Vitality variation and population structure of a riparian forest in the lower reaches of the Tarim River,NW China

Maierdang Keyimu •U¨mu¨t Halik, •Florian Betz •Choimaa Dulamsuren

Introduction

Water is the driving force of the composition,development and stability of oasis ecosystems and the key ecological factor in hyper arid areas(Chen et al.2004a;Thomas et al.2006;Halik et al.2009;Disse et al.2012).Exploitation and utilization of water resources around the Tarim basin in Northwest China is increasingly counterproductive to protecting the ecology of the area(Chen et al.2004c;Rumbaur et al.2014;Keilholz et al.2015).The Tarim River is located in Xinjiang Uyghur Autonomous region(Xinjiang)in Northwest China(Fig.1),with its 1320 km mainstream it is the longest inland river in the country and the 5th longest inland river in the world(Keyimu et al.2014;Ling et al.2015).In the Tarim river basin,Populus euphraticaOliv.(Euphrates poplar)is the dominant tree species of the riparian forest(Huang 2002),and as a ‘‘green corridor’’(Gao et al.2007;Thomas et al.2006,2016)plays an indispensable role in maintaining balance in the regional ecosystem,environment,sand fixation,regulating the oasis climate and forming fertile forest soil(Halik et al.2006;Aishan et al.2013;Betz et al.2015).Since the 1950s,due to the excessive usage of water resources for agricultural activities in the upper and middle reaches of the Tarim river,water flow to the lower reaches was cut off(Halik et al.2008;Abdurahman et al.2013).Extreme water shortage then caused signi ficant changes in the riparian ecosystem in the lower river(Li et al.2008;Liu et al.2008;Chen et al.2010;Disse et al.2012):decline in groundwater level,withering of herbaceous plants,deterioration of the regional ecosystem,decrease in biodiversity,most importantly,serious degradation of theP.euphraticaforest,the constructive species of the riparian ecosystem and ecological processes in this region(Halik et al.2009;Aishan et al.2013;Keyimu et al.2014).Therefore,the deserti fication process was exacerbated,severely threatening people’s lives around the river(Wang et al.2009;Halik et al.2011b;Betz et al.2015;Keyimu et al.2016).

Fig.1 Map of the lower reaches of the Tarim River in Xinjiang,Northwest China with its two main ecological water diversion routes via the Qiwenkol River and the Old Tarim River(modi fied from Ling et al.2015).Field investigations were conducted at the Arghan transect

For the restoration of the degraded poplar forest and the damaged ecosystem in the lower Tarim River,Chinese government invested ca.US$1.6 billion in a project for emergency water diversion to the lower Tarim(Xu et al.2003;Ye et al.2009;Chen et al.2010;Aishan et al.2015a).With an increase in the water transferring times and the accumulated water since the water diversions started in May 2000,the groundwater level has risen,and the damaged riparian vegetation downstream has undergone different levels of recovery(Halik et al.2009;Aishan 2016;Keyimu et al.2016).The various effects of the water transfer on the regional ecosystem and the riparian vegetation has been evaluated in a number of studies(Liu and Chen 2002;Xu et al.2003,2004a,b;Chen et al.2004a,b,c;Li et al.2006;Thevs 2007;Halik et al.2009;Lam et al.2011;Liu et al.2013;Keyimu et al.2014,2016).For instance,Xu et al.(2007)and Chen et al.(2010)comparatively analyzed the groundwater table and salt content before and after the water diversion.Chen et al.(2004c)analyzed the physiological response of vegetation to the increased groundwater level after the water transfer.Li et al.(2004)and Yan et al.(2008)examined the growth status and species abundance of natural vegetation in response to the changing groundwater and soil moisture.Aishan et al.(2013,2015a)explored the variation in groundwater table in the vicinity of the river bed in different transects of the lower Tarim River in detail and the response of major eco-morphological variables of the riparian forest to variations in groundwater depth.

However,little is known about the consistency of the effect and ef ficiency of the water diversions on the restoration process of riparian forests.In this paper,we contribute to this area of knowledge with a comparative analysis of variation in the vitality ofP.euphraticaand its population structure using field investigation data on the tree growth parameters(tree height:TH,diameter at breast height:DBH,crown diameter:CD,crown lose:CL.,etc.)from 2005 and 2010 at Arghan on the lower Tarim River.Tree vitality is a theoretical concept that usually refers to the growth status of the tree stand and is often associated with the physiology and morphology of the tree stand(Halik et al.2009;Aishan et al.2015a;Aishan 2016).It is a key parameter to assess the overall health of a forest(Schulz and Hartling 2003).

The results of this research provide a scienti fic basis for the quantitative assessment of the ecological restoration along the lower Tarim River and the volume,period as well as interval of water delivery.

Materials and methods

Study area

Study area Arghan(40°08′50′′N,88°21′28′′E)is located in the lower reaches of the Tarim River between the Taklamakan and Kuruk-Tagh Desert in southern part of Xinjiang,Northwest China(Fig.1).The area is extremely arid,and the ecological environment is very fragile(Song et al.2000;Gao et al.2007;Abdurahman et al.2013;Keyimu et al.2017a).The annual precipitation is about 17–42 mm,and therefore,one of the most arid places in China(Song et al.2000;Aishan et al.2015b;Thomas et al.2016).From the 1970s,due to the cut-off of water flow and lack of surface water recharging,the groundwater level declined,diversity of the natural vegetation fell off,the population size of and the reproductive capacity(regenerative and vegetative)ofP.euphraticawas decreased on a large scale(Westermann et al.2008;Lam et al.2011;Eusemann et al.2013;Zhu et al.2016).The reduced vegetation density and ecosystem service lose led to increased sand movement,a potential hazard for road infrastructure in the region(Chai et al.2008;Halik et al.2011a;Betz et al.2015;Aini 2016).

The desert riparian vegetation has a very simple structure along the lower reaches of the Tarim River.The main tree species arePopulus euphraticaOliv.,Elaeagnus angustifoliaL.,and the main shrubs areTamarix hispidaWilld.,Tamarix elongataLedeb.,Tamarix ramosissimaLedeb.,Halimodendron halodendron(Pall.)Voss.,Lysium ruthenicumMurr.,Karelinia caspica(Pall.)Less.,Hexinia polydichotoma(Ostent.)H.L.Yang,Inulasalsoloides(Turcz.)Ostrnf.,Phragmites communis,Poacynum hendersonii(Hook.F.)Woodson.,Halostachys caspica(M.B.)C.A.Mey.,Alhagi sparsifolia(B.Keller et Shap.)Shap.andGlycyrrhiza in flataBat.(Xu et al.2004b;Yang and Guo 2004;Halik et al.2006,2009,2011a;Keyimu et al.2017b).The study site is located at the con fluence of two main waterways,the old Tarim River and the Qiwinkol River in the mid-lower part of the lower Tarim River.As a consequence,the natural vegetation coverage of this area is relatively high,and restoration ofP.euphraticaafter water transfer can be seen.Therefore,this area is regarded as the ideal place for evaluating the rehabilitation of the degraded forest community.

Site selection and measurements

In study area,for long-term monitoring the response of poplar forest to the water diversion,100 plots with an area of 100 ha were established(each plot is 100×100 m)in 2005(Fig.2).Considering the relation between groundwater depth and the distance from the river,all plots were divided into 10 fixed measuring belts,from 0 to 20 m(A,included 15 plots),＞20–50 m(B,12 plots),＞50–100 m(C,10 plots),＞100–200 m(D,10 plots),＞200–300 m(E,10 plots),＞300–400 m(F,10 plots),＞400–500 m(G,10 plots),＞500–600 m(H,11 plots),＞600–800 m(I,7 plots)and＞800–1000 m(J,5 plots).The plots included 4777P.euphraticatrees,and each tree was labeled with a permanent identity number.Major growth variables ofP.euphratica,e.g.,tree height(TH),diameter at breast height(DBH),crown diameter(CD)and crown loss(CL)were measured during the growth season(Musha et al.2009;Aishan et al.2015a).In addition,digital photos and geographical coordinates of each tree stand have been taken for the systematic analysis on DBH and population structure.To illustrate the effect of water diversion,the number of newly generatedP.euphraticatrees within 50 m distance to the river has been recorded in 2010.Tree vitality can’t be measured directly,but in forestry,crown transparency or crown loss can be used instead for ranking tree vitality(Dobbertin 2005),in this study,we used the vitality classi fication criteria of Aishan et al.(2015a).Information about the 12 water diversions and data for six groundwater monitoring wells located in the study area were provided by the Tarim River Administration Bureau.

Fig.2 QuickBird satellite image acquired on July 5,2005,showing(A)location of the long-term monitoring plots(40°08′25′′N–40°08′54′′N and 88°20′34′′E–88°21′42′′E)and(B)location of random monitoring plots(40°07′52′′N–40°08′25′′N and 88°21′31′′E–88°22′40′′E)

Data processing

The number of trees for each different vitality,within different DBH groups and distance to the river course,groundwater variation was processed by Excel 2015 software(Microsoft,Redmond,Washington,USA).Nonlinear regression analysis was used to reveal the population structure of trees within each measuring belt.Attest was done for the Gaussian fit results(atP≤0.05 level),and cluster analysis has been carried out using a near distance algorithm for the composition study of theP.euphraticacommunity.The statistical analysis and plotting were carried out with SPSS statistics 20.0(IBM,USA)and Origin 8.5(OriginLab Corp.2011).

Results and analysis

In this study,theP.euphraticatrees were classi fied into different DBH groups,more than 90%of the poplar trees had a BDH less than 72 cm,so they were categorized in 18 groups,each separated by 4 cm.Only a small number of poplar trees had DBH greater than 72 cm,so they were in one group;however,the measurement data for 2010 revealed that vitality variation of trees in this group was less obvious;therefore,they were excluded during data processing.

Vitality variation among trees of P.euphratica

Establishment and growth ofP.euphraticamainly depend on the groundwater level(Song et al.2000).A groundwater depth between 3.5 to 5.0 m is needed to ensure the succession and natural regeneration of the poplar community in the lower Tarim River(Xu et al.2003).From the 1970s to 2000s,the water flow cut off to the lower Tarim River decreased the groundwater level and soil moisture on the two sides of the riverbed,and the vitality ofP.euphraticacorrespondingly declined.Under the in fluence of 10 years of water diversion,the groundwater level rose,and soil moisture increased to some degree(Grashey-Jansen et al.2014),positively impacting the revitalization process of poplar trees.In total,4777 individual trees within six different vitality classes(Table 1;Aishan et al.2015a)at Arghan were investigated to assess the effect of man-made water transferring on vitality variation among the population ofP.euphratica.The results showed that the number ofP.euphraticatrees in the healthy,good and medium vitality classes increased in 2010,while the number that were senescing,dying or dead decreased.This increase in vitality was more obvious in A,B,C,D and E measuring belts that were within 300 m of the river(Table 2).The groundwater level in areas more than 300 m from the river course did not respond well,however,because the areas received less water for short durations during the diversions and was lower than the level needed to maintain the good growth of the poplars(Fig.5).Therefore,the growth of poplar trees in belts F,G,H,I and J was still restricted,and their vitality slightly declined;the number ofP.euphraticatrees in the healthy,good and medium vitality classes decreased,while the number in the senescing,dying and dead classes increased(Table 2).

Population structure of P.euphratica

The DBH,one of the most common dendrometrics,can re flect the structure of the forest population and the growth status of tree stands(Condit et al.2000;Di et al.2014).P.euphraticawithin 0–15 cm DBH are young trees,16–40 cm are medium trees and DBH ＞40 cm are large trees(Wang et al.1996).

The population structure ofP.euphraticatrees in the measuring belts that were established based on the horizontal distance to the river course were analyzed based on the terrestrial investigation data in 2010.In the nonlinear regression analysis usingP.euphraticaDBH data to demonstrate its population structure(Manabe et al.2000;Di et al.2014),the DBH distribution ofP.euphraticain belta showed a reverse J-distribution (logistic fit,R2=0.93),which means thatP.euphraticain this area was expanding trend(Di et al.2014).In belt B,C,D,G,H and J,the distribution ofP.euphraticaapproximated a Gaussian fit(R2=0.86,0.96,0.89,0.78,0.85 and 0.94,respectively)(Fig.3).In belt G,although the curve showed a Gaussian fit,the normal distribution test result was not reliable(P＞0.05),but the curve peak covered medium trees(16≤DBH≤36 cm).In other belts(E,F and I),the distribution showed a polynomial fit.As seen in Fig.3,that the curve peaked around the medium DBH class trees,which means that were relatively more mediumP.euphraticatrees than in the other DBH classes in these belts.

Cluster analysis on the composition ofP.euphraticain the different DBH classes showed that all the trees in sampling site can be classi fied into three different population groups:the first group includes trees between 20 to 36 cm DBH(medium trees,total of 2794 trees);the second group includes trees with DBH of 0–16 cm and 32–40 cm(mostly young trees,total of 1498 trees);the third group includes trees between 40 to 72 cm DBH(large,total of 485 trees)(Fig.4).The medium poplar community was most abundant in the sampling site,4013P.euphraticatree stands(accounted for 84%of total number of individuals)in the study area had a DBH less than 36 cm(young and medium).

Impact of groundwater level on poplar forest distribution and generation

In the lower reaches of the Tarim River,the threshold of groundwater depth for sustaining the normal growth of vegetation is 4.5 m(Wang et al.2008).Considering the current water shortage and main vegetation types(mainlyP.euphraticaandTamarixspp.)in this region,the most reasonable groundwater depth range is 3.5 and 5.0 m(Chen et al.2003),less than this causes the weakening ofP.euphraticagrowth and its eventual death(Chen et al.2005).Data for groundwater wells in this region showed that groundwater level decreases with increasing distance from the river(Fig.5).

In Arghan,the density ofP.euphraticatrees on the two sides of the river diminished with increasing distance from the river;yet its density can be higher depending on topographic features,human activity and old waterways in the middle section of the investigation area.We found that 80%of the trees ofP.euphraticaare located within 200 m to the river.According to the DBH structural characteristics,juvenileP.euphratica,considered the decisive indicator for the steady development of a poplar community(Keyimu et al.2014)are present in the highest proportion in the A and B measuring belts(0–50 m to the river)where the groundwater level and soil moisture content are highest.At more than 400 m from the river,due to the sharp decline in the groundwater level,the growth and regeneration needs ofP.euphraticaare not met;thus its density begins to decrease greatly,with less than 5%of the total trees,and most have low vitality(Fig.6).

After recent years of water diversions to the lower Tarim River,numerous juvenileP.euphraticain the area close to the river were newly generated due to the replenished groundwater and increased soil moisture.During field investigations in 2010,we recorded theseP.euphraticawithin 50 m of the riverbed where the higher soil moisture is most favorable for establishment and growth of juvenile trees.In 2005,there were 307 juvenileP.euphraticain this area in 2005(12.13%of allP.euphraticain this area)and in 2010,there were 762(accounting for 25.52%of the total).This further investigation demonstrated that the continuous water diversion activity had a positive effect on the generation of young poplars.

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Discussion

In the lower reaches of the Tarim River,the vitality ofP.euphraticahighly depends on the groundwater level and soil moisture(Halik et al.2006,2009;Wang et al.2008;Ginau et al.2013;Aishan et al.2015a).When the groundwater level is suf ficiently high for the development of tree stands,the vitality ofP.euphraticawill be good(Aishan et al.2013).Since the 1970s,when the flow cut off in the lower reaches of the Tarim River,groundwater level on the both sides of the riverbed decreased sharply,eventually diminishing tree vitality.Halik et al.(2009)and Aishan et al.(2015a)found that the water diversion enhanced the revitalization process of the degraded riparian forest and the poplar forest ecosystem.However,as the levels of groundwater decrease farther from the river,the vitality of poplar forest in these areas also decreases.The present study showed that the poplar community within 300 m of the riverbed showed a signi ficant positive revitalization trend,but the revitalization trend far from the riverbed was negative,corresponding with the groundwater level variation in the study area.

Fig.3 DBH distribution of P.euphratica and the curve fit at different distances from the river(a–j).a 0–20 m,b 20–50 m,c 50–100 m,d 100–200 m,e 200–300 m,f 300–400 m,g 400–500 m,h 500–600 m,i 600–800 m,j 800–1000 m

Sustainable forest management requires a basic knowledge of forest population ecology(Kimmins 2004).Our study demonstrated that there were numerous young poplar trees close to the river,and the population structure had a reversed J-curve indicative of an uneven-aged forest structure(Wittwer et al.2004).For maintenance of this forest structure,new seedlings need to regenerate every year.Figure 7 shows that in 2005 and 2010,the population structure of the forest in the first measuring belt generally remained the same(both logistic fits),and in 2010 in the second belt due to the newly generated poplar seedlings,the population structure had a logistic fit in contrast to the Gaussian fit in 2005,indicating that water diversion activity to the lower Tarim River is promoting forest regeneration of this area.However,poplar trees in other measuring belts(B,C,D,F,H and J)farther from the river had nearly a bellshaped DBH distribution,which means that the forest structure has an even-aged status(Wittwer et al.2004).Relatedstudieshaveshownthatnaturalforestsoftenhavean even-aged forest population structure(Ashton 1976),and development of the forest community is stable(Oliver and Larson 1990).Based on the cluster tree analysis of the DBH compositionofP.euphratica,thediameterofmorethan80%ofthetreesis＜40 cm(youngandmediumtrees),whichalso suggests stable development of tree population.

Fig.4 Clustering tree of P.euphratica for different DBH classes(investigation data of 2010 on 4777 P.euphratica trees were used to create the graphic)

Fig.5 Groundwater depth in relation to the distance from the river in three different years

Fig.6 Number of poplar trees at different distances from the Tarim River

Fig.7 DBH distribution and its fitting curve of poplar trees in the belt a and b in 2005 and 2010

During the water diversion activity,the groundwater level rises to a certain level.Particularly near the river,groundwater and soil moisture were raised to level that can sustain normal growth and development of juvenileP.euphratica(Chen et al.2010;Cao et al.2012)and the establishment of young poplar seedlings.JuvenileP.euphraticaaccounted for 25.52%of the total trees near the river belts(A and B).AlthoughP.euphraticahas drought resistance characteristics,its seed has very low vitality,without a dormancy feature(Wang et al.1996).Thus,a set of conditions must be met for the germination and survival of young seedlings(Thevs et al.2008).In natural conditions,P.euphraticaseed loses 70%of its vitality after 24 h,so it needs soil with higher moisture content(Wang et al.1996)and is the main reason that so many juvenileP.euphraticaare near the river.As groundwater level decreases farther from the river,the generation capacity of poplar forests in the far regions from the river decreases(Chen et al.2006);the number of juvenileP.euphraticaalso diminishes,while the amount of trees within larger DBH classes increase.Most of theP.euphraticatrees farther from the river are over-mature and with low vitality due to groundwater levels that are insufficient for the longterm maintenance of the riparian vegetation.

Conclusions

After the past 10 years of waterdiversion,theP.euphraticacommunity has recovered at certain degree due to the raised groundwater level.The poplar trees within 300 m of the river showed a positive revitalization trend,while the vitality of trees farther from the river showed a negative trend,highlighting limitations in the water diversion practices for the lower Tarim River.Population structure and composition analysis on poplar forest showed that more than 80%of the trees belong to the small and medium tree groups(＜36 cm DBH).Thus,the development of the poplar forests in the lower reaches of the Tarim River is generally stable.In the vicinity of river,the density ofP.euphraticawas greater than farther from the river.Similarly,more juvenileP.euphraticatrees grew close to the river where soil moisture was better for regeneration as opposed to farther from the river where few juvenile poplar trees regenerated,due to less water from the diversions.Therefore,to realize the wide scale rehabilitation of the degraded riparian forest,the water diversion practices need to be improved,and at the same time,young seedlings ofP.euphraticaneed to be better protected.More importantly,exploitation of water resources in the upper and middle reaches needs to slow to ensure that suf ficient water can be allocated to the lower reaches of the Tarim River.Acknowledgements This research work was supported by National Natural Science Foundation of China(Grant Nos:31360200,31270742)and the German Volkswagen Foundation within the framework of EcoCAR project(Az.:88497).We express our gratitude toward Prof.Dr.Alishir Kurban,Dr.Tayierjiang Aishan,Dr.Abdulla Abliz,Dr.Aliya Baidourela and Ms.Gulpiye Omer from Chinese Academy of Sciences and Xinjiang University for their dedication during the fieldwork.In addition,we thank the Tarim River Basin Administration Bureau for providing hydrological data and the Forestry Department of Qarkilik(Ruoqiang)for logistical support during our fieldwork in Arghan.We also thank the editors and anonymous reviewers who helped to improve the manuscript.

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