The easternmost parts of the Caspian forests in Iran, in which Golestan National Park is located, provide a transition zone between the Hyrcanian forests, the juniper woodlands, the montane and the Artemisia steppes. The diverse climatic, topographic and edaphic features of the area provide a wide spectrum of biotope diversity suited well for ecological investigations, and particularly for studying ecological distribution of C-4 and C-3 plants. Using the carbon isotope composition method, the delta(13)C values for 103 grasses occurring in the Park were determined, Additional data are given for the life forms, the chorotypes, the habitat types and the altitude ranges of the respective species. According to the results, 13 taxa exhibit C-4 and 90 taxa C-3 types of Photosynthetic pathways, respectively. The C-3 plants are abundantly growing in the montane steppes, Juniperus excelsa-woodlands and Artemisia-steppes. The majority of C-4 species are growing in the rocky outcrops and cliffs of the lowland forest zone of the Park. The vegetation types and communities of C-4-dominated rocky outcrops and cliffs and some surrounding parts have been studied using the Braun-Blanquet method of phytosociology. Combining the phytosociological table, multivariate analysis and the ecological criteria, 8 plant communities are described for the first time. These communities are classified into three major groups: 1) Irano-Turanian cliff communities (incl. Stachys subaphylla-Perovskia abrotanoides and Parietaria judaica-Scrophularia variegata communities). 2) Lowland Hyrcanian rocky outcrops and cliff communities associated with C-4 grasses (incl. Cleistogenes serotina-Ceterach officinarum, Cleistogenes serotina-Zelkova carpinifolia, Cleistogenes serotina-Heteropappus altaicus, Cleistogenes serotina-Juniperus communis and Cleistogenes serotina-Eriocycla ghafooriana communities. 3) Hyrcanian highland wall cliffs (incl. Laser rechingeri community). The distribution maps of the characteristic C-3 and C-4 species are provided showing their cover-abundances. Carpinus orientalis, an east Mediterranean, Euxino-Hyrcanian element, is the main component of steep rocky outcrops of the Hyrcanian area which in altitudes under 1200 m is associated with late-flowering C-4 grasses such as Bothriochloa ischaemum, B. bladhii, Cleistogenes serotina and Heteropogon contortus. The occurrence of such thermophilous plants is closely related to the high temperature of the S-facing sunny rocks and the presence of summer rain in the area. In spite of some floristical similarities between the Hyrcanian and the Mediterranean rock flora, the floristic composition and the ecological conditions separate them from the hitherto described higher syntaxa of the Euro-Mediterranean rock vegetation. The rocky and cliff vegetation of the Irano-Turanian parts of the area and the communities of higher altitudes (over 1200 m) in the Hyrcanian area are almost devoid C-4-grasses. These communities can be classified into the Asplenictea trichomanis. According to the present study, the rocky outcrops of the East Caspian forest zone show a high diversity, compared to the other surrounding vegetation types, This is mainly due to the occurrence of different microhabitats and microclimates which are affecting various plants with different seasonal, climatic and substrate requirements, Furthermore, it can be observed that the tertiary relict species not only include arboreal species of the southern Hyrcanian forests but also a considerable number of non-arboreal species. Some of them were discovered during this research.
