| The semiarid region of Loess Plateau in China is characterized by low precipitation, strong evaporation, serious soil and water loss, and weak eco-environment. The improvement of eco-environment in the Loess Plateau is the basic approach to resolve the contradiction between insufficient water resource and vegetation construction and to realize the sustainable development of artificial vegetation in this area. With the thermal dissipation probe(TDP) sap flow measuring system and automatic meteorological station, the stem lever water use of Robinia pseudoacacia and Platycladus orientalis and related environment factors were monitored.The influencing factors of sap flow were analyzed systematically. The main results are as follows:1. Under typical weather conditions, R. pseudoacacia and P. orientalis have a similar trend in sap flux density. In sunny days, both the diurnal sap flux density variations of R. pseudoacacia and P. orientalis are shown as single-peak curves, which are in step with the changing patterns of photosynthetically active radiation (PAR) and vapor pressure differential (VPD), and the order of the sap flux density is sunny days> cloudy days> overcast days. In the early leaf frushing period, the sap flux density of R. pseudoacacia increases gradually with the growth of leaves, and it presents a single-peak curve in this stage. During the growing seaon, diurnal sap flux density variation process of R. pseudoacacia appears as a wide-peak curve; it reaches the peak at 11 o'clock in the morning and reduces with the decrease of solar radiation after that. While for P. orientalis, the diurnal sap flux density variation shows a single-peak curve whose shape is steep and narrow; it increases sharply in the beginning of growth stage and falls down significantly after getting to its maximum. In the deforliation period, the sap flux density of R. pseudoacacia declines gradually, while that of P. orientalis almost keeps stable, and there is an obvious rise during the night time.2. In the late leaf frushing period and the rapid growth period, sap flux density of R. pseudoacacia positively correlates with PAR, VPD, air temperature(T) and wind speed(Vs), while negatively correlates with relative humidity(RH). The order of the correlation degree is: PAR> T> VPD> RH> Vs. For P. orientalis, the correlativity between sap flux density and meteorological factors in the growth stage is the same with R. pseudoacacia in the corresponding period.3. There is a power exponent relationship between diameter at breast height (DBH) and sapwood area for R. pseudoacacia, and so is ground diameter and sapwood area for P. orientalis. Both the diurnal transpiration consumption per wood of R. pseudoacacia and P. orientalis increase as DBH (ground diameter) increases. In different growing seasons, the accumulative monthly water consumption per wood of these two species has clear seasonal characteristics. For R. pseudoacacia, the water consumption is relatively less in spring and autumn, but the most in August (280.11 L in 2009 and 238.32 L in 2010). While P. orientalis has the maximum transpiration consumption in May and June (238.21 L and 198.80 L respectively in 2009), the transpiration consumption decreases gradually during the whole growing season. There is a relatively big difference in total transpiration consumption per wood in the growing season between R. pseudoacacia and P. orientalis in 2009 and 2010. However, the monthly transpiration consumption presents stable ratios of the total transpiration consumption during the whole growing season in different years. The total transpiration consumption of R. pseudoacacia forest in the growing season is 103.25mm in 2009 and 96.75mm in 2010, and for P. orientalis forest, it is 194.97mm in 2009 and 181.90mm in2010.4. The root system of R. pseudoacacia and P. orientalis mainly distribute in the soil layer of 0-60 cm. The average effective root density of P. orientalis is as 2.07 times as that of R. pseudoacacia, and they are respectively 0.14 mg·cm-3 and 0.29 mg·cm-3. The effective root (diameter≤1mm) of R. pseudoacacia mainly distribute in the 20-30 cm layer, and 10-20 cm for P. orientalis. In the top 30 cm layer, soil moisture content of both R. pseudoacacia and P. orientalis decrease rapidly with soil depth, and that is because the most distributed effective root uptake a large amount of soil moisture. The soil moisture content of P. orientalis is lower than that of R. pseudoacacia in the 0-30 cm layer because of the higher effective root density in P. orientalis plantation. In the soil layer deeper than 30 cm, R. pseudoacacia and P. orientalis have a similar trend in soil moisture content which increases slightly with soil depth, and also similar moisture content values in same soil layers. |
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