Runoff and Sediment Response to Different Fire Intensities in a Hyrcanian Mixed Broadleaved Forest Ecosystem

Abstract

Wildfires are increasingly recognized as a critical driver of ecosystem degradation, with post-fire hydrological and soil impacts posing significant threats to biodiversity, water quality, and long-term land productivity. In fire-prone regions, understanding how varying fire intensities exacerbate runoff and erosion is essential for guiding post-fire recovery and sustainable land management. The loss of vegetation and changes in soil properties following fire events can significantly increase surface runoff and soil erosion. This study investigates the effects of varying fire intensities on runoff and sediment yield in the Kheyrud Educational Forest. Controlled burns were conducted at low, moderate, and high intensities, along with an unburned plot serving as the control. For each treatment, three replicate plots of 2 m² were established. Runoff and sediments were measured over the course of 1 year under natural rainfall. In addition, key soil physical properties, including bulk density, penetration resistance, and particle size distribution (sand, silt, and clay fractions), were assessed to better understand the underlying mechanisms driving hydrological responses. The results revealed that bulk density and penetration resistance were lowest in the control and highest for the high-intensity fire treatment. A significant correlation was observed between bulk density, penetration resistance, and both runoff and sediment production. However, no significant correlation was found between runoff and soil texture (sand, silt, and clay content). Fire intensity had a pronounced effect on runoff and sediment, with the lowest levels recorded in the control and low-intensity fire treatment, and the highest in the high-intensity fire treatment. The total annual erosion rates were 0.88, 1.10, 1.57, and 2.24 tons/ha/year for the control, low-, moderate-, and high-intensity treatments, respectively. The study demonstrates that high-intensity fires induce substantial changes in soil structure and vegetation cover, exacerbating runoff and sediment loss. To mitigate post-fire soil degradation, proactive forest management strategies are essential. Preventive measures—such as reducing fuel loads (e.g., removing uprooted trees in beech stands), minimizing soil compaction and vegetation damage during logging operations, can help reduce the ecological impact of wildfires. These findings provide a scientific basis for adaptive management in fire-prone forests, addressing urgent needs to balance ecological resilience and human activities in wildfire-vulnerable landscapes.