| The soil water balance and the optimization of system functions of forestlands are always the key contents of research on vegetation restoration and reconstruction in the Loess Plateau. In this study, the hydrological processes and ecological functions of typical forestlands in the Caijiachuan watershed in Jixian, Shanxi province, where the Chinese National Ecosystem Observation and Research Station is located, was analyzed on the basis of a great deal of field investigations and observations. The main results of this study can be summarized as follows.Based on the observations of 39 rainfall events from 2011 to 2013, the canopy interception rates of Rohinia pseudoacucia and Pinus tabulaeformis forests were calculated as in the ranges of 9.1% to 33.3% and 12.8% to 36.4%. respectively. The trunk stemflow rates of them were calculated as in the ranges of 2.7% to 6.1% and 1.5% to 3.0%. respectively. The throughfall rates of them ranged respectively from 60.6% to 88.3% and 60.6% to 85.6%, and litters interception rates of them ranged from 15.0% to 36.4% and from 13.9% to 30.3%. respectively. In contrast, the litters interception rates of waste grassland ranged from 9.3% to 27.7%. The rainfall interception and water conservation effects of forest lands were better than waste grassland.The runoff generated from the 39 rainfall events of the R. pseudoacacia forestland. P. tabulaeformits forestland, and waste grassland ranged from 0.027 to 1.059 mm.0.032 to 1.471 mm, and 0.057 to 2.374 mm, respectively. And the corresponding runoff coefficients of them were 0.818% to 4.179%.0.97% to 5.805%, and 1.73% to 10.79%. respectively. The amounts of soil erosion of different land use types were ranked as. waste grassland (3.64 g·L-1·m-2)>P tabulaeformis forestland (1.44 g·L-1m-2)> R. pseudoacacia forestland (1.37 g·L-1·m-2). Covering the land with a humus and litter layer could effectively alleviate soil erosion. The more the hairy roots (d< 1mm) in the surface layer, the less soil erosion.Analyzed the soil physical properties and soil water vertical movement of different land-use types, the soil bulk density of R. pseudoacacia forestland, P. tabulaeformis forestland, and waste grassland were 1.15 to 1.23 g·cm-3,1.16 to 1.23 g·cm-3, and 1.29 to 1.42 g·cm-3, respectively, and the total porosity of them were 53.43% to 56.60%,53.47% to 56.23%, and 46.38% to 51.32%, respectively. In the surface soil, the soil bulk density was smaller and the total porosity was more than lower layers, which was beneficial to the soil water moving. With soil depth increasing, the soil viscosity and the soil suction increased, which lead to the soil water movement slowly.Based on the long-term monitoring results of stem sap flow and soil water from 2009 to 2012, the annual water consumption of R. pseudoacacia forest and P. tabulaeformis forest were 314 mm and 256 mm, which accounted for 68.25% and 55.25% of the annual rainfall respectively. The annual water consumption of both forest were serious under the densities of both 1300 trees per hectare. The soil water deficit degrees of them were 67.09% and 63.08% respectively, while the soil water compensation degrees were only 4.79% and 19.48% respectively. The soil water deficit degrees were much larger than the compensation degrees, which indicated that larger forest density was an important reason to cause deep soil drought and it was in an urgent need to cut down the density of plantation in order to keep the soil water balance.Using the soil water balance method, the vegetation carrying capacity for a 20 years old R. pseudoacacia forestland and P. tabulaeformis forestland were estimated as 967 and 1163 trees per hectare respectively. In addition, we proposed the vegetation carrying capacity of 220 and 317 trees per hectare respectively for 20 years old R. pseudoacacia forestland to resist aridity with return periods of 20 years and 10 years in this area, while the value for 20 years old P. tahulaeformis forestland were 236 and 357 trees per hectare respectively. Consequently, reasonable density of plantation lands is the foundation to avoid excessive consumption of deep soil water, and the basics for biodiversity and water conservation ability improvement. |
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