Extended Abstract
Introduction: In forest ecosystems, throughfall, a pivotal component of the hydrological cycle, exhibits spatial variations that have received scant attention. The redistribution of rain beneath the forest canopy gives rise to distinct rainfall patterns, resulting in substantial spatial disparities across diverse forest ecosystems. Forests assume a pivotal role in the water balance of the Zagros ecosystem. However, the increasing trend of afforestation using both native and non-native species, particularly fast-growing ones, necessitates an assessment of its impact on rainfall and its components compared to natural stands. This assessment is crucial as it affects the water cycle significantly. The present study aims to estimate the spatial variability of throughfall in natural stands of Persian oak (Quercus brantii) and afforested areas with Pinus eldarica and Cupressus arizonica in Zagros forests, with a specific focus on Chaghasbez Forest Park in Ilam county.
Materials and methods: This study was conducted in Chaghasbez Forest Park in Ilam Province, with the aim of measuring rain components, including throughfall and stemflow, in stands of Persian oak, Pinus eldarica (Eldar pine) and Cupressus arizonica (Silver cedar). Rainfall was measured using five rain gauges located in the open space (outside the canopy) adjacent to the studied stands. Throughfall was measured using 27 collectors in Persian oak stands underneath the canopy, and 36 collectors in each Eldar pine and Silver cedar stand. To minimise measurement errors in throughfall at the stands, after every five rain events, four of the collectors were randomly relocated under the crown of the stand. The remaining collectors were stationed at fixed points throughout the study period. This approach increased the number of samples and reduced the measurement error of throughfall. Geostatistical methods were employed to investigate the spatial patterns and distribution of rainfall, and statistical analyses were conducted using GS+ software (version 5.1.1).
Results and Discussion: The mean throughfall depths for oak, Eldar pine, and Silver cedar were calculated as 207.32 mm, 129.21 mm, and 152.47 mm, respectively. The results indicate that the throughfall percentage of oak is higher than that of Eldar pine and Silver cedar. The average leaf area index (LAI) and the percentage of canopy gap of the studied stands were 1.4 m²/m² (CV= 53%) and 43.85% for Persian oak stands, 1.04 m²/m² (CV= 88.78 %) and 57.04% (CV= 46.26%) for Eldar pine, and 1.2 m²/m² (CV= 80.78%) and 54.71% (CV= 52.47%) for Silver cedar. The spatial pattern analysis of throughfall using variogram analysis revealed that under the canopy of Persian oak (RSS= 0.021, C0= 0.001, and r²= 0.51) and Silver cedar (RSS= 0.0, C0= 0.125, and r²= 0.92), the distribution is anisotropic with a strong spatial structure. Conversely, under the canopy of Eldar pine, the distribution exhibited isotropic characteristics with an average spatial structure (RSS= 0.102, C0= 0.54, and r²= 0.66). The analysis identified the exponential, linear, and spherical variogram models as the most suitable for Persian oak, Eldar pine, and Silver cedar, respectively.
Conclusion: This study found that the spatial continuity range of throughfall was estimated to be 10.8 metres in eldar pine, 8 metres in silver cedar, and 2.4 metres in Persian oak. A review of the literature revealed that the type of forest, tree density, and biomass can affect the spatial change and correlation structure of throughfall. Consequently, it is imperative to take into account the distinct characteristics of the forest type when investigating the spatial patterns of throughfall and their ecological ramifications. Subsequent research, which involves a comparison of the spatial correlation structure of throughfall in evergreen and deciduous forests, is expected to provide a more precise understanding of this subject.