The objective of this study was to determine plant species diversity in the understory of loblolly pine plantations in different stages of growth. To conduct this study, random-systematic sampling method was used, and plot area was obtained using Whittaker’s nested plots and species / area curve. Within each plot, woody and herbaceous species were identified, and percentage cover was estimated according to abundance-dominance criterion of Braun-Blanquet. In order to data analysis, Shanonn-Wiener’s diversity index and Smith and Wilson evenness and Jacquard similarity indices were used. Results indicated that the number of plant species (richness) were 25, 38, 48, 42 and 32 in seedling, small sapling, large sapling, bole tree of growth stages and natural forest, respectively. The highest and lowest of plant species diversity were found in the large sapling (non-thinned forest) and seedling growth stages, respectively. Also, plant species diversity was higher in large sapling than bole tree (thinned forest) plantations, but there was no significant difference between them (P> 0.05). The jacquard similarity index in the large sapling and bole tree plantations were higher than seedling and natural forest.
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