Effect of physiographic factors and land-use units on dieback frequency of oak (Quercus persica) in Zagros Region, Iran

Background and Objective: The oak (Quercus sp.) dieback phenomenon in the western Zagros forests is an environmental disaster. Although climate change is the most important factor in this phenomenon, its intensity under the same climatic conditions depends on geological factors, latitude, topographic factors (slope, aspect, and altitude above sea level), and human activities such as land-use change, forest floor plowing, arson fires, and civil constructions (roads, open-pit mines, gas networks, etc.). The main issue of this research, which was carried out in parts of the forests of Kermanshah province, was to investigate whether the severity of the Zagros forests dieback is affected by topographic and physiographic factors. If so, which factor or factors have the greatest effect? The aim of this study was to evaluate the role of topographic factors and physiographic units in intensifying the dieback phenomenon through a field-based assessment.
Methodology: Based on field surveys and considering the diversity of topographic conditions and dieback severity, two main sites were selected in Kermanshah province: in the vicinity of Kermanshah city (Site A) and Islamabad-Gharb (Site B). For this purpose, 31 geomorphological facies were identified, and according to slope conditions, 2 to 4 plots (100 m² each) were randomly established within each facies. In each plot, topographic characteristics (slope, slope aspect, and elevation), visible symptoms of dieback, and the number of dried and healthy trees were recorded. Finally, the major human-induced factors contributing to the intensification of dieback were identified and prioritized.
Results: The results showed that most of the dieback hotspots occurred on slopes of 10–30%. Moreover, southern slopes were more severely affected by dieback than northern slopes, and about 70% of the affected stands were distributed within the elevation range of 1500–2000 m above sea level. The average canopy cover of understory vegetation and bare soil in both sites were 47% and 30%, respectively. Field observations revealed that in areas with weaker understory vegetation cover, complete dieback of tree bases was more evident. The ratio of dried to healthy trees in sites A and B was approximately 47% and 42%, respectively. Since oak is the dominant species in the Zagros forests, its dieback frequency was higher compared to other species, reaching 70% and 80% in sites A and B, respectively. In addition, most of the dried trees were sprout-origin with diameters of 10–15 cm, and signs of pest and disease damage were more frequent in completely dried individuals. Human-induced factors that increased evaporation and soil moisture loss, thereby intensifying dieback, included (in order of importance): land-use conversion to agriculture with plowing, arson fire and charcoal extraction, intentional tree drying and branch cutting, civil constructions, soil erosion and landslides, forest fragmentation, and livestock grazing.
Conclusion: Dieback severity was higher on southern slopes than on northern slopes. Field assessments confirmed that anthropogenic factors play an equally critical role as climatic factors in threatening the forests. Among these, land-use change was the most important driver. In some areas, intentional arson fires for agricultural expansion, deep plowing around trees to accelerate drying, and severe erosion accompanied by landslides not only intensified dieback but also directly caused forest destruction. Field surveys indicated that land-use change, particularly on northern slopes and in low-slope areas, is rapidly expanding. Overall, the intensity and frequency of oak dieback were found to be a function of topographic, geological, and unsustainable human activities.