| By in situ measurements this paper studied the water transportation characteristics in the Pinus tabulaeformis and Quercus variabilis mixed forest of north China in the growth seasons under different water conditions. Water use patterns and scales of Pinus tabulaeformis and Quercus variabilis were fully studied under both normal and water stress conditions. By analysis water transportation patterns and scales in SPAC, the importance, probability and reasonable tree density in the process of vegetation rehabilitation and construction on the basis of water balance philosophy in semi-arid and arid areas were discussed. It was aimed at to provide a theoretical foundation for selecting tree species, improving survival rate, maintaining water balance in the process of afforestation while vegetation rehabilitation and construction was performed in semi-aired and arid regions.Study showed that the soil had a comparatively fine physical properties relating water use by plant. Although the soil had a total water holding capacity of 418.77 mm of which theoretically available water is 314.61 mm, the virtual available water is only account about 30%. The soil moisture was between 5%—30% in the whole growing season with May the lowest and July, August the highest. All layers were tending to have comparatively equal moisture except the 0-20 cm layer which was much higher. The recorded rainfall was 470 mm of which 378.8 mm reached to the ground in the study period. The total evapo-transpiration is 391.4 mm from April to October and this induced soil water loosed by 12.4 mm.Both the sap flow velocity of Pinus tabulaeformis and Quercus variabilis had very close linear relationships with the soil moisture and soil water potential on a monthly average basis. The sap flow velocity of Pinus tabulaeformis was between 0.0004~0.0015 cm/s(4~10 times higher than that of the dry period) taking the shape mono-peak which was similar with the patterns of total solar radiation in sunny day in humid period. The sap flow velocity of Quercus variabilis was about 0.0015 cm/s(2.5 times higher than that of the dry period) taking the shape multi-peaks. The sap flow velocity of Quercus variabilis was 5~10 times higher than that of the Pinus tabulaeformis on a monthly average basis. The sap velocity of Pinus tabulaeformis was mainly influenced by weather conditions in July and August while it was mainly influenced by soil water regime in May. In the whole season, total forest stand transpiration is 347.68mm, with Pinus tabulaeformis 208.48 mmand Quercus variabilis 139.2 mm, account 88.8% of the total evapo-transpiration.The water potential of the atmosphere was between -300 MPa ~-5 MPa in the study period. May was the driest month with the water potential of the atmosphere between -300 MPa ~ -100 MPa without any rainfall. While July and August were the humid months with strong rainfall and the water potential of the atmosphere was between -50 MPa ~ -5 MPa taking the shape of multi-peak.The soil water potential was between -0.5 MPa ~ -3.0 MPa with the highest potential(-0.5 MPa~-1.0 MPa, average -1.2 MPa) happened in July and August while the lowest fell on May. The water potential did not recorded apparent daily change in all soil layers. In dry period all layers were tended to have same water potential around -1.5 MPa.The leaf potential of Pinus tabulaeformis was between -1.0 MPa~-2.0MPa while that of the Quercus variabilis was between -2.2 MPa~-2.7MPa. There existed apparent potential gradient in the SPAC. From soil to plant the water potential was lowered by 2.0 MPa or less. But the water potential was lowered by hundred MPa from plant to atmosphere. It is the much lower water potential in the atmosphere than that of the soil and plant that of the pulling force of water moving form soil and plant to the atmosphere. The leaf water potential of Pinus tabulaeformis had a very close linear relationship with the soil water potential(P<0.001, R=0.971) while that of the Quercus variabilis had a very close relationship with the water potential of the atmosphere...
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