Functional Characteristics of the Arborescent Genus Polylepis Along a Latitudinal Gradient in the High Andes

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FUNCTIONAL CHARACTERISTICS OF THE ARBORESCENT GENUS Polylepis ALONG A LATITUDINAL GRADIENT IN THE HIGH ANDES Aura Azócar, Fermín Rada and Carlos García-Núñez SUMMARY Polylepis is a genus restricted to the Andean Mountain Range, naturally occurring above the upper continuous forest limit. The purpose of this work was to integrate and compare functional characteristics in terms of water and carbon rela‑ tions and low temperature resistance mechanisms in different Polylepis species along a latitudi
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  OCT 2007, VOL. 32 Nº 10 663 KEY WORDS / Andean Mountain Range / Carbon Relations / Gas Exchange / Low Temperature Resistance / Photosynthesis / Water Relations /  Received: 05/02/2005. Modifed: 09/14/2007. Accepted 09/20/2007. Aura Acar . Biologist, Universidad Central de Veneuela (UCV). Dr. in Ecolog, Universit o Montpellier, France. Proessor, Universidad de Los Andes (ULA), Veneuela. Dirección: Laboratorio de Ecofsiología, Instituto deCiencias Ambientales  Ecológicas (ICAE), ULA, Mérida, Veneuela. e-mail: aaocar@ula.ve Fermn Rada . Biologist, Universit o Marland, USA. Dr. in Tropical Ecolog, ULA, Vene-uela. Proessor, ULA, Veneuela. e-mail: rada@ula.ve Carlos Garca-Ne. Forestr Engineer and Dr. in Tropical Ecolog, Proessor, ULA, Veneue-la. e-mail: cgarcia@ula.ve FUNCTIONAL CHARACTERISTICS OF THEARBORESCENT GENUS  Polylepis ALONG ALATITUDINAL GRADIENT IN THE HIGH ANDES AURA AzóCAR, FERMíN RADAand CARLOS GARCíA-NúñEz limate is an impor-tant actor or plantgrowth; it governs dis-tribution and sets limits or survival.One important characteristic o highmountain environments is the low airtemperature, which limits plant growthand survival in these habitats (Sakaiand Larcher, 1987). On the other hand,tropical high mountain environmentspresent large dail variations andsmall seasonal changes in temperature,and reeing temperatures occur al-most ever da o the ear (Hedberg,1961; Aócar and Monasterio, 1980).Temperature and rost occurrence varwith latitude, altitude and topograph. 0378-1844/07/10/663-06 $ 3.00/0 Thus, minimum absolute temperaturesbecome an important variable in or-der to establish the distribution limitso the maor vegetation tpes (Wood-ward and Williams, 1987; Prentice et al. , 1992). In spite o this, the genus Polylepis does not seem to respondin their distribution to this regime o low temperatures, since it presents aunique altitudinal distribution reachingmuch higher elevations than those o an other angiosperm tree in the world(Liberman-Cru, 1986). Additionall,it is an endemic genus in the AndeanMountain Range, ound rom Vene-uela in the north to Central Argentinain the south. Kessler and Schmidt-Leb-unhn (2006) reported 26 species, withthe highest diversit ound in Peru (14species, 3 endemic) and Bolivia (13species, 4 endemic); onl one specieswas recorded or Veneuela, 3 in Co-lombia (1 endemic), 7 in Ecuador (2endemic), 2 in Chile and 4 in Argen-tina (1 endemic).Regional orest distribu-tion seems to be related to areas wheresolar irradiance is controlled b theslope aspect (direction) and inclination(angle), and sun elevation inluences lo-cal distribution (Braun, 1997). The ge-nus creates a natural treeline well abovethe timberline (continuous orest line),in some cases orming orest patches SUMMARY  Pollepis is a genus restricted to the Andean Mountain Range, naturally occurring above the upper continuous orest limit. The purpose o this work was to integrate and compare unctional characteristics in terms o water and carbon rela‑tions and low temperature resistance mechanisms in dierent  Pollepis species along a latitudinal gradient. The studied species were P. sericea in Venezuela, P. tarapacana in Boliviaand  P. australis in Argentina. Seasonal measurements o lea water and osmotic potentials, stomatal conductance, CO 2 as‑similation and respiration rates, and injury and reezing tem‑ peratures were compared. There is a gradient, in terms o  unctional attributes, along the environmental range. P. tarapa-cana is the most tolerant species to water stress, while P. seri-cea avoids the less harsh conditions o its habitat through os‑motic adjustments and cell wall elasticity changes. Mean CO 2  assimilation rates were higher in P. australis (9µmol·m ‑2 ·s ‑1 )campared to P. sericea (5µmol·m ‑2 ·s ‑1 ) and  P. tarapacana  (3·µmol·m ‑2 ·s ‑1 ). Mean night lea respiration rates were similar  or all species (1‑2·µmol·m ‑2 ·s ‑1 ). In terms o low temperatureresistance, P. sericea shows daily osmotic adjustments and amoderate supercooling capacity (‑9ºC). The other two spe‑cies rely on reezing tolerance in order to survive the moreextreme low temperature conditions. The unctional attributesdescribed in this study or the dierent species in a wide en‑vironmental range may explain some aspects o their successalong the latitudinal and altitudinal gradients.  664 OCT 2007, VOL. 32 Nº 10 between 4000 and 5200masl. In gen-eral, Polylepis orests are restricted tosteep slopes in mid to low topographicpositions, although some species arealso ound on lat terrains. Polylepis  dominates the upper strata o woodlandsand shrublands, together with other lessabundant wood species o the genera  Berberis, Gaultheria , Valeriana , amongothers. These wood species occasion-all orm dense stands that cover mosto the surace, oten called orests, thatare usuall intermingled with patches o tussock grasslands, erns or rock out-crops (Kessler, 2006).Despite their relativelsmall area cover, these orests representunique ecological islands o biodiver-sit that are vanishing rapidl; Feldsaand Kessler (1996) indicate that onl2% o the srcinal Polylepis orests re-main in Perú and 10% in Bolivia, sug-gesting that human occupation in theAndean highlands has led to consider-able destruction o these orests.All species o the ge-nus are exposed to ver harsh climaticconditions in both their altitudinal andlatitudinal distribution, indicating thattheir survival depends on ver specialadaptive characteristics that allow themto sustain reeing temperatures and adistinct water stress due to precipita-tion regimes and dring winds. Nor-mall, these conditions would preventtree growth; however, the presence o man Polylepis species above the tim-berline, orming orest islands alongbroad temperature and water availabilitgradients, suggests a high diversit o unctional attributes, which in turn plaa undamental role in their survival.In this work, someunctional attributes o three Polylepis species along a wide latitudinal gra-dient are integrated and compared, inorder to establish unctional responsesthat help explain their wide geographi-cal distribution. Functional charac-teristics are compared in terms o re-sistance mechanisms to reeing tem-peratures, lea gas exchange and waterrelations. In order to carr out thiscomparison, a revision and re-inter-pretation o results obtained through-out the last two decades (Rada et al. ,1985, 1996, 2001; García-Núñe et al. ,2004) was made, and more recent un-published results used. Materials and Methods Studied speciesand site characteristics All species o the Polylepis genus have a tree or shrublie orm (Simpson, 1979), character-ied b twisted shapes, a thick, dense-l laminated and lak bark with red-dish color (Simpson, 1986), and smallgreen-gra leaves. The trees are 1-6min height and have crown diameterso 3-5m. With respect to their distri-bution, the species occur in dierentecological habitats associated with el-evation and humidit.Three species were com-pared a latitudinal distribution gradient: P. sericea Wedd, in Veneuela, P. tara‑ pacana Philippi, in Bolivia and P. aus‑tralis Bitter, in Argentina. Table I showsthe environmental characteristics andgeographic coordinates or the dierentstud sites. P. sericea , in the Ven-euelan Andes, ma reach an altitudeo 4600m (Arnal, 1983), well abovethe treeline (3200masl) in this tropi-cal area (Monasterio, 1980). Its dis-tribution is alwas associated to moreavorable thermal conditions in areaswhere large boulders accumulate to-wards the base o glacial cirques (Wal-ter and Medina, 1969; Aócar andMonasterio 1980). This species has thewidest latitudinal distribution, romVeneuela to northern Bolivia. Appar-entl, this species has spread throughthe Andes in the last million earsas a consequence o climatic changes(Simpson, 1986). P sericea ´s stud siteshows a 2.7ºC dierence between thecoldest and the warmest month. Dailtemperature luctuations range rom -3º to 15ºC during the rain season and-14º to 18ºC in the dr season (Monas-terio, 1986). P. tarapacana orms o-rests with an amaing altitudinal dis-tribution (4200-5200m) in Bolivia. Thelowest altitude represents the maximumelevation or the growth o an shrub ortree lie-orm, while the high extreme ispracticall the limit or plant growth inthe Andean region (Braun, 1997, Liber-man-Cru et al. , 1997). The stud siteis located at Nevado Saama Nation-al Park, where P. tarapacana ormsthe world’s highest wood plant orest(Liberman-Cru, 1986; Braun, 1997)around the Saama Volcano. Althoughthe stud site is considered a semiaridtropical high mountain environment, itslatitudinal position determines a markedseasonal pattern in temperature and pre-cipitation regimes, with a cold dr sea-son during the austral winter (Ma-Oct)and a wet warmer season (Nov-Apr;Liberman-Cru et al. , 1997). Absolutemaximum and minimum temperaturesregistered at 4200m are 21 and -19ºC,respectivel (Liberman-Cru, 1986). P. australis is oundat the north extreme o Sierra Grandein Córdoba, Argentina, mainl associ-ated with rock outcrops. This specieshas to cope with altitude conditions ina temperate environment. Most (83%)rainall occurs in the summer (Renison et al., 2002). Absolute dail maximumand minimum temperatures o 27.1 and-12.8ºC were registered during thisstud. Field and laboratory studies Mean air temperaturevalues were measured with copper-constantan thermocouples (n=3) con-nected to a digital thermometer. Rela-tive humidit was recorded with a dig-ital hgrometer (n=3). Plant responseparameters were obtained rom dailmeasurements o three mature leaveso each o three individual trees atthe dierent studied sites. These pa-rameters were CO 2 assimilation (A)and transpiration rates (E), stomatalconductance (Gs), night respiration(R) and minimum lea water potential( Ψ Lmin ) or both dr and wet seasons.Photosntheticall active radiation wasobtained rom the lea chamber inte-grated to the gas exchange sstems.TABLE IENVIRONMENTAL CHARACTERISTICS AND COORDINATESOF THE STUDy SITES Stud siteSpeciesCoordinatesAltitude(masl)Meanannualtemperature(°C)Meanannualprecipitation(mm)Piedras Blancas,Veneuela P. sericea 9°N - 70°W42003.7800Nevado Saama,Bolivia P. tarapacana 18°S - 68°W43003.4347Los Gigantes,Argentina P. australis 32° S - 66°W21008.0854  OCT 2007, VOL. 32 Nº 10 665 Three 24h courses were carried out oreach season and at ever site, with 7measurements during da hours and 3to 4 at night. Furthermore, water re-lations parameters such as turgor losspotentials ( Ψ π , n=5) obtained rompressure-volume curves and low tem-perature resistance mechanisms (ree-ing and inur temperatures) werealso obtained in the laborator or allspecies. A portable gas exchange ss-tem (LCA-2 or P. sericea and LCA-4 or P. tarapacana and P. australis ,ADC, Hoddesdon, England) was usedor gas exchange measurements com-prising lea conductance, transpira-tion, CO 2 assimilation and respirationrates. Water relation parameters weremeasured with a pressure bomb (PMSInstruments Co., Oregon, USA). Da-time CO 2 assimilation and nighttimerespiration rates were used to obtainintegrated curves or the assimilation/ respiration ratio (A/R) at lea level.Freeing temperature (initiation o theice nucleation process) was determinedas the appearance o a low tempera-ture exotherm in ten to iteen samplessubected to thermal analsis in a con-trolled bath; the sstem recorded exo-therms as plant tissue was cooled at arate o 9ºC·h -1 rom 5 to -25ºC. Lowtemperature inur was determinedquantitativel with the triphenl tetra-olium chloride method (TTC) in threedierent samples; respiring tissues re-duce TTC, changing rom clear to ared color that was quantiied throughspectrometr. A ull description o themethods used ma be ound in Rada  et al. , (1985, 1996, 2001) and García-Núñe et al. , (2004).Non-parametric testsor comparison between two samplesand between three samples were used.For the comparison o ecophsiologicalparameters between seasons and oravoidance or tolerance mechanisms theU-Mann-Whitne test was used. Forcomparison between the three speciesa Kruskall-Wallis test was used. Results Freezing temperature resistancemechanisms Inur temperatures weresimilar in P. tarapacana or both drand rain seasons (Table II), but thetemperature at which the reeing pro-cess began showed signiicant dierenc-es, with values o -3.5 and -9°C duringwet and dr seasons, respectivel. P.australis showed lower inur tempera-tures or the dr season but the ree-ing temperatures were similar in bothseasons. Dierences between rost or-mation (i.e. supercooling capacit) andinur temperature or these two speciespoint to reeing tolerance in lea tis-sues. In contrast, P. sericea showed nosigniicant dierences between reeingand inur temperatures during eitherseason, indicating rost avoidance as itsresistance mechanism to reeing tem-peratures. Water and carbon relations The three species re-sponded dierentiall to temperatureTABLE IIMINIMUM REGISTERED AIR TEMPERATURE, INjURy ANDFREEzING TEMPERATURES AND LOW   TEMPERATURERESISTANCE MECHANISMS DURING WET AND DRy SEASONSFOR THE THREE Polylepis STUDIED SPECIES SpeciesDr season temperatures (ºC)Wet season temperatures (ºC)RMT min InurFreeingT min InurFreeing P. sericea -4.5-9.0 ±0.5 a-8.5 ±0.5 a-2.0-8.0 ±0.9 a-8.0 ±0.3 aA P. tarapacana -13.0-20.0 ±0.5 a-3.5 ±0.5 b-6.0-21.0 ±1.2 a-9.2 ±0.6 bT P. australis -13.5-24.0 ±1.0 a-7.0 ±0.2 b-2.0-18.0 ±2.0 a-6.0 ±0.2 bT Minimum registered air temperature (Tmin), inur and reeing temperatures and low temperatureresistance mechanisms (RM, A = avoidance through supercooling capacit and T = reeing tolerance)during wet and dr seasons or the three Polylepis studied species. Dierent letters represent signif-cant dierences (P<0.05) between inur and reeing temperatures or each o the species. TABLE IIIMEAN PHOTOSyNTHETICALLy ACTIVE RADIATION, AIR TEMPERATURE,RELATIVE HUMIDITy AND PLANT RESPONSE PARAMETERS FOR WET AND DRy SEASON DAILyCOURSES IN THREE POLyLEPIS SPECIES SpeciesSeasonPARTaRHGsARA/R P. sericea Wet1042 ±168 a 1 11.6 ±1.2 a 1 84.2 ±1.4 a 1 90.6±15.4 a 1 4.60±0.6 a 12.02.3(1700)(21.5/-0.7)(100-64)(213)(7.4)Dr1146 ±256 a 1 13.4 ±1.7 a 1 52.2 ±2.1 b 1 63.3±6.0 b 1 3.6±0.5 a 11.6 ±0.3 12.3(1900)(24.5/-2.7)(98-32)(93)(5.8)(2.7) P. tarapacana Wet-warm721 ±182 a 2 7.9 ±1.1 a 2 73.4 ±4.0 a 2 58.9±10.4 a 1 2.8±0.4 a 21.3 ±0.1 12.2(1369)(12.5/-4.5)(92-46)(128)(6.8)(2.6)Dr-cold1443 ±152 b 1 7.5 ±1.2 a 2 38.6 ±5.2 b 2 33.5±4.6 b 2 2.5±0.4 a 1--(2052)(12.5/-14.0)(76/15)(65.0)(4.7) P. australis Wet-warm1034 ±126 a 1 7.7 ±0.8 a 2 56.0 ±3.0 a 3 65.5±10.6 a 1 7.3±0.4 a 31.8 ±0.1 a 24.1(1943)(15.3/-2.8)(82/29)(92)(14.4)(2.3)Dr-cold1246 ±133 a 1 8.5 ±1.4 a 2 53.4 ±5.7 a 1,2 43.7±8.5 a 2 9.0±0.3 b 21.6 ±0.1 a 15.6(2231)(15.3/-12.8)(89/25)(75)(12.3)(1.9)  PAR: mean photosntheticall active radiation, Ta: mean air temperature (°C), RH: relative humidit (%), Gs: stomatal conductance (mmol·m -2 ·s -1 ), A: CO 2 assimilation rate (μmol·m -2 ·s -1 ), and R: respiration rate (μmol·m -2 ·s -1 ). Values in parenthesis are maximum or PAR, maximum and minimum or Ta and RH,and maximum or Gs and A. Dierent letters correspond to signifcant dierences (P<0.05) or the measured parameters between seasons or each species.Dierent superscript numbers correspond to signifcant dierences (P<0.05) or measured parameters between species during the same season.  666 OCT 2007, VOL. 32 Nº 10 and/or seasonal water availabilit (Ta-ble III). P. sericea showed clear di-erences between seasons; mean airtemperature increased b 2ºC, whilelea conductance and CO 2 assimila-tion were lower during the dr season.Air temperatures below ero occurredin both seasons. P.   tarapacana alsoshowed some dierences in gas ex-change characteristics between seasons(Table III). Even though a large (43%)drop in lea conductance was oundrom the wet to the dr season, onl asmall (11%) drop in CO 2 assimilationwas detected during the latter. Meanair temperatures were similar or bothseasons and lower compared to P. seri‑cea ; however, minimum temperatureswere much lower during the dr sea-son. P. australis had a substantialldierent response to the two previousspecies; means or most o the ana-led parameters showed no signiicantdierences between the two measure-ment periods. It is interesting to notethat the assimilation rate increased to-wards the dr-cold season, even thoughlea conductance tended to decrease(no signiicant dierences).In general, mean CO 2  assimilation rate (A) was higher in P.australis during both seasons, as com-pared to the other two species. On theother hand, dark respiration rates werelower in P. tarapacana during the wetseason, as compared to P. australis  and P. sericea (Table III). All threespecies showed a positive diurnal lea carbon balance throughout the ear, al-though more avorable in P. australis .cal high altitude environments, butsome authors (Sakai and Larcher, 1987;Goldstein et al., 1994) indicate that itis rather improbable that rost damageplas a decisive role in the survivalo trees in these regions. The threestudied species respond in contrastingmanners to the dierent thermal con-ditions o their speciic habitats. P.sericea , growing in less extreme envi-ronments, showed no signiicant di-erences between inur and reeingtemperatures, indicating that when thereeing process begins, inur will si-multaneousl occur and, thereore, thisspecies does not resist ice ormation inits tissues. However, although ree-ing takes place at moderate tempera-tures (-9ºC), this suprcooling avoid-ance mechanism seems to be enoughto withstand mild, short duration lowtemperatures present in the VeneuelanAndes throughout the ear (Rada et al., 1985). In contrast, P. tarapacana and P. australis show signiicant di-erences between reeing and inur,indicating that once the reeing pro-cess occurs, the tissues remain undam-aged down to much lower tempera-tures. This corresponds to a reeingtolerance mechanism, as a response tomore extreme temperature conditionso their habitats (Aócar et al., 1988;Squeo et al., 1991).Dierences between sea-sons in supercooling capacit in P.tarapacana suggest dierences in re-sistance mechanisms when thermalconditions change (Rada et al. , 2001).During the cold-dr period, when airtemperature ma be below -10ºC, tis-sue reeing occurs at relativel hightemperatures (-3.5ºC) determining areeing tolerance range. During thewet and warmer season, night air tem-peratures never reach tissue reeingtemperatures (-9ºC). This seasonaldierentiation is a consequence o anincrease in total soluble carbohdratesand proline during the wet warm sea-son, determining an increase in super-cooling capacit (Rada et al. , 2001).Freeing tolerance provides plants abetter protection against cold inurand this increase in supercooling ca-pacit ma be advantageous to thisspecies since it would permit an ear-lier start in photosnthetic activit.In spite o the dier-ent degrees o water stress ound atthe sites, there are no marked eectson carbon gain at the lea level. Ingeneral, mean CO 2 assimilation ratesor the three species studied are com-parable to or higher than those re-ported or other tropical alpine plants(Schule et al., 1985; Goldstein et    al. ,1994) and timberline species (Meiner et al., 1984; Alett, 1985). The stud-ied species showed a coupling o gasexchange characteristics to the extremedail and seasonal conditions o theirrespective environments.In terms o water re-lations, even though P. tarapacana issubected to the most extreme condi-tions, minimum lea water potentialsor the dr season are similar to thoseo  P. australis and P. sericea . This isan indication that under the arid char-acteristics o its habitat, this speciesrelies on a greater stomatal control.At the other end, P. sericea avoids theless harsh water stress conditions o its habitat through osmotic adustmentsand wall elasticit changes (Rada et al., 1996).The results support thestatement b Goldstein et al. (1994)that high photosnthetic eicienc androst resistance are the main phsi-ological explanations or the Polylepis  phenomenon. However, controvers stillpersists about its restricted distribution,alwas at altitudes above the naturalaltitudinal limit o tree growth, asso-ciated with rock outcroppings, alongstreams and, in the case o  P. tarapa‑cana , around a volcano suggesting theneed o special soil conditions. Thisapparent preerence has been explainedb the existence o a avorable micro-climate at these sites, which permitstree survival in an unavorable envi-ronment (Walter and Medina, 1969;Troll, 1973; Simpson, 1979, 1986; Aó- Figure 1. Minimum lea water potentials ( Ψ Lmin ) or dr and wet seasonsand osmotic potential at turgor loss point ( Ψ π 0 ) during dr and wet sea-sons or the three Polylepis species along a latitudinal gradient. Dierentletters represent signifcant dierences (P <0.05) between species. In relation to mini-mum lea water potentials(Figure 1) P. sericea and P. tarapacana showeda decrease towards thedr season. P. austra‑lis showed a tendenc tomaintain similar poten-tials between seasons.With respect to turgorloss, both P. tarapacana  and P. australis main-tain similar values be-tween seasons. On theother hand, P. sericea  decreased its turgor losspoint during the dr sea-son, indicating a signii-cant capacit or osmoticadustment. Discussion Freeing damage ma oc-cur an night o the earin tropical and subtropi-
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