Variations of understory vegetation composition and diversity in pure and mixed beech stands (Case Study: Kheyrud Forest- Noshahr)

This study aims at comparing the understory vegetation composition and diversity in pure and mixed beech stands in the Hyrcanian forests. Nine pure beech and 15 mixed beech stands were selected. In each stand, 400 square meter (20×20 m) sample plots were established to determine the cover and abundance of all understory plants using Braun-Blanquet scale. As a biotic environmental parameter, beech tree-layer proportion of each sample plot was determined. At the center of each releve, two soil samples were taken from 0-10 and10-30 cm depths to analyze pH, organic matter, lime, available phosphorous, total nitrogen and exchangeable potassium. For measuring plant diversity within each sample,species richness, Evenness, Shannon and Simpson diversity indices were calculated. Non-metric multidimensional scaling (NMS) was applied to assess the rate and direction of changes on the understory vegetation in the different stands and t-test was used to test the differences in the species diversity indices between pure and mixed beech stands. The NMS result showed that the understory vegetation differed in pure and mixed beech stands and were categorized in distinct groups. The main variation gradient was firstly associated with beech proportion in the tree layer and then related to organic matter and total nitrogen in the first soil depth. All diversity indices and understory species frequency were higher in mixed beech stands than pure ones. Frequency of many species such as Carex divulsa,Calystegia sylvestris and Brachypodium sylvaticum decreased while increasing the proportion of beech in the tree layer. However, shade tolerant species such as Cardamine bulbifera and Mercurialis perennis were observed with higher frequency in pure beech stands than mixed ones.