| Forest ecosystem in North China is an important ecological barrier in North China Plain.The relationship between forest ecosystem and water resources is a key issue in forest vegetation construction in North China.In order to study the process and mechanism of forest water movement in North China,the dominant afforestation tree species of Platvcladus oriental is and Quercus variabilis in Beijing mountainous and Populus in plateau of Zhangbei were selected as the research objects in this paper.Based on the stable hydrogen and oxygen isotope technology,the water absorption ratio of trees at different time scales was quantitatively analyzed by precipitation gradient control experiment.In addition,in order to reveal the water movement mechanism of forests in North China and provide reference and basis for forest vegetation construction in North China,the long-term data of TDP,HRM and meteorological were used to studying the water transport,utilization and balance process of main forests in North China.The main results of this paper are as follows:(1)The annual precipitation in Beijing mountainous area from 2015 to 2017 was 513.0 mm.The precipitation increases first and then decreases with the season.The precipitation in June-September(rainy season)was more than 75%of the annual precipitation.In the treatment area of no precipitation,the variation of soil water content(SWC)in each layer was relatively stable.The order of average SWC in each layer was 80-100 cm>60-80 cm>40-60 cm>0-20 cm>20-40 cm.In the treatment with precipitation,the response of SWC to different rainfall was different.The SWC in shallow layer of 0-20 cm and 20-40 cm were sensitive to rainfall of<10 mm,while that in middle layer of 40-60 cm and 60-80 cm were no significant to rainfall of<10 mm and that in deep layer of 80-100 cm was not sensitive to rainfall of<10 mm.The 10-20 mm precipitation could significantly increase SWC of 40-60 cm and 60-80 cm in the middle layer,but had no significant effect on SWC of 80-100 cm in the deep layer.The>20 mm precipitation can penetrate 80-100 cm deep soil and increased 80-100 cm soil water content.There was no obvious seasonal variation of ?2H and ?18O of soil water in the no precipitation treatment.The ?2H and ?18O of soil water soil water in five layers fluctuate around the mean.The ?2H and ?18O of soil water were increased with the increase of soil depth in the treatments of half precipitation,natural precipitation and twice of precipitation.The values of ?2H and ?18O of soil water in dry season were higher than those in rainy season.The values?2H and ?18O of in spring water were relatively stable,and their ranges of variation were-73.51‰ to 68.18‰ and-10.84‰ to-9.79‰,respectively.(2)In the treatment area of no precipitation,P.oriental is was mainly absorbed groundwater and 60-100 cm deep soil water,and the absorption ratios of these two water sources were 28.2-38.3%and 22.1-39.6%,respectively.The seasonal variation of P.orientalis's water source was not obvious in this treatment;Q.variabilis was absorbed water from all soil layers and the absorption ratios of these water sources were uniform.In the dry season of half precipitation and natural precipitation treatments,P.orientalis was mainly absorbed water from 60-100 cm soil water and groundwater.The average proportion of absorption to these water sources were 26.6-30.1%and 26.3-30.6%,respectively.In rainy season,P.orientalis were sensitive responded precipitation.It was not only utilized 60-100 cm soil water form deep layer and groundwater in natural precipitation area,but also absorbed water from 0-20 cm soil water in surface layer.The utilization ratio of these water sources were 23.7-30.5%,21.6-26.8%and 25.5-26.6%,respectively.The main water source of Q.variabilis gradually from surface soil layer transfers to deep soil in rainy season.During the whole observation period,60-100 cm soil water and groundwater were the main water sources of Q.variabilis.In the twice of precipitation treatment,0-20 cm,20-40 cm,60-100 cm and groundwater are the main water sources of P.orientalis.With the increase of precipitation,the water source of P.orientalis was gradually transferred from deep water to shallow water,while Q.variabilis were used more groundwater and 60-100 cm deep soil water.(3)With an increased in the degree of degradation,the SWC was decreased and the ?2H and ?18O of soil water was increased.The root system of Populus was "umbrella" shaped.Its root system can extend 240 cm in vertical direction and 200 cm in horizontal direction,respectively.The root system of Populus has a"dimorphism" structure,which can absorb soil moisture from multiple water sources.With an increased in the degree of degradation,the utilization ratio of deep soil water by Populus was decreased,and the utilization ratio of surface soil water by Populus increased.Populus tends to absorb and utilize deep soil and water sources with high water content under normal growth conditions,but Populus in moderate and severe degradation tends to absorb water from shallow soil.However,the surface soil moisture was changeed greatly and the soil moisture content was low.The lack of water supply may be the main reason for the degradation of Populus.(4)In the growing season,the sap flow rates of P.orientalis and Q.variabilis increased first and then decreased in typical sunny days.The sap flow rate of broad-leaved forest in growing season was higher than that of coniferous forest,and that of rainy season was higher than that of dry season.Under natural conditions,the sap flow rate was about three times higher than that in drought conditions.The sap flow rate at the main root was less than that in the breast diameter,but greater than that in the lateral root.The sap flow rates of P.orientalis and Q.variabilis were significantly correlated with air temperature,solar radiation and air humidity,respectively,but no significantly correlated with wind speed.Under natural conditions,the reverse flow of the lateral roots of P.orientalis occurred only in the last few days of August.The reverse flow of the lateral roots was higher than that of the forward flow under drought conditions.(5)The time lag of sap flow of P.orientalis and Q.variabilis was different in the same month.The time lag of radiation and VPD dependence of P.orientalis was longer than Q.variabilis.The time lag curve of sap flow fitting with VPD and radiation was single peak curve.The photosynthetic rates of P.orientalis and Q.variabilis were similar in each month.P.orientalis showed a double-peak "M-shaped" change in the dayily,reaching the first and second peaks at 10:00 and 14:00,respectively.Q.variabilis showed a single-peak "convex" curve which increased first and then decreased in the dayily.The peak photosynthetic rates of P.orientalis and Q.variabilis in rainy season were higher than those in dry season.The transpiration rates of P.orientalis and Q.variabilis were positively correlated with radiation and VPD,and the photosynthetic rates of P.orientalis and Q.variabilis were positively correlated with radiation and temperature.(6)The night sap flow of P.orientalis and Q.variabilis were both used water trunks to store water.The water storage of trunks showed seasonal variation.The water storage in rainy season was larger than that in dry season,and that in sunny days was larger than that in non-sunny days.The average proportions of reverse liquid flow,transpiration water consumption,photosynthetic water consumption and trunk water storage of P.orientalis to total sap flow were 2.26%,91.16%,8.84%and 6.37%,respectively.Because there was no reverse flow in Quercus variabilis,the reverse flow was neglected in Q.variabilis.The transpiration water consumption,photosynthetic water consumption and trunk water storage of Quercus variabilis account for 94.63%,5.37%and 6.09%of the total flow,respectively.The photosynthetic water consumption and water storage of P.orientalis and Q.variabilis only account for a small part of the total flow,while the transpiration water consumption of P.orientalis and Q.variabilis variabilis were the main way of water use. |
PDF Full Text Request