Pourmajidian M.R., Kavian H. (2017): Assessment of plant biological diversity and soil characteristics in the pure ash tree stand and in mixture with beech (a case study of Lavij-Noor, Iran). J. For. Sci., 63: 443-448.The present study has been done to evaluate the impact of pure ash stand and mixed stand with beech on the herb layer biodiversity and soil properties in the forests of Noor city. There is a unique stand of ash in the forests of Noor city and at 1,900-2,100 m a.s.l., which is rarely seen like that in the northern forests of Iran. Shannon-Wiener diversity indices (H'), Simpson index of dominance (D), Margalef richness (R 1 ) and Pielou's evenness were used to analyse biodiversity. Sampling was also conducted to investigate physical and chemical properties of soil (bulk density, acidity, electrical conductivity, soil moisture, soil lime, nitrogen and organic carbon) in each sample plot and at two depths (10 and 20 cm). The total number of 26 soil samples (13 soil samples at either depth) in pure ash stand and 24 soil samples (12 soil samples at either depth) from this type of stand mixed with beech were taken. The results of plant species biodiversity showed that between the pure ash stand and the stand mixed with beech there is a significant difference at the 99% probability level in Shannon-Wiener diversity, Margalef richness, and Simpson dominance. There is also a significant difference at the 95% probability level between the two stands under study in Pielou's evenness index. The results of soil factors also showed that at the depth of 10-20 cm acidity factor showed a significant difference from its adjacent stand at the 99% probability level. There is also a significant difference at the 99% probability level in acidity (0-10 cm) and soil moisture at the depth of 10-20 cm between the two areas, but there is not any significant difference between the areas under study in electrical conductivity and lime factors at the two depths and also in moisture content at the depth of 0-10 cm. It should be noted that between the stand and soil nitrogen, organic carbon and bulk density parameters at both depths (0-10 and 10-20 cm) a significant difference at the 95% confidence level is shown.