journal of Arid Environments

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1 journl of Arid Environments 83 (212) Contents lists vilble t SciVerse ScienceDirect c; ' , journl of Arid Environments journl hom epg e: octe/j ri denv Hydrulic redistribution by two semi-rid shrub species: mplictions for Shelin gro-ecosystems F. Kizito., M.. Drgil b, M. Sene c. j.r. Brooks ct, F.C. Meinzer e.. Diedhiou c, M. Dioufc, A. Luff f, R.P. Dickg, j. Selkerh, R. Cuenc h 'nterntionl Wter Mngement nstitute, PMB CT 112 Accr, Ghn b Deprtment of Crop nd Soil Science, Oregon Stte University, ALS Building 317, Corvllis, OR 97331, USA '/nstitut Seneglis de Recherches Agricoles (SRA)/CERAAS, B.P 332 Thies-Escle, Senegl d Western Ecology Division, US EPA/NHEERL, 2 SW 35th St., Corvllis, OR 97333, USA 'USDA Forest Service, PNW Reserch Sttion, 32 SW jefferson Wy, Corvllis, OR 97331, USA rthe World Bnk, 1818 H Street, NW Wshington, DC 2433, USA school of Environment nd Nturl Resources, Ohio Stte University, Columbus, OH 4321, USA h Oregn Stte University. Wter Resources Engineering, Deprtment of Bioengineering, Gilmore Hll, Rm. 116, Corvllis, OR 97331, USA ARTCLE NFO Article history: Received 11 Jnury 211 Received in revised form 7 November 211 Accepted 14 Mrch 212 Avilble online 4 My 212 Keywords: Agro-ecosystems Annul food crops Hydrulic redistribution Shel Shrubs ABSTRACT Hydrulic redistribution is the process of pssive wter movement from deeper moist soil to shllower dry soil lyers using plnt roots s conduits. Results from this study indicte tht this phenomenon exists mong two shrub species (Guier seneglensis nd Piliostigm reticultum) tht co-exist with nnul food crops in Shelin gro-ecosystems. Rel-time mesurements were conducted for soil wter content, soil wter potentil nd microclimte vribles notbly; ir temperture, reltive humidity, wind speed, precipittion nd solr irrdince. Additionlly, sp flow mesurements were conducted in shrub roots using the therml dissiption technique on intct nd coppiced shrubs. Monthly predwn lef wter potentil ws mesured using portble pressure chmber. Soil wter potentil (lj 5 ) t the 2 em depth declined significntly during the dry seson with die! chnges in lj 5 of -.6 to -1.1 MP. These vritions were ttributed to pssive wter relese from shrub roots resulting in overnight rewetting of drier upper soil lyers. Sp flow mesurements on tp nd lterl shrub roots indicted dily reversls in the direction of flow. During the pek of the dry seson, both positive (towrd shrub) nd negtive (towrd soil) flows were observed in lterl shrub roots with sp flow in the lterl roots frequently negtive t night nd rpidly becoming positive soon fter sunrise. The negtive sp flow t night in superficil lterl roots nd the periodic positive flow in the descending tp roots were indictive of hydrulic redistribution. Hydrulic redistribution my be n importnt mechnism for drought stress voidnce while mintining plnt physiologicl functions in both shrubs nd neighboring nnuls in wter-limited environments. 212 Elsevier Ltd. All rights reserved. 1. ntroduction The productivity of rid nd semi-rid ecosystems is controlled primrily by wter dynmics. The reltively recent discovery of hydrulic redistribution (HR) hs importnt implictions for understnding ecosystem functioning in these dry environments (Cldwell et l., 1998). Hydrulic redistribution (HR) is the process of pssive wter movement from reltively moist to drier regions of soil using plnt roots s conduit (Cldwell, 199; Dwson, 1995; Richrds nd Cldwell, 1987; Xu nd Blnd, 1993). t is driven by soil Corresponding uthor. Tel.: : fx: E-mil ddresses: fredkizito@gmil.com, f.kizito@cgir.org (F. Kizito ). mtric potentil grdients nd modulted by resistnce to reverse flow through roots nd by rhizosphere resistnce to trnsfer of wter from roots to soil. Hydrulic redistribution usully occurs t night when trnspirtion hs diminished sufficiently to llow the wter potentil in roots to exceed tht in the drier upper portions of the soil profile (Scholz et!., 22). Although movement of wter vi HR is usully upwrd from deep, moist soil lyers to shllow, dry lyers, both downwrd (Burgess et l., 21; Smith et!., 1999) nd lterl (Bleby et l., 21; Brooks et l., 22, 26) HR hve been documented. Downwrd HR my ply n importnt role in deep soil moisture rechrge in rid regions (Leffler et l., 25; Rye! et l., 23). Associted benefits of HR for the plnts involved my include delying loss of root xylem conductivity in shllow roots /$ - see front mtter 212 Elsevier Ltd. All rights reserved. doi: /j.jridenv

2 7 F. Kizito et/. journl of Arid Environments 83 (212) (Domec et l., 24, 26) nd prolonging their nutrient uptke (Cldwell et l., 1998) during dry periods. Now tht more thn 6 cses of HR hve been reported in woody nd herbceous plnts Uckson et l., 2), there is reson to expect tht its existence is widespred wherever conditions re conducive to its occurrence. Evidence for HR consists lrgely of time fluctutions of soil wter potentil showing prtil overnight recovery in drier soil lyers (Millikin lshiikw nd Bledsoe. 2; Richrds nd Cldwell, 1987), movement of deuterted wter s trcer from roots to soil nd neighboring plnts (Brooks et l., 22, 26; Moreir et l., 23 ), nd reverse sp flow in roots (Burgess et l., 1998; Scholz et l., 22; Smith et l., 1999). Verifiction of HR requires close observtion of other biophysicl processes tht could ccount for ltered soil moisture regime in the upper soil lyers in the presence of woody shrubs. For instnce, ner-surfce therml vribility cn ffect wter movement within the vdose zone in liquid nd vpor phses (Chill nd Prlnge, 1998; Milly, 1996; Prlnge et l., 1998; Philip nd de Vries, 1957). These studies hve demonstrted tht the presence of temperture grdients in unsturted soils my lso induce wter fluxes in gs nd liquid phses tht cn significntly contribute to the wter nd energy trnsport processes. However, in modeling study Rye! et l. (22) concluded tht during summer drought, the mount of wter moved by unsturted flow ws typiclly negligible compred to tht moved by HR in n Artemisi tridentt stnd. Trnsfer of wter from plnts conducting HR to surrounding plnts hs been documented (Brooks et l., 26; Cldwell nd Richrds, 1989; Moreir et l., 23), but the potentil fcilitting effects of HR on neighboring plnts remin to be evluted. Although recent studies in the Est Africn svnn indicted tht the deeply rooted tree Acci tortilis conducts HR. the fcilittive effects of hydruliclly lifted wter tken up by surrounding grsses were uncertin becuse soil wter potentil ws consistently lower under the tree crowns thn outside the crowns (Ludwig et l., 23). Luff et l. (29) hve shown tht two semi-rid shrub species (Guier seneglensis nd Piliostigm reticultum) co-exist within row crops nd re widely distributed in Senegl. Previous work (Kizito et l., 26, 27) hs demonstrted tht during the dry seson, the soil surrounding the shrub shllow roots is substntilly moister thn soil distnt from the shrubs, but these shrubs hve not been investigted for their potentil to perform HR. The existence of HR mong these shrubs could lter hydrologicl processes enough to significntly impct wter blnces nd improve or stbilize crop productivity by reducing drought stress. We therefore hypothesized the presence of unique interction nd exchnge of wter pulses between the semi-rid shrubs nd co-occurring nnul row crops. Our objectives in this study were to: 1) evlute the existence nd mgnitude of hydrulic redistribution by G. seneglensis nd P. reticultum; nd 2) quntify the mgnitude of other bio-physicl processes potentilly responsible for soil wter trnsport during the dry seson. 2. Mterils nd methods 2.1. Study sites The study ws conducted in two different gro-ecologicl zones in the Penut Bsin of Senegl, West Afric. The region is chrcterized by temporlly nd sptilly vrible unimodl rinfll with episodic droughts nd frequent crop filures (Centre de Suivi Ecologique, 2). The riny seson lsts from july to September, generlly s scttered, high intensity, short-durtion showers. Air temperture exhibits high diurnl nd nnul vribility. The min nnul crops re millet (Pennisetum glucum (L.) R. Br.), groundnuts (Archis hypoge L.), sorghum (Sorghum bicolor (L.) Moench), cowpes (Vign unguicult (L.) Wlp.) nd corn (le mys) in the southern prt of the Penut Bsin. The plnting density for penut is bout 13, plnts h- 1 h (sown s one seed per hole) while tht of corn is bout 5 plnts h- 1 The reserch ws conducted t two sites. One study site, Keur Mtr Arme (KMA), is locted in the northern region (N14 46W 16 51; 43 m bove se level, slope rnge -1%) of the Penut Bsin with men nnul unimodl precipittion of 45 mm (Fig. 1 ). The wter tble t this site lies t 15m. G. seneglensis is the dominnt shrub, chrcteristiclly 1 m tll with shrub cnopy crown of bout 2 m nd highly spreding shllow rooting system in the top.45 m nd nrrow leding unbrnched root to depth greter thn 2.5 m (Kizito et l.,26). The site hs shrub stnd density of 24 shrubs h- 1. The minimum men nnul mbient temperture is 2 oc nd mximum men nnul mbient temperture of 33 "C. The re lies on leched nd disturbed ferruginous snd soil clssified s ferruginous Oxisol (FAO, 1998). The top horizon (-.7 m), is sndy with frible continuous structure nd no distinct horizontion with low cly contents of bout 5% nd scnty orgnic mtter content of.5%. The second study site is locted t Nioro, in the southern region (N13 45 W 15 47; 18m bove se level, slope rnge of-2%) of the Penut Bsin, with totl men nnul unimodl precipittion of 75 mm (Fig. 1 ). P. reticultum is the predominnt shrub occurring t density of 185 shrubs h- 1 with well developed nd firly deep rooting system (Did< et l., 2; Kizito et l., 26). The wter tble lies t bout 12 m below the soil surfce. This site hs rndomly scttered Cordyl pinnt nd Prosopis fricn trees in the lndscpe. The men nnul minimum mbient temperture is 2 oc nd the men nnul mximum mbient temperture is i..:: "' b -- 5<:'-- x {) \ x Q L_ j- - R R> ' Yers Fig. l. Decdl cumultive nnul rinfll for rin guges t () KMA nd (b) Nioro. Dotted horizontl lines show the decdl verge vlue (After Kizito et l.. 27).

3 F. Kizito et l. journl of Arid Environments 83 (212) oc. The re lies on fine-sndy, mixed Hplic Ferric Lixisol, leched ferrugeneous tropicl Ultisol (Did< et!., 2). t is chrcterized by leched light brown ferrugeneous top horizon (-.6 m). This grdully blends into red brown lteritic color ( m) with ferric spots interspersed in the profile Field nd lbortory methods Experimentl design nd tril lyout The experimentl re t both sites (KMA nd Nioro) ws 64 m x 44 m. Ech site hd 18 plots with dimensions of 6 m x 8 m in rndomized block design with two tretments within three blocks replicted three times (Appendix 1, electronic version only). Tretment 1 served s the control: bre plots tht hd previously hd n nnul crop (penut or millet). Tretment 2 ws comprised of shrubs with one shrub monitored below shrub center; 5 em wy from shrub center (t the 2 em depth); nd 2 em wy from shrub center (t the 2 em depth). For ech tretment, soil wter content nd soil wter potentil mesurements were conducted t 2, 4, 6, 8, 1 nd 12 em depths Soil wter mesurements Volumetric soil wter content (8) ws continuously monitored over two-yer period (23-24) below three replicte individuls of ech species using dielectric soil wter content sensors (ECHzO Probe; Decgon Devices nc, Pullmn, WA, USA). These were clibrted grvimetriclly in the field nd were instlled t 2, 4, 6, 8, 1 nd 12 em depths. Probe dt were logged t hourly intervls with dt logger (CR1X; Cmpbell Scientific, Logn, UT, USA). Soil wter potentil (lj!s) ws mesured using screen-cge soil psychrometers (PST-SS,Wescor, nc., Logn, UT, USA). These were individully clibrted using stndrd slt solutions following the procedures of Brown nd Brtos (1982). Psychrometers were instlled t depths of 2, 4, 6, 8, 1 nd 12 em. The psychrometers mesured both soil wter potentil nd soil temperture mesurements on n hourly bsis with 3 s cooling time for the Peltier effect. Psychrometer dt were logged nd downloded from wter potentil dt logger (PYSPRO; Wescor, nc.). Soil psychrometers were instlled in close proximity to the dielectric probes to permit concurrent comprison with soil wter content mesurements. The role of HR in shrub-crop interction ws investigted by mesuring ljf s t 2 em nd 4 em depths for three distnces from the shrub center: directly below the shrub, 5 em wy from shrub center nd 2 em wy from shrub center outside cnopy periphery. The ljf s were conducted to investigte the effect of prtil soil profile rewetting. Totl dily wter use ( mm dy- 1 ) ws clculted s soil moisture chnges mesured by the dielectric probes locted t depths of 2 nd 4 em. This ws the difference between the dily mximum nd minimum moisture content vlues. The instruments locted t the 2 em nd 4 em depths were ssumed to represent soil moisture conditions over control volume tht spnned pproximtely 1-5 em depth. Bsed on field observtions, it ws estimted tht the 1-5 em depth would encompss the ner-surfce rooting zone for both nnul crops nd ntive shrubs. Hydrulic redistribution ws clculted s the difference between the dytime minimum soil wter storge nd the following dy's mximum, which represents overnight recovery of soil wter storge (Brooks et l., 22). n order to derive the net effect of HR. moisture increses between the mxim of consecutive dys, t the 2 nd 4 em depth were considered to be dul contribution of HR nd vpor Tble 1 Men predwn wter potentil (soil wter potentil before sunrise) (MP) for the 2 em depth during pek of dry seson (My-june). Microsite Guier seneglensis Piliostigm reticultum Keur Mtr Arme Nioro Beneth -2.84(.2)' -3.32(.12)' -1.99(.11 )' -2.3(.8)' shrub center 5 em from -2.92(.6)'b -3.46(.8)'b -2.45(.14)'b -2.47(.2)'b shrub center 2 em from -3.26(.18)' -4.31(.26)' -2.92(.1 )be -2.93(.4)bc shrub center Bre soil site -5.41(.12)" -5.25(.9)" -3.71(.5)" -3.74(.3)" Lef wter -.54(.4) -.71(.16) -.5(.8) -.86(.13) potentil 'Numbers in prenthesis represent stndrd errors while letters in superscript represent Fisher's protected lest significnt difference (LSD) test where differences were considered sttisticlly significnt t P <.5. flux deposition. The moisture increse ws thus termed "pprent HR" (Tble 2 ). The net HR ws clculted s the difference between pprent HR nd estimted vpor flux deposition. Cumultive pprent HR ws clculted s the minimum soil wter content for one dy subtrcted from the mximum soil wter content t the commencement of the following dy Environmentl mesurements Microclimte vribles monitored included ir temperture, reltive humidity, wind speed, precipittion nd solr irrdince. Air temperture nd reltive humidity were mesured with combined probe (Visl HMP 45C, Cmpbell Scientific). This ws plced in one of the study plots t two-thirds of the men cnopy height of the shrub stnd. Wind speed nd direction were monitored using wind sensor (RM Young 513-L, Cmpbell Scientific, Logn, UT, USA) plced 2 m bove the soil surfce. Solr irrdince ws mesured with silicon diode pyrnometer (L-COR L2X, Lincoln, NE, USA). A tipping bucket rin guge (TE525-L, Texs Electronics, Dlls, TX, USA), with tips t.1" increments (bout.254 mm of rinfll per tip) ws used to monitor precipittion. All the bove prmeters were recorded every 15 min nd dt subsequently verged t hourly intervls with dt logger (CR1X; Cmpbell Scientific). Tble 2 Sesonl vritions t 2 em depth of soil wter sttus, vpor flux nd HR (Vlues in brckets represent stndrd error of the men). Vrible Guier seneglensis Piliostigm reticultum Keur Mtr Arme Nioro Soil wter content (m 3 m 3 ) Strt of dry.18(.1 )'.16(.2)'.64(.4)'.82(.1 )' seson End of dry.26(.1)b.21(.1)b.96(.3)b.114(.l)b seson Soil wter potentil (MP) Strt of dry -5.6(.5)' -4.98(.3)' -4.32(.7)' -4.11(.5)' seson End of dry -4.89(.3)b -4.2(.4)b -4.19(.6)'b -3.92(.2 )'b seson Vpor flux (mm) 1(.1).8(.2).12(.2).2(.1) Apprent HR (mm) 36(.2) 48(.1) 21(.1) 18(.1) NetHR(mm) 35(.1) 47.2(.2) 2.88(.1) 17.8(.2) 'Numbers in prenthesis represent stndrd errors while letters in superscript represent Fisher's protected lest significnt difference (LSD) test where differences were considered sttisticlly significnt t P <.5.

4 72 F Kizito et/. journl of Arid Environments 83 (212) Vpor trnsport dynmics Vpor flux ws deduced from soil temperture nd 1Jf 5 mesurements tken long the profile. Atmospheric mesurements were used to compute the vpor pressure deficit, component used in vpor trnsport derivtion under field conditions (Milly, 1996). The rte of diffusion of wter vpor is directly proportionl to the grdient of vpor pressure or vpor density (Hillel, 1998). The density of wter vpor.in soil under sturtion is function of temperture s expressed by (Ahrens. 1994): Pst = Tc x1-3 Tz x1-4 Tf (1) Where, Pst is the sturted wter vpor density in soil under sturtion (gm m- 3 ) nd Tc is the temperture ( C). Since soil temperture fluctutes with depth diurnlly, the mount of vpor tht cn be held in ir chnges. Decresing temperture will therefore cuse vpor to condense, this condition ws ssumed to pply t the rnge of wter potentil vlues exhibited t our study sites. The shpe nd depth of the soil therml profile will be influenced by the therml conductivity nd het cpcity, which depend on soil moisture, orgnic nd minerl content (Hillel, 1998). Movement of the therml signl down the soil profile will determine the vlue of the sturted vpor density, nd therefore the reltive humidity impcting condenstion, evportion nd vpor flux processes through the soil profile (Ahrens, 1994). Fick's Lw is generlly used for stedy stte diffusion of vpor in soil ir -D vvpor -Ds'l - Vu ]g = s - ZJ- Zu z Where,]g is the diffusive flux of gs (g m- 2 s- 1 ), Dis the diffusion coefficient (generlly hving dimensions of re per unit time), vvpor is the chnge in vpor pressure nd z is the chnge in depth. Assuming tht the reltive humidity is 1% ll the time for ny prticulr soillyer,]g cn be clculted in the finl eqution where, v 1 is the lower depth vpor pressure, Vu is the upper depth vpor pressure, 21 is the depth of the lower lyer from the soil surfce, Zu is the depth of the upper lyer from the soil surfce. The diffusion coefficient for soil cn be clculted from the diffusion coefficient in free-ir (Penmn, 194): Ds = Do.66f (3) Where, Do is the diffusion coefficient of wter vpor in ir,.66 is the tortuosity coefficient, nd f is the ir-filled porosity. Air-filled porosity Cf) ws derived from hourly mesured field wter content Where 8 is the mesured field moisture content, Pb is the site specific soil bulk density (g cm- 3 ), p 5 is the prticle density (g cm- 3 ). Vpor movement in the 2-4 em depth includes fluxes deposited in this lyer cross the boundry between lyers. n order to ccount for the vpor flux within the 2-4 em lyer, the flux ws clculted for ech depth nd ws subsequently summed for both depths over the seson Sp flow mesurements nd coppicing tretments Sp flow mesurements were performed simultneously on the shrub tp nd lterl roots to scertin the reltive contribution of ech of these roots to the HR process. Root sp flow ws mesured using the therml dissiption technique (Grnier, 1987), which ws modified to permit the direction of flow to be detected (Brooks (2) (4) et l., 22, 26). As shown (in Appendix 2, electronic version only), the schemtic portrys the modifiction to the probe tht permitted determintion of sp flow direction. n its originl form, this technique relies on probe tht indictes the mgnitude of flux by mesuring the difference between heted temperture sensor nd single unheted reference temperture sensor. t ws insensitive to the direction of flow nd unsuitble for monitoring reversl of flow in roots ssocited with hydrulic redistribution (Burgess et l.. 2). However. n dditionl directionl probe ws instlled where two thermocouples were plced up nd downstrem with the differentil between these two thermocouples indicting the direction of flow. As shown in Appendix 2, electronic version only, the sensor hs symmetricl design centering on the heter probe, with one sensor upstrem nd one sensor downstrem reference probe (plced 1 em xilly from the heter) s well s one upstrem nd one downstrem fine-wire thermocouples (7 mm wy from the heter). Together, this ssembly generted three different temperture mesurements recorded in mv with which to clculte root xylem wter flux (g m- 2 s- 1 ). Dt from the sp flow sensors were smpled t 1-s intervls with mens logged hourly. The sensors were connected to dt logger (CR1X, Cmpbell Scientific) vi 32-chnnel multiplexer, with both the heter source nd dt logger being powered by 12-V bttery. Sp flow sensors were instlled on one superficil lterl root nd lrger descending tp root on three shrubs per site. For ech root monitored, flux nd directionl probe were instlled. The probes were inserted t bout 15 em soil depth on ech of the roots (tp nd lterl). The men root spwood res for the instrumented shrubs in KMA site were 8.4 cm 2 for the lterl roots nd 11.5 cm 2 for the tp root. The men root spwood res for the instrumented shrubs in Nioro site were 1 cm 2 for the lterl roots nd 16.5 cm 2 for the tp roots. For both shrub species, it ws ssumed tht wood in the root ws predominntly spwood. Bseline sp flow dt ws collected. from intct shrubs before conducting the coppicing tretments (only conducted in 24). On dy 198, shrubs were prtilly coppiced by removl of portion of their bove ground biomss which ws bout 8% of the cnopy including shoot tips. For ech of the coppicing tretments, rel-time sp flux dt were monitored for bout 13 dys t both shrub sites. On dy 22, complete removl of the bove ground biomss ws conducted leving only the shrub bse close to the soil surfce. Sp flow dt ws collected before, during nd fter coppicing tretments Lef wter potentil mesurements Predwn lef wter potentil ws mesured on monthly bsis using portble pressure chmber (PMS 6, PMS nstruments, Corvllis, OR, USA). n ddition to predwn wter potentil, mesurements were performed t severl dditionl times to determine shrub diurnl trends up to just before midnight when the shrubs were ssumed to hve recuperted to their predwn vlues. For ll smpling dtes, mesurements were performed on the sme shrubs to minimize errors (Turner, 1988). These mesurements spnned over three consecutive sesons, two dry sesons nd the trnsition into the wet cropping seson. Mesurements were conducted on the third rnk of fully opened well-exposed upper cnopy lefy twig. These were excised with shrp bldes nd immeditely trnsferred to the pressure chmber for mesurement. Rel-time monitoring of soil wter potentils (s described in Section 2.2.2) permitted close comprison with lef wter potentil mesurements. n ddition, for ech seson, three replictes of lef wter potentil of neighboring nnul weeds Amrnthus species (pig weed) in close proximity to the shrubs were mesured.

5 F. Kizito et l. j journl of Arid Environments 83 (212) Dt nlysis Since smpling ws conducted on the sme shrubs over time, sp flow nd lef wter potentil dt were nlyzed using repeted mesures model (Lindsey, 1993 ). Pre-plnned comprisons between tretment (shrub res) nd control (bre soil) mens were mde using Fisher's protected lest significnt difference (LSD) test nd differences were considered sttisticlly significnt t P <.5. Differences in field mesurements between shrub res nd bre soil res were determined using one-wy ANOVA nd to ssess the difference t ech dte, two-wy ANOVA with one repeted mesure fctor ws mde for soil wter content. All sttisticl procedures were conducted with Sttisticl Anlysis Systems softwre, SAS V8 (21). 3. Results 3.1. Soil wter sttus The J!s observed t ll three distnces from the shrub center to 2 em wy ws consistently higher nd significntly different thn those in the bre soil plots (Tble 1 ). Soil wter potentil vlues before dwn were slightly higher directly beneth the shrub thn further wy (Tble 1) though this ws not consistent over the entire dry seson. Predwn lef wter potentils were higher thn soil wter potentils before dwn indicting the wter potentil grdient ws from the plnt to the soil. The minimum for ech dy ws minly between 13 h nd 14 h (1 p.m-2 p.m) while the mximum ws between 24 h nd 1 h (midnight nd 1.m). The KMA site depicted considerbly higher net HR vlues compred to the Nioro site (Tble 2). Results from Tble 2 depicted significnt differences for both soil wter content nd soil wter potentil between the strt nd end of the dry sesons for both sites. A consistent die! prtil rewetting of portions of the profile occurred below nd round the shrub root zone during the dry seson observed s overnight increses in soil J!s t the 2 nd 4 em depths (Fig. 2, b). The 6 em depth lso exhibited grdul increse but with differing diurnl trends to the 2 em nd 4 em depths (Fig. 2, b). The J!s below nd round shrubs t the 2 em depth rnged from -3.5 to -6.3 MP t KMA nd from bout -1.8 to -4.2 MP in Nioro, resulting in similr die! mplitude of bout 2.5 MP t both sites. n contrst, J!s t the 2 em depth bre soil remined low with smll die! fluctutions of bout.2 MP occurring during the dy round noon. The mgnitude of HR seemed to pek t both sites between April-My nd declined in the subsequent two months (Fig. 3). For the -2 em depth rnge, verged over the entire 23 dry seson, the verge dily HR in KMA ws bout.13 mm dy-\ replenishing 3% of the dily depletion nd for the 24 dry seson, the verge dily HR in KMA ws bout.17 mm dy-\ replenishing 42% of the dily depletion. For the -2 em depth rnge, verged over the entire 23 dry seson, the verge dily HR in Nioro ws bout.7 mm dy-1, replenishing 15% of the dily depletion nd for the 24 dry seson, the verge dily HR in Nioro ws gin.6 mm dy-1, replenishing 22% of the dily depletion. There were notble differences in the mgnitudes of HR nd the bove results show tht HR peked month erlier in 24 t both study sites nd in 23, replenishment of soil wter through HR ws lower thn in 24 t both study sites. Replenishment of soil wter through HR ws lso lower t the Nioro site thn t the KMA site. Bsed on the rinfll dt, for KMA, there ws less rinfll in 23 thn 24 (Fig. 1) nd the HR pek mgnitude ws higher in 23 thn Cil (ij :p c: Gl... () ;:::... r:l ::!!: b Cil ::!!: (ij :p c: Gl - () ;:::... r:l ::!!: -3 J l -i l -7 r--, =Nighttime t=.j Dytime ----Q-- 2cm below shrub G E) 4 em below shrub A. A A 6 em below shrub A em bre soil --"--""'N" H ' 1 f\ 4 \? 1 \. J \ \ : r -..) n - i \ c::=:;::::::::j Dy of yer Dy of yer Fig. 2. Dily soil mtric potentil (MP) vrition with dy of yer. 24 below shrubs for the 2-6 em depths nd t bre soil plots for the 2 em depth for both sites () KMA; (b) Nioro. Stndrd error brs re computed from men vlues of three shrub replictes per mesurement. 24(Fig. 3).ln Nioro, there ws less rinfll in 24thn 23 (Fig.1) nd the HR pek mgnitude ws higher in 24 thn 23 (Fig. 3) Vpor flux dynmics Soil vpor flux entering or exiting the control volumes ws computed for the 2 nd 4 em depths nd showed substntil diurnl vrition with the gretest mgnitudes occurring between 1 h nd 12 h t both study sites (Appendix 3, electronic version only). This my be indictive of the peks observed in

6 74 F. Kizito et l.f journl of Arid Environments 83 (212) ().15 --o o ;< ;;..., :1: 't) = 2 =.1.5 /.::;:; ;:::.2... :: "' b --o- 23,... QJ --o-- 24 u.15 (,... 't) ;;.., ::.1 (}.(}5 ::t'/ if /' ---- / " / ";" () 3 2 "t E.3 >< ::l t;: ll en b lntd shrub <.,':----::;. 196 Prtil coppicing <- ----> Dy of Yer (24) 26 Feb Mr Apr My.Jun.Jul Month 3 1:rtilcoppking Fig. 3. Net hydrulic redistribution during dry sesons of 23 nd 24 for the 2-4 em depths with: () Gueir seneglensis t the KMA site; (b) Piliostigm reticultum t the Nioro site (Stndrd error brs re mens for HR from three shrub replictes s shown on ech curve) /) "t E 2 Appendix 3 for the 2 em bre soil tht were occurring round 12 h (noon). Positive vlues of vpor flux signify movement of vpor into ech control volume, with negtive numbers implying vpor flux leving the control volume. Sesonl vpor flux in the G. seneglensis dominted site t KMA over the two yers verged bout.9 mm while in the P. reticultum dominted site t Nioro verged bout.16 mm. Cumultive sesonl vpor deposition ws roughly n order of mgnitude less thn the observed sesonl increses in soil moisture within the 2-4 em lyer ttributble to HR t both study sites (Tble 2) Sp jlow mesurements.:!) >< ::l -= -1 c. ll ) Dy of Yer (24) 26 Reversls in the direction of sp flow were detected in the instrumented roots of both shrub species (Fig. 4, b). During the pek of the dry seson, when shrubs were still intct (uncoppiced) both positive (towrd shrub) nd negtive (towrd soil) flows were observed in the lterl shrub roots. Sp flow in the lterl root ws frequently negtive (towrd soil) t night nd rpidly becme positive (towrd shrub) soon fter sunrise, with the lrger descending tp root remining positive t this time nd with consistently more flow thn the lterl (Fig. 4, b). For the KMA site, uncoppiced men sesonl root sp velocity for the lterl t night time (19 h-6 h) nd volumetric sp flux were bout -2 em h- 1 nd.9 gm- 2 s-1, respectively while for Nioro these were bout -5 em h- 1 nd 1 gm- 2 s- 1 (Negtive sp flow velocity indictes tht flow reversls towrd soil). There ws slightly higher sp flow t Nioro though it hd lower replenishment through HR thn KMA. Sp flow sensors showed consistent nd firly similr results mong the different coppicing regimes hence the smll brely visible stndrd error brs. Fig. 4. Spflow dynmics mimicking trditionl mngement t study sites () KMA; (b) Nioro. Stndrd error brs re computed from men vlues of three shrub replictes per mesurement. With prtil copptcmg (Fig. 4, b), there were pronounced chnges for both tp nd lterl root flow. Dytime sp flow in the lterl roots towrd the plnt remined constnt however, s1gnificnt increse in nighttime sp flow towrd the soil ws observed soon fter prtil coppicing, with n pprent return to norml vlues fter three dys. Tp roots continued to show miniml downwrd reversl t night, but s expected, reduced the mount of upwrd flux during the dy. Lterl flow remined negtive (towrd soil) t night with n increse in reverse flow mgnitude nd decrese in positive flow (towrd shrub) during the dy. Tp root flow remined positive (towrd shrub) but with 48% decrese in flow for the KMA site nd 3% decrese t Nioro compred to the flow before coppicing.

7 F. Kizito et l./ journl of Arid Environments 83 (212) After complete cnopy removl (Fig. 4. b), tp roots exhibited trends of negtive flow (towrd soil) hence redistribution t night but minly remining positive (towrd shrub) during the dy though with very low mgnitudes (Fig. 4, b). Lterl root flow vritions indicted tht complete coppicing hlted positive flow (towrd shrub) t both sites during the dy, but t night both species exhibited negtive flow (towrd soil) nd hence continued to redistribute during this time but with lower mgnitudes thn when shrubs were either intct or prtilly coppiced. Night time reverse flow in the tp root ws not significntly different from zero in the coppiced shrub Lef wter potentil Men monthly lef wter potentil t the pek of the dry seson showed tht shrubs t both sites hd higher predwn wter potentil vlues thn either nnul herbs in ssocition with shrubs or those tht were fr wy from shrubs (Fig. 5). However, minimum lef wter potentil of nnul herbs ws consistently more negtive thn tht ttined by their shrub counterprts. Lef wter potentil of shrubs reched its minimum between My nd june with grdul rewetting in the trnsition into the riny seson. 4. Discussion With reference to the study objective. it ws hypothesized tht unique interction of hydrulic redistribution existed between shrubs nd nnul crops. Study results support this hypothesis s indicted by pronounced die! fluctutions in lj!s nd sp flow mesurements. During the study period t both sites. increses in ljf s over the dry seson indicte tht the soil wter content t 2 nd 4 em depths hd lso incresed. Since there ws no rin during this period, the increses indicte upwrd HR from lower lyers with greter lj!s. To the best of our knowledge, this is the first report of HR by roots of G. seneglensis nd P. reticultum. Support for this interprettion is provided by the lck of die! chnges in ljf s in the bre plot sites t the sme depth, mnsistent with die! fluctutions in lj!s in the vicinity of both shrub species being the result of uptke of wter by deep roots nd exudtion of wter from shllow roots to the surrounding dry soil. Peks in HR were observed in the mid-dry seson which could be ttributed to lj! 5 before dwn hving fllen sufficiently below predwn lef wter potentil to llow the upper dry soil lyers to effectively compete with the bove ground portion of the plnt for wter tken up by deep roots. This is consistent with observtions reported for Dougls-fir nd ponderos pine (Brooks et!., 26; Domec et!., 24) in the Pcific Northwest, U.S.A. nd for Brzilin svnn trees (Scholz et!.. 22). Results from the study enbled the quntifiction of the mgnitude of other bio-physicl processes potentilly responsible for wter trnsport. Soil moisture vribility within the profile is the result of HR. vpor trnsport nd plnt uptke. n comprison with bre plot results t the sme sites s shrubs, the increses in ljf s t the 2 nd 4 em depths in shrub res seem not to be solely from hydrulic redistribution through root systems. Estimtes from our dt re consistent with the presence of vpor trnsport nd deposition t both study sites which confounds estimtion of net HR mgnitudes. Vpor trnsport processes operte slowly nd re principlly driven by extreme temperture grdients (Mrshll et!., 1996; Hillel, 1998) s those prevlent t our study sites temperture dt (Kizito et!., 26, 27). The slight diurnl increses in ljf s in bre soil res, t both sites. during the dy suggest vpor trnsport nd deposition in the upper soil lyers. Though the existence of vpor trnsport hs been refuted in mesic regions (Leffler et l.. 25 ). our results indicte tht the process is -.5 co ll "i c:: Annul herb in close proximity to shrub --Annul herb fr from shrub Shrub ltl 3: ltl...j b -.5 co ll. ::!: -1 "i c:: ltl -2 3: -t...j -2.5 KMA Mrch April My June July August Nioro Month Mrch April My June July August Month Fig. 5. Monthly vritions in lef wter potentil (MP) of shrub, nnul herb close to shrub nd nnul herb wy from shrub for both study sites () KMA; (b) Nioro. Stndrd error brs re computed from men vlues of three shrub nd nnul replictes per mesurement. existent though t smll mgnitudes nd ws more pronounced in the upper soil lyers. This is consistent with findings elsewhere in rid ecosystems (Scnlon nd Milly, 1994; Milly, 1996). Though smll, vpor trnsport nd deposition frctionlly contributed to replenishing the depleted soil moisture pool in the upper soil lyers. Previous work by Kizito et!. (26) noted fcilittive interctions where soil below shrubs ws ssocited with lower soil temperture extremes thn djcent bre soil which ws ttributed to shde provision by shrub cnopies. This hs n effect of reducing both ir nd soil tempertures giving shrub-imprted ltertion in microclimte nd lower soil evportion below shrub cnopies thn bre soil which in turn lters wter potentil trends below shrubs compred to bre soil res. The vpor flux trends nd the forementioned fcilittive effects of shrubs indicte potentil contribution of other fctors nd is not solely HR driven.! :r:

8 76 F Kizito et l.( journl of Arid Environments 83 (212) The observtions of slightly higher sp flow t Nioro thn KMA yet with lower replenishment through HR thn KMA my be ttributed to the vrying rinfll regimes t both study sites. Nioro hs higher rinfll nd the soils hve higher cly content thn those in KMA (sndier soils) with lower rinfll. The mtric potentil grdients between the drier sndy soil nd plnt roots would subsequently be greter in KMA thn those in Nioro hence the lower HR levels in Nioro. Complete removl of shrubs seemed to hve lower effect on reverse sp flow thn prtil coppicing. t could be tht the completely coppiced plnts experience reduced trnspirtion during the dy with little or no depletion of soil wter in the shllow lyers to crete sufficient grdient necessry for HR. The differences in J-R mgnitude in the different yers (23-24) s well s differences in J-R peking month erlier in 24 t both study sites re lso likely due to differences in rinfll ptterns nd soil types s explined previously. When the soil profile is drier due to lower rinfll, this induces greter grdient between the drier soil nd plnt roots hence the shrub roots compenste for the diminished wter reserves in the upper soil profile by extrcting wter from deeper moist lyers nd in turn perform the HR process. Hence HR mgnitudes were higher for ech of the yers with lower rinfll t the study sites. Monthly chnges in plnt wter potentil reveled tht the soils were driest between April nd july which resulted in higher mtric potentil grdients between the dry soil nd shrub roots. This my explin the observed peks in HR during these months. Additionlly, t both study sites, plnt wter potentil of nnul herbs in close proximity to the shrubs tended to recover to vlues closer to the shrub wter potentil thn the nnul herbs tht were fr from the shrubs (Appendix 4, electronic version only). This indictes tht shrubs rewetted the upper profile nd the nnul herbs tpped some of those wter pulses. Other shrub fcilittive benefits beyond lef wter potentils for shrub-nnul crop interctions hve been demonstrted where shrub lef nd stem mendments improved crbon, nitrogen nd phosphorus minerliztion (Doss et!., 29). nitilly, sp flux ws positive (towrd shrub) during the dy in the tp roots nd negtive (towrd soil) t night in the lterl roots, which is consistent with redistribution of wter from tp to lterl roots. After prtil cnopy removl, with incresed soil drying during the dy (less shrub shde, higher soil moisture evportive losses) nd decresed trnspirtion grdient due to reduced cnopy volume (s'me root volume), shrub roots my hve ttined higher turgor pressures. Hence, root wter potentil would gretly exceed tht of the surrounding desiccted soil, which might explin the greter HR observed t night soon fter coppicing. The negtive sp flow t night (towrd soil) in the surfce lterl root nd the periodic positive flow (towrd root) in the tproot t night re indictive of hydrulic lifting of wter from the lower to the upper soil lyers which re drier. With complete cnopy removl, there ws slight positive tp root flow (towrd shrub) suggesting there could hve been some leking stems or uncoppiced twigs tht contributed to the trnspirtion grdient. The lower tp root sp flow vlues for dy 198 t Nioro were due to. cloud cover which gretly reduced the vpor pressure deficit hence the lower sp fluxes. Ther e ws some unexplined behvior by the tp roots in this study s it would be expected tht the tp roots should hve shown smll mount of positive flow t night when the lterls re showing reverse flow. The flow from the tp roots ws not sufficiently positive but hovered slightly bove zero. This suggests tht the wter for reverse flow my not be coming entirely from tp roots but from some lterl sinker roots. Previous studies conducted by Kizito et l. (26) on root rchitecture for both shrub species indicted tht lterl roots hve numerous sinker roots brnching from them some of which extend to depths beyond 5 m. The dt suggest tht prtil coppicing my trigger slight increses in HR. However, longer period of prtil coppicing is needed before this suggestion cn be tested conclusively. Results indicte tht the HR process provides mechnism for prtil rechrge ofthe upper soil profile in the bsence of new precipittion, nd since rechrge is driven by grdients in 1Jf 5, HR my significntly retrd the rte of soil drying in the upper profile nd reduce the number of severe drought stress dys for the shrub or ny opportunistic neighboring nnuls. This my lso be relted to other fctors such s shde, temperture, lower soil wter evportion under the shrub cnopy s noted by fcilittive effects reported by Kizito et l. (27) nd Cllwy, 27. Bsed on personl field observtions, the recovery of sp flow rtes in the lterl roots t KMA to pre-coppicing vlues on dys 21 nd 22 my hve been response to production of new, rpidly trnspiring shoots. Coppicing s mngement prctice could hve significnt implictions on shrub physiologicl behvior with prticulr reference to HR. The current study suggests tht intercropping of nnul crops with shrub stnds my provide benefits to griculturl productivity since the roots of nnul food crops overlp with the wetting zone provided by the shrub's HR process. Work conducted by Kizito et l. (26) on shrub root systems in the study re indictes tht the shrubs exploit the first 4 em where most of the rewetting is nd this is the sme zone where shllow rooting nnul herbs thrive. Recent findings show tht deeper-rooted 'nurse plnts' if subjected to complete shoot removl cpture wter tht is out of rech of other crops in 'wter sfety-net' role nd mximize hydrulic redistribution (Burgess, 211 ). We suggest tht s opposed to complete shrub removl, prtil coppicing my be fesible mngement regime in these semirid environments. Prtil coppicing reduces overll shrub wter consumption nd trnspirtion which in turn dmpens the recovery of the redistributed wter by the shrubs during dytime trnspirtion to the benefit of nnul crops. Prtil coppicing lso reduces the shrub cnopy size hence incresing vilble crege for nnul row food crops, yet preserves the shrub-imprted benefits tht provide fvorble microclimte for griculturl productivity under hrsh semi-rid conditions. However, frmers prefer complete cnopy removl nd sometimes uproot the entire shrub cluster to increse griculturl crege on their frm plots. While this might seem ppeling in the short term for the frmers, other rmifictions such s reduced profile rechrge, incresed rindrop impct nd run-off, higher surfce soil tempertures nd incresed soil evportion (Kizito et!., 26) which could culminte in long-term detrimentl impcts to griculturl productivity. 5. Conclusion This study demonstrted tht HR ws beneficil component in Shelin crop-shrub gro-ecosystems with G. seneglensis nd P. reticultum. Results indicte tht the HR process ws highest during the prolonged dry sesons in both study yers. The process ws strongly governed by Jf 5 grdients nd consumptive wter use by shrub species. The quntities of wter moved by HR in this study were reltively smll. However, these mounts cn hve significnt benefits to the plnt lifting the wter nd to neighboring nnul plnts to the shrubs. The HR process provided prolonged stored wter for nnul herbs survivl in the dry seson with potentil of preventing or delying loss of wter trnsport cpcity in shllow roots. Hydrulic redistribution of wter by plnt roots hs importnt implictions for both locl nd regionl hydrologicl cycles nd

9 F. Kizito et/. j journl of Arid Environments 83 (212) functioning of ecosystems. The process my ply significnt role in the rtio of ecosystem evportion nd trnspirtion on both diurnl nd sesonl scles. This process could lter soil wter nd groundwter distributions t levels tht my significntly impct regionl wter blnces nd improve crop productivity in wterscrce environments through shrub-imprted fcilittive effects. Acknowledgments This mteril is bsed upon work supported by the Ntionl Science Foundtion under Grnt No Any opinions, findings, nd conclusions or recommendtions expressed in this mteril re those of the uthor( s) nd do not necessrily reflect the views of the Ntionl Science Foundtion. Specil grtitude goes to jon Sndeno for proof reding this mnuscript nd to Rob Coulombe for his technicl ssistnce on the construction of sp flow sensors. Apprecition goes to the EPA tem in Corvllis, CERAAS nd SRA technicins in Senegl for ll the field support rendered during the course of this reserch effort. Appendix. Supplementry mteril Supplementry dt ssocited with this rticle cn be found, in the online version, t doi:1.116/j.jridenv References Ahrens, D., An ntroduction to Wether, Climte nd the Environment. Meteorology Tody, fifth ed. West Publishing Co. Bleby, T.M., McEirone, A.j., jckson, R.B., 21. Wter uptke nd hydrulic redistribution cross lrge woody root systems to 2 m depth. Plnt, Cell nd Environment 33, Brooks, j.r., Meinzer, F.C., Coulombe, R., Gregg, j., 22. Hydrulic redistribution of soil wter during summer drought in two contrsting Pcific Northwest coniferous forests. Tree Physiology 22, Brooks, j.r., Meinzer, F.C., Wrren, j.m., Domec, j.c., Coulombe, R., 26. Hydrulic redistribution in Dougls-fir forest: lessons from systems mnipultion. Plnt, Cell nd Environment 29, Brown, R.W., Brtos, D.L., A Clibrtion Model for Screen-cged Peltier Thermocouple Psychrometers. USDA Forest Service, lntermountinforest nd Rnge Experiment Sttion, Ogden, UT. 155pp. Burgess, S.S.O., Adms, M.A., Turner, N.C., Ong, C.K., The redistribution of soil wter by tree root systems. Oecologi 115, Burgess, S.S.O., Adms, M.A., Bleby, T.M., 2. Mesurement of sp flow in roots of woody plnts: commentry. Tree Physiology 2, Burgess, S.S.O., Adms, M.A., Turner, N.C., White, D.A., Ong, C.K., 21. Tree roots: conduits for deep rechrge of soil wter. Oecologi 126, Burgess, S.S.O., 211. Cn hydrulic redistribution put bred on our tble? Plnt nd Soil Chill. A.T., Prlnge, M.B., On wter vpor trnsport in field soils. Wter Resources Reserch 34 (4), Cldwell, M.M., Richrds, j.h., Hydrulic lift: wter efflux from upper roots improves effectiveness of wter uptke by deep roots. Oecologi 79, 1-5. Cldwell, M.M., Dwson. 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