Evaluation of Understory Plant Species Biodiversity in Buaxus hyrcana Habitats within Hyrcanian Forests

Extended Abstract Background: The plant biodiversity of any ecosystem is directly affected by its vegetative characteristics and diversity of plant species, which always guarantees the ecosystem’s stability against variable environmental and biological factors. Biodiversity indices are measured to compare the biodiversity of different masses and estimate the changes in biodiversity over time at the mass level. The Hyrcanian forests of northern Iran are a legacy left from the third geological period, and today a large part of them has been destroyed in the plains due to destructive human activities. These forests have a special importance for the protection and management in the south of the Caspian Sea. Hyrcanian boxwood (Buxus hyrcana Pojark.), which belongs to the Buxaceae family, is the only species of boxwood in the Hyrcanian forests of Iran. It has unique values from various aspects, including economics, tourism, preservation of biodiversity, etc. This valuable species has been threatened by human and natural factors for a long time and its area has decreased in the country. The present research investigates the floristic-physiognomic status of boxwood habitats in Cheshme Bulbul, Sangdeh, Si-Sangan, and Shafarood regions. Methods: In this research, the important habitats of Hyrkanian boxwood were sampled from the protected area of Cheshme Bulbul, Bandar Gaz, Golestan province, as the easternmost distribution area of this species in Hyrkanian forests to Shafarood forests in Gilan province. In total, 125 sample plots of 400 m2 (20 × 20 m) were planted in the four identified areas (35, 30, 40, and 20 sample plots in Bandar Gaz (Cheshme Bulbul), Frame (Sangdeh), Si Sangan, and Shafarood, respectively). To measure the herbaceous cover of the forest floor in each sample plot, five small plots of four m2 (2 × 2 m) were walked in the center and four corners of each sample plot to harvest their herbaceous cover. Biodiversity indices, including Shannon, Simpson, and Fisher diversity, Margalf and Menhinik richness, and Pilou, Simpson, and Shannon uniformity, were measured for each of the sample plots. The plants were identified very carefully using the Persian Flora of Iran, the Flora of Iranica, the Flora of Türkiye, and the Flora of Europe. The biological form of any plant was determined and recorded based on the Rankier method. The range of geographical distribution was determined according to the mentioned flora. Then, the belonging of each species to the existing phytochorions was determined and the geographical distribution diagram of the regional plants was drawn using floristic data in the division of the vegetative regions of the earth's surface.  Species diversity between habitats was compared with the one-way analysis of variance. Means were compared using the Tukey-HSD test in SPSS software. Graphs were drawn using Excel software (2013). Results: In total, 186 plant species belonging to 68 plant genera were identified in the studied regions. The hemi-cryptophytes (65 species, 34.94%), phanerophytes (40 species, 21.5%), geophytes (37 species, 19.90%), trophytes (36 species, 19.35%), and cryptophytes (5 species, 69.2%) dominated the vegetation composition of Hyrkanian boxwood habitats. Kamephytes were the rarest biological form in the region with a percentage (3 species). The results of the geographical distribution of plant species in the region showed that the entire flora was mainly of European Siberian (55 species) and multi-regional (38 species) elements, which accounted for more than 50% of the species (93 species). This was followed by Europe-Siberian/Turanian Iran/Mediterranean vegetation areas (25 species, 13.44%), Europe-Siberian/Turanian Iran (25 species, 13.44%), Turanian Iran (16 species, 60.8%), Europe-Siberian/Mediterranean (13 species, 6.70%), and cosmopolitan (8 species, 4.3%) species. The lowest presence in the region was observed for the vegetation areas of Mediterranean/Turanian Iran (5 species, 2.70%), considering that the northern forests are geographically located in the Auxin-Hyrcania state of the Pontic sub-region, which is a large vegetation area belonging to Europe-Siberia. The presence of species belonging to this geographical spectrum in the flora of these regions is not far from expected. Moreover, the presence of species in other geographical areas is a result of the natural patterns of the accidental presence of plant elements from other vegetation areas of the world. Conclusion: The results of this research show higher indices of diversity, uniformity, and species richness in Shamshad Frame and Shafarood habitats, which have a higher average height above sea level than Cheshme Bulbul and Sisangan habitats. The review of the sources shows that physiographic factors play an important role in the indicators of species richness and diversity. Changes in altitude above sea level often lead to changes in environmental conditions, such as temperature, precipitation, and soil characteristics. This diversity in habitat conditions increases its heterogeneity and allows a wider range of species to coexist. As a result, more habitat heterogeneity leads to higher species diversity, which is also reflected in the biodiversity index.