Research Article Effects of Drought Frequency on Growth Performance and Transpiration of Young Black Locust (Robinia pseudoacacia L.

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1 Interntionl Forestry Reserch, Article ID , 11 pges Reserch Article Effects of Drought Frequency on Growth Performnce nd Trnspirtion of Young Blck Locust (Robini pseudocci L.) Drio Mntovni, 1,2 Mik Veste, 3 nd Dirk Freese 2 1 Interntionl Grdute School, Brndenburg University of Technology Cottbus-Senftenberg, Konrd-Wchsmnn-Allee 6, 346 Cottbus, Germny 2 Chir of Soil Protection nd Recultivtion, Brndenburg University of Technology Cottbus-Senftenberg, Konrd-Wchsmnn-Allee 6, 346 Cottbus, Germny 3 Centre for Energy Technology Brndenburg e.v. (CEBr), Friedlieb-Runge-Strße 3, 346 Cottbus, Germny Correspondence should be ddressed to Drio Mntovni; mntdr2@gmil.com Received 19 November 213; Revised 28 Jnury 214; Accepted 28 Jnury 214; Published 17 Mrch 214 Acdemic Editor: Kihchiro Kikuzw Copyright 214 Drio Mntovni et l. This is n open ccess rticle distributed under the Cretive Commons Attribution License, which permits unrestricted use, distribution, nd reproduction in ny medium, provided the originl work is properly cited. Blck locust (Robini pseudocci L.) is drought-tolernt fst growing tree, which could be n lterntive to the more common tree species used in short-rottion coppice on mrginl lnd. The plsticity of blck locust in the form of ecophysiologicl nd morphologicl dpttions to drought is n importnt precondition for its successful growth in such res. However, dpttion to drought stress is detrimentl to primry production. Furthermore, the soil wter vilbility condition of the initil stge of development my hve n impct on the tree resilience. We imed to investigte the effect of drought stress pplied during the resprouting on the drought tolernce of the plnt, by exmining the blck locust growth ptterns. We exposed young trees in lysimeters to different cycles of drought. The drought memory ffected the plnt growth performnce nd its drought tolernce: the plnts resprouting under drought conditions were more drought tolernt thn the well-wtered ones. Blck locust tolertes drstic soil wter vilbility vritions without ltering its wter use efficiency (2.57 g L 1 ), evluted under drought stress. Due to its constnt wter use efficiency nd the high phenotypic plsticity, blck locust could become n importnt species to be cultivted on mrginl lnd. 1. Introduction Summer drought, s observed during extreme events in Centrl Europe in 23 [1],isoneofthemjorbiotic stress fctors tht limit plnt growth nd hve drstic effects ontheecosystemproductivity.theongoingclimtechnge mplifies the internnul climte vribility nd chnges thesesonldistributionofrinfllincentrleurope[2]. Within Centrl Europe prticulrly the southern prts of Brndenburg nd Polnd will be highly vulnerble to climte chnge nd decrese in summer precipittion is forecsted. Consequently, drought periods during the growth seson re to be expected [3]. In this region the reduced soil wter vilbility (SWA), in combintion with sndy soils, hs led to negtive effects on the productivity of the ecosystem [4]. An integrted concept of ctive species selection nd n pproprite mngement of tree stnds could mitigte the effect of the climtic stress to certin extent. Therefore, it is importnt to ssess the plsticity of the species to drought stress by understnding the plnt response in terms of wter consumption, growth performnce, nd production [5]. This is prticulrly importnt for short-rottion coppice (SRC) systems, where the primry production is driven minly by the SWA [6, 7]. Blck locust (Robini pseudocci L.), s drought tolernt species suitble for SRC, cn be grown even on reclimed post-mining sites, where the edphic conditions re extreme [8]. Vrious studies emphsize the blck locust morphologicl nd ecophysiologicl dpttions in coping with long-term drought stress [9 11]. Hence, the ecologicl stress memory, defined by Wlter et l. [12] snyresponse of single plnt fter stress experience, which improves the response of the sme plnt towrds future stress experiences,

2 2 Interntionl Forestry Reserch could be n importnt fctor when evluting the drought stress tolernce of the species. Depending on the drought stress severity, reduction of the primry production due to these morphologicl nd physiologicl djustments is to be expected [13 15]. Stomtl closure is typicl temporry rection to drought stress, leding to reduced CO 2 uptke nd finlly influencing crbon budgets nd growth performnces [16, 17]. Besides physiologicl djustments, drought stress occurring during the tree development induces morphologicl dpttions from the roots to the crown, which willchrcterizethehydrulicrchitectureofthetree[18 23]. As consequence the trnspirtion of the plnt will be influenced, long with its tolernce to drought stress [18 2]. Hence the recovering process fter drought stress in terms of growth increment could be ffected by the SWA t the initil stge of resprouting. In spite of the importnce of this topic for coppicing prctices, up to this dte there is still considerble informtionl gp: the effect of the drought occurring during the resprouting period on the drought tolernce, biomss production, nd wter use under drought stress hs not been investigted. In our study, we imed to ssess the impct of drought cycles, pplied during the resprouting phse on the drought resilience of blck locust trees. We defined the physiologicl conditions in terms of plnt wter sttus t which the growth performnce is compromised ndnlyzedtheinfluenceofdroughtppliedduringthe resprouting phse on the hydrulic rchitecture of the tree, evluting the reltion between root system nd the totl trnspiring surfce. The effect of the different root weight: lef re rtio ws evluted by nlyzing (i) growth performnce, (ii) trnspirtion rte, (iii) boveground biomss production, nd (iv) the wter use efficiency under cyclic drought stress. The informtion gthered enlightened the importnce of blcklocustsnppropritetreeformrginllndndthe strong impliction tht the SWA during the resprouting phse hs on the lter drought tolernce nd productivity of the species. 2. Mteril nd Methods 2.1. Plnts nd Drought Stress Tretment. The study ws crried out with three-yer-old blck locust trees in lysimeter experiment, under light trnsmissive rin-out shelter, rrnged t the Brndenburg University of Technology Cottbus-Senftenberg. The wter ws supplied solely by n utomtic irrigtion system. The plnt mteril ws collected from short-rottion plnttion in the postmining re of Welzow-Süd, Brndenburg, Germny (N , E ), pproximtely 25 km south from the lysimeter fcility. The climte t the collection site nd t the lysimeter fcility is comprble. It is trnsition from the Atlntic to the continentl climte, with men nnul rinfll of 556 mm nd men nnul temperture of 9.3 C( , Deutscher Wetterdienst). During the experiment, the highest ir temperture in Cottbus ws reched in August 212 with more thn 36.5 C. The monthly men temperture ws +.7 C bove the long-term verge, while the erly summer ws.5 C (June) nd.3 C (July) colder thn the verge. However, extreme conditions with ir tempertures bove 3 Cndeven35 Csobservedin212recommonin Brndenburg. All the selected trees presented comprble crown rchitecture, with three min brnches (men rdius of 1.6 ± 1.9 cm) connected to the trunk (men rdius of 13.4 ± 1.9 cm, height pprox. 3 cm). At the beginning of the experiment, ll the primry brnches were cut bck t 1 cm from the trunk. After period of estblishment (Mrch 2 to June 1, 212), during which the plnts were well wtered, the drought cycle experiment strted on June 1 until October11withtwotretments:short-termdroughtstress () nd long-term drought stress (), ech executed in three replics (n = 3). In our context we defined drought stress s condition where the men soil wter content (θ) ws mintined t vlues of 7%, which is the vlue t which the growth rte drsticlly decreses to vlues close tozero.thewell-wteredconditioninstedcorresponded to the hydrosttic stte of the lysimeter, where the men of the θ ws set t 2%, with full vilbility of wter. Since ll tretments strted developing new leves during the lst week of My, we considered the resprouting phse the period fromthe23rdtothe26thweekoftheyer(woy).the cycling of drought nd full vilbility of wter ws scheduled s follows: the plnts experienced drought conditions throughout the four weeks of the resprouting period. Insted, the plnts were exposed only to one week of drought (24thWOY)duringthesmeperiod.Aftertheresprouting phse, three weeks of well-wtered conditions followed for both tretments. From the 3th WOY on both tretments were exposed to drought stress for two weeks. From the 33rd WOY until the end of the experiment (41st WOY), the plnts were subjected to short-term drought stress (one week of drought nd two well-wtered ones), while the plnts were subjected to long-term drought stress (two weeks of drought stress, followed by one week of well-wtered condition) Lysimeter nd Soil Wter Sttus. In order to control the SWA nd to clculte the wter budget, the plnts were grown in lysimeters (5 cm dimeter, 5 cm height) under light trnsmissive rin-out shelter [11]. The dily trnspirtion ws clculted by the difference between the mesured irrigtion nd the sum of the estimted dily wter storge vrition nd the lechte collected. Evportion from bre soil ws minimized by covering the soil with double leflet. To fcilitte the infiltrtion rte, the sndy lom soil column ws filled t bulk density 1.3 kg m 3. The soil, reltively low in orgnic crbon (1.3%) nd totl nitrogen (.8%), presented the oxlte-extrctble Al, Fe, nd Mn content vlues of.65, 1.53, nd.8 mg g 1, respectively. The irrigtion system ws utomticlly controlled by dt logger (GP1, Delt-T Devices, Cmbridge, UK) nd the wter ws dispensed from four drippers per lysimeter. The volume of wter ws supplied in reltion to the volumetric soil wter content, mesured t 2 cm depth by frequency domin reflectometry probe (SM-2, Soil Moisture Sensor Delt-T Devices, Cmbridge, UK). To evlute the storge vrition of the weekly wter budget, the θ ws mesured t four depths (1, 2, 3, nd

3 Interntionl Forestry Reserch 3 Plnt wter potentil (MP) Soil moisture (vol-%) Soil mtric potentil Plnt wter potentil Figure 1: Predwn lef wter potentil nd soil mtric potentil in reltion to the soil moisture nd soil wter retention curve. 4 cm) with profile probe (PR2/4w-2, Delt-T Devices, Cmbridge, UK). The climtic vribles ir temperture nd reltive humidity were monitored on n hourly bsis by using thermistor-hygrometers (HC2-S, Rotronic AG, Bssersdorf, Germny). The vpor pressure deficit (VPD) vlues were clculted for the dytime by the ir temperture nd reltive humidity recorded from 9: HR to 18: HR locl summer time Lef Wter Potentil. We linked the physiologicl condition of the trees in terms of plnt wter sttus to the growth rte by mesuring the lef wter potentil (Ψ L )t different soil moisture conditions. The lef wter potentil ws mesured t predwn time on three fully developed leves for ech tree by using the Scholnder pressure chmber (Plnt Wter Sttus Console 3, Soilmoisture Inc., Snt Brbr,CA, USA)fter Veste et l. [24]. The wter potentil of the distl prt of the rchis, pproximtely 1 cm completed with leflets, ws mesured immeditely fter the excision. We linked the Ψ L to the SWA, by using the soil wter retention curve of the lysimeter soil column fter Mntovni et l. [11] to convert the soil θ vlues red by the sensors into soil mtric potentil (Ψ S ) vlues (MP) (Figure 1). The correltions between Ψ S nd Ψ L nd between Ψ L nd θ re significnt, with coefficient of correltion of.749 nd.757, respectively (Figure 1). Hving this wy both vribles expressed in the sme unit (MP), we used the liner reltion (Ψ L = 9.13 Ψ S, r 2 =.86) described in Figure 2, to set the indirect plnt-bsed irrigtion control nd monitoring system Growth Rte nd Biomss Production. The response to drought stress in terms of growth increment throughout the vegettion period ws evluted on different time scle by using dendrometers (DD-S, Ecomtik, Dchu, Germny) nd cliper. The dendrometers were instlled round the trunk of ech plnt t 2 cm height from the soil nd the rdil vritions were recorded dily t dwn time 5: HR,tovoidbismesurementsduetoexcessivetrunk shrinkge. To minimize the effect of the different initil trunk Soil mtric potentil (MP) Plnt wter potentil (MP) Soil mtric potentil (MP) Figure 2: Reltion between the mesured predwn lef wter potentil nd the clculted soil mtric potentil. rdii, the dt series ws compred in reltive terms, s percentge of the growth. The monthly growth rte insted ws evluted by mesuring the trunk rdius t 25 cm height from the soil with the cliper nd the min brnch t pproximtely 1 cm from the trunk. Also the monthly growth rte is expressed s percentge of the growth. Furthermore, the dt series is plotted fter normliztion. The biomss productionwsmesuredttheendofthevegettionperiod. The totl boveground biomss (secondry brnches nd leves) ws hrvested before the defolition (October 11, 212) nd oven-dried seprtely t 65 Cuntilconstnt dry weight ws reched. The grvimetric increment of the trunk nd the primry brnches ws estimted by multiplying the sesonl volumetric increment recorded throughout the experiment, mking use of the clculted specific weight of the blck locust dry wood (.76 g cm 3 )estimtedfrom thesecondrybrnches.thewteruseefficiency(wue) under different drought stress durtions ws clculted fter the hrvest by the rtio between the dry totl boveground biomss produced during the vegettion period nd the wter used. On the other hnd, the economicl wter use efficiency (EWUE) under different drought stress durtions ws clculted considering only the dry boveground wood production Lef Are nd Root Verticl Distribution. The totl lef re (m 2 )forsingletreewsclcultedtthehrvest,by multiplying the blck locust specific lef re (g m 2 )bythe weight of the totl dry leves collected from ech tree. The specific lef re ws clculted from the rtio between the mesured re of 1 fully developed single leves for ech tree nd their single dry weight. In order to evlute the verticl sptil distribution of the root weight, without the stump, root smpling ws performed t 5 cm, 2 cm, nd 4 cm soil depth t the end of the experiment (October 11, 212). We used six stel rings of known volume (5 cm rdius 1 cm height) t ech soil depth. To homogenize the opertion, three smples were collected t 1 cm from the center of the lysimeter nd three smplest1cmfromthelysimeterwllforechlyer.all the roots contined in the rings were seprted nd the soil ws removed from the roots with brush. The roots were

4 4 Interntionl Forestry Reserch Men ir temperture ( C) Jun. 8-Jun. 15-Jun. 22-Jun. 29-Jun. 6-Jul. 13-Jul. 2-Jul. 27-Jul. 3-Aug. 1-Aug. 17-Aug. 24-Aug. 31-Aug. 7-Sep. 14-Sep. 21-Sep. 28-Sep. 5-Oct. 12-Oct. () Dte Wter (L) Irrigtion Trnspirtion () Storge Lechte Men reltive humidity (%) Jun. 8-Jun. 15-Jun. 22-Jun. 29-Jun. 6-Jul. 13-Jul. 2-Jul. 27-Jul. 3-Aug. 1-Aug. 17-Aug. 24-Aug. 31-Aug. 7-Sep. 14-Sep. 21-Sep. 28-Sep. 5-Oct. 12-Oct. Dte Figure 3: Men of the dily temperture nd reltive humidity mesured on hourly bsis, clculted from 9: HR to 18: HR, recorded throughout the vegettion period. Wter (L) Irrigtion Trnspirtion Storge Lechte Figure 4: Men of the weekly experimentl wter blnce clculted throughout the vegettion period. wshed crefully in dish of wter to remove ny residul soil. Subsequently, they were oven-dried t 65 Ctillstedyweight. The totl root biomss (g) for ech depth ws determined by weighing the roots of ech smple nd multiplying the men of the root weight density (g cm 3 )bythesoilvolumeofech depth, mounting to cm Sttisticl Anlysis. The nonprmetric Spermn rho correltion (P <.5) ws pplied to correlte (i) the weekly wter use, growth rte, nd the VPD, (ii) the Ψ S nd the Ψ L, nd (iii) the Ψ L nd the θ. The non-prmetric nlysis Mnn-Whitney U test (P <.5) ws performed to compre (i) the men weekly growth rte, (ii) the cumultive wter use, (iii) the trunk nd brnch rdil increments, (iv) the boveground biomss production, (v) the root weight verticl distribution, (vi) the totl lef re, nd (vii) the WUE nd the EWUE under drought stress, between nd within the tretments. All the nlyses were executed by using the IBM SPSS version 21 (SPSS Inc, Chicgo, IL, USA). 3. Results 3.1. Wter Use. The men of the dily ir temperture nd reltive humidity, recorded during the vegettion period, is shown in Figure 3. During the vegettion period, for 18 dys themenofthedilytemperturewsbove3 C, nd 7 dys bove 34 C, with pek of 36.5 CinAugust.Thelst dy with temperture 3 C ws September 1. After tht the temperture slowly decresed till reching the minimum vlues of 12.5 C t the end of the experiment. In order to clculte the weekly trnspirtion, the experimentl wter blnce ws performed (Figure4). During the periods when the plnts were well wtered, the lechte ws minimized by mintining the soil column t the hydrosttic stte, in order to void excessive soil prticle trnsloction. During the trnsition phse from well-wtered to drought condition, the irrigtion ws stopped nd the plnts used the wter stored into the soil, till the predefined vlue of 7% ws reched.ontheotherhnd,duringthetrnsitionphsefrom drought condition to well wtered, the irrigtion supplied

5 Interntionl Forestry Reserch 5 Cumultive trnspirtion (L) VPD () VPD (kp P 1 ) Weekly trnspirtion (L) Wter use Mtric potentil Soil mtric potentil (MP) Weekly trnspirtion (L) Wter use Mtric potentil (c) Figure 5: () Men nd stndrd devition of the weekly vpor pressure deficit nd cumultive wter use, short-term stress tretment men nd stndrd devition of the weekly wter use nd men of the soil mtric potentil, nd (c) long-term stress tretment men nd stndrd devition of the weekly wter use nd men of the soil mtric potentil. Drought stress periods re mrked with blck rectngle beneth the brs Soil mtric potentil (MP) dditionl wter to rechrge the storge till the predefined vlue of 2% ws reched. The men trnspirtion from the tretment during the vegettion period mounted to 26. ± 7.6 L, corresponding to 57% of the wter used by the tretment (456.8 ± 25.3 L) (Figure 5()). The men of the weekly wter useofthetretmentwsmoreffectedbythemen of the weekly VPD s compred to the tretment. The correltion ws significnt for both tretments, with coefficients of.68 nd.499 for nd, respectively. From the nlysis of the weekly wter blnce, we cn visulize the different plnt responses to the SWA vrition in terms of wter use (Figures 5 nd 5(c)). During the first four weeks of our investigtion (WOY 23 26), the wter usewsonly26%,compredtothe.forthefollowing three weeks (WOY 27, 28, nd 29) the trees were lredy differentitedndtheusedonly5%ofthewterused by the plnts. When imposing the drought regime (WOY 3 nd 31), both tretments reduced their wter use. However, the still trnspired 52% more wter thn the. After the drought period (WOY 32), the plnts of both tretments recovered quickly nd the wter use rose up by 5% nd 74% for the nd, respectively. During WOY 33 both tretments drsticlly dropped their wter consumption to vlues comprble to the vlues recorded during the previous phse of drought stress (WOY 31). The trees did not recover their functions fully, since the wter consumption incresed only by 14% (WOY 34) nd ws followed by further decrese of 25% (WOY 35). During the subsequent stress period (WOY 36) defolition strted nd the wter consumption ws reduced by 34%, not to recover nymore, till it reched the minimum men weekly vlue of 6.9 ±.3 L during the lst week of the investigtion (WOY 41).Theinsteddidnotchngeitswterconsumption substntilly during the lst two drought stress cycles, till reching the minimum men weekly vlue of 7.9 L Growth. The correltion between the men weekly reltive trunk rdil increment nd the men weekly Ψ L ws

6 6 Interntionl Forestry Reserch Cumultive trunk increment (μm) Weekly trunk increment (%) Cumultive trunk increment (μm) Weekly trunk increment (%) Dily Weekly Dily Weekly () Cumultive trunk increment (%) (c) Figure 6: Men dily trunk increment nd men stndrd vrition of the weekly reltive trunk increment: () short-term stress, longterm stress, nd (c) men cumultive reltive trunk increse of nd. not significnt. The tretment decresed the trunk rdil increment by 58% fter WOY 24 nd only recovered slowly before the following drought stress (WOY 28) (Figure 6()). The men rdil increment (Figure 6) fter the stress (WOY 23 26) on the other hnd incresed drmticlly by 119%ndkeptonincresingforthefollowingthreeweeks (WOY 27, 28, nd 29). After one week of stress (WOY 3), we hd drstic reduction of the men weekly rdil increment of72%nd61%forthend,respectively.however, duringthesecondweekofstress(woy31),wherethemen weekly VPD reched vlues of 2.93 kp P 1,thementrunk rdil increment incresed by 26% nd 36% for the nd, respectively. By rewtering (WOY 32), the plnts of both tretments recovered. The men of the trunk rdil increment incresed by 6%, while the incresed by 48%. During the third drought stress period (WOY 33) the men of the trunk rdil increment decresed nd theplntsstoppedgrowingltogetherinwoy36.themen of the trunk rdil increment rte insted still incresed by 29% fter the third drought stress term (WOY 33 nd 34) nd decresed slowly till the end of the experiment (WOY 41), with the minimum weekly vlues of.15%. The effect of the VPD on the men weekly rdil increment mesured by dendrometers ws more incisive for the thn the tretments, with coefficient of correltion of.619 nd.548, respectively (Figure6(c)). Anlyzing the men wter use, clculted during vrious phses of growth, the difference between the nd decresedfrom6%duringthefirstperiod(woy23to27)to 7% during the lst period (WOY 35 to 41) (Figure 7()). The frequency of the drought stress neither significntly ffected the men primry brnch rdil increment mesured with the cliper (Figure7), nor ws significnt difference found between the tretments by normlizing the dtset (Figure 7(c)) Root Verticl Distribution nd Totl Lef Are. The men of the totl root biomss production ws not significntly different between the tretments; however, the men of the root weight (34.8 g) ws 13% more, compred to the root weight of the tretment (298.4 g). For both tretments the men of the root weight t 5 cm ws significntly different, compredtothemenrootweightestimtedt2nd4cm

7 Interntionl Forestry Reserch 7 Normlized rdil increment Wter use (L) () Figure 7: () Men weekly wter use nd men nd stndrd devition of the primry brnches reltive rdil increments mesured with cliper nd (c) men primry brnch rdil increments normlized, mesured with cliper. depth. The portion of the root developed t 5 cm depth ws 66% nd 74% of the totl, for the nd tretment, respectively (Figure8). Notwithstnding the fct tht the difference between the tretments in terms of totl lef re ws not significnt (Figure 9()), the estimted men of the totl lef re (6.2 m 2 ) ws 17% higher thn the men of the totl lef re (5.2 m 2 ). The men of the rtio root weight: lef re ws significntly different between the tretments, being for (33.6) only 31% compred to the tretment (16.4) (Figure 9) Production. The totl dry boveground biomss produced during the vegettion period ws comprble between the tretments (Figure 1()).However,themenofthe dry boveground biomss mounted to ± g, which ws 36% more thn with the production (73.2 ± 76.6 g). The men dry wood production mounted to ± 39.8 g, which ws 44% higher thn the dry wood production (462. ± 71.1 g) (Figure 1()). The tretments produced the higher mount of boveground biomss, even though its brnch rdil increment ws lower thn the one of the. This incongruence is due to (c) Rdil increment (%) Depth (cm) b b b b Dry weight (g) Figure 8: Men nd stndrd devition of the root weight, t 5, 2, nd 4 cm depth. Nonprmetric nlysis Mnn-Whitney U test. Different letters indicte significnt difference (P <.5). the brnch rdius dimension of the two tretments. The men primry brnch rdius of the t the beginning of the experiment mounted only to 78% compred to the. In terms of comprble reltive growth, we hve more substntil bsolute increse of biomss. The lef : wood rtio ws.41 for the nd.59 for the tretment. The WUEofthewsslightlyhighercompredtothe, but the difference ws not significnt between the tretments (Figure 1). The coefficient of correltions between wter use nd dry totl biomss (.997) nd wter use nd dry wood (.928) ws significnt. For both the reltion is liner; the WUE nd the EWUE under drought condition mounted of 2.57 g L 1 (r 2 =.84) nd 1.79 g L 1 (r 2 =.99),respectively(Figure 11). 4. Discussion 4.1. Lef Wter Potentil nd Growth. The severity of the drought stress is tightly relted to the soil chrcteristics, rther thn the bsolute vlues of the θ. Therefore, the Ψ L is effective s drought stress indictor to evlute the plnt sensitivity to the reduction of the soil wter vilbility [24 27]. Unlike indirect methods (e.g., soil moisture or soil mtric potentil mesurements), plnt-bsed direct method presents the dvntge of evluting the true wter sttus of the plnt, rther thn the soil conditions. Under the simulted soil moisture conditions in the lysimeter, the tree growth ws not influenced by the plnt wter sttus, nd no noteworthy reltion between the men of the weekly growth increse nd themenoftheweeklyψ L could be found Wter Use. Blck locust is ring-porous tree with wide erly wood pores nd, if wtered well, it demonstrtes high wter consumption, s generlly recorded for trees with comprble hydrulic rchitecture rrngement [28]. However, blck locust demonstrted high plsticity, since thespeciescncopewithwiderngeofenvironmentlconditions [1, 29]. As the wter vilbility decreses, stomtl

8 8 Interntionl Forestry Reserch Totl lef re (m 2 ) 6 4 Root weight : lef re b Tretment Tretment () Figure 9: Men nd stndrd devition of the () totl lef re nd root weight:lef re rtio t 5, 2, nd 4 cm depth. Non prmetric nlysis Mnn-Whitney U-Test. Different letters indicte significnt difference (p <.5) B 5 4 Dry biomss (g) 8 4 A b 3 2 Wter use efficiency (g L 1 ) 1 Totl Wood Tretment Tretment () Figure 1: () Men nd stndrd devition of the bove ground dry totl biomss nd dry wood biomss produced during the vegettion period; men nd stndrd devition of the wter use efficiency under different drought stress durtion. Nonprmetric nlysis Mnn- Whitney U test. Different letters indicte significnt difference (P <.5).

9 Interntionl Forestry Reserch 9 Dry biomss (g) Wter use (L) Totl biomss Wood biomss Figure 11: Wter use efficiency under different drought stress durtions, for the dry wood nd the dry totl boveground biomss. regultion tkes plce in order to minimize the trnspirtion t the expense of the photosynthetic crbon gin [3]. If the stress is protrcted, physiologicl djustments [13, 31, 32] nd hydrulic rchitecture cclimtion occur [18 2, 22, 23] in order to mintin the functionlity of the vsculr system [33, 34]. As found in our studies, morphologicl dpttionsoccurredinresponsetotheinduceddrought stress. Due to the lower totl lef re the tretment resulted to be more tolernt to drought stress. A higher rtio root weight: totl lef re clculted for enhnced its efficiency in using the wter resources during the drought stress periods. Furthermore, due to the reduction of the totl lef re, its potentil trnspirtion decresed, reducing the wter loss during the dys with reltively high VPD nd limited wter vilbility. As mtter of fct, the plnts tht resprouted under wter constrint fter the winter were more tolernt to the imposed drought stress extreme het conditions in August 212 thn the plnts. Also for other brod lef species such s Fgus sylvtic, it hs been found tht the previous summer drought stress hd effects on the trnspirtion nd photosynthesis of the following yers nd influenced the growth performnce [35]. On the other hnd, the underwent only temporry djustments to cope with the short-term drought stress pplied during the resprouting phse. The plnts evidently cclimtized to well-wtered conditions in terms of potentil trnspiring surfce, rther thn through the xylem hydrulic cpcity differentition [33] demonstrted by the higher wter consumption lredy t the erly stges. A lrger trnspiring lef surfce mens higher potentil trnspirtion, which is beneficil only when the wter supply is sufficient. However, by reducing the soil wter vilbility or incresing the VPD, stomtl regultion tkes plce [1], nd if the stress persists, reduction of the trnspiring surfce occurs to minimize the embolism hzrd [36] Growth Increment nd Biomss Production. The soil wter vilbility reduction t the initil stge of the plnt development ws reveled to be criticl fctor to enhnce the bility for recovery fter subsequent drought stress, s reported in previous studies [23, 37]. Higher biomss production ws chrcterized by higher wter consumption, independent of the growth rte nd the drought stress durtion. The WUE vlue of 2.56 g L 1 ws clculted for blck locust under drought stress, this vlue lying within the sme rnge of the typicl vlues estimted for trees such s Populus nd Slix growing in temperte climte zones, nmely, between 1.42 nd 6.66 g L 1 [38 41]. Therefore, the link between the drought stress, trnspirtion nd primry production is cler: independently from the drought stress frequency nd durtion, the primry production is directly proportionl to the wter used. 5. Conclusions The experiment emphsized the importnce of the combintion of preconditions nd drought cycles on the drought tolernce of Robini pseudocci L. Prestressed trees hve lredy lower totl lef re nd therefore lower wter consumption. Under incresed trnspirtion demnds with incresing summer temperture they tolerte the pplied drought stress better thn the with lrger lef re. In the trees the potentil wter demnd is higher thn the soil wter vilbility. The wter imblnce, in combintion with the high tempertures, leds to drstic lef fll nd fully stops of the growth under such conditions. A more extended root system, together with the reduction of the totl lef re s n dptive strtegy, ppers to be effective in order to enhnce the drought tolernce of blck locust. However, its growth performnce is strongly ffected by the soil wter vilbility. The initil conditions of resprouting ply centrl role in defining the recovery processes fter drought stress, which is of fundmentl importnce for the growth performnce of blck locust on mrginl lnds t the edges of its distribution re in Centrl Europe, where wter is limiting fctor nd summer droughts nd tempertures bove 3 C occur. This hs importnt impliction for the mngement of short-rottion forestry systems. The high plsticity of Robini nd its recovery potentil fter stress mkes the tree species relevnt for the production of biomss s rw mteril in the drier regions of Centrl nd Estern Europe. Conflict of Interests The uthors declre tht there is no conflict of interests regrding the publiction of this pper. References [1] P. Ciis, M. Reichstein, N. Viovy et l., Europe-wide reduction in primry productivity cused by the het nd drought in 23, Nture,vol.437,no.758,pp ,25. [2] U. Cubsch nd C. Kdow, Globl climte chnge nd spects of regionl climte chnge in the Berlin-Brndenburg region, Erde,vol.142,no.1-2,pp.3 2,211. [3] N. Schller, I. Mhlstein, J. Cermk, nd R. Knutti, Anlyzing precipittion projections: comprison of different pproches

10 1 Interntionl Forestry Reserch to climte model evlution, Geophysicl Reserch D: Atmospheres, vol. 116, no. 1, Article ID D1118, pp , 211. [4] A.Holsten,T.Vetter,K.Vohlnd,ndV.Krysnov, Impctof climte chnge on soil moisture dynmics in Brndenburg with focus on nture conservtion res, Ecologicl Modelling,vol. 22, no. 17, pp , 29. [5] R.Monclus,E.Dreyer,M.Villretl., Impctofdroughton productivity nd wter use efficiency in 29 genotypes of Populus deltoides x Populus nigr, New Phytologist, vol. 169, no. 4, pp , 26. [6]E.Sevigne,C.M.Gsol,F.Brunetl., Wterndenergy consumption of Populus spp. bioenergy systems: cse study in Southern Europe, Renewble nd Sustinble Energy Reviews, vol. 15, no. 2, pp , 211. [7] R.S.Zlesny,D.M.Donner,D.R.Coyle,ndW.L.Hedlee, An pproch for siting poplr energy production systems to increse productivity nd ssocited ecosystem services, Forest Ecology nd Mngement,vol.284,pp.45 58,212. [8] H. Grünewld, C. Böhm, A. Quinkenstein, P. Grundmnn, J. 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11 Interntionl Forestry Reserch 11 [36] R. Tognetti, A. Longobucco, nd A. Rschi, Vulnerbility of xylemtoembolisminreltiontoplnthydrulicresistnce in Quercus pubescens nd Quercus ilex co-occurring in Mediterrnen coppice stnd in centrl Itly, New Phytologist, vol. 139, no. 3, pp , [37] T. J. A. Bruce, M. C. Mtthes, J. A. Npier, nd J. A. Pickett, Stressful memories of plnts: evidence nd possible mechnisms, Plnt Science, vol. 173, no. 6, pp , 27. [38] H. S. Grip, S. Hlldin, A. Lindroth, nd G. Persson, Evpotrnspirtion from willow stnd on wetlnd, Ecology nd Mngement of Forest Biomss Production Systems, vol.15,pp , [39] A. Lindroth, T. Verwijst, nd S. Hlldin, Wter-use efficiency of willow: vrition with seson, humidity nd biomss lloction, Hydrology,vol.156,no.1 4,pp.1 19,1994. [4] C. Yin, X. Wng, B. Dun, J. Luo, nd C. Li, Erly growth, dry mtter lloction nd wter use efficiency of two symptric Populus species s ffected by wter stress, Environmentl nd Experimentl Botny, vol. 53, no. 3, pp , 25. [41] Z.-S. Ling, J.-W. Yng, H.-B. Sho, nd R.-L. Hn, Investigtion on wter consumption chrcteristics nd wter use efficiency of poplr under soil wter deficits on the Loess Plteu, Colloids nd Surfces B: Biointerfces,vol.53,no.1,pp , 26.

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