Comparison of Runoff and Soil Erosion in Natural Stands of Iranian Oak (Quercus brantii Lindl.) and Afforested with Cupressus (Cupressus sempervirens var. horizontalis) in the Zagros Forest Ecosystems

Introduction and Goal
Investigating the factors affecting runoff and soil erosion is one of the main measures for proper soil management in natural areas. Water and soil resources in arid and semi-arid areas are of particular importance as the foundation of life and sustainability of human societies. On the other hand, the limitation of these resources in these areas, unsustainable exploitation, and lack of attention to the scientific principles of their management have caused increasing destruction and increased the risk of desertification. Soil erosion in these areas, as one of the most important environmental threats, has created serious challenges for natural resource management; therefore, identifying the factors affecting soil erosion and developing soil conservation programs is essential. Given the lack of appropriate statistical data, the difficult physical working conditions in watersheds, and unpredictable climatic fluctuations such as long-term droughts, it is difficult to collect information related to the processes affecting soil loss in natural areas. Accordingly, in many erosion and sediment studies, a rainfall simulator is used. Given the importance of the subject, in the present study, the effect of afforestation on changes in runoff and sediment was investigated in comparison with natural oak forests and areas outside the canopy cover. This study aimed to investigate the volume of runoff and soil erosion in the understory of natural coppice oak (Quercus brantii Lindl.) and planted cupressus (Cupressus sempervirens var. horizontalis) forests after rainfall simulation.
Materials and Methods
The study area, Choghasabz Forest Park, is part of the Ilam city watershed in Ilam province. The climatic characteristics and diversity of vegetation cover provide unique conditions for investigating the relationships between vegetation, runoff, and soil erosion in this area. In order to compare the effects of afforestation of the cypress species and natural oak coppice forest on runoff and sediment, a portable rainfall simulator was used. In this study, rainfall was simulated with a specific intensity and duration based on intensity-duration-frequency (IDF) diagrams, and sampling of sediment and runoff under the trees of these stands was carried out using 2 m² plots (1×2 m). Rainfall simulation was carried out for two periods of 15 and 30 minutes and with a rainfall intensity of 80 mm/h. Then, the rainfall simulator was installed in three random replicates in both the afforested and natural stands and the control plot in a position outside the canopy and adjacent to the stands, and runoff, sediment, and soil samples were collected in each stand and control area. Then, statistical analysis of the data was performed using SPSS version 25 software. The differences among treatments were evaluated using one-way, and mean comparisons were performed using Duncan's test. In addition, the mean values obtained at the 15- and 30-min rainfall durations were compared using the independent samples T-test.
Results and Discussion
The results of scatter analysis showed that the effect of different vegetation covers on some physical properties of soil was significant. In the cypress afforestation, no runoff and sediment were observed, which may be due to the dense vegetation cover and high soil permeability. In the area outside the canopy, the highest amount of runoff and sediment was observed, indicating the positive effect of vegetation covers on reducing runoff and sediment. For the sediment variable, the differences in sediment rates in the cypress afforestation area with the other two areas were not significant, but the differences between the natural oak stand and the area outside the canopy were statistically significant. According to the results of this study, the runoff coefficient increased with increasing rainfall duration. However, the rate of increase in the runoff coefficient from 30 min to 15 min of rainfall was higher in the control area (1.82 times) than in the oak afforestation area (1.36 times). This finding indicates the importance of vegetation cover and show that under conditions of continued rainfall, the amount of runoff and soil loss will in areas without vegetation cover. Based on the results of this study, the presence of tree canopy, the presence of tree canopy was the reason for the decrease in the rate of litter decomposition, the increase in the volume and depth of litter on the ground surface, and significantlydecrease runoff under the canopy of this tree species. On the other hand, poor soil permeability in areas without canopy cover due to low organic matter and litter content, high bulk density, and compact soil structure led to increased runoff in these areas. An important feature of the vegetation canopy, especially trees, is the creation of a suitable substrate on the ground surface and the development of a root network that helps improve soil gradation, maintain soil structure, and stabilize soil aggregates. In such conditions, runoff is so-called filtered and sedimentation is reduced.
Conclusion and Suggestions
The findings of this study indicate that vegetation is of great importance in regulating soil hydrological characteristics and reducing erosion and sedimentation. Accordingly, it can be concluded that maintaining and restoring vegetation in forest stands, especially in the Zagros regions, improves soil physical properties and reducing problems caused by erosion. Therefore, it is suggested that appropriate management measures such as planting native trees and shrubs in degraded and sparsely vegetated areas should be taken to cconservation soil and water resources. Overall, these measures can help reduce runoff and soil erosion in watersheds, sustainable management of natural resources and reducing environmental problems. It is suggested that greater attention be paid to the development of afforestation and preservation of vegetation cover as an effective solution to reduce erosion and preserve natural resources in arid and semi-arid regions.