Background and Objective: The Zagros forests, as one of the most important forest ecosystems in Iran, play a vital role in water and soil conservation, biodiversity, and the regional ecological balance. However, in recent decades, oak decline and dieback have emerged as major ecological challenges in these forests. Exacerbated by a complex of biotic and abiotic stressors—including climate change, recurrent droughts, pests, diseases, and human interventions—this phenomenon severely threatens the health and vitality of Persian oak stands. Continuous assessment of tree crown health indicators and the investigation of their phenotypic and morphological responses to environmental conditions, such as alterations in seed dimensions and leaf area, serve as effective tools for understanding decline dynamics and monitoring the ecological status of habitats. Accordingly, the present study aims to comparatively analyze the health and vitality indicators of oak trees in two forested areas of the Northern Zagros (Marivan County, Kurdistan Province), namely the “Garan” and “Doleh-Naw” sites, to monitor and evaluate the trend of changes over a four-year period.
Material and Methods: This study was conducted in two one-hectare sample plots within the Garan (northern aspect) and Doleh-Naw (southern aspect) forest sites located in Marivan County. In each site, a plot with a floating area encompassing 100 oak trees was established. Crown health monitoring and the measurement of morphological traits were carried out over four consecutive years (2019 to 2022). The vitality and health status of the tree crowns were classified into four categories based on the percentage of crown dieback. To examine morphological traits, 20 leaves and 25 seeds were collected annually from each geographical aspect of every tree. The collected data included leaf area and seed dimensions. For data analysis and mean comparisons between different years and the two sites, following normality checks, parametric and non-parametric statistical tests, including One-way ANOVA, Two-way ANOVA, independent t-test, Kruskal-Wallis test, and Mann-Whitney U test, were applied at a significance level of p<0.05.
Results: The results indicated significant differences in morphological indicators and crown health between the two sites and over the studied years. Regarding seed traits, findings revealed a significant difference between the Garan and Doleh-Naw sites across all four consecutive years. Generally, seeds collected from the Garan site exhibited larger dimensions compared to those from Doleh-Naw. Furthermore, an examination of the annual trends demonstrated that climatic conditions, particularly the severe drought of 2021, had a pronounced negative impact on the reproductive performance of the trees, leading to a substantial decrease in seed dimensions during that year. Concerning leaf area, Two-way ANOVA results showed a significant effect of the site on the mean leaf area; overall, the mean leaf area of trees in the Doleh-Naw site was greater than in Garan. However, no significant differences were observed in leaf area among different years within each site. In the Garan site, the highest leaf area was recorded in 2020 and the lowest in the drought year of 2021. Conversely, in the Doleh-Naw site, the maximum leaf area belonged to 2019 and 2020, while the minimum pertained to 2021 and 2022. The increase in leaf area can be explained by the “Resource Allocation” theory; implying that stressed trees reduce the number of leaves and allocate their resources towards producing larger leaves to maintain photosynthetic capacity. Regarding crown health and vitality, the Kruskal-Wallis test indicated significant differences among various health classes in the Garan site. Additionally, the Mann-Whitney U test confirmed a significant difference in crown health status between 2019 and 2022 in this site, highlighting fluctuations in tree vitality. Meanwhile, in the Doleh-Naw site, the trend of changes in crown health remained more stable over the four years, with no significant differences observed among the various years.
Conclusion: The findings of this study suggest that although the phenomenon of oak decline and dieback has not been fully established with high intensity in the studied sites (Garan and Doleh-Naw), the trees in these areas are under stress, particularly due to climatic fluctuations such as the 2021 drought. The observed differences in seed dimensions and leaf area between the two sites reflect the impact of microclimatic conditions and slope aspect (northern versus southern) on the growth strategies and resource allocation of oak trees. Compensatory phenotypic responses, such as changes in leaf area in stressed stands, indicate the tree’s effort to adapt to adverse environmental conditions. Given the ecological significance and value of these forest stands, it is recommended that field monitoring and the assessment of crown health indicators and morphological traits continue in the coming years. Furthermore, a more precise evaluation of ecological indicators, studying the drivers of dieback, and investigating the role of anthropogenic degradation factors are essential for a better understanding of decline mechanisms. Implementing scientific, evidence-based management programs can significantly contribute to mitigating environmental pressures, enhancing stand health, and ensuring the long-term sustainability of the Zagros forests.