In Arid and semiarid regions trees have noticeable effects on soil resources distribution and its biochemical processes. Organic materials are naturally much more in the understory than out of it. Acacia nilotica and Prosopis juliflora are two nitrogen fixing trees (NFTs) which mainly grew in arid and semiarid tropics and tropics. In Arid and semiarid regions trees have noticeable effects on soil resources distribution and its biochemical processes. Organic materials are naturally much more in the understory than out of it. Acacia nilotica and Prosopis juliflora are two nitrogen fixing trees which mainly grew in arid and semiarid tropics and tropics. These Nitrogen fixing trees are multipurpose by their ability in producing fodder, timber and fuel and since they are resistant to aridity, they are suitable for rehabilitation of degraded lands in arid and semiarid tropics. This study was done to reveal and compare the effects of Acacia and Prosopis trees on the physical and chemical properties of the soil and their results on the growth of understory plants. The studied trees were selected quiet randomly, in a range of age about 12 years old. Considering the fact that the provender quality under the tree shadows is higher comparing with the open areas, the under-study area is selected of those covered with the trees. Regarding the works referred in the history section of the present work, the Nitrogen stabilizer trees, have the capability of stabilizing nitrogen and converting it into the inorganic Nitrogen which is usable for the plants. From this point of view, there is deference in the Nitrogen amount in the areas dominated by the Acacia nilotica and Prosopis Keywords: Prosopis, Acacia, Nitrogen fixing trees, Canopy effects, Understory Plants, Abadan.
