Assessing and classification of tree species diversity in the Arasbaran forests of Iran

Background and objectives: Biodiversity loss, increase in invasive species, and climate change are challenging issues of this century leading to decreased plant diversity and negative impacts on the sustainability and stability of ecosystems such as forests. Arasbaran forests have significant changed in recent years due to degradation and human interventions. Therefore, it is essential to assess the biodiversity of Arasbaran forests in terms of species composition and species diversity to improve the forest stands stability. The aim of this research was to evaluate and determine the tree and shrub species diversity in the Arasbaran forests using biodiversity indices and classifying forest stands based on their species similarity.
Methodology: Mountainous broad-leaved forests of Arasbaran located in the northwest of Iran belonging to the main Euro-Siberian region and the Hyrcanian sub-region is known as a biosphere reserve of Iran. However these forests are protected, but intense human activities in recent decades have severely changed the appearance, composition, and structure of Arasbaran forests.To conduct this research, 12 forest stands were randomly selected all over the area based on suitable distribution of stands in different forest types and elevation gradients using a two-stage sampling method. Then, 347 circle permanent sample plots of 300 and 1000 m2 were established using a systematic-random sampling approach and all tree parameters were measured. Afterward, species composition, basal area distribution, mean cross-sectional area, density per hectare, tree height, and proportion of sprout and seedling bases were estimated based on the collected data.To assess the species diversity of the stands, Margalef and Menhinick richness indices, Pielou and Heip evenness indices, and Shannon-Wiener and Simpson diversity indices were used. Furthermore, the Jaccard index was applied for classifying forest stands based on species similarity or dissimilarity, and a dendrogram was prepared based on Jaccard distances. Statistical differences among groups regarding biodiversity indices were examined using analysis of variance.
Results: Based on the results, 25 tree species were identified in the study area, including four dominant species of Carpinus spp L. with 40%, Quercus spp L. with 29%, Juniperus spp L. with 12%, and Acer spp L. with 11%. The quantitative characteristics of the studied stands indicated that the forests of Arasbaran are young forests with small tree diameters (average diameter of 10 cm) and high tree density, with the dominance of coppice and seedling bases, that it is likely due to historical clear-cutting of forests for charcoal production, strong resprouting ability of species such as hornbeam and oak, and human interventions. The obtained six biodiversity indices showed that the species richness, evenness and biodiversity were moderate, very low, and relatively moderate respectively in the Arasbaran forests. Based on the Jaccard index, forest stands were divided into five groups, which analysis of variance results indicating significant statistical differences between groups in terms of biodiversity indices. The highest biodiversity indices related to mixed broad-leaved stands with values of 1.15, 0.66, and 0.21 for richness, evenness, and diversity indices respectively, while the lowest indices associated with pure stands of Quercus macranthera with values of 0.28, 0.06, and 0.12 for richness, evenness, and diversity indices respectively. Furthermore, the Margalef, Pielou and Shannon-Wiener indices were recognized as the best indices for the region based on the coefficient of variation.
Conclusion: According to the results, biodiversity of tree species is more influenced by species richness, and species richness plays a significant role in the variation of biodiversity in the Arasbaran region. These forests are dominated by two or three tree species, that leading to a reduction in evenness. This research revealed that classifying tree species based on biodiversity using the Jaccard index, which effectively differentiated groups, is feasible and can be useful in conservation and management decisions. Therefore, it is necessary to assess the biodiversity indices and identifying factors caused to reduction in biodiversity in the region in order to improve them to achieve sustainability. Successful management plans depend on proper conservation practices for the forests, preventing livestock grazing in the forest stands, avoiding tree cutting, identifying seed-trees and creating space for seed production by removal of the coppice sprouts and investigation of the reasons for lack of establishment of regeneration in the region.