Water relations of tree species growing on a rock outcrop in the Parque Estadual de Itapuã , RS

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Water relations of the tree species Myrsine umbellata Mart. ex A. DC., Dodonaea viscosa Jacq. and Erythroxylum argentinum O. E. Schulz, growing on a rock outcrop in the "Parque Estadual de Itapuã" (RS), were studied. Environmental

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  Revista Brasil. Bot., V.30, n.4, p.703-711, out.-dez. 2007 Water relations of tree species growing on a rock outcrop in the“Parque Estadual de Itapuã”, RS LUIZ GUSTAVO RABAIOLI DA SILVA 1  and LÚCIA REBELLO DILLENBURG 1,2 (received: September 9, 2004; accepted: October 24, 2006) ABSTRACT  – (Water relations of tree species growing on a rock outcrop in the “Parque Estadual de Itapuã”, RS). Water relationsof the tree species  Myrsine umbellata  Mart. ex A. DC.,  Dodonaea viscosa  Jacq. and  Erythroxylum argentinum  O.E. Schulz,growing on a rock outcrop in the “Parque Estadual de Itapuã” (RS), were studied. Environmental (precipitation, temperature, soilwater) and plant (water potential, vapor pressure deficit, stomatal conductance, transpiration, leaf specific hydraulic conductance,osmotic potential and cell wall elasticity) parameters were collected in five periods and pooled into two sets of data: wet anddry periods.  Myrsine umbellata  showed great stability of the plant parameters, including the maintenance of high pre-dawn(  wpd ) and mid-day (  wmd ) water potentials in the dry period (-0.48 and -1.12 MPa, respectively), suggesting the presence of a deep root system.  Dodonaea viscosa  and  E. argentinum  reached lower  wpd  (-1.41 and -1.97 MPa, respectively) and a greaterdegree of stomatal closure in the dry period, suggesting a shallower root system .  Differential exposure to soil drought wasalso corroborated by differential drought effects on the whole-plant leaf specific hydraulic conductance (G t ). Correlation analysispointed to weak correlations between  wpd  and g s .  Erythroxylum argentinum was the only species to show osmotic adjustmentin response to drought. It is suggested that  M. umbellata  has low tolerance to water deficits, adopting an avoidance behavior. Themuch lower values of  w  reached by  D . viscosa and  E. argentinum  suggest a greater tolerance to drought by these species.Key words -  Dodonaea viscosa ,  Erythroxylum argentinum ,  Myrsine umbellate , rock outcrops, water relations RESUMO  – (Relações hídricas de espécies arbóreas em um afloramento rochoso no Parque Estadual de Itapuã, RS). Foramexaminadas neste estudo as relações hídricas das espécies  Myrsine umbellata  Mart. ex A. DC.,  Dodonaea viscosa  Jacq. e  Erythroxylum argentinum O.E. Schulz, ocorrentes em um afloramento rochoso no Parque Estadual de Itapuã, RS. Parâmetrosambientais (precipitação, temperatura, água no solo) e das plantas (potencial hídrico, déficit de pressão de vapor, condutânciaestomática, transpiração, condutância hidráulica, potencial osmótico e elasticidade da parede celular) foram coletados emcinco períodos e agrupados em dois conjuntos de dados: períodos úmido e seco.  Myrsine umbellata mostrou grande estabilidadedos parâmetros vegetais, incluindo a manutenção de altos potenciais hídricos de base (  wpd ) e de meio-dia (  wmd ), mesmo noperíodo seco (-0,48 e -1,12 MPa, respectivamente), sugerindo a presença de um sistema radicular profundo.  Dodonaeaviscosa  e  E. argentinum  apresentaram menores  wpd  (-1,41 and -1,97 MPa, respectivamente) e maior fechamento estomáticono período seco, sugerindo um sistema radicular mais superficial. Exposição diferenciada à seca do solo foi também corroboradapelos efeitos diferenciados da seca na condutância hidráulica folha-específica da planta. Análises de correlação indicaramfracas correlações entre  wpd  e g s .  Erythroxylum argentinum foi a única espécie a mostrar ajuste osmótico em resposta à seca.Sugere-se que  M. umbellata  seja pouco tolerante a déficits hídricos, adotando uma estratégia de evitação. Os menores  w alcançados por  D . viscosa e  E. argentinum  sugerem maior tolerância destas espécies à seca.Palavras-chave - afloramentos rochosos,  Dodonaea viscosa ,  Erythroxylum argentinum ,  Myrsine umbellata , relações hídricas Introduction Rock outcrops (inselbergs) are places where bedrockis exposed at the surface (Skinner & Porter 1992) andare widely distributed in tropical and subtropical regions(Porembski & Barthlott 2000, Hunter 2003a). Rockswhich are close to the surface tend to be brittle and, then,to be cut by innumerable fractures. This may significantlyincrease the usual low porosity of igneous rocks, suchas granite. If these fractures are highly interconnected,a high-quality aquifer (a body of highly permeable rockor regolith lying in the zone of saturation, which yieldssignificant quantities of water) may be present (Skinner& Porter 1992, Thompson & Turk 1993).Vegetation in these outcrops differs considerablyfrom the surrounding one, due to peculiar soil (thin layer)and microclimatic (high irradiance and evaporation)conditions (Porembski et al.  1998, 2000, Hunter 2003b), and it displays interesting patterns of distribution andsubstrate affinity (Meirelles et al . 1999). In theseenvironments, plant species commonly arrange themselvesto form elliptical soil-island plant communities, whichvary in size from a few square centimeters to several 1.Universidade Federal do Rio Grande do Sul, Departamento deBotânica, Avenida Bento Gonçalves 9500, 91501-970 Porto Alegre,RS, Brazil2.Corresponding author: lucia.dillenburg@ufrgs.br  L. G. R. da Silva & L. R. Dillenburg: Tree water relations in a rock outcrop 704 hundreds square meters (Meirelles et al.  1999).The slowpace of intemperization processes and high declivityresult in the lack of water and of an adequate substratefor root development and also in a high degree of surfaceheating, such that a herbaceous or shrubby vegetationmight be the less successional stage in such environments(Shure & Ragsdale 1977). The presence of aquifers andthe access of deep-rooted species to this source of groundwater may affect species composition and vegetationdynamics in rock outcrops.The stomatal response to water stress is essentialfor a plant’s success, because it results in the control of water vapor exit from the leaf. When stomatal regulationbalances transpiration with the efficiency of water supplyto the leaves, leaf desiccation is avoided. It is then intuitivethat maximum stomatal aperture and conductance shouldbe limited by the water supply to the leaves. Plant hydraulicconductance then becomes an important characteristicdetermining operational values of stomatal conductance(Hubbard et al . 2001, Meinzer 2002). Mechanisms thatcontrol water loss may also be expressed at the cellularlevel, like those related to osmotic adjustment and cellwall elasticity (Colombo & Teng 1992), which will haveimportant effects on turgor maintenance and waterabsorption (Cheung et al.  1975, Abrams 1988, Jensen &Henson 1990).Tropical rock-outcrop soil islands have not beenexplored in the same detail as those in temperate regions.Although important plant ecology studies have beenconducted in the past few years in rock outcrops in Brazil(e.g., Mattos et al . 1997, Meirelles et al.  1999, Porembsky& Barthlott 2000), they are basically inexistent for thestate of Rio Grande do Sul. Ecophysiological studies insuch environments are not only extremely interestingbecause of their peculiar conditions, but are also of majorimportance for the understanding of the vegetation dynamicsand for the characterization of the morphological andphysiological mechanisms that allow plant species tothrive in the somewhat extreme conditions of rock outcrops.This study aimed to evaluate the soil and atmospherewater conditions of a granitic outcrop in southern Brazil,under wet and dry conditions, to relate such conditionsto the water use patterns of selected tree species, and toanalyze and compare the contributions of stomatalaperture regulation and of osmotic and cell wall elasticityadjustments to the drought resistance of these species. Materials and methods Study site – This study was conducted in a rock outcrop of the “Parque Estadual de Itapuã”, located in the town of Viamão,Rio Grande do Sul, Brazil (30°22’ S and 51°02’ W). Thisoutcrop has a little pronounced westward inclination. Theclimate of the region is characterized by mean annualprecipitation and temperature of about 1.200 mm and 17.5°C,respectively, by the lack of a dry season and by high atmospherichumidity, which is associated to air masses of coastal srcinand to the proximity of big lagoons (Universidade Federal doRio Grande do Sul 1982). The litolic soils of the Park are of recent srcin and classified as Litolic Neosoils (Streck et al . 2002). These are well-drained, dark-colored soils, derivedfrom granitic rocks (Rio Grande do Sul 1997). Soil depth atthe study site varied from about 10 to 30 cm, and some of its chemical characteristics are presented in table 1.Table 1. Chemical characteristics of the soil in the studiedrock outcrop. Analyses were made in a compound samplemade up of soil collected in five of the six vegetation islandsselected in this study (two adjacent vegetation islandsprovided only one soil sample), from a depth between 0 and30 cm. Clay content determined by density analysis. pHmeasured in water solution (1:1; v:v). Organic matter (OM)obtained by multiplying the organic carbon (measured byhumid digestion) content by 1.72. Determination of P andK based on the Mehlich I method. Exchangeable Ca, Mg.Al, and Mn extracted with KCl 1 mol L -1  Analyses wereperformed by the Analyses Lab of the Soil Department of the Federal University of Rio Grande do Sul.Soil parameterValueClay (%)19.0pH4.3P (mg L -1 )8.4K(mg L -1 )97.0OM (%)3.9Al .  (cmol c  L -1 )2.8Ca . (cmol c  L -1 )0.7Mg .  (cmol c  L -1 )0.6Al + H (cmol c  L -1 )8.0Cation Exchange Capacity (cmol c  L -1 )9.6Al saturation16.0Base saturation29.2Selected species –  Myrsine umbellata Mart. ex A. DC.(“capororoca”),  Dodonaea viscosa  Jacq. (“vassoura-vermelha”)and  Erythroxylum argentinum  O.E. Schulz (“cocão”) wereselected for this study. They are all evergreen species withwide distribution in the State of Rio Grande do Sul.  Myrsineumbellata  and  E. argentinum  are reported to grow in varioustypes of soil, are characteristic of the initial stages of succession,can tolerate shade (Amaral 1980, Reitz et al.  1988, Ferrer1999), but little is known about their drought tolerance.  Dodonaea viscosa  is a sun, drought-tolerant species that isparticularly abundant in human-disturbed areas (Reitz 1980).In the “Parque Estadual de Itapuã”,  M. umbellata  and  E.  Revista Brasil. Bot., V.30, n.4, p.703-711, out.-dez. 2007 705 argentinum  have a very wide distribution, and they can appearas either shrubs or trees, depending on soil conditions. Inthe rock outcrops of the State Park, they usually co-habitthe same vegetation islands, with heights that range fromone to three meters. At the edges of these islands, and alwaysunder high irradiances,  D. viscosa  thrives, with heights thatdo not exceed three meters. Table 2 summarizes some growthand morphological characteristics of the selected species inthe study site, and shows that  M. umbellata  has leaves whichare much larger and have about twice the specific mass whencompared to the other two.Sampling design – Six individuals of each species wereselected for measurements of the water relations parameters.Each one of these individuals was located on a differentvegetation island. The vegetation islands were treated asblocks in the experimental design and each contained oneof the six individuals of each species, totaling six vegetationislands (hereon treated as sampling points) and 18 individuals(experimental units). The size of the vegetation islandsranged from 7.85 to 500 m 2 , and the individuals sampled inthis study were always located at the edges of the islands(  E. argentinum  and  M. umbellata ) or just outside them (  D.viscosa ). Data were collected in five different days: March19, May 03, August 20, and October 11 of 2003, and January03 of 2004. Measurements taken on these dates were groupedinto two sets of days, according to the total precipitation alongthe 10 days that preceded them: wet period (data collectedin March, May and October) and dry period (data collectedin August and January). Mean values of maximum andminimum temperatures for the sampling dates plus the 10preceding days were 25.2 °C and 18.2 °C for the wet periodand 24.9 °C and 13.3 °C for the dry period. Total precipitationaveraged 59.4 mm for the 10 preceding days in the wet period(34.8, 64.4 and 79.1 mm for the months of March, May andOctober, respectively), and was null for the dry period (figure1). This data pooling procedure provided a better and cleanerpicture of the effects of soil drought on the water relationsof the species. A previous analyses of the data revealed that thedrought of the atmosphere, as measured by the leaf-to-airvapor pressure deficit at mid-morning, did not differ significantlybetween the two dates associated to the dry period (1.20 kPain August and 1.62 kPa in January), but differed significantlybetween measurements taken in March (2.11 kPa) and thosetaken in May (0.70 kPa) and October (1.00 kPa). Someimplications of such difference within the wet period willbe latter discussed.Table 2. Some growth characteristics of the selected tree species in the study site. Values of leaf size and leaf mass per areaare means (± SE) of compound leaf samples collected from each of the six individuals of each species sampled in this study.SpeciesGrowth formHabitat lightSize of matureLeaf mass perconditionsleaves (cm 2 )area (g m -2 )  Myrsine umbellata Small tree or shrubSun150.60 ± 6.85211.29 ± 7.89  Dodonaea viscosa Small treeSun024.23 ± 1.74102.64 ± 6.62  Erythroxylum argentinum Small treeSun031.86 ± 2.43120.12 ± 4.01Figure 1. Precipitation and air temperature in the ten dayspreceding the sampling date (day 11). A. Wet period (values areaverages of data related to sampling dates in March, Mayand October). B. Dry period (values are averages of data relatedto sampling dates in August and January). Precipitation; Maximum temperature; Minimum temperature.Shoot water potential (  w ) – Due to the destructive natureof these measurements, only one shoot per individual wasmeasured. Measurements were taken between 4 h 30 minand 6 h (predawn shoot water potential –  wpd ) and between12 h and 13 h (mid-day shoot water potential –  wmd ). Forboth measurements, sun-exposed leafy shoots of similar size(15-20 cm) and position within the crown among individualsand species were sampled. A Scholander-type pressure chamber(Series 3000, Soil Moisture Equipment Corp, Santa Bárbara,CA) was used for water potential evaluations of the leafyshoots, and the values presented are averages of six individualsmeasured in three different days for the wet period ( n  = 18)and in two different days for the dry period ( n  = 12).Leaf stomatal conductance (g s ) and transpiration (E) – Forthese measurements, three fully-expanded, sun-exposed  L. G. R. da Silva & L. R. Dillenburg: Tree water relations in a rock outcrop 706 leaves per plant were sampled and treated as sub-samplesof the experimental units. The abaxial surface of each leaf (there was no measurable conductance in the adaxial surface)was measured with a steady-state porometer (Li-1600, Li-Cor,Inc., Lincoln, NE) at two different times during the day:between 9 h and 10 h 30 min (morning stomatal conductance– g smor  – and transpiration – E mor ) and between 14 h and 15 h30 min (afternoon stomatal conductance – g saft  – andtranspiration – E aft  –). The values g s  and E reported in thisstudy are averages of six individuals measured in threedifferent days for the wet period (n = 18) and in two differentdays for the dry period ( n  = 12).Whole-plant leaf specific hydraulic conductance (G t ) – Itwas estimated as the ratio between E aft  and the drop of  w from pre-dawn to mid-day (  wpd  –  wmd ).Microclimate conditions – At each time a leaf was sampledfor g s  and E, leaf temperature (T l ) was also recorded by aleaf thermocouple located inside the porometer chamber.Temperature and relative humidity of the air around theislands was recorded every thirty minutes along each day of measurement with and electronic data-logger equipment(LogBox, Novus). Based on these air temperature and humiditydata and on T l , the leaf-to-air vapor pressure deficit (VPD)was calculated for the two times of the day when g s  and Ewere measured (morning – VPD mor  – and afternoon – VPD aft ).Pressure-volume curves – These curves were constructedbased on three shoots of each species (each shoot collectedfrom one individual growing in three previously selectedislands), which were always collected from sun-exposedregions of the crown and had similar positions amongindividuals and species. Shoots were cut in the field between8 h and 9 h on the same sampling dates already reported,had their the cut stems immediately placed under distilledwater, and were covered wit black plastic to preventtranspirational water loss. Back in the lab, they were recutunder water (Parker et al.  1982), and let to rehydrate underdarkness for 20 h. Each shoot was then weighed (turgidweight) and immediately placed inside the pressure chamber.The pressure inside the chamber was slowly increased untilthe water potential of the turgid shoot could be recorded.Pressure was then slowly released (-0.02 MPa s -1 ) to avoiddamage to the plant tissues. The shoot was then releasedfrom the chamber and placed on a bench top for naturaldrying (free-transpiration method, Parker & Pallardy 1988).Successive measurements of fresh weight and water potentialof the shoot were made until there was only about 40% of the water present in the turgid state. The plant material wasthen oven-dried (80 °C) and weighed. The relative watercontent (RWC) associated to each measurement of waterpotential was calculated as (Fresh weight – Dry weight) / (Turgid weight – Dry weight] x 100. From the mathematicalrelationship between RWC and the inverse of water potential(Sigmaplot 5.0, SPSS, Science), estimates were made of theosmotic potentials at full turgor (  (100) ) and zero turgor(  (0) ), and of the bulk modulus of elasticity (  ) (Tyree &Hammel 1972, Ritchie & Hinckley 1975, Jensen & Henson1990) The region of the curve between full and zero turgorwas used on the estimation of  . Values reported for thedifferent parameters extracted from these curves are averagesof three individuals measured in three different days for thewet period ( n  = 9) and in two different days for the dryperiod ( n  = 6).Soil water content (SWC) – A compound soil sample madeup of five sub-samples collected with an auger, between 0and 30 cm of soil depth, in each vegetation island (two adjacentvegetation islands provided only one soil sample) wasweighed, air-dried, and then reweighed for estimation of the gravimetric soil water content [(fresh weight-dry weight) / dry weight * 100]. The values reported are averages forthe wet ( n  = 15) and dry ( n  = 10) periods.Statistical analysis – Mean values of shoot water potential,stomatal conductance and transpiration for each period (wetand dry) and species were statistically compared using analeatorization test, based on the Euclidean distance betweenexperimental units, and taking into account block, periodand species as sources of variation. For statistical analysisof SWC, sources of variation included only block and period.These analyses were made with the aid of the statistical programMultiv minor 2.1.1 (Pillar 2003). Correlation analyses betweenparameters were made using Sigmaplot 5.0 (SPSS Inc.). Results Soil water content was about 58% greater in thewet than in the dry period (figure 2A). Differences inSWC resulted in  wpd  and  wmd  values which were alsogreater in the wet than in the dry period (figure 2B, C).An important exception was the  wmd  of  M. umbellata ,which did not vary significantly between the two periods.In the wet period,  wpd  was greater in  M. umbellata than in  D. viscosa and the  wmd  was greater in  M.umbellata  than in both  D. viscosa  and  E. argentinum .In the dry period, both  wpd  and  wmd  of  M. umbellata were greater than the corresponding values of  D. viscosa and  E. argentinum .Significant differences in leaf-to-air VPD betweenthe wet and dry periods were present only formeasurements taken in the afternoon, with the wet periodshowing the smaller values for all three species (figure3A, B). There were no differences among species. Themean values of g smor  and g saft  were greater in the wet thanin the dry periods Differences among species were onlypresent in the dry period, when  M. umbellata  showedgreater values of both g smor  and g saft  than  D. viscosa  and  E. argentinum  (figure 3C, D). Variations in E were mostlysimilar to those of g s : E mor  and E aft  were greater in thewet than in the dry period for both  D. viscosa  and  E.argentinum . Contrary to g s , E mor  and E aft  of  M. umbellata did not vary between the two periods, and reached quite  Revista Brasil. Bot., V.30, n.4, p.703-711, out.-dez. 2007 707 high values, especially in the afternoon (figure 3E, F).In the dry period, E mor  and E aft  of  M. umbellata  weregreater than in the other two species. All species hassimilar values of G t  in the wet period, and drought led toa significant reduction in G t , except in  M. umbellata ,which then exhibited significantly higher values than theother two species (table 3).Daily variations in leaf-to-air and air-to-air VPDwere poorly correlated to variations in E and g s  for allthree species ( r  2     0.14). Values of  b  also showed a weak linear correlation with g smor  ( r  2    0.14), but a bit higherwith g saft . In this last case, the coefficient of determinationwas much lower for  E. argentinum  ( r  2   = 0.03) than for  M. umbellata  ( r  2   = 0.32) and  D. viscosa  ( r  2   = 0.38).Both estimates of shoot osmotic potential,   (100) and   (0) , reduced in the dry compared with the wetperiod only for  Erythroxylum argentinum . The speciesdiffer from each other only in the wet period, with  M.umbellata  showing more negative values of both   (100) and   (0)  than  D. viscosa  and  E. argentinum . The cellwall elasticity parameter, e, did not vary between thewet and dry period but was significantly greater in  M.umbellata  than in  D. viscosa  and  E. argentinum  in thewet period (table 3). Discussion Estimating rooting depth from values of  wpd  –Measurements of  wpd  have commonly been used asestimates of soil  w , based on the assumption that, duringthe night, the plant  w  reaches an equilibrium with the  w  of the soil explored by the absorbing roots (Reich &Hinckey 1989, Boyer 1995, Donovan et al . 2003). Whencomparing individuals that co-habit a particular space,those that reach the greatest values of  wpd  most likelyhave a greater access to soil water, commonly indicatinga greater rooting depth. The distinction of  wpd  amongindividuals with different rooting depths becomes moreevident as the soil profile dries. Thus, during the wetperiod, values of  wpd  were similarly high in all species.However, in the dry period, when soil water between 0and 30 cm depth fell to about half the values measuredin the wet period, both  D. viscosa  Jacq. and  E  . argentinum O.E. Schulz exhibited major reductions in  wpd , whilethat of  M. umbellata  Mart. ex A. DC was little affected.It is then suggested that  D. viscosa  and  E. argentinum have a greater dependence on the water present in the Table 3. Whole-plant leaf specific hydraulic conductance(mmol m -2  s -1  MPa -1 ) Values are means ± SE. Different capitaland small letters indicate significant mean differencesbetween periods and species, respectively (P   0.05).Period  MyrsineDodonaeaErythroxylumumbellataviscosaargentinum Wet5.55 ± 1.040Aa3.41 ± 0.32Aa4.95 ± 1.42AaDry4.16 ± 0.874Aa1.26 ± 0.25Bb1.57 ± 0.35BbFigure 2. Measurements of soil and plant water availabilityin the wet and dry periods. A. Soil water content (SWC). B.Pre-dawn shoot water potential (  wpd ). C. Mid-day shootwater potential (  wmd ). Vertical bars indicate the standarderror of the means, and an asterisk and different small lettersindicate significant mean differences between periods andspecies, respectively ( P     0.05).  Myrsine umbellata ; 12341234  Dodonaea viscosa ; 123123    Erythroxylum argentinum . não encontrei no texto citação para table 4 (Suzete) ABC
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