Woody rangelands are a combination of tree plants in the upper stratum and herbaceous plants in the sub-stratum, both of which are interconnected. Upper stratum coatings have different effects on subterranean plants. The aim of this study was to investigate the effect of tree cover crown on the diversity of subspecies in the Nejdarre reserve. For this purpose, three tree species, Acer monspessulanum, Pistacia atlantica and Crataegus azarollus were selected, which have relative importance in the herbaceous composition of the upper stratum. Using one-square-meter plots, the percentage of vegetation covered under trees and rangeland open space between trees (micro sites) was recorded. Species diversity indices were then calculated for each of them. Data were analyzed by two-way ANOVA and Duncan tests. According to the results, open tree-free micro-sites have higher species diversity compared to tree-lined micro-sites. In addition, the north and south directions of the slopes were not significantly different in terms of species diversity. P. atlantica had allelopathic and depressive effects compared to other species and C. azarollus increased the diversity indices in some cases. The results of regression analysis between species diversity indices under morphological cover and morphological characteristics of trees showed that with increasing trees height and decreasing their shading diametre, the under-species diversity increased significantly. Therefore, in order to increase the diversity of under-species species, tree management should be carried out to increase height of tree. In breeding projects, measures should be taken to increase the species without allopathy in the tree plant community.