and Animal Sciences, University of Udine, v. delle Scienze 206, 33100 Udine (Italy) @ Gholamreza Salehi Jouzani (gsalehi@abrii.ac.ir) Received: Apr 09, 2020 - Accepted: Nov 02, 2020 Citation: Bayranvand M, Akbarinia M, Salehi Jouzani G, Gharechahi J, Alberti G (2021). Dynamics of humus forms and soil characteristics along a forest altitudinal gradient in Hyrcanian forest. iForest 14: 26-33. – doi: 10.3832/ifor3444-013 [online 2021-01-10] Communicated by: Maurizio Ventura Research Article Research Article doi: doi: 10.3832/ifor3444-013 10.3832/ifor3444-013 vol. 14, pp. 26-33 vol. 14, pp. 26-33