The study was conducted in dune restoration sites introduced with maritime pine (MP, Pinus pinaster Aiton) and stone pine (SP, Pinus pinea L.) at different development stages (diameters at breast height (DBH) in which small-diameter forests (SDF) = 0-8 cm, medium-diameter forests (MDF) = 8-20 cm, large-diameter forests (LDF) = 20-36 cm, and upper large-diameter forests (UDF) = > 36 cm). There were 15 replicated plots in each stage of both species and 25 dune sites; thus, a total of 145 sample plots were selected. Soil samples were taken from six different depth layers (0-5, 5-15, 15-30, 30-50, 50-70, and 70-100 cm). Forest floors were sampled with five replicates in each plot, and they were separated into leaf + fermentation and humus layers to determine unit mass and carbon concentration. Forest floor mass is significantly increased (17-34 t/ha in MP and 28-57 t/ha in SP) with the development stage. Low organic carbon (0.09-0.36% in MP and 0.13-0.84% in SP) was found in the top soil layer despite a significant accumulation of forest floor. The soil organic carbon density varies between 3 and 34 t C/ha. As the stand development stage increases, clay concentrations in every depth layer increased and soil pH and calcium carbonate values tend to decrease. Results indicated that both species have capability to grow on sandy material within poor nutrient and water capacities in a 50-year restoration process. However, the accumulation of forest floor increased and organic matter storage in the topsoil (0-5 cm) remained quite low due to the slow decomposition process.
Keywords: Afforestation; Clay; Organic matter; Sand; Soil pH.