Impact of Canopy Gap Size on Plant Species Diversity and Composition in Mixed Stands, case study: Reserve Area, District No.3 Asalem Forests

In order to study the effect of canopy gap size on species diversity and composition in mixed stands، a reserved area which located in Asalem forests was selected. All of gaps in this area identified and were divided into four categories: Small gaps (<200 m2)، Medium gaps (200-500 m2) large gaps (500-1000 m2) and Extra large gaps (>1000 m2). In each category، 21 of gaps with a relatively homogeneous in terms of an aspect and slope were selected. The major and minor diameter of the gaps were measured and the area of the gaps was calculated based on an ellipse. Then، 2 m×2 m sampling micro-plots were systematically taken along two diameters of each gap. The number of individuals of tree seedlings and saplings and shrubs were counted and coverage percent of herbaceous species were estimated in each sampling plot based on Braun-Blanquet criterion. Number of species were counted and the Simpson (1-D)، Shannon-Wiener (H'') indices، species richness، Camagaro and Smith- Wilson’s evenness indices were calculated in different vegetation layers. The Kolomogrov-Smirnov test was used to study the normality of diversity، richness and evenness data in different gaps، then One-Way and Kruskal- Wallis tests were performed using SAS software. The results showed that mean expanded gap sizes were 442. 6 m2. Also، the results revealed that there was not significant differences among different size gap categories in terms of diversity، but the composition of regeneration was significant among different gaps. Therefore، based on the results of this study، the medium gaps which area 200- 500 m2 in these mixed-species forests are recommended the species diversity.