Hydrological Effects of Forest Litter and Soil on Different Density Plantations of Pinus sylvestris L. Var. mongolica Litv. in Mu Us Sandland, Northwest China

A pilot study of hydrological effects of litter and soil layers was carried out on different density plantations of Pinus sylvestris L. Var. mongolica Litv. in the southeast of Mu Us Sandland. For the purpose of evaluating the hydrological function of litter and soil quantitatively, 30 standard plots (10 types of forest×3 replications) were investigated. The soil physical characteristics and water-holding capacity were determined. The results showed that: (1) The total storage capacity of litter among 10 forest types, generally followed by the sequence of forest densities, were P_III = P_IV > P_X > P_VII > P_IX > P_VI > P_VII > P_V > P_II > P_I; the maximum water-holding capacity and effective water-holding capacity of litter, for 10 forest types, were 2.46-8.23 t/hm² and 0.163-6.42 t/hm², respectively. (2) There were no significant differences in water content, bulk density and the maximum water-holding capacity of soil amongst these forest types, with the variation range of 8.94-16.54%, 1.10-1.66 g/cm³ and 200.43-266.43 t/hm², respectively. However, the non-pore porosity of soil among these 10 forest types varied greatly with a variation range of 0.99-4.32%. The hydrological effects of soil were the highest in the P_III and P_IV plots, and the lowest in the P_I plot. (3) The power function model can better indicate high correlation coefficients between the water absorption rate of litter and its immersion time, and also between the water-holding capacity of litter and its soaking time. The stable infiltration rates of soil in 10 different forest stands were consistent, while they varied greatly with a range of 0.11-0.89 mm/min after 20 minutes. (4) In this study, the hydrological effects of forest litter and soil in P_III and P_IV plots were the best, and the optimal afforestation density was 1500 to 1800 plants per hectare. The results show that thinning should be done timely and appropriately in this study area to prevent the recession of a forest stand. Besides, it is unapproachable whether Pinus sylvestris Var. mongolica can be planted in large scale or in high density, and further research is needed.