The purpose of this study was to quantify plant species diversity in three plantations composed of loblolly pine (Pinus taeda) (two stands) and Sugi (Cryptomeria japonica) (one stand) species, located in western Guilan, Iran. Sampling procedure was systematic-random method, and with regard to the homogeneity of plantations 20 sampling plots were taken in each plantation and the surrounding natural forest. The size of sample plot was 20 20 m to survey woody species. The diameter of breast height of all trees was measured and individuals of shrubs and saplings were enumerated within the plot. In order to survey herbaceous layer, sampling plot area was determined using nested plot sampling at the center of the large plot and species/area curve was plotted in each forest. Coverage percent of herbaceous species was estimated using Domin criterion and type of herb species were identified. In addition, Crown cover percentage was visually estimated in each large sampling plot. Litter thickness was measured in four selected points of large plot corners. Several diversity indices and Jaccard’s similarity index were used for data analyses. Results indicated that basal area in Sugi plantation was the highest amongst the studied forests. The highest degree of similarity of woody species was found between P. teada 72 (that is planted in 1972 year) and P. taeda 91, and between Sugi and P. taeda 91 plantations. Moreover, the highest degree of similarity of herbaceous species was between P.teada72 and Sugi plantations. P. taeda 91 plantations showed the highest values of species diversity in woody and herbaceous layers, while P. teada 72 and Sugi plantations had the lowest diversity in woody and herbaceous layers, respectively. The highest and lowest values of litter thickness were found in the P. taeda 91 plantation and the nearby natural forest, respectively.
Key words: Plant species diversity, Pinus taeda, Cryptomeria japonica, plantation, Guilan
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