Study On The Ecological Benefits In High-cold Areas On Loess Plateau

Depending on the national tenth-five year plan of state key task of scientific and technological research, in this paper, the vegetations of converting cropland to forest with different ages and vegetation types were taken as research object, and the ecosystem plotting position was applied in high-cold zone of Datong county, Qinghai province, which was associated with the artificial vegetation recovery and reconstruction for Datong county. The following factors and their interactions were studied in the process of converting cropland to forest, including soil physicochemical property, water utilization efficiency and potential productivity of different vegetations, biomass of major vegetation and biodiversity. The purpose of this paper is to provide evidences vegetation construction of converting cropland to forest, which has great significance in theory and practice.In the soil genetic horizon of 0-20 cm, with the age increasing of converting cropland to forest; the porosity, capillary porosity, and the minimum water-holding capacity took on the uniform trend of "S" curve, and the organic content and carbon content showed "V" curve, the maximum occurred in the arable land and 18a and 32a natural forests of converting cropland to forest; the C/N took on down trend, indicating that the N was in under-supply in the converting cropland to forest; the root density increase from 4,560 roots/m~3 of arable land (0 year) to 111,940 roots/m~3 of 18 a, which brought out the3.57 times increasing of anti-scour capacity and 71.98% improvement of anti-scour strengthen value; and the order of stable infiltration velocity was natural forest of 32a>artificial forest of 18a>artificial forest of 5a>artificial forest of la>arable land. In addition, in the soil genetic horizon of 0-20 cm, the soil density is negatively related to the N content and C content, and positively related to clay content; however, in the 20-40 cm soil horizon, the soil density is significant positively to the content of N, C and clay particle. When the soil erosion occurred by the different exogenic forces, the soil damage degree increased with the soil horizon deepening. Due to the content of clay particle is positively correlated with the content of N and the content of C, so the decreasing of clay led to the attenuation and stability falling of N.The transpiration process was measured with the Li-1600. The diurnal transpiration order of 6 vegetation types was: Triticum aestivum (0.405mm/h·cm~2)>Populus cathayana (0.256 mm/h·cm~2)>Betwla platyphylla (0.242 mm/h·cm~2)>Hippophae rhamnoides (0.115 mm/h·cm~2)>Larix Prlncipis-rupprechi (0.045 mm/h·cm~2)>Picea crassifolia (0.029 mm/h·cm~2); and the transpiration of Triticum aestivum, Larix Prlncipis-rupprechi of 18a, Picea crassifolia of 32a was closed to the local average potential evapotranspiration, about 80.5～95.0%; artificial juvenile Populus cathayana+Betwla platyphylla and Picea crassifolia+Hippophae rhamnoides forest was relatively lower, about 52.8～66.8%. The order of monthly transpiration was: Aug.>Jul.>Sep.>May>Oct., and the total transpiration from July to September was up to the 70%-80% of growing season transpiration.The Thornthwaite model and Miami model were employed to calculate the potential productivity of climate, the results showed that the Miami model gave the bigger approximate value and Thornthwaite model gave the lower approximate value, and the value with Miami model was more closed to the local reality. The average theoretical value of local potential productivity of climate is 8436.900 kg/hm~2·a in 1988-2005. The order of average efficiency of water utility is: 32a natural Picea crassifolia (89.4%)>Triticum aestivum (87.2%)>18a artificial Larix Prlncipis-rupprechi (85.6%)>5a artificial Picea crassifolia+Hippophae rhamnoides (53.1% and 57.1%) >la artificial Populus cathayana+Betwla platyphylla (30.5% and 26.5%).The species richness indices of Gleason and species diversity in arbor layer, shrub layer and herbaceous layer were increasing, that is: herbaceous layer>shrub layer>arbor layer; species dominance and evenness indices were: arbor layer>shrub layer>herbaceous layer. Coenology characteristics took on significant differences in different vegetation types and different ages of converting cropland to forest. The minimum of species richness indices and species diversity was in the arable land and natural 32a Picea crassifolia forest, and the maximum of species dominance and evenness indices was in the natural 32a Picea crassifolia forest.Generally considering the trees growing, efficiency of water utility, productivity and communities succession, etc., in the converting cropland to forest, Prlncipis-rupprechi, Populus cathayana and Betwla platyphylla species should be mixed with other trees for shortening the process of communities succession and increasing ecological and economy efficiency.