Effect of different thinning intensities on the disposition of precipitation, soil moisture and some chemical properties of soil water in black pine plantation

This study examined how different intensities of thinning treatments influence hydrological and chemical processes in black pine (Pinus nigra) planting sites. The research focused on throughfall, stemflow, rainfall reaching the soil, interception, forest floor flow, soil water content, and the chemical properties of water samples from these pathways. It also aimed to assess potential changes in soil temperature and moisture caused by thinning. Two thinning treatments were applied: moderate thinning, removing 21% of the basal area, and intensive thinning, removing 40%, along with an untreated control plot. Each treatment had three replicates, totaling nine plots. In each plot, standard totalizer rain gauges, stemflow and forest floor flow collectors, and soil water samplers were installed, with soil temperature and moisture continuously monitored. The study spanned from October 2020 to September 2023. During this period, open areas received 349 mm of annual rainfall. Of this, 74.7%, 80.2%, and 83.4% reached the forest floor in control, moderate, and intensive thinning plots. Increased thinning intensity resulted in more throughfall, rainfall reaching the soil, forest floor flow, and soil water content, while interception rates decreased. Water samples from beneath the canopy had slightly more acidic pH values (6.23–6.28) than open-area rainfall (6.35). Stemflow had the lowest pH values (4.88–5.09), whereas forest floor flow (7,24-7,34) and soil water (7,53-7,55) samples were more alkaline. Total organic carbon (TOC) levels varied among treatments across all sample types, with the highest TOC observed in forest floor flow. Annual TOC input ranged from 32 to 40.8 kg/ha via throughfall and from 83.5 to 118.8 kg/ha via forest floor flow. Total nitrogen (TN) levels varied among treatments only in stemflow and forest floor flow. In forest floor flow, annual TN input was 6.0 kg/ha in the control, 8.6 kg/ha in moderately thinned plots, and 7.9 kg/ha in intensively thinned plots. Throughfall samples showed higher deposition of base cations (Na+, K+, Ca2+, Mg2+) compared to open-area rainfall. Among these, Ca2+ was the dominant base cation in both forest floor flow and soil water, with the highest flux in the intensively thinned plots. Soil temperature data revealed significant differences among treatments. The average maximum temperatures were 14.37°C in intensively thinned plots, 14.36°C in control plots, and 14.05°C in moderately thinned plots. Monthly and seasonal analysis indicated that soil moisture at 20 cm and 40 cm soil depths was significantly higher in the intensively thinned plots than in the control and moderate plots. Overall, the results suggest that thinning treatments under semi-arid conditions increased the amount of rainfall reaching the forest ecosystem, with the greatest effect observed in the intensively thinned plots. Therefore, in black pine planting areas where improving soil moisture is a management goal, intensive thinning may be an effective silvicultural practice or wider planting distances than those traditionally used in humid and sub-humid planting sites may be worth considering.