Study On The Water Use Strategy Of Different Forest Age Of Populus Euphratica In The Lower Reaches Of Tarim River

Water availability is one of the most important factors affecting the growth and reproduction of plant in terrestrial ecosystems particularly in arid regions.The research of water sources on Populus euphratica of different ages in the lower reaches of Tarim River by using the stable hydrogen and oxygen isotope technology helps to understand the water circulation and utilization strategies of desert riparian forest under the background of ecological water conveyance,which can provide the scientific basis for ecological water conveyance engineering and provide the reference for the ecological restoration of similar areas.In the research,we studied the P.euphratica of different ages in the same habitat and measured hydrogen and oxygen stable isotope ratio(?D and?¹⁸O)values of xylem water and potential water sources?soil moisture and ground water?.We used multi-isotope mass balance analysis?IsoSource?to calculate the possible contributions of potential water sources to total water content in plants.In addition,we took the leaves of different ages of Populus euphratica in the experimental sample and made samples,and used the stable carbon isotope technique to measure stable carbon isotope ratio(?¹³C)of three ages of P.euphratica in the sites where parallel to riverway and perpendicular to riverway respectivelyThe conclusions were as follows:?1?The climate is dry and the surface soil is strongly evaporated in the lower reaches of the Tarim River,which causes the water content is lower than deep soil.The soil of 0-340 cm of three kinds of populus euphratica were divided to the water dispersed layer,the moisture change layer and the moisture stable layer.That was related to the soil texture types at different depths of the Populus euphratica.For The young forest,the water content for 0⁸0cm is low and the degree of variation is low;soil moisture content for 80¹40cm increased significantly and the degree of variation was higher,soil water content for 140³40 cm tended to be stable,and the variation degree decreased significantly.For mature forest of Populus euphratica,Soil water content for 0-40 cm was low and the coefficient of variation was low,soil water content for 40-140 cm increased obviously and the coefficient of variation was higher,soil water content for 140-340 cm was in stable state and the degree of variation was low.For The over-mature forest of Populus euphratica,Soil water content for 0-60 cm was lower and has a smaller variation coefficient.The increase of water content in 60-100 cm was obvious with a large coefficient of variation.The water content in 100-340cm was in a stable state with a lower variation degree.With the increase of forest ages,the salt content,total nitrogen and total potassium all increased first and then decreased.The total nitrogen and total potassium reached the highest value for the mature forest stage.pH and total phosphorus were not affected by the age of forest.?2?There were significant differences of?¹⁸O value in the soil moisture of different depths,while?D values were no distinctly different.The?¹⁸O values of young-growth forest,mature forest and over-mature forest were-7.04‰±0.07,-8.53‰±0.11 and-9.36‰±0.21,which were also significant different;The shallow soil water in 0–60 cm was significantly affected by evaporation.The isotopic composition of soil water had experienced a strong evaporation and fractionation process.And soil water content was small,while?¹⁸O values were large.The?¹⁸O values of soil water in 60-340cm were lower and became lower while the distance was deeper from soil surface and gradually approached to ground water;The water utilization of Populus euphratica in different ages came from different sources.The average utilization ratio of soil water and underground level for 80–140 cm,140–220 cm,220–340 cm were 16.2%,21.4%,24.6%and 24.5%in turn.The mature wood of Populus euphratica mainly used the soil water in 220–340 cm and the groundwater with the utilization rate of 36.9%and 42.3%;The over-mature wood mainly used the soil water in 140–340 cm and groundwater,with the utilization rate of 32.8%and 49.3%.?3?Water use efficiency?wue?of the three ages of populus euphratica maintained in lower level,water use efficiency of young-growth was maximum,the mature forest was second,and over-mature forest was minimum.But carbon stable isotope ratio of the mature approximated to the over-mature,which may be associated with root system distribution of p.euphratica,young-growth forest adapt to drought stress by increasing water use efficiency due to the shallow root system distribution.As the distance increases from the river,whether young-growth forest,mature forest or over-mature,water use efficiency of populus euphratica were raised correspondingly.In the range of 0-300m from the river,the growth trend of water use efficiency of three ages p.euphratica were relatively flat,and the site of400m from the river,the the value changed opposite bigger compared to before,the growth trends were obvious,may be due to the increase of the distance,the influence of river water to plants reduce,underground water level fall,leading to plants adapt to the environment by increasing water use efficiency.