Spatial variability of throughfall in heavily fogged old-growth Fagus orientalis forests is controlled by fog precipitation and stand structural characteristics

Ecological impacts of the spatial heterogeneity of throughfall (TF) in forest ecosystems are recognized in ecohydrology. The objective of this study was to evaluate the spatial variability of TF and to quantify the effects of forest cover, fog, and gross rainfall (GR) characteristics on TF. Data were collected from May 2016 to November 2017 on an event basis in an intact, old-growth oriental beech (Fagus orientalis L.) stand in northern Iran. Over the measurement time period, 22 rainfall events were observed (total rain = 648 mm). Because the region experienced considerable fog, TF was greater than 100% of GR in 14 rainfall events with negative interception loss (total = -24.1 mm), causing low interception values, and 8 rainfall events were found with positive I (total = 25.5 mm). The low TF spatial variability and increasing TF depth observed in this study were related to increases in rainfall depth and intensity. TF was significantly and positively correlated with LAI and significantly and negatively correlated with canopy openness for rainfall events of more than 50 mm. The high amount of TF depth highlighted the role of LAI to enhance fog interception and the role of fog precipitation to increase TF in the study site. Spatial continuity at an effective range of 3-8 m was observed by variogram analyses. The increasing spatial continuity of TF with increasing GR observed in this study, along with the larger effective range for GR >50 mm and correspondingly similar LAI range, suggest an effect of LAI on the spatial patterning of TF for larger GR. Therefore, this heavily fogged ecosystem received considerable TF due to fog interception, and higher LAI increased TF for larger rainfall events. Our findings can optimize forest management practices by accounting for the effect of Fagus orientalis trees on the local hydrological processes of the Hyrcanian mountain forests.