| The loess plateau is our country and even the world’s most serious soil erosion areas, mild erosion area of 454000 km2 and the average years of data input of the Yellow River silt up to 1.6 billion tons, including contribution of about 0.975 billion tons of natural erosion. Vegetation restoration is the most direct, effective and economic means in the Loess Plateau. For a long time in the loess plateau region will plant trees grow grass as the main measures of ecological construction and has achieved great success, but there are still a low survival rate, low productivity is lower than the survival rate of afforestation tree species phenomenon, investigate its reason is not based on the characteristics of the loess plateau soil and water resources reasonable scientifically build artificial vegetation. Since the "six-five", the loess plateau ecological environment problems and vegetation restoration has become research hotspot. Through long time research, scholars have the basic ecological environment problems and of the loess plateau vegetation restoration scheme has been basically reached a consensus, namely, according to local conditions, suitable to the principle of optimal tree suitable grass appropriate, appropriate irrigation, irrigation, appropriate grass, grass, agroforestry, halosols deserts, scientifically establish forest vegetation and ecological environment are the most appropriate. There are "three lows" problems in the course of vegetation resuming. So you need to the various components of water balance of artificial vegetation and vegetation net primary productivity, for reasonable and effective allocation of water resources, vegetation restoration and reconstruction of loess plateau has far-reaching significance.We took the Pinus tabuliformis, Robinia Pseudoacacia, Caragana korshinskii, Hippophae rhamnoides, Populus tomentosa, Prunus armeniaca, grass land and spring wheat as the studied objects. The main contents were as follow:(1) The observation and simulation of canopy interception in the shrub lands;(2) The observation and simulation of evapotranspiration in the shrub lands;(3) The observation and simulation of water potential and photosynthesis in the shrub lands;(4) The distribution of fine roots in all land covers and the construction of root water uptake model;(5) The observation of runoff in all land cover types;(6) The simulation of water balance and NPP in all land cover types; The main results were as follow:1. The canopy interception of Caragana korshinskii, Hippophae rhamnoides accounts for 28% and 18.3% of rainfall, respectively, Caragana korshinskii and Hippophae rhamnoides through accounted for 59.7% and 73.3% of rainfall, rainfall and trunk stem flow respectively accounted for 12.3% and 8.4% of rainfall. Through the rain and the change of the trunk stem flow coefficient is similar to other shrub ecosystem. When rainfall< 10 mm, through dramatic changes in the rain change coefficient, for large rainfall events, however, through the rain change coefficient will be stable. Trunk stem flow variation coefficient has similar laws. When withholding amount is small, the simulation value slightly higher than that of observations, when large amounts of intercept simulation value are lower than the observed value.2. We extrapolated measurements of water use by individual plant to determine the area-averaged transpiration of the shrublands. There was a good agreement between transpiration estimated by the Penman-Monteith method and sap flow method, suggesting that sap flow method can provide reliable estimates of shrub transpiration at the stand level. Stand transpiration was mainly influenced by environmental factors such as photosynthetically active radiation, vapor pressure deficit and soil water content. When soil water content was sufficient, photosynthetically active radiation and vapor pressure deficit were the dominant factors, but soil water content was the primary factor under low soil moisture levels. Stand transpiration ranged from 0.52 to 4.21 mm day-1 with an average of 1.42 mm day-1 for C. korshinskii and from 0.57 to 3.99 mm day-1 with an average of 1.94 mm day-1 for H. rhamnoides. During the experimental period (from June to September 2013), cumulative transpiration were 173.4 and 236.6 mm for C. korshinskii and H. rhamnoides, which accounted for up to 88.2% of the rainfall registered during this period. We calculated soil water balance and measured water potential of stems and leaves for C. korshinskii and H. rhamnoides. H. rhamnoides had lower net soil water storage, indicating it consumed more soil water than C. korshinskii. There were some negative water potential drops between stems and leaves for H. rhamnoides, suggesting the lack of a safety margin for H. rhamnoides. Our results indicated that C. korshinskii is more suitable for afforestation than H. rhamnoides in the Loess Plateau. The results showed that the response of sap flow began about 1-2 h earlier, and increased two to three times after rainfall, the response to rainfall was larger in branches than in stems. The sap flow increased significantly with increasing rainfall classes, then gradually decreased. The response of sap flow was different among rainfall, species, position (branch and stem) during the pulse period, and the interactive effects also differed significantly (P<0.0001). The response pattern followed the threshold-delay model, with lower rainfall thresholds of stem and branch were 5.2, 5.5 mm and 0.7,0.8 mm for C. korshinskii and H. rhamnoides. Therefore, small rainfall events had a positive effect on the survival and growth of shrubs in semi-arid regions.3. More than 50% of fine root length was concentrated at depths between 0 and 40 cm in vertical direction, In horizontal direction, most of fine roots concentrated near the trunk. Results showed a significant negative correlation between vertical distribution of FRLD and soil water content, a positive correlation between FRLD and organic matter and total N is significant, and a negative correlation with bulk density. No relationships were found with total C and particle size distribution in any soil layer for the six tree species. Stepwise multiple linear regression confirmed that changes in different soil properties significantly affected the variation in FRLD for each tree species, total N had strong and positive relationships with FRLD.4. The soil water replenishment by rainfall during rainy seasons was not sufficient to fully recharge the soil water storage. Landscape restoration through shrub planting may help retain more soil water than other land cover types. Secondly, the ratio of actual evapotranspiration(ET) and pan evaporation (Ep) generally declined for all land cover types during the study period. Shrub lands had the largest ET/Ep ratio, followed by native grassland, cropland/alfalfa, and pine woodland. The ET/PET ratio of native grasslands declined fastest, followed by pine woodlands, shrub lands, alfalfa, and croplands. Pine woodland’s low ET/Ep ratios were mainly caused by its higher runoff due to soil compaction resulted from soil desiccation. Lastly, we found that the soil water storage at the beginning of growing season was important in determining the ET/Ep ratios. This study suggested that pine plantations may not be appropriate for landscape restoration in such a semi-arid loess hilly area while shrubs may be highly recommended.5. P. tabulaeformis and H. rhamnoides belong to high-low transpiration types, high WUE, but with smaller effective utilization rate of water; NPP of spring wheat is very high, but also consume large amounts of water, high WUE also, generic highlights-high transpiration types; Grass generic low photosynthesis-low transpiration type, has the lowest WUE and effective utilization rate of water; C. korshinskii and H. rhamnoides both shrubs, NPP value center, through transpiration consume large amounts of water, low WUE, water utilization is higher. Through five years of simulation, the vegetation type soil moisture basically can reach balance, but the degree of different species of deficit. Weeding, other 5 vegetation types are soil moisture in different degree were decreased, including P. tabulaeformis, H. rhamnoides soil water deficit is larger.Soil erosion and desertification is directly related to the national ecological security. Serious soil erosion and desertification is the concentrated reflection of ecological deterioration, to become one of the most prominent problems in the ecological environment in our country. Soil erosion and desertification is not only leading to land degradation, destruction of cultivated land, mountain area of social and economic development, make people urban subsistence, and intensifying river lake sedimentation and flooding, increasing poverty, threat to national food security and ecological security; This not only influence the development of contemporary society, but also affect the survival of future generations. Therefore, strengthening soil and water conservation and desertification control basic theory research, to find a scientific and reasonable methods and measures for prevention and control, to carry out the ecological environment construction, resource and society theory research and practice of sustainable development, can for the country’s ecological security and sustainable economic and social development to provide solid and reliable foundation. |
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