Water status, drought responses, and growth of Prosopis flexuosa trees with different access to the water table in a warm South American desert

Water status, drought responses, and growth of Prosopis flexuosa trees with different access to the water table in a warm South American desert
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  Water status, drought responses, and growth of   Prosopis  flexuosa  trees with different access to the water tablein a warm South American desert Carla V. Giordano  • Aranzazu´ Guevara  • Herna´n E. Boccalandro  • Carmen Sartor  • Pablo E. Villagra Received: 15 March 2010/Accepted: 27 December 2010/Published online: 8 January 2011   Springer Science+Business Media B.V. 2011 Abstract  Prosopis flexuosa  trees dominate wood-lands in the Central Monte Desert (Mendoza, Argen-tina), with \ 200 mm rainfall, exploiting the watertable recharged by Andean rivers, and also growing indunes with no access to the water table.  Prosopis woodlands were extensively logged during develop-ment of the agricultural oasis, and surface andgroundwater irrigation could lower the depth of thewatertableinthefuture.Weevaluatedtreepopulationswith decreasing access to the water table: valley adulttrees, valley saplings, and dune adult trees, in order toassess their ecophysiological response to water tableaccessibility.Highandseasonallystablepre-dawnleaf water potentials ( - 2.2  ±  0.2 to  - 1.2  ±  0.07 MPa)indicated that valley adults utilize larger and morestable water reservoirs than valley saplings and duneadults ( - 3.8  ±  0.3to - 1.3  ±  0.07 MPa),with highermidday leaf conductance to water vapor (valley adults * 250; dune adults \ 60 mmol m - 2 s - 1 ), potentiallyhigher CO 2  uptake, and increased radial growth rate(valley adults 4.1  ±  0.07; dune adults 2.9  ±  0.02mm year - 1 ). Trees with poor access to the water tableexhibited drought tolerance responses such as middaystomata closure, leaflet closure, and osmotic adjust-ment.Stomatadensitydecreasedinresponsetodroughtwhen leaf expansion was restricted. The combinationofphreatophytismanddrought tolerance would enlarge P. flexuosa  habitats and buffer populations againstchanges in rainfall dynamics and water table depth. Electronic supplementary material  The online version of this article (doi:10.1007/s11258-010-9892-9) containssupplementary material, which is available to authorized users.C. V. Giordano ( & )    A. GuevaraInstituto Argentino de Investigaciones en Zonas A´ridas(IADIZA), Consejo Nacional de InvestigacionesCientı´ficas y Te´cnicas (CCT-Mendoza CONICET), Av.Ruiz Leal s/n, Parque General San Martı´n, CP 5500,Mendoza, Argentinae-mail: cgiordano@mendoza-conicet.gov.arH. E. BoccalandroInstituto de Biologı´a Agrı´cola de Mendoza (IBAM),Universidad Nacional de Cuyo-CONICET, AlmiranteBrown 500, 5505, Chacras de Coria, Mendoza, ArgentinaH. E. BoccalandroInstituto de Ciencias Ba´sicas, Universidad Nacional deCuyo. Ciudad Universitaria, Parque General San Martı´n,CP 5500, Mendoza, ArgentinaC. Sartor    P. E. VillagraFacultad de Ciencias Agrarias, Universidad Nacional deCuyo, Almirante Brown 500, 5505, Chacras de Coria,Mendoza, ArgentinaP. E. VillagraInstituto Argentino de Investigaciones en Nivologı´a,Glaciologı´a y Ciencias Ambientales (IANIGLA), ConsejoNacional de Investigaciones Cientı´ficas y Te´cnicas(CCT-Mendoza CONICET), Av. Ruiz Leal s/n, ParqueGeneral San Martı´n, CP 5500, Mendoza, Argentina  1 3 Plant Ecol (2011) 212:1123–1134DOI 10.1007/s11258-010-9892-9  Keywords  Dunes    Groundwater    Monte   Phreatophytes Introduction Woody phreatophytes play particular ecological andeconomic roles in deserts, related to their ability touse constantly saturated soil water reservoirs inenvironments where rainfall is intra and inter-annu-ally variable and scarce (Noy-Meir 1973). They oftenpossess dimorphic root systems where deep rootsaccess groundwater, while lateral roots search forsurface moisture and nutrients. They produce higherannual biomass than that predicted by precipitation(Sharifi et al. 1982), and are dominant components of their community, with central roles in ecosysteminteractions (Rossi and Villagra 2003; Zou et al.2005; Simmons et al. 2008), biogeochemical (Schade and Hobbie 2005; Alvarez et al. 2009), and hydro- logical cycles (Burgess et al. 2000; Hultine et al.2004). Prosopis flexuosa  D.C. (Fabaceae, Mimosoidae, ‘algarrobo dulce’ ) is a phreatophytic tree thatdominates open woodlands in the Central MonteDesert, Argentina (Morello 1958), whose presence ina warm arid region is supported by the contributionof groundwater derived from Andean rivers (Torresand Zambrano 2000).  Prosopis  woodlands havehistorically provided local indigenous people descen-dant from the  Huarpes  with goods for survival, andwere extensively exploited in the twentieth centuryfor vineyard and urban development in the oasis.They were also indirectly affected by human activ-ity, as irrigation with river water has reduced riverflows into barren lands and riparian woodlands(Abraham 2000). Increased irrigation supplementedby groundwater extraction could lower the depth of the water table beneath  Prosopis  woodland areas inthe future.Despite the ecological and economic importanceof   Prosopis  species, information about their water useand drought responses to surface and groundwateravailability in arid South America is scarce (Mooneyet al. 1980; Villagra and Cavagnaro 2006; Villagra et al. 2010) compared to the information on  Prosopis species from North American deserts (e.g., Nilsenet al. 1984; Ansley et al. 1992; Stromberg et al. 1992; Resco et al. 2009). It has been suggested thatreceiving less than 350 mm mean annual rainfall P. flexuosa  needs access to constantly saturated soilwater reservoirs (Morello 1958; Gonza´lez Loyarteet al. 2000). However, it has been recently demon-strated from the analysis of stable isotopes of xylemwater that it can grow without access to groundwater,using only rainwater from the unsaturated soil of 20-m-high dune slopes receiving  \ 200 mm meanannual rainfall in the Central Monte Desert (Jobba´gyet al. 2010), although there is no physiological orgrowth-related evidence of ground and rainwater useby this species.In this paper, we ask what consequences watertable accessibility and rainfall dynamics have onwater use, drought tolerance responses and growth of  P. flexuosa  trees at the mostly arid end of its range of distribution. We compare three  P. flexuosa  popula-tions that, due to their topographical position (valleyor dune) or size (adult or sapling), could havedifferent access to the water table and, consequently,differing dependence on rainfall. We determinephysiological and growth indicators of water tableaccessibility in valley and dune adult trees based onprevious xylem water isotopic composition data thatindicated differential use of groundwater by bothpopulations (Jobba´gy et al. 2010). We also inferwhether or not young valley trees (for which we haveno xylem water isotopic information) access thewater table, based on their physiological behavior.Finally, we discuss the likely effects on  P. flexuosa populations of variations in water table depth andrainfall dynamics. Materials and methods Study siteIt is located in the Central Monte Desert on the easternfoothills of the Andes, NE Mendoza city, Argentina,within the Telteca Natural Reserve (32  20 0 S;68  00 0 W). The climate is arid with mean annualrainfall of 156 mm (1972–2007), mostly concentratedin the austral summer (October–March), with meanannual temperature of 18.5  C (Alvarez et al. 2006).The region comprises a NNW–SSE oriented valley-dune system, with a 6–15 m deep subterraneanwatershed that is remotely replenished by Andean 1124 Plant Ecol (2011) 212:1123–1134  1 3  river infiltration, with local recharge by rainfalldrainage being negligible (Jobba´gy et al. 2010; Arani-bar and Gomez, personal communication).Experimental designOur experimental site comprised a valley and anadjacent 20 m high dune in Puesto La Penca(32  25 0 42 00 S 68  00 0 33 00 W). The depth of the watertable was 7.1 m below the valley surface (assessed bya hand-dug well) with 25% w/w water in the 1.5-mwide capillary fringe (Jobba´gy et al. 2010). The duneheld  * 3% w/w rainwater in 1–4 m deep waterreservoirs, and both landscape units had a homoge-neous sandy soil profile in the root zone (Jobba´gyet al. 2010; Guevara et al. 2010).We studied three populations of   P. flexuosa  trees that due to theirtopographical position (valley-dune) or size (adult-sapling) could have variable access to the water tableand thus, varying dependence on rainfall: adult treesin the valley and the dune, for which we had isotopicevidence indicating different access to the water table(Jobba´gy et al. 2010), and saplings in the valley [non-reproductive tree-type individuals, base diameter1.5–4.5 cm (Table 1), 4–10 years old], for whichwe have no isotopic data with which to distinguishwater sources. We selected five similar-sized adultindividuals at each landscape unit, and four saplingsin the valley (14 individuals in total; Table 1). Leaf water status, conductance, osmolality and leafletclosure were measured in two consecutive growingseasons on dates: 21/12/2007, 08/02/2008, 13/03/ 2008, 17/12/2008, and 05/03/2009.RainfallRainfall was recorded with a data logging rain gauge(Hobo Event, Onset Computer Corporation, Bourne,MA, USA).Leaf water statusWe measured pre-dawn (PD, 3:00 to 5:00 h) andmidday (MD, around local solar noon: 13:30 h) leaf water potential ( W w ) in two branches  B 2 mm diam-eter that supported expanded sun-exposed leaves witha pressure chamber (PMS Instruments Co., Corvallis,OR, USA) based on Scholander et al. (1965). Cutbranches were placed in sealed nylon bags in the dark and measured within 1–2 min.Leaf conductance to water vaporWe measured adaxial and abaxial leaf conductance towater vapor ( g l ) with a steady-state diffusion porom-eter (SC-1, Decagon Devices, Pullman, WA, USA).Daily  g l  profiles were obtained from two adult trees,one from the valley and one from the dune in wet anddry periods. Seasonal midday  g l  was measured in the14 selected individuals from all three populations. Inall cases we measured three expanded sun-exposedleaves per tree, added up adaxial and abaxial  g l , andused the averaged data for further analyses.Leaf osmolalityWe measured osmolality (mol kg - 1 ) in three frozenleaves per tree collected at midday. Because thawedleaves yielded a low volume of cellular juice bypressing, we disrupted the cells by smashing them inliquid N 2  and sonicating (50 mg) in 200  l l of potassium phosphate buffer 50 mM (pH 7), in anultrasonic bath at room temperature (Bransonic1510RMT, 42 kHz, Branson Ultrasonic, Danbury,CT, USA) for 7 min. We centrifuged the samples at10,000 rpm for 5 min, and measured osmolytes in thesupernatant with a vapor pressure osmometer(Wescor, Logan, UT, USA). Leaf osmolality wascalculated by correcting for the dilution by thepotassium phosphate buffer and for the amount of liquid in a subsample of smashed leaves after dryingat 60  C for 2 days. Table 1  Dimensions of individuals selected for fieldmeasurementsPopulation DB a (cm) Height (m) Crown area(m 2 )Valley sapling 3.0 (0.65) 1.53 (0.30) 2.27 (1.13)Valley adult 14.54 (1.36) 4.08 (0.29) 12.91 (1.81)Dune adult 15.35 (2.7) 3.34 (0.32) 13.56 (3.9) P  value b 0.82 0.13 0.88Values are means  ±  s.e.m. between brackets a Diameter at the tree base b From the two tailed  t  -test between valley and dune adulttrees,  N   =  10Plant Ecol (2011) 212:1123–1134 1125  1 3  Leaf movementsWe measured  P. flexuosa  leaf movements that alterthe fraction of total leaf area and stomata exposed toair and sun (three leaves per tree).  Leaflet closure We measured the angle between the adaxial faces of opposite leaflet laminas with a portable protractor.  Leaf orientation Using a compass, we recorded the cardinal directionof an imaginary vector perpendicular to the planedetermined by the adaxial surface of leaflets.Stomatal density and amphistomyStomata were counted in optical microscope (Axio-star Plus, Carl Zeiss International, Go¨ttingen) photo-graphs of epidermal prints (Boccalandro et al. 2009)obtained at 400 9 magnification from middle portionsof leaflet laminas. We evaluated three mid laminasper leaf and three leaves per tree, and used averageddata for further analyses.Stomata density SD ð Þ¼  no :  of stomata per leaf area mm  2   Amphistomy  ¼  SD adaxial  =  SD abaxial : Early (harvested in December) and late (harvested inMarch) cohorts of leaves were evaluated.Leaf surface areaWe harvested five expanded sun-exposed leaves,scanned them with a reference area, and calculatedsurface area per leaf (without the petiole) usingAdobe Photoshop (v. 7.0). Early and late leaf cohortswere evaluated.Tree growthWe took transverse core samples at the base of sevenadult trees from the valley and six from the dune witha gas-powered drill (TED_262R, Tanaka Kogyo Co.Ltd, Chiba, Japan) in June 2008. We measured thewidth of all growth rings from the pith to theoutermost ring with a precision of 0.01 mm using aVelvex system, and cross-dated them using theCOFECHA program (Holmes 1999a), followingFritts (1976). We constructed curves of cumulativeradial growth vs age for each sampled tree with DPL-AGE program (Holmes 1999b) and estimated indi-vidual growth curves by linear regression. Wecalculated the rate of radial growth per year, as theslope of each individual curve. Growth curves foreach population (valley adults and dune adults) wereconstructed using the data of all measured trees andadjusting to a line by linear regression.Statistical analysisDimensions of trees from the dune and the valleywere compared with a two tailed  t  -test. Leaf   W w  (PDand MD),  g l , osmolality, and leaflet angle meancomparison in time among the three populations(valley adults, dune adults, and valley saplings) wereanalyzed with repeated measures (rm) ANOVA.Comparison of population means on each samplingdate was done with Tukey post-hoc test on the rm-ANOVA. Variation in  g l  and leaflet angles as afunction of   W wPD  or midday  g l  was evaluated withlinear regression. SD, amphistomy, and leaf area of valley and adult trees and two leaf cohorts wereanalyzed with two-way ANOVA. We evaluateddifferences in annual growth rates between bothpopulations comparing the slopes of the growthcurves of dune and valley adult trees with  t  -test. Weused Infostat (v.2008, InfoStat, Co´rdoba, Argentina)and Statistica (v.6, StatSoft, Tulsa, OK, USA). a  =  0.05. Results Climate conditions and rainfall dynamicsThe 2007–2008 growing season received more rain-fall (250.6 mm) than the 2008–2009 growing season(154.9 mm), exceeding the annual average for theregion, and with contrasting dry (October–December)and wet (January–March) periods (Fig. 1 a). In2008–2009, rainfall was close to the average for theregion and concentrated in two huge events inDecember and January (Fig. 1b). 1126 Plant Ecol (2011) 212:1123–1134  1 3  Leaf water statusPre-dawn leaf water potential ( W wPD ) varied season-allywithtreepopulation(interaction P \ 0.0001,time P \ 0.0001, population  P \ 0.0001 rm-ANOVA;Fig. 2). Dune adult trees showed the lowest  W wPD valuesandhighestseasonalfluctuationassociatedwithrainfall dynamics (e.g., low  W wPD  in dry periods andhigh  W wPD  following rains), indicating a strongdependence on rainfall as a source of water. Valleyadult trees showed the lowest seasonal fluctuation andhighest values of   W wPD , associated with the exploita-tion of a groundwater reservoir of greater magnitudeand seasonal stability than rainfall.  W wPD  values forvalley saplings were intermediate between those of valley and dune trees during the relatively dry periods,indicating access to an additional water reservoirbesides current rainfall, although of lower magnitudethan that exploited by valley adult trees. On the otherhand, midday leaf water potential ( W wMD ) varied intime with similar values among tree populations(interaction  P  =  0.58, time  P \ 0.0001, population P  =  0.56), indicating that  W wPD  is a better physiolog-icalindicatorofgroundwateraccessibilitythan W wMD .Leaf   W w  morning fluctuation ( W wMD  -  W wPD ) duringdry periods (December 2007 and March 2009) can bearranged as: valley trees [ valley saplings [ duneadults, indicating higher water movement throughtrees and gas exchange in populations with higheraccessibility to the water table. In December 2007, W w morning fluctuation of dune trees was nearly 0,suggesting poor or null water transport and gasexchange in dunes during the driest period.Leaf conductance to water vapor Prosopis flexuosa  trees showed the highest leaf conductance to water vapor ( g l ) during the morning,with peak values approximately 2.5 h followingsunrise and a steep decrease toward midday andafternoon (Online Resource 1). The dune tree showedlower  g l  than the valley tree, with greater differencesduring the driest period, and midday stomata closureclearly promoted leaf rehydration after the steepdecrease of about 2.5 MPa in leaf   W w  from pre-dawn values after morning transpiration (OnlineResource 1).Midday  g l  evaluated across seasons showed afluctuation that varied with tree population 250300350 233.6 mm7 d 506070 156.0 mm P r  e c i   pi   t   a t  i   on ev  en t   (  mm )   2007-2008 a 050100150200 37.8 mm39 d 010203040    C  u  m  m  u   l  a   t   i  v  e  p  r  e  c   i  p   i   t  a   t   i  o  n   (  m  m   ) 20025030035040506070 62.3 mm4 d140.5 mm24 d P r  e c i   pi   t   a t  i   on ev  en t   (  mm )   2008-2009 b 0501001500102030 4Oct Nov Dec Jan Feb Mar    C  u  m  m  u   l  a   t   i  v  e  p  r  e  c   i  p   i   t  a   t   i  o  n   (  m  m   ) Fig. 1  Rainfalls during 2007–2008 ( a ) and 2008–2009( b ) growing seasons. Cumulative rainfall ( solid line ) andrainfall events ( dotted line ) are plotted from 1st October to 31stMarch.  Vertical arrows  indicate measurement dates;  captionsabove  indicate cumulative precipitation and days (d) since thelast precipitation event  C 10 mm 2007 2008 2009 -2-10 Dec Feb Mar MarDec 37.8 mm 156 mm 39 d 16 d 233.6 mm 7 d 62.3 mm 4 d 140.5 mm 24 d  aa    M   P  a   ) aabb -5-4-3 cabb     ψ   w    ( Fig. 2  Pre-dawn ( solid line ) and midday ( dotted line ) leaf water potential ( W w ) of valley adult trees ( closed circles )valley saplings ( triangles ) and dune adult trees ( open circles )during two consecutive growing seasons. Accumulated rainfalland days (d) since the last precipitation event  C 10 mm areindicated.  Symbols  are means and vertical lines  ±  s.e.m.  Different letters  indicate differences among tree populationson each sampling date and type of measurement (pre-dawn ormidday), tested by Tukey’s post-hoc test on the rm-ANOVAPlant Ecol (2011) 212:1123–1134 1127  1 3
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