Fog Precipitation Plays a Significant Role in Providing Moisture to the Caspian High Latitude Forests of Northern Iran

Fog precipitation is likely essential for the mountainous Hyrcanian forests of northern Iran, which is experiencing water stress. It is a source of moisture that is often overlooked. The impact of fog on some temperate deciduous forests, such as these Hyrcanian stands, is believed to be significant, but there are still limited quantitative assessments of this contribution. This study directly addresses this knowledge gap by quantifying fog precipitation within a pure natural stand of oriental beech (Fagus orientalis Lipsky) at 2000 m asl during the growing seasons (foliated periods) of 2022 and 2023. The study site had a tree density of 217 trees ha⁻¹, mean height of 19.1 m and DBH of 41.0 cm. The site was equipped with 50 throughfall, 10 open field rainfall and 6 stemflow collectors. Rainfall and rainfall-fog events were visually separated. During the measurement period, 76 rainfall and rainfall-fog events were recorded. Rainfall-fog events accounted for 80% of all events. The cumulative amounts of rainfall and rainfall-fog events were 23.1 and 287.4 mm, respectively. Half of the events (all rainfall-fog events) exhibited negative interception. Despite having the same number of events, negative I events generated higher amounts of cumulative throughfall (238.7 mm) and stemflow (8.1 mm) and lower amount of cumulative interception value (−40.1 mm). Fog precipitation was estimated 20.3 mm in our measurement site during two leaf-out seasons. This fog capturing potential of the beech forests during the growing season was equal to roughly 100 m³ ha⁻¹ per growing season. Our findings demonstrated that the Hyrcanian beech forests play a crucial role in enhancing water availability, particularly during dry periods, through effective fog capturing potential. Integrating fog water inputs into regional water resource management and forest conservation strategies is needed for ensuring the sustainability of these valuable ecosystems.