Comparison of runoff and soil erosion in natural stands of Iranian oak (Quercus brantii Lindl.) and afforested with cupressus (Cupressus sempervirens var. horizontalis) in 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 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 protection 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 rain 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 rain 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 rain 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. In order to examine the differences, one-way analysis of variance and comparison of means with Duncan's test were performed. Also, because the samples were examined at two rainfall times, comparison of the data means at these times was performed using independent T-test.
Results and Discussion
The results of the analysis of variance showed that different vegetation covers have a significant effect on some physical soil properties. 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, with increasing rainfall time, the runoff coefficient increased, but the rate of increase in the runoff coefficient during the rainfall period of 30 minutes to 15 minutes was higher in the control area (1.82 times) than in the oak afforestation area (1.36 times), which indicates the importance of vegetation cover and show that if rainfall continues, the amount of runoff and consequently soil loss in uncovered areas will occur more rapidly. Based on the results, the presence of tree canopy, which is associated with characteristics such as a decrease in the rate of litter decomposition and an increase in the volume and depth of litter on the ground surface, has caused a decrease in runoff under the canopy of this tree species. In contrast, areas without canopy cover face a lack of soil permeability due to the low amount of organic matter and litter, as well as the high bulk density and compactness of the soil structure, which has caused an increase in 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
These findings indicate the high importance of vegetation cover 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, can have a significant impact on improving soil physical properties and reducing soil 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 protect soil and water resources.