Introduction: In the high-elevation areas of the Hyrcanian forests, frequent fog events and subsequent fog precipitation occur during the growing season, due to the high humidity of the Caspian Sea and complex topography. In addition to supplying water, fog precipitation plays an important role in ion deposition and the nutrient cycling of the forest. However, changes in the chemical composition of fog precipitation after passing through the oriental beech canopy (Fagus orientalis Lipsky) and its contribution to nutrient inputs to the forest floor have not yet been studied. Therefore, this study aimed to measure the acidity (pH), electrical conductivity (EC), and concentrations of potassium (K⁺), sodium (Na⁺), calcium (Ca⁺⁺), and magnesium (Mg⁺⁺) in fog precipitation of pure beech stands located at the highlands of the Hyrcanian forests and to compare its nutrient contents with those of open-field rainfall.
Materials and Methods: This study was conducted in a pure beech stand in the Hyrcanian forests during the growing season, coinciding with four rainfall events. For chemical analysis, rainwater of four events accompanied with fog were sampled. To achieve this, 50 fog precipitation collectors were installed under the canopy. In addition, 30 open-area rainfall collectors and six stemflow collectors were installed in an open area, and on the trunk of selected trees, respectively. In total, for each rainfall event, five fog precipitation and three open-field rainfall samples were analyzed for pH, EC, K⁺, Na⁺, Ca⁺⁺, and Mg⁺⁺. After confirming normality with the Shapiro–Wilk test, the data were compared using independent t-tests and Mann–Whitney tests at the 5% significance level.
Results: The mean (±standard deviation: SD) pH of fog precipitation (6.5±0.6) beneath the beech canopy did not differ significantly from that of open-field rainfall (6.0±0.3) across the studied events (p > 0.05). However, the EC of fog precipitation in the second (112.4 µs cm^-1), and fourth (79.6) events (p < 0.05) was significantly higher than those of open-field rainfall (40.8, and 21.8 µs cm^-1, respectively). K⁺ concentrations in fog precipitation were significantly greater than those in open-field rainfall across all events (p < 0.05), with the highest value recorded in the third event at 21.5 ± 1.6 ppm, while the maximum concentration in open-field rainfall was 4.0 ± 0.1 ppm. Na⁺ concentration of fog precipitation also exhibited significant differences from that of open-field rainfall in all four events (p < 0.05). Third event showed the maximum concentration of Na⁺ for both fog precipitation (3.4 ppm ± 0.7) and the open-field rainfall (5.8 ppm ± 0.1). Nevertheless, the Na⁺ concentration of fog precipitation in the second (1.35 ppm) and fourth (0.55 ppm) events was higher than that of open-field rainfall, which differed from the concentration observed in the first and third events. Ca⁺⁺ and Mg⁺⁺ showed significant differences in all events (p < 0.05). The maximum Ca⁺⁺ concentration was observed in the third event for both fog precipitation (14.5 ppm ± 2.7) and open-field rainfall (24.2 ppm ± 0.5). Likewise, the maximum Mg⁺⁺ concentration was measured for the third event; 3.1 ppm ± 0.5 for fog precipitation and 5.7 ppm ± 0.1 ppm for open-field rainfall.
Conclusion: This study indicated that fog precipitation is a significant source of nutrient input into the ecosystem of upper-montane Hyrcanian beech forests. While fog precipitation did not significantly affect pH, it altered EC and nutrient concentrations. K⁺ showed a significant increase in fog precipitation, indicating intense leaching, whereas Ca⁺⁺, Mg⁺⁺, and Na⁺ exhibited different patterns. These findings could be instrumental in improving the precision of nutrient input estimations, monitoring the health of beech stands, and predicting the consequences of climate change on water and nutrient cycles in forest ecosystems.