Measuring and modelling precipitation components in an Oriental beech stand of the Hyrcanian region, Iran

Abstract

Summary Interception loss from the canopy is a major pathway for the loss of water from forest ecosystems. This study was conducted in an Oriental beech stand, neighboring Gorgan, representing typical forest characteristics of the Hyrcanian region. The Hyrcanian region is situated to the south of the Caspian Sea and covers approximately 1.8 million ha of the northern foothills of the Alborz Mountains in northern Iran. This region is characterised by temperate deciduous forests with Oriental beech stands, formed mainly of Fagus orientalis. Because these beech stands occupy 80% of the Hyrcanian region, rainfall interception via the tree canopy is an important pathway for water loss in this region. The main objectives of this study were to determine and model the precipitation components including stemflow, throughfall, net precipitation, and interception loss using gross precipitation and to understand how the diameter classes influence precipitation partitioning by comparing precipitation components across the tree diameter classes. A total of 31 beech trees with the following classes of diameter were randomly chosen: 11 trees of 30–60 cm (young), 10 trees of 60–100 cm (middle-aged), and 10 trees of 100–130 cm (old) of Diameter at Breast Height (DBH). Field measurements of gross precipitation, stemflow, and throughfall were made for 33 rainfall events over a period of 12 months from November 2005. Then, based on these measurements, net precipitation and interception loss were calculated. The value of gross precipitation was approximately 827 mm. Interception loss estimated to be about 53%, 57%, and 60% of gross precipitation corresponding to the tree diameter classes of 30–60, 60–100, and 100–130 cm, respectively. ANOVA results show that the values of the mean of precipitation components were significantly different across the diameter classes. There was an indirect relationship between tree diameter and the volumes of stemflow, throughfall, and net precipitation. Thus, a direct relationship between tree diameter and interception loss was observed. Since the values of the precipitation components were strongly dependent on per event variations in the gross precipitation, the power law regression model was parameterised to predict the values of precipitation components by the value of gross precipitation for each rainfall event. The models explained the variability in the precipitation components very well and are a suitable tool to predict these components of the water balance of forest ecosystems. The best model was obtained for interception loss across the different tree diameter classes. Additionally, some ecological implications of the results are discussed.