معرفی فلور، شکل زیستی و پراکنش جغرافیایی گیاهان جنگل‌های پست نور و سیسنگان

Introduction

Hyrcanian (Caspian) forests extend from Talish area of Azerbaijan Republic in the west to Golestan National Park of Golestan in the east and constitute a green cover across the northern slopes of the Alborz Mountains. These forests are related to an unique climate with high annual precipitation (ranging from 600 to 2000 mm) and considered to be largely compatible with the Euro-Siberian forest structure (Frey and Probst, 1986). The forests of the south Caspian area have been severely degraded and deforested; particularly, in the alluvial and lowlands where only small remnants of the forests exist now. Due to this land conversion, many plant species were restricted to isolated remnants of a formerly more widespread lowland habitats (Ghahreman et al., 2006). A considerable number of vegetation and floristic investigations have been conducted especially in the mountain and sub-mountain forests of Hyrcanian area (e.g. Djazirei, 1965; Mobayen and Tregubov, 1970; Dorostkar and Noirfalise, 1976; Assadollahi, 1980; Mossadegh, 1981; Hamzeh'ee, 1994; Akhani, 1998; Klein, 2001; Akbarinia et al., 2004; Esmailzadeh et al., 2007; Atashgahi et al., 2009; Naqinezhad et al., 2010). However, necessity of an extensive floristic and vegetation studies in the remained lowland patches is felt more than ever. There are some specific investigations particularly on the lowland areas (Rastin, 1983; Tabari et al., 2002; Ghahreman et al., 2006; Hamzeh'ee et al., 2008; Naqinezhad et al., 2008; Ghahremaninejad et al., 2011; Asadi et al., 2011). For instance, Tabari et al (2002) provided some information on the distribution and vegetation structure of Fraxinus excelsior in the lowland Hyrcanian forests. Likewise, a detailed vegetation and floristic survey was done particularly on the Alnus glutinosa subsp. barbata patches in northern Iran (Ghahreman, et al., 2006; Hamzeh’ee et al., 2008).

Noor and Sisangan are two large patches of such lowland forests classified as “natural forest parks” in the context of “Iranian Natural Resources” (Mazandaran Natural Resources Office, 2012). It may be proclaimed that these forests are the only remnants of Caspian lowland forests, due to the destruction and severe damage by animals and humans in recent years (Barzehkar, 1994; Hamzeh’ee et al., 2008). Sisangan is known by the occurrence of pure stands of Buxus hyrcana, an endemic Hyrcanian woody species which has been largely destroyed from the other parts of the Hyrcanian forest. Likewise, one of the main habitats of Populus caspica, a rare tree in the area, is located in Noor forest. Detailed floristic and vegetation studies should be conducted to provide a basic framework for further ecological and conservational studies on these highly threatened ecosystems. By now, no comprehensive study has been accomplished in these two areas particularly on all parts of the forests and forest margins. The aimes of the current investigation are (1) representing a complete and updated checklist of all plant species of these two areas; (2) assessing some species-related characters such as life form and chorology in the areas; and (3) comparing the flora of the two forest areas with each other and with other forest areas studied in the Hyrcanian area.

Materials and Methods

Study sites

Noor forest is located between 52° 00´ to 52° 06´ E and 36° 32´ to 36° 34´ N with a 3600 hectare surface area. The forest is surrounded by main transitional Noor-Nowshahr road in the north, Noor-Chamestan road in the west, Afraseyabkola village in the south and Hashemrud river and Izdeh forests in the east. The area is generally flat. Sisangan forest is located between 51° 47´ to 51° 49´ E and 36° 33´ to 36° 34´ N, in 27 km east of Nowshahr, Mazandaran. The total area of the forest is approximately 602 hectare and is generally flat. The forest faces Caspian Sea and main transitional road in the north, Tooskatook village in the west, Salahedinkala in the east and south (Figure 1).

Figure 1. Locations of the Noor and Sisangan forests in the Hyrcanian lowland area

Data collection

Data was collected during March to November of 2011. Floristic data were collected by using 62 relevés with the area of 400 m² surface in Noor and 20 relevés with the area of 100 m² in Sisangan forests. Different sizes of relevés were allocated to different vegetation physiognomy and plant density and richness. Sisangan forest was covered mainly by mono-dominant Buxus hyrcana trees or shrubs with low herbaceous ground vegetation. Other species found beyond the relevés were also considered for our floristic survey. Plant determination was carried out using Rechinger (1963-2010), Assadi et al., (1988-2011), Davis (1965-1988), Komarov (1968-1980), Townsend et al., (1966-1985), Ghahreman (1979-2003). Moreover, the ferns were identified using Khoshravesh et al., (2009). All plant names and their authors were checked by IPNI website (www.ipni.org). The classification system of The angiosperm phylogeny group (2009) was used for family names. The life form of each species followed Raunkiaer’s classification system (Raunkiaer, 1934). The terminology and delimitation of the main phytochoria was based on the concepts applied by Zohary (1973), Léonard (1988) and Takhtajan (1986). In this article, PL (Pluriregional elements) are plants ranging in distribution over three phytogeographical regions, SCOS (sub-cosmopolitan elements) are plants ranging in distribution over most continents but not all of them and COS (cosmopolitan elements) referring to plants that have a broad worldwide distribution. Floristic similarity between two forests was evaluated using Sørensen’s (1948) similarity index.

Results

Floristic study in Noor forest showed the occurrence of 185 plant species belonging to 149 genera and 68 families while the number of determined plants in the Sisangan forest was 137 species belonging to 111 genera and 57 families. Totally, 225 plant species from 176 genera and 77 families were collected from these two forests. Among the 77 plant families, nine families were Pteridophytes and 68 were Angiosperms (Table 1). Eudicots with 56 families, 123 genera and 160 species were the richest group, while monocots had 12 families, 40 genera and 50 species in the studied flora (Table 2).

Table 1.Checklist of identified plant species in Noor and Sisangan lowland forests

Table 2. Number of families, genera and species in the main taxonomic groups

The largest families in terms of number of genera were Poaceae (24 genera), Asteraceae (15 genera) and Lamiaceae (8 genera) (Figure 2). Poaceae (28 spp.), Asteraceae (18 spp.) and Rosaceae (9 spp.) showed the highest species richness respectively (Figure 3). The genera with the largest number of species were Carex (6 spp.), Veronica (5 spp.) and Euphorbia, Polygonum and Solanum (each with 4 spp.) respectively (Figure 4).

Figure 2. The richest families in terms of number of genera

Figure 3. The richest families in terms of number of taxa

Figure 4.The genera with the largest number of species

Life form and chorotype spectrum

In the assessment of life form spectrum in the two forests, the dominant life forms were therophytes, which constituted 30.2% of the studied flora, followed by the geophytes (27.1%), hemicryptophytes (20.9%) and phanerophytes (18.2%) (Figure 5). Detailed surveys of life form spectrum in the two studied forests showed that the dominant life forms within the Noor forest were geophytes (29.7%), therophytes (29.2%) and hemicryptophytes (19.5%) followed by phanerophytes (17.3%), hydrophytes (2.1%), chamaephytes and helophytes (1.1%). Nevertheless, the dominant life forms in the Sisangan forest were therophytes (29.9%), geophytes (29.6%) followed by hemicryptophytes, phanerophytes (21.9%) and chamaephytes (0.7%).

The total flora was composed mostly of pluriregional elements with 60 taxa (27.3%), followed by Euro-Siberian/Irano-Turanian/Mediterranean elements with 43 taxa (19.5%) (Figure 6). The ratio of endemism was 6.4% and included 14 taxa in the two studied forests. The flora of both forests was mostly composed of pluriregional elements with 52 taxa (28.1%) in Noor and 33 taxa (24.6%) in Sisangan forest, followed by Euro-Siberian (16.8%), Euro-Siberian/Irano-Turanian/Mediterranean (14%), Euro-Siberian/Mediterranean (9.2%), Euro-Siberian/Irano-Turanian (8.2%), endemics (7%), sub-cosmopolitan (5.4%), cosmopolitan (4.9%), Irano-Turanian/Mediterranean (4.3%), Irano-Turanian (1.6%) and Mediterranean (0.5%) elements in Noor forest and Euro-Siberian/Irano-Turanian/Mediterranean (18.7%), Euro-Siberian (18%), Euro-Siberian/Irano-Turanian (9.7%), ES/M (8.2%), endemics and sub-cosmopolitan (5.2%), Irano-Turanian/Mediterranean (4.5%), cosmopolitan (3.7%), Irano-Turanian (1.5%) and Mediterranean (0.7%) in Sisangan forest.

Figure 5. Life form spectrum of studied flora of Noor and Sisangan forests

Figure 6.Percentage of main chorotypes of plants studied in Noor and Sisangan forests

Discussion

The knowledge of the floristic composition of an area is a prerequisite for any ecological and phytogeographical study and conservation management (Siadati et al., 2010). Noor and Sisangan forests are considered as remnants of Caspian lowland forests. For the first time, Barzehkar (1994) conducted a preliminary study on vegetation of Noor forest, but detail floristic accounts of this area is still lacking. On the other hand, Sisangan forest is characterized as a unique lowland forest due to occurrence of Hyrcanian endemic plant, Buxus hyrcana. The latter species constitutes some rare pure stands across Hyrcanian lowland and submountain forests (Zohary, 1973; Rastin, 1983; Hamzeh’ee et al., 2008; Asadi et al., 2011). Likewise, Noor forest is characterized by the occurrence of Populus caspica, a rare and endangered tree species in Iran. Sisangan forest has lower plant diversity compared to Noor forest due to high density of box trees and shrubs in the former. According to a subjective observation, soil texture of the two forests is relatively different from each other and it might be considered as the main cause of differences in the floristic composition and vegetation structure between the two areas.

Based on Sørensen’s formula, the obtained similarity between the two forests was about 60 % which indicates rather high similarity of floristic compositions between the forests due to their placement within the lowlands and lacking altitudinal gradients. However, the occurrence of Buxus as mono-dominant woody species in some parts of Sisangan forest makes its floristic composition slightly different from Noor forest Park.

Since the life form classification is based essentially on plant reaction to climate, the individual spectrum should tell us much about macroclimatic patterns at field sites (Pears, 1985). Although, therophytes occur abundantly in desert areas (Archibold, 1995), more or less high occurrence of this life form indicates some anthropogenic and over-grazing effects in the study areas (Grime, 2001; Naqinezhad et al., 2006). Similar proportion of therophytes has been previously observed in some other studied ecosystems (Ghahreman et al., 2006, Naqinezhad et al., 2006, Ghahremaninejad et al., 2011). The high percentage of therophytes in the life form spectrum were also encountered elsewhere (Ozen and Kilinch, 2002; Severoglu et al., 2011). The occurrence of therophytes in the Sisangan forest is more prominent than in the Noor forest due to more anthropogenic effects in the former. Following therophytes, geophytes are next dominant life forms. The high proportion of geophytes is consistent with the results of some floristic studies in some other forest areas in the Hyrcanian district (e.g. Ghahreman et al., 2006; Akbarinia et al., 2004; Razavi, 2008; Siadati et al., 2010).

Similar to previous investigations (Ghahreman et al., 2006; Naqinezhad et al., 2010; Ghahremaninejad et al., 2011), pluriregional species constitute a remarkable proportion of the studied flora. These elements can be observed in the lower altitudes of some mountainous systems (Hegazy et al., 1998). Euro-Siberian elements constitute the large proportions of both total flora and flora of each studied forest separately. The occurrence of these elements reflects the phytogeographical link of the studied area with the Euro-Siberian region (e.g. Zohary, 1973; Takhtajan, 1986; Akhani et al., 2010).

Noor and Sisangan forests are the last remnants of the lowland Hyrcanian forests. These highly threatened ecosystems possess two rare and endemic/subendemic species (Populus caspica and Buxus hyrcana) which have been drastically exteriminated from other areas of the Hyrcanian forests. Conservation policies upon the areas should be applied seriously in order to decrease further damaging effects.

Acknowledgement

We are indebted to Dr. H. Zare, Botanical Garden, Nowshahr for his helps during the study. Mr. A. Dehghani and Mr. H. Gholizadeh, University of Mazandaran are thanked for their helps during the field studies.