Abstract: Plant species diversity is one of the most widely used indicators in ecosystem management. The relation of species diversity with the size of the sample plot has not been fully determined for Oriental beech forests (Fagus orientalis Lipsky), a widespread species in the Hyrcanian region. Assessing the impacts of plot size on species diversity is fundamental for an ecosystem-based approach to forest management. This study determined the relation of species diversity and plot size by investigating species richness and abundance of both canopy and forest floor. Two hundred and fifty-six sample plots of 625 m² each were layout in a grid pattern across 16 ha. Base plots (25 m × 25 m) were integrated in different scales to investigate the effect of plot size on species diversity. The total included nine plots of 0.063, 0.125, 0.188, 0.250, 0.375, 0.500, 0.563, 0.750 and 1 ha. Ten biodiversity indices were calculated. The results show that species richness in the different plot sizes was less than the actual value. The estimated value of the Simpson species diversity index was not significantly different from actual values for both canopy and forest floor diversity. The coefficient of variation of this index for the 1-ha sample plot showed the lowest amount across different plot sizes. Inverse Hill species diversity was insignificant difference across different plot sizes with an area greater than 0.500 ha. The modified Hill evenness index for the 1-ha sample size was a correct estimation of the 16-ha for both canopy and forest floor; however, the precision estimation was higher for the canopy layer. All plots greater than 0.250-ha provided an accurate estimation of the Camargo evenness index for forest floor species, but was inaccurate across different plot sizes for the canopy layer. The results indicate that the same plot size did not have the same effect across species diversity measurements. Our results show that correct estimation of species diversity measurements is related to the selection of appropriate indicators and plot size to increase the accuracy of the estimate so that the cost and time of biodiversity management may be reduced.

Key words: Species diversity, Oriental beech forest, Sample plot size, Richness, Evenness