Nitrogen accumulation in forest floors with introduced Pinus pinea and Pinus pinaster in dune site

Introduced stone pine (Pinus pinea L.) and maritime pine (Pinus pinaster Aiton) stands were sampled 60 years after plantation in Istanbul-Durusu (Terkos), Turkey. Sampling was conducted at four different developmental stages (mean diameter of trees at 1.3-m height (DBH) in stands: SDF = < 8 cm, MDF = 8–20 cm, LDF = 20–36 cm, and UDF > 36 cm), with 15 replicated sample plots for each species and developmental stage, for a total of 120 sample plots. The forest floor was sampled in 5 replications in each sample plot. The forest floor samples were divided into two layers: L + F (litter + fermentation) and H (humus), and the oven dry mass and nitrogen (N) content were determined. As a result, the masses of the total forest floor and of both layers were significantly different among the development stages with an increasing trend in maritime pine stands despite no significant difference found in the mass of the humus layer in stone pine stands. However, total forest floor accumulation significantly increased with the development stage (28–60 t/ha in stone pine and 17–64 t/ha in maritime pine). In both species, the N concentrations in the forest floor layers differed significantly among the development stages, and the N concentrations tended to increase as the development stage increased in the L + F layer, whereas a fluctuating trend was observed in the H layer. Although the N content of the humus layer of stone pine did not show a significant difference among the development stages, the N content in the total forest floor was determined to be 0.1–0.5 t/ha in maritime pine and 0.2–0.5 t/ha in stone pine. The relationship between the mean stand DBH and the N stock of the total forest floor was determined to have a higher correlation in maritime pine (R² = 0.8) than stone pine (R² = 0.4). In conclusion, the nitrogen concentrations and nitrogen contents of the forest floor were remarkably different in stands introduced with different tree species, indicating the accumulation of forest floor nitrogen.