Stidues On The Water Consumption Characteristics And Water Balance Of The Stands Of Populus Simonii And Robinia Pseudoacacie In Yangqin, Beijing

Aiming at the water consumption of forest trees and the shortage of water resources during the construction of forests in semi-arid areas for water conservation,during the rain seasons in July to October of 2003,5 typical stand sample plots of 2 locally prior species, Populus simonii and Robinia pseudoacacia L., in the little watersheds of Shangxinzhuang Village,Yanqing of Beijing which is located in the Guihe Basin of Gaungting Reservoir,were selected.Heat pulse measurement technology and water balance method were used for the research on the rules of plant transpiration and water consumption, the impacts of environmental factors and the composition of water-balance typical stands at whole-tree and stand scales.The research findings may provide certain scientific justification for the selection of drought-resistant and water-conserving tree species for the area and the rational planning of constructing forests for water conservation.The major findings obtained are as follows:(1)Characteristics of transpiration and water consumptionThe stem sap flow velocities of Populus simonii and Robinia pseudoacacia L.are differentiated in different directions (in the east,west,south or north),however,no interrelationship to tree crown structure and sapwood width has been found.The sap flow velocities in different relative points of stem section appear a rule of increase from inside to outside.The sap flows in deeper inside start later and stop earlier.The activating time of the stem sap flow is longer when it is closer to the bark.The stem sap flow velocities and sap flow of Populus sirnonii and Robinia pseudoacacia L.are changed obviously according to the change of day and night, and seasons.The stem sap flows weakly in the night and flows in a single-wave curve in daytime. Solar radiation intensity,air temperature,air humidity, soil temperature and wind speed are in remarkable correlation with the stem sap flow velocities.From July to September,2003,the stem sap flows in a relatively high peak value,but in an obviously lower one in October.The sap flow velocity of Robinia pseudoacacia L.is remarkably bigger than that of Populus simonii, their respective average peak values in the growth season are 12.1 cm/h and 7.6 cm/h.But due to the smaller area of sapwood of Robinia pseudoacacia L.than that of Populus sirnonii (the respective sapwood rate are 14.8％and 81.8％),the sap flow turnover and daily water consumption of a whole tree of Robinia pseudoacacia L are lower than Populus simonii,based on the same diameter class. The peak values of whole-tree stem sap flow velocities of Populus simonii at different diameter classes varies without a certain rule.Robinia pseudoacacia L.has a unobvious tendency to heighten as the increase of diameter class. The stem sap flow turnover and daily transpiration water consumption of the two species can be estimated from the sapwood width at breast height, which is directly estimated from the diameter at breast height.The sap flow turnover and daily transpiration water consumption are increased as the increase of the stem diameter at breast height. The depth of daily average transpiration water consumption of the typical Populus simonii plot is 1.37 mm/d and of the Robinia pseudoacacia L.plot is 1.46 mm/d.(2)Research on the water balanceThe canopy rainfall interception and forest runoff yield of both Populus sirnonii and Robinia pseudoacacia L.are significantly influenced by rainfall characteristics and stand structure.The canopy rainfall interception is prominently influenced by rainfall capacity and secondly by rainfall inttensity.The rate of canopy rainfall interception is increased as the increase of canopy density and leaf area index.The forest runoff yield has a positive relation with rainfall capacity and rainfall intensity,but it is not so obviously correlated between the runoff coefficient and rainfall characteristics.Meanwhile the forest water runoff and runoff coefficient are decreased as the reduction of ground coverage,but the impacts of soil properties and topography (gradient and slope position) are not obvious.The transpiration water consumption of arbor wood is the major content of forest stand water balance.The forest transpiration amounts of Populus sirnonii and Robinia pseudoacacia L.stands from July to October are respectively 128.2～138.4 mm and 92.1～187.0 mm.The transpiration amount of vegetation under tree of stands is much lower than that of arbor wood transpiration,with an average amount of below 10 mm every month.However,the transpiration amount of vegetation under Robinia pseudoacacia L.is bigger than that of vegetation under Populus sirnonii.The water transpiration amounts of the soil in all stands in July to October have not exceeded 50 mm and they will decrease as the increase of the canopy density.The depth changes of soil moisture in all stands are similar,with the largest variation of 5％～28％in the soil layer of 0～20 cm and the common variation of 7％～12％in the soil layer under 60 cm.The time order of the amount of soil water content in all stands is October >September >July >August.The order of the amount of soil water content of different forestland is Robinia pseudoacacia L.on slow slope > Populus simonii on steep slope > Robinia pseudoacacia L.on steep slope > Populus simonii on slow slope > Robinia pseudoacacia L.on mesa.The rainfall capacity of July to October of 2003 is 231.8 ram,lower than the average (about 350 mm) of multiple years in the same place.Thus 2003 can be counted as a light dry year.The accounting on the water balance of all stands in July to October shows the natural rainfall cannot meet the demand of tree growth.In particular,the shortage of soil moisture in the soil layer of 0-100 cm for Robinia pseudoacacia L.on mesa reached 138.2 mm.Populus simonii is also faced with a serious moisture shortage.The moisture shortage for Populus simonii on slow slope and Populus simonii on steep slope stands are respectively 37 mm and 53.5 ram.Generally, the moisture shortage of Populus simonii is bigger than that of Robinia pseudoacacia L.Therefore,Populus simonii is not an ideal selection for the afforestation in mountainous areas of the locality.Although the transpiration of Robinia pseudoacacia L.is small at its youth, its water consumption will increase to even more than the rainfall capacity as its growth and thus cause to the shortage of water. As a result,water-conserving management measures should be adopted to maintain the forestland water balance and the forest water runoff yield.