Effects of Stand Density on Tree Growth, Diversity of Understory Vegetation, and Soil Properties in a Pinus koraiensis Plantation

The regulation of stand density has been studied in specific aspects of plantations with different age categories. A clearer understanding is still required of the extent to which stand density impacts multiple plantation attributes such as tree growth, understory vegetation diversity, and soil properties in Korean pine (Pinus koraeinsis Sieb. et Zucc) plantations. This study utilized a 43-year-old middle-aged Korean pine plantation in Qingping Forest Farm in northeast China to answer the research question posed. Three stand density levels, low stand density (LSD, 716 trees/ha), medium stand density (MSD, 850 trees/ha), and high stand density (HSD, 916 trees/ha) were studied for their effects on plantation attributes enumerated above. The results revealed a significant (p < 0.05) effect of stand density on mean stand volume (m3). Medium stand density had the highest mean stand volume of 26.16 (±0.27) m3 while the lowest stand volume was recorded for the low stand density at 14.90 (±1.72) m3. Also, significant differences in total nitrogen, total potassium, available nitrogen, available potassium, and percentage soil moisture content were observed among stand density levels. Additionally, stand density was found to have significant effects on species richness, Shannon–Wiener, and Simpson’s diversity indexes for the shrub and herb layers of the plantation. This study shows that a medium stand density is the most optimal for promoting tree growth and understory biodiversity, as well as enhancing the status of certain soil nutrients. Thus, medium density achieves a balance between growth, nutrient availability, and biodiversity in middle-aged Korean pine plantations after a short period of thinning. These findings provide valuable knowledge for forest management, emphasizing the significance of continuous, long-term, and site-specific research.