Effects of enclosure on the soil seed banks of sand-fixed Pinus sylvestris var. mongolica forests

The effects of enclosure on the properties of sand-fixed Pinus sylvestris var. mongolica forests were examined. A total of four forests were chosen as study sites: one that had been semi-enclosed for 5 years (E₅), one that had been fully enclosed for 10 years (E₁₀), and two unenclosed control forests (CK₁, and CK₂). Species composition, density, vertical distribution characteristics, and species diversity of the soil seed bank of the different P. sylvestris forest sites were analyzed by combining field investigations, sampling, and indoor germination tests. The results showed that there were 20 species belonging to 9 families and 18 genera in forests that were semi-enclosed for 5 years, 15 species belonging to 7 families and 13 genera in CK₁, 17 species belonging to 6 families and 15 genera in forests that were enclosed for 10 years, and 15 species belonging to 6 families and 13 genera in CK₂. In enclosed forests, the annual plants, perennial plants and shrubs and semi-shrubs accounted for 47.83%, 39.13% and 13.04% of all species, respectively. While the annual plants, perennial plants and shrubs and semi-shrubs accounted for 50%, 38.89% and 11.11% of all species in the control forests, respectively. The soil seed bank densities of the understory vegetation in CK₁, E₅, CK₂, and E₁₀ plots were 6 741.5, 8 529.7, 11 559.7, and 7 836.9 grains·m^–2, respectively. The density of the soil seed bank was greatest in the 0 – 2 cm soil layer, suggesting surface aggregation. The vertical distribution of the soil seed bank density in enclosed forest sites was consistent with that of unenclosed forest sites. The Margalef richness index of the enclosed forest site was higher than that of the control forest site. The Shannon-Wiener index, Simpson dominance index, and Pielou evenness index of the E₅ forest site were lower than those of the control forest sites, but the opposite was true for E₁₀ sites. The similarity index for the species composition of the soil seed bank between enclosed and control site was 0.88. Leguminous plants were very different between enclosed and control sites. In conclusion, 5 years of semi-enclosure can increase the species richness of communities and increase the proportion of perennial plants and shrubs as well as increase the density of the soil seed bank. At the same time, enclosure can increase the community development potential and improve ecosystem stability, so as to ensure the stability and development of sand-fixing P. sylvestris forests.