Transpiration and forest structure in relation to soil waterlogging in a Hawaiian montane cloud forest

Transpiration and forest structure in relation to soil waterlogging in a Hawaiian montane cloud forest
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  Summary  Transpiration, leaf characteristics and foreststructurein  Metrosiderospolymorpha Gaud.standsgrowinginEast Maui, Hawaii were investigated to assess physiologicallimitations associated with flooding as a mechanism of re-duced canopy leaf area in waterlogged sites. Whole-tree sapflow, stomatal conductance, microclimate, soil oxidation–re-duction potential, stand basal area and leaf area index (LAI)weremeasuredonmoderatelysloped,drainedsiteswithclosedcanopies(90%)andonlevel,waterloggedsiteswithopencan-opies(50–60%).TheLAIwasmeasuredwithanewtechniquebased on enlarged photographs of individual tree crowns andallometricrelationships.Sapflowwasscaledtothestandlevelbymultiplyingbasalarea—normalizedsapflowbystandbasalarea. Level sites had lower soil redox potentials, lower meanstand basal area, lower LAI, and a higher degree of soil avoid-ance by roots than sloped sites. Foliar nutrients and leaf massper area (LMA) in  M.polymorpha  were similar between leveland sloped sites. Stomatal conductance was similar for  M.polymorpha  saplings on both sites, but decreased with in-creasing tree height ( r  2 = 0.72; P  < 0.001). Stand transpirationestimates ranged from 79 to 89% of potential evapotrans-piration (PET) for sloped sites and from 28 to 51% of PET forlevel sites. Stand transpiration estimates were strongly corre-latedwithLAI( r  2 =0.96; P <0.001).Whole-treetranspirationwasloweratlevelsiteswithwaterloggedsoils,butwassimilaror higher for trees on level sites when normalized by leaf area.Trees on level sites had a smaller leaf area per stem diameterthan trees on sloped sites, suggesting that soil oxygen defi-ciency may reduce leaf area. However, transpiration per unitleaf area did not vary substantially, so leaf-level physiologicalbehavior was conserved, regardless of differences in tree leaf area. Keywords:evapotranspiration,leafareaindex,Metrosideros polymorpha, redox potential, stomatal conductance. Introduction Wet montane forests in Hawaii are dominated by the canopytree species,  Metrosiderospolymorpha  Gaud. (Myrtaceae),which undergoes natural cycles of decline and regeneration(Mueller-Dombois 1986). This pattern, which leads to a re-duction in canopy leaf area during cycles of decline, has beencorrelated with poorly drained soils (Akashi and Mueller-Dombois 1995). Reduced canopy leaf area of   M.polymorpha forest is evident on windward mountain slopes in Hawaii(Mueller-Dombois 1986), yet little is known about how struc-tural and physiological changes during canopy decline affectcanopy processes. The present study focuses on the effects of soil waterlogging on forest structure and water utilization of   M.polymorpha .Becauseoverabundanceofsoilwatercanleadto physiological limitations in Hawaiian wet forests (Canfield1986), control of water use in relation to the degree of soilwaterlogging was used to assess possible mechanisms of can-opy decline.Leaf area index (LAI) and canopy structure are importantdeterminantsofmassandenergyexchangefromforestcanopysurfaces (Running and Coughlan 1988, Granier et al. 1996 a ).Transpiration in forested ecosystems is positively correlatedwith LAI (Bréda and Granier 1996, Granier et al. 1996 a ),which is considered an important structural determinant of transpiration (Running and Coughlan 1988). Canopy struc-ture, including the shapes and distribution of tree crowns, isalso important in determining water fluxes from forest cano-pies because of its influence on boundary layer conductance(McNaughton and Jarvis 1983, Meinzer et al. 1997). Canopyproperties of   M.polymorpha  are influenced by soil fertilityduring ecosystem development in Hawaii (Vitousek et al.1993, Herbert and Fownes 1995, Vitousek et al. 1995). TheLAI of   M.polymorpha  stands increases with addition of limit-ing nutrients on Kauai and Hawaii (Herbert and Fownes 1995,Harrington and Fownes, unpublished data). Soil oxygen defi-ciency has the potential to limit LAI (Kozlowski 1976) and Tree Physiology  20, 673–681 © 2000 Heron Publishing—Victoria, Canada Transpiration and forest structure in relation to soil waterloggingin aHawaiian montane cloud forest LOUIS S. SANTIAGO, 1,2 GUILLERMO GOLDSTEIN, 1 FREDERICK C. MEINZER, 3 JAMES H.FOWNES 4,5 and DIETER MUELLER-DOMBOIS 1 1  Department of Botany, University of Hawaii, Honolulu, HI, 96822, USA 2 Present address: Department of Botany, University of Florida, 220 Bartram Hall, P.O. Box 118526, Gainesville, FL 32611-8526, USA 3  Hawaii Agriculture Research Center, 99-193 Aiea Heights Drive, Suite 300, Aiea, HI 96701-3911, USA 4  Department of Agronomy and Soils Science, University of Hawaii, Honolulu, HI 96822, USA 5 Present address: Department of Forestry and Wildlife Management, University of Massachusetts, Amherst, MA 01003, USA Received February 12, 1999 Downloaded from gueston 13 March 2018  may contribute to the mechanism of canopy decline.The chemical environment of waterlogged soils can affectthe physiological functioning of plants and overall plant struc-ture(ZimmermannandBrown1971,Kozlowski1976,Chapin1991). Oxygen deficiency associated with soil waterloggingmay cause plants to exhibit physiological stress responsesincluding reduced transpiration and stomatal closure (Koz-lowski and Pallardy 1984, Chapin 1991, Sojka 1992). Water-logged soil may also be associated with thicker leaves and lessnegative  δ 13 C values signifying stomatal closure and greaterwater-use efficiency (Meinzer et al. 1992). Avoidance of an-oxic horizons through the production of aboveground roots isa common strategy of plants adapted to waterlogged soils(Armstrong et al. 1991) and has been reported in high precipi-tation montane forests (Gill 1969). However, abovegroundand shallow rooting may increase susceptibility to drought(Holbrook and Putz 1996), especially in cloud forest vegeta-tion adapted to humid conditions (Jane and Green 1985).Most estimates of transpiration from tropical montane for-ests are derived from top-down approaches such as water bal-ance calculations (Bruijnzeel and Proctor 1993). Such resultsare difficult to extrapolate to smaller scales because they donot account for the contributions of individual plant species orfunctional groups that can be important for comparing the ef-fects of vegetation on hydrological cycles (McNaughton andJarvis 1983). Alternatively, bottom-up models are determinis-tic, process-based approaches that predict how systems mayfunction at large scales, based on investigations of small spa-tial and short temporal scales (Jarvis 1993). Hawaiian mont-aneforestsareoptimumsystemsforinvestigatingtheutilityof bottom-up models because the single canopy species simpli-fiesscalingfromleavesorwhole-plantmeasurestothecanopylevel. We have examined the effects of waterlogging on tran-spiration and forest structure by comparing stands growing intopographically level, waterlogged soils with stands on mod-erately sloped, drained soils, to obtain physiological informa-tion on  M.polymorpha  canopy decline in waterlogged soils.Specificobjectiveswereto:(1)investigatephysiologicallimi-tation of water use by trees in waterlogged soil; (2) determinehow canopy water fluxes are affected by differences in LAIand stand structure; (3) develop a model that predicts  M. polymorpha  canopy transpiration based on climatic parame-ters; and (4) examine the effects of tree size and rooting me-dium on stomatal conductance. Materials and methods Study site The study was conducted in the East Maui watershed on thenortheast, windward slope of Haleakala, a 3055 m dormantvolcano on the island of Maui, Hawaii. Sites were located atapproximately 1200 m altitude, along the Waikamoi stream(20°48 ′  N, 156°13 ′  W). The climate is primarily determinedby the northeast trade winds, which are persistent for most of the year and result in frequent enveloping fog and orographicrainfall of approximately 5000 mm per year (Giambelluca etal. 1986). The soils are derived from Kula series lava flows of approximately 410,000 years (Macdonald et al. 1983) and areclassified as Inceptisols (Aquertic Humitropept; Schuur1999). Waterlogging has been a major factor in the develop-ment of these soils, resulting in an organic horizon in the top20 cm subtended by a waterlogged mineral horizon whereplant roots are absent or very rare (Kitayama andMueller-Dombois 1994 a ). These soils are high in extractablealuminum, acidic and hypoxic (Kitayama and Mueller-Dombois 1994 a , Kitayama et al. 1997). The topography con-sists of narrow, flat to moderately sloped ridges formed by re-sistant portions of the shield volcano, dissected by steepravines. Soil waterlogging varies with topography and is mostevident in level areas creating a mosaic of depressions andslopes on broad ridges.Moderately sloped areas support communities dominatedby a closed canopy of   M.polymorpha , which is 11–13 m tall,and an open sub-canopy shrub and tree fern layer, which is1–3 m tall. Level, poorly drained areas support open canopy  M.polymorpha  stands of approximately the same height, butwith smaller individual tree crowns, and an understory domi-nated by the sedge, Carexalligata  Boott, an indicator of wet-land conditions (Kitayama and Mueller-Dombois 1994 a ,1994 b ). Six 10  ×  10 m plots were chosen on the basis of slopeangle(Table1).Levelandslopedsiteshadcanopycoverageof approximately60and90%,respectively,asdeterminedwithaspherical densiometer.  Micrometeorological measurements Climatic conditions at the study sites were monitored at can-opy height from September 16, 1996 until February 10, 1997.Net radiation was measured with a net radiometer (Model Q7,Radiation and Energy Balance Systems, Seattle, WA). Rela-tive humidity and air temperature were measured with ashielded temperature/humidity sensor (Model HMP35C,Vaisala, Helsinki, Finland). Windspeed was measured with acup anemometer (Model 014A, Met One Instruments, GrantsPass, OR). Photosynthetic photon flux density (PPFD) wasmeasured with a quantum sensor (Model Quantum, Li-Cor, 674 SANTIAGO, GOLDSTEIN, MEINZER, FOWNES AND MUELLER-DOMBOISTREE PHYSIOLOGY VOLUME 20, 2000Table 1. Stand characteristics of pairs of level and sloped study sites.Soil water is expressed as the temporal mean of three measurementperiods.Site Slope angle Basal area LAI Soil water(°) (m 2 ha –1 ) (g g –1 )Mean  ±  SEPair 1 0 21.04 0.61 8.0  ±  0.410 29.89 5.85 7.5  ±  0.4Pair 2 0 25.86 1.28 11.0  ±  0.912 52.42 5.19 6.5  ±  0.1Pair 3 0 26.54 0.90 10.8  ±  1.214 39.84 5.74 6.5  ±  0.8 Downloaded from gueston 13 March 2018  Inc.,Lincoln,NE).Datawererecordedevery5sandaveragedevery10minwithadatalogger(Model21X,CampbellScien-tific, Logan, UT). Potential evapotranspiration (PET) was cal-culated by the Penman equation (Penman 1948).  Redox potentialand soilwater  Soilredoxpotentialsweremeasuredwithplatinumelectrodes,acalomelreferenceelectrodeandaportablemVmeter(Model290A, Orion, Inc., Boston, MA). Electrodes were inserted to10- and 40-cm depths at 11 randomly selected points withineach 10 × 10 m plot and allowed to equilibrate for 45 min be-fore measurements. Values were corrected to a standard hy-drogen electrode (+244 mV) and to a pH of 7 (–59 mV per pHpoint below 7; Bohn et al. 1985) from a soil pH of 3.7(KitayamaandMueller-Dombois1994 a ).Soilwaterwasmea-suredgravimetricallythreetimesthroughoutthemeasurementperiod. In each 10 × 10 m plot, six soil cores were taken to adepthof10cm,weighed,thendriedinanovenat70°Ctocon-stant weight. Water content of the soil was expressed in gramsof water per gram of oven-dried soil (g g –1 ). Stand structure, leaf area index and leaf chemistry The diameters at 1.4 m height (DBH) of all  M.polymorpha stems ≥ 2 cm were measured in each 10 × 10 m plot. To evalu-ate the degree of soil avoidance by roots, the rooting habits of   M.polymorpha  individuals were assessed by observingwhere, in relation to the soil surface, rooting initiated. A plantwas considered to be rooted in the soil if there were adventi-tious roots and the shoot to root boundary occurred belowground. If the shoot to root boundary occurred above ground,the plant was considered aerially rooted. A four-class systemwas used to distinguish rooting habit: (1) soil (stem enteredsoil intact with little or no aboveground rooting); (2) nurse log(roots penetrated an aboveground horizontal log); (3)epiphytic(rootsweredependentontheboleofanothertreeforsupport); and (4) aerial (shoot to root boundary occurredabove the soil surface, either exposed or in aboveground,moss-covered root mats).In each plot, four trees with a diameter greater than 6 cmwere randomly selected for leaf area and transpiration mea-surements. The LAI was estimated by a new technique con-sisting of counting the number of leaf clumps in photographsof individual tree crowns. Because leaf arrangement is tightlyclumped in  M.polymorpha , mean leaf area per clump was de-termined and the numbers of clumps observed in photographswere used to estimate individual tree leaf area. The relation-ships between DBH and individual tree leaf area were thenused to predict leaf area for other trees. Leaf area index wascalculated as the sum of predicted leaf areas (m 2 ) for trees in a10 × 10 m plot as: LAI =  Σ (predicted tree leaf area)/100.The canopy photo method for estimating LAI has an advan-tage over other methods because it considers leaf clumping.Methods based on gap-fraction analysis (Welles and Norman1990) underestimate LAI when leaf area is spatially clumpednonrandomly. In  M.polymorpha , this error may be quite large(Herbert and Fownes 1997). Because the  M.polymorpha  can-opy at these sites is sparse, individual shoots of clusteredleaves could be identified consistently, and thus errors in leaf area determination are likely to be small. The LAI of theunderstory sedge C.alligata  was estimated with a portablearea meter (Model 3100, Li-Cor, Inc.) by destructively har-vesting four randomly selected 0.5 × 0.5 m plots along a 20-mtransect in an area of continuous C. alligata  groundcover.Canopysunleaveswereobtainedfromthethreetallestmea-surement trees in each plot with a telescopic pruning hook.Leaf area was determined with a portable area meter (Li-Cor,Inc.). Leaves were then dried in an oven at 70 °C to constantweightforthedeterminationofleafmassperarea(LMA).Thedried leaves were finely ground and a subsample was sent tothe Agricultural Diagnostic Service Center, University of Ha-waiiatManoaforchemicalanalysisofleafnutrientconcentra-tions. An additional subsample was sent to the University of California at Berkeley where the relative abundances of   13 Cand  12 C were analyzed by mass spectrometry. Transpirationand stomatal conductance Between September 1996 and February 1997, transpirationwas measured by a paired plot technique where simultaneousmeasurements of the four measurement trees in one level plotand one sloped plot were allowed to run for 15–20 days. The5 days with highest evaporative demand from each measure-ment period were used to compare both sets of measurementtrees. Transpiration was estimated from sap flow measuredwith the constant heating method (Granier 1987). One pair of 30-mm long probes (Dynamax Inc., Houston, TX) was in-serted into the north-facing side of each measurement tree at1.4 m height. Measurements of mean temperature differencesalong the lengths of the upper, heated probe, and the lower,reference probe were taken every 5 s and averaged every 10min by a data logger (Model CR10X, Campbell Scientific).Sap flow velocity was calculated from the temperature differ-ence between probes by the empirical equations developed byGranier (1987). Transpiration (  E  ) was determined by multi-plyingsapflowvelocitybysapwoodareaattheheightofmea-surement.Sapwood area was determined by dye injection and the dif-ference in wood color. Toluidine blue dye (Sigma Chemicals,St. Louis, MO) was inserted into the bole of the tree at mea-surement height. A core was taken with an increment borer2 cm above the point of insertion several hours later and sap-wood thickness was observed as the distance from the innerbark toward the center of the trunk that was stained blue. Be-cause long periods of low evaporative demand made this tech-nique difficult, sapwood thickness was also determined by thechange in color between the lighter, conducting sapwood andthe darker heartwood. On trees where both methods were suc-cessful, the border of color change between the sapwood andheartwood corresponded well to the extent of stained tissue.The relationship between basal area and sapwood area of treesin all plots fell on the same line, so a single log–log regression(Sprugel 1983) was used to predict sapwood area from basalarea in cm (SA = e –0.18 BA 0.92 , r  2 = 0.98, n  = 22). In all but four TREE PHYSIOLOGY ON-LINE at http://www.heronpublishing.comCLOUD FOREST STRUCTURE AND TRANSPIRATION 675 Downloaded from gueston 13 March 2018  of the 24 trees measured, the sapwood was thicker than thelengthoftheprobe(30mm)andinthosecasesonlyabout10%of the probe was in nonconducting wood. Therefore, the erroris probably less than 15% in those trees (unpublished observa-tions).Transpiration from individual trees was scaled to the standlevel (  E  stand ) by dividing measured sap flow by basal area andthen multiplying this value by stand basal area. Because a sig-nificant portion of the understory is exposed to incoming radi-ation in open-canopied, level sites, the contribution of  C.alligata  to transpiration was modeled by the Penman equa-tion as modified by Monteith (1965). In this model, the contri-bution of all other understory species was ignored and theamountof  C.alligata perunitgroundareawasassumedtocor-respond to the area not covered by  M.polymorpha  canopy.The canopy resistance term in the Penman-Monteith equationwas estimated based on the LAI of  C.alligata  and averagemeasures of stomatal conductance with a steady state poro-meter(Model1600,Li-Cor,Inc.).Theaerodynamicresistanceterm was estimated as a function of canopy height andwindspeed (Monteith 1965).Stomatal conductance ( g s ) of   M.polymorpha  saplings(1–2 m tall) growing on level versus sloped topography wasmeasured with a steady state porometer (Model 1600, Li-Cor,Inc.). Four saplings from each site were selected and measure-ments were made during sunny and cloudy conditions. Four tofive series of measurements were conducted between 0900and 1400 h on five leaves from each individual when leaveswere dry. Values of  g s  were also measured on  M.polymorpha treesofdifferentheightsbetween1and6.4mtallthatwereac-cessible from an aqueduct structure. Generally, four series of measurements were conducted between 0900 and 1200 h onfive leaves from each individual during dry conditions. Results  Micrometeorological measurements Rainfall totaled 1666 mm during the 23-week measurementperiod (State of Hawaii, Department of Land and Natural Re-sources 1997) and greatly exceeded PET, which totaled151 mm. A mean relative humidity of 94% contributed to theoverall low evaporative demand. The relative humidity re-mained at 100% for 56% of the total measurement period andusually coincided with canopy wetting. Air temperature aver-aged 14.5 °C with a maximum of 25.1 °C in October and aminimum of 5.5 °C in January. Incoming radiation, air satura-tion deficit (ASD) and transpiration were constantly changingthroughout most days as clouds moved over the study site al-lowing for transient high light and relatively high evaporativedemand conditions (Figure 1). Daily windspeed averaged1.24ms –1 andvariedbetween0.06and7.45ms –1 .InSeptem-ber and October, clear conditions were more common in themorning, with orographic clouds leading to fog formation asearly as mid-morning. From December through February, thetrade winds relaxed at times, leading to clear conditions andperiods of no precipitation for as long as 10 days.  Redox potential and soilwater  Soil redox potentials near the surface were higher on slopedtopography than on level topography, but this difference wasless pronounced at greater depths (Figure 2). At 10 cm belowthesurface,redoxpotentialvariedfrom–9to67mVonslopedsites, and from –253 to –230 mV on level sites, indicating amore reduced chemical state and lower oxygen availability onlevel topography. At 40 cm below the surface, standard errorsof the two sites overlapped, but the means remained distinctwithvaluesrangingfrom–180to–107mVinslopedplotsand from –240 to –216 mV in level plots. Soils on level topogra- phywereslightlylessreducedat40cmthanat10cm,whereason sloped topography soils were increasingly reduced withdepth. The organic horizons of soils on level sites were morewaterlogged than those on sloped sites (soil water content of  676 SANTIAGO, GOLDSTEIN, MEINZER, FOWNES AND MUELLER-DOMBOISTREE PHYSIOLOGY VOLUME 20, 2000Figure 1. Diurnal courses of environmental conditions and transpira-tion estimated from sap flow. In b and d, transpiration rates per unitleaf area for a tree in a level, waterlogged site (solid line) and a tree of similarsizeinaslopedandbetter-drainedsite(dottedline)areshown.Figure2.SoilredoxpotentialsnormalizedtopH7.0forpairedsitesof level (open symbols) and sloped (closed symbols) topography. Sites:(  ,  ) pair 1; (  ,  ) pair 2; (  ,  ) pair 3. Downloaded from gueston 13 March 2018  8.0–11.5 g g –1 versus 6.5–7.5 g g –1 ; Table 1). Superficialpooling of water was observed only on level topography. Stand structure, leaf area index and leaf chemistry The rooting habits of   M.polymorpha  trees were associatedwithtopographyanddegreeofsoilwaterlogging.Treesrootedin soil comprised between 14 and 20% of total basal area onslopedsitescomparedwith7%orlessonlevelsites(Figure3).Trees rooted on nurse logs constituted an average of 48% of thebasalareaatlevelsitesandonly23%atslopedsites.Over-all, better-drained substrates with higher redox potentials sup-ported a mean live  M.polymorpha  basal area of 40.7 m 2 ha –1 ,whereas substrates with lower redox potentials supported amean basal area of only 24.5 m 2 ha –1 .Leaf clumps were divided into large (944 cm 2 ) and small(502 cm 2 ) size classes based on analyses of 19 clumps fromsix trees. Multiplying the number of small and large clumps incrown photographs by these values resulted in a canopy LAIof   M.polymorpha  that was close to 1 in level plots and be-tween 5 and 6 in sloped plots (Table 1). Leaf areas of individ-ual  M.polymorpha  trees increased with sapwood area onslopedsites,butdidnotincreasewithincreasingsapwoodareaon level sites (Figure 4). This difference in leaf area to sap-wood area ratios (LA/SA; Meinzer et al. 1997) reflects poten-tial differences in water transport capacity relative totranspirational demand. Sapwood from trees on sloped sitessupplied up to 7 times the amount of leaf area that was sup-pliedbythesameamountofsapwoodfromtreesonlevelsites.CanopyLAIwassixtimesgreateronslopedsitesthanonlevelsites even though basal area was only 1.6 times greater (Ta-ble 1).Mean  δ 13 C values of trees from level and sloped sites were–28.09 ± 0.41 SE and –28.68 ± 0.13 SE, respectively, but were not significantly different ( t   = 1.38; P  = 0.24). TherewerenodifferencesinmeanLMA(181gm –2 ±4SE),foliarN(1.13%±0.03)orfoliarP(0.07%±0.001)inleavesfromlevel and sloped sites. Transpirationand stomatal conductance Transpiration in three pairs of sloped and level sites was com-pared over 5-day measurement periods (Table 2). Trees onslopedsitestranspiredmorewaterthantreesonlevelsitesonapertreebasisandperunitofbasalarea(  E  BA ),whereastranspi-ration per unit leaf area (  E  LA ) was higher in the open canopiesonlevelsites,andthistrendwasmorepronouncedduringperi-ods of high PET. Stand transpiration (  E  stand ), including onlythe canopy of   M.polymorpha , varied from 0.17 to 1.17 mmday –1 on sloped sites and from 0.05 to 0.31 mm d –1 on levelsites depending on prevailing PET. Climate-normalized standtranspiration (  E  stand  /PET) during the same measurement peri-ods, varied from 0.79 to 0.89 on sloped, closed-canopy sitesand from 0.16 to 0.24 on level, open-canopy sites (Table 2).Total leaf surface area was the morphological trait mostclosely correlated with maximum sap flow rate of individualmeasurementtrees(Figure5).Itwasalsotheonlymorpholog-icaltraitthathadthesamerelationshipwithmaximumtranspi-ration rates in trees from both site types.At the stand level, leaf area was also the structural parame-ter most closely correlated with transpiration. There was a lin-ear relationship between  E  stand  /PET and LAI and this linearrelationship was maintained when the contribution of  C.alligata  transpiration was included (Figure 6). Including tran-spiration of  C.alligata  in the model indicated that understoryvegetation was a significant contributor to evaporative flux inlevel, open-canopy stands, but its influence was negligible insloped, closed-canopy stands.There was no significant difference between stomatal con-ductances( g s )of   M.polymorpha  juvenilesgrowingonlevelor TREE PHYSIOLOGY ON-LINE at http://www.heronpublishing.comCLOUD FOREST STRUCTURE AND TRANSPIRATION 677Figure 3. Basal area of   Metrosiderospolymorpha  in study plots dif-fering in topography. Proportion of total basal area in each class of rooting habit is shown.Figure 4. Relationship between individual tree total leaf surface areaand the area of conducting sapwood at 1.4 m height for trees on leveland sloped sites. Lines indicate the best-fit linear relationship forsloped (LA = 0.19·SA + 9.38, r  2 = 0.36; P  = 0.045) and level (LA =0.019·SA + 5.57; r  2 = 0.10; P  = 0.292) sites. Downloaded from gueston 13 March 2018
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