A Study On The Hillslope Ecohydrological Processes And Vegetation Carrying Capacity In The Small Catchment Of Diediegou, Liupanshan Mountain

Considering and optimazing the interrelationship between forest/vegetation and water resources is one of the important subjects for ecological reconstruction and vegetation restoration in the semi-arid zone of Northwest China. In this paper the growth and water use regularities were studied for main vegetation type which is one of the main plantation tree species on north side of the Liupan Mountains. This study is helpful to understand the interrelation between plantation growth and water condition in the semiarid area, and to promote the development of theories and techniques of the restorationan of forest/vegetation in semiarid area. The research conclusions could offer scientific guidance for the harmonious and integrated management of forest and water. The main conclusions were as follows:1. Analysis of soil hydrological-physical properties in different typical plots:Soil physical properties of different vegetation types: The results show obvious difference of the soil physical properties between different vegetation types. Soil bulk density showed the following order: Ostryopsis davidiana stands(1.16 g/cm3) > semi-shady grassland(1.15g/cm3) > sunny grassland(1.12g/cm3) > Hippophae rhamnoides stands(1.03g/cm3) > Larix principis-rupprechti stands(0.92 g/cm3). The size order of soil non-capillary porosity is sunny grassland(10.45%) > Larix principis-rupprechti stands(7.49%) > Ostryopsis davidiana stands(5.86%) > Hippophae rhamnoides stands(4.86%) > semi-shady grassland(4.48%). The size order of capillary porosity is Hippophae rhamnoides stands(52.9%) > Larix principis-rupprechti stands(51.1%) > semi-shady grassland(48.38%) > sunny grassland(46.85%) > Ostryopsis davidiana stands(44.65%). The size order of total porosity is Larix principis-rupprechti stands(58.59%) > Hippophae rhamnoides stands(57.82%) > sunny grassland(57.29%) > semi-shady grassland(53.22%) > Ostryopsis davidiana stands(50.51%).Soil water hoding capacity of different vegetation types: Results of soil water holding capacity showed obvious difference between different vegetation types. The size order of soil capillary capacity is Larix principis-rupprechti stands(511.01mm) > semi-shady grassland(492.73mm) > Ostryopsis davidiana stands(485.69mm) > Hippophae rhamnoides stands(476.60mm) > sunny grassland(468.46mm). The size order of non-capillary capacity is sunny grassland(104.49mm) > semi-shady grassland (78.74mm) > Larix principis-rupprechti stands(74.81mm) > Ostryopsis davidiana stands(52.17mm)>Hippophae rhamnoides stands(43.74mm). The maximum water capacity showd the following order: Larix principis-rupprechti stands(585.82mm) > sunny grassland(572.94mm) > semi-shady grassland(571.47mm) > Ostryopsis davidiana stands (537.87mm) > Hippophae rhamnoides stands (520.34mm).The influence of landform upon soil physical properties: By analyzing influence factors of soil physical properties in plots with different slope position and aspect, it was concluded that there was a passive correlation between the soil depth and slope position, and also a passive correlation between the soil depth and slope gradient. Soil total porosity and capallary porosity decreased with soil depth increasing, and total porocity increased with vegetation coverage increasing. The non-capallary porocity was influenced by many environmental factors such as biological factors, stone content, etc, but in terms of slope aspect , the non-capallary porocity in shady slope was higher than sunny slope since mainly of influence of vegetation root and soil biology. Generally speaking, the soil physical properties of shady slope and semi-shady slope were better than sunny slope.2. Study on soil water dynamicsSoil water dynamics during different periods and between different layers: Considering the precipitation during the research period, the soil water seasonal dynamic in different typical plots(0~180cm) can be divided into accumulating period and consuming period and restoring period; soil water dynamics in vertical direction can be divided into soil water variable greatly layer, soil water using layer, soil water subactive using layer and water stable layer.The law of soil water variation in different position on slope: the variance coefficient of soil (0~100 cm) water of Larix principis-rupprechti ahowed a fluctuant rising, and the biggest(35%) appeared on the upper slope,which was unfavorable for vegetation growth. But the variance coefficient of soil water was also very big in 3-3 plot, since the plot is located at the forest edge. The change of other plots showed a similar and quite smooth tendency. The variance coefficient ranged from 16% to 22%. The variance coefficient of sunny slope soil water showed a fluctuant rising and the lowest value (22%) appeared on the down slope, and the biggest(38.5%) on the upper slope. The variance coefficient ranged from 22.1% to 38.5%. The variance coefficient of soil water of semi-shady slope showed single peak and the biggest value (43%) appeared in 2-3 plot at the middle slope, and the variance coefficient ranged from 23.5% to 43%.The soil was in water deficit under all vegetation types: this is because the precipitation is the only source of soil water in the study area and soil water consumption (including transpiration water consumption and soil evaporation) was much larger than precipitation. This has resulted in long-term soil water deficit under almost all vegetation types. The water deficit ranking were: Hippophae rhamnoides stands > semi-shady grassland > Ostryopsis davidiana stands > Larix principis-rupprechti stands > sunny grassland, and soil water availability ranking were: sunny grassland> Larix principis-rupprechti stands > Ostryopsis davidiana stands > semi-shady grassland > Hippophae rhamnoides stands.3. Characters of sap flow of Larix principis-rupprechtii and the influencing factorsDaily and seasonal variation of sap flow velocity (SFV) : . Sap flow velocity decreased gradually in the whole season. the highest value(0.22 cm/min) was observed in May, and the lowest value(0.013 cm/min) was observed in October. The daily change of sap flow velocity(starting, rising, peaking, falling and reachingthe low valley) tend to uniform and that showed a single-peak curve. The average sap flow velocity in the whole growing season showed the following order: May(0.27 cm/min) >June(0.25 cm/min)> August(0.21 cm/min)> July(0.21 cm/min) > September(0.13 cm/min) > October(0.054 cm/min).The effect of soil water: The soil water effected not only the peak value of SFV but also the diurnal course (starting time, rising process and falling process). Under drought condition, peak value and staring time of SFV delayed one hour than wet condition, and the dynamic curve was more abrupt. The trend of daily accumulated sap flow amounts of sapwood were similar in soil drought and wet period, and the entire process of daily sap flow presented a S-shape curve. Enough soil water would promote the vegetation transpiration water consumption, while reduced the transpiration water consumption in drought period to maintain normal growth.The effect of tree growth characteristics: the varying process of SFV was quite consistent between sample trees with different diameter, and the average value(0.113 cm/min) was similar, too. However, the sap flux density different is quite obvious since the sapwood area difference between the different diameter sample trees is significant. The maximum sap flux density and sapwood area were used to get the regression equation (R2=0.8848) . According to the fact that there were not simple proportional relation between daily transpiration rate and leaf area index, the daily transpiration rate increased with the leaf area index when leaf area index is less than 2 m2/m2. But if the leaf area index kept increasing after that, the daily transpiration rate would decrease.The effect of meteorological factors: During the period from May to September, solar radiation showed very significant effect on SFV, and could be regarded as the dominant influencing factor. Air temperature was obviously and negatively related with SFV only in May. The effect of soil temperature was less significant than that of air temperature. There was a significant negative correlation between relative air humidity and SFV. During the precipitation process, the instantaneous SFV would markedly decrease to a very low level; But duringintermittent rainfall, the daily average SFV was positively related with the precipitation, and this could be the response to the increased soil wetness caused by rainfall infiltration. A positive correlation was observed between the SFV and air water potential as well as air saturation vapor pressure deficit, but it was not significant in June and August. There was no significant effect of wind speed on SFV.Simulation of SFV and the accumulated sapflow (SFC): A regression equation (R=0.832) was established for relating daily SFV with daily solar radiation, soil water potential within 1 m depth, and air temperature.5. rainfall interception of typical vegetationLarix principis-rupprechti: The result shows that the rainfall interception ratio of Larix principis-rupprechti was 1.25%~63.5%, and the total canopy interception was 57.22 mm, accounting for 15.5% of total rainfall and ranging from 0.06 to 9.20 mm. Total interception kept at a high level while the rainfall class was low, then decreased sharply and headed for a stable value gradually untill the highest interception capacity was reached. Relationship between rainfall inside forest and rainfall outside forest was simple linear regression. The average stemflow accounted for 0.2% of total rainfall, ranging from 0.001 to 0.383 mm, and increased linearly (R2=0.5347) with the rainfall.Shrub: The canopy interception ratio of rainfall of Hippophae rhamnoides was 36.8%, ranging from 1.25% to 63.5%, and gradually reducing with rainfall class increasing. The average stemflow accounted for 8.52% of total rainfall, ranging from 0.00 to 2.95 mm; The canopy interception ratio of rainfall was 53.5% for Hippophae rhamnoides, ranging from 0.511% to 20.661%. The average stemflow accounted for 14.64% of total rainfall for沙棘灌丛, ranging from 0.00 to 5.26 mm. The stemflow of shrub was an important part of water quantity balance.The litter existing amount: the litter existing amount of different vegetation type showed the following order: Hippophae rhamnoides stands(2985 g/m2) > Larix principis-rupprechti stands(948.5 g/m2)> semi-shady grassland(567.4 g/m2) > Ostryopsis davidiana stands(498.1 g/m2) > sunny grassland(204.5 g/m2). Distribution of litter existing amount of different slope: the biggest litter value (567 g/m2) appeared in 2-1 plot for the semi-shady slope, and the lowest (110 g/m2) in 2-8 plot; The biggest litter existing amount (398 g/m2) appeared in 1-1 plot for the sunny slope; The biggest litter existing amount (1486 g/m2) appeared in 3-5 plot for the shady slope, and the lowest (82 g/m2) in 3-9 plot and similar to Larix principis-rupprechti biomass.6. Water balance of typical plots After integrating the research results of interception, evapotranpiration, soil water variation etc, the results show that the Larix principis-rupprechti have the maximum transpiration rate, accounting for 50%~80% of total rainfall, except for the August and October. Forest soil evaporation, undergrowth vegetation transpiration and canopy interception accounted for 12%,10% and 10% of rainfall, respectively. The stem-flow and runoff caused by rainfall is relatively small, accounting for less than 1%, respectively. Soil water content deficit existed during May, September and October, and the water demand for vegetation growth could be meet during June, July and August. In general, the rainfall during whole growing season can meet the vegetation transpiration water consumption. The community evapotranspiration rate in sunny grassland and semi-shady grassland was 237.8 mm and 204.2 mm, respectively. The water consumption in sunny grassland was more than semi-shady grassland.7. Analysis of soil hydrological-physical properties in different typical plots:Soil physical properties of different vegetation types: The results show obvious difference of the soil physical properties between different vegetation types. Soil bulk density showed the following order: Ostryopsis davidiana stands(1.16 g/cm3) > semi-shady grassland(1.15g/cm3) > sunny grassland(1.12g/cm3) > Hippophae rhamnoides stands(1.03g/cm3) > Larix principis-rupprechti stands(0.92 g/cm3). The size order of soil non-capillary porosity is sunny grassland(10.45%) > Larix principis-rupprechti stands(7.49%) > Ostryopsis davidiana stands(5.86%) > Hippophae rhamnoides stands(4.86%) > semi-shady grassland(4.48%). The size order of capillary porosity is Hippophae rhamnoides stands(52.9%) > Larix principis-rupprechti stands(51.1%) > semi-shady grassland(48.38%) > sunny grassland(46.85%) > Ostryopsis davidiana stands(44.65%). The size order of total porosity is Larix principis-rupprechti stands(58.59%) > Hippophae rhamnoides stands(57.82%) > sunny grassland(57.29%) > semi-shady grassland(53.22%) > Ostryopsis davidiana stands(50.51%).