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Sapwood as the scaling parameter- defining according toxylem water content or radial pattern of sap flow?

Jan Cermak, Nadezhda Nadezhdina

To cite this version:Jan Cermak, Nadezhda Nadezhdina. Sapwood as the scaling parameter- defining according to xylemwater content or radial pattern of sap flow?. Annales des sciences forestières, INRA/EDP Sciences,1998, 55 (5), pp.509-521. �hal-00883217�

Original article

Sapwood as the scaling parameter -defining according to xylem water content

or radial pattern of sap flow?

Jan Cermak Nadezhda Nadezhdina

Institute of Forest Ecology, Mendel’s Agricultural and Forestry University, 61300 Brno,Zemedelska 3, Czech Republic

(Received 11 April 1997; accepted 23 April 1998)

Abstract - Sapwood cross-sectional area is a simple biometric parameter widely used for scal-ing up the transpiration data between trees and forest stands. However, it is not always clearhow the sapwood can be estimated and considered, which may cause scaling errors. We exam-ined the sapwood depth according to xylem water content and more precisely according to radialpatterns of sap flow rate in five coniferous and four broad-leaved species of different diameter,age and site conditions. Sapwood estimated by the two methods was almost equal in some species(e.g. Cupressus arizonica), but differed significantly in other species (e.g. Olea europaea, Pinuspinea). Radial pattern of sap flow rate is a more reliable indicator of sapwood then xylem watercontent for sap flow scaling purposes. Percentage of sapwood along radius changed with tree diam-eter and age. Sapwood also changes substantially under severe drought (e.g. in spruce, Piceaabies, up to 1:3 in the course of several months). Sapwood should be used for upscaling sapflow data from measuring points to the whole trees and from trees to stands only for the periodwhen it was actually measured, or the radial profile of sap flow should be measured continu-ously to avoid possible scaling errors. (© Inra/Elsevier, Paris)

woody species / sapwood / radial pattern / sap flow / xylem water content / scaling

Résumé - Le bois d’aubier : paramètre de changement d’échelle défini en relation avec lecontenu en eau du xylème ou avec le type radial de flux de sève ? La surface de la section debois d’aubier est un paramètre biométrique largement utilisé pour effectuer des changementsd’échelle concernant la transpiration des arbres et des peuplements forestiers. Cependant, lafaçon dont le bois d’aubier est évalué peut être la cause d’erreurs dans les changements d’échelle.L’épaisseur du bois d’aubier est ici examinée en relation avec la teneur en eau du xylème et plusprécisément en relation avec le type radial de densité de flux de sève (cinq conifères et quatrefeuillus) de diamètre, âge et situation différents. Le bois d’aubier estimé à l’aide de deux méthodes

* Correspondence and reprintsE-mail: cermak@mendelu.cz

était presque identique chez quelques espèces (Cupressus arizonica) mais diffère significative-ment chez d’autres espèces (Olea europaea, Pinus pinea). Le type radial de densité de flux de sèveest un meilleur indicateur de bois d’aubier que la teneur en eau du xylème pour un objectif de chan-gement d’échelle du bois de sève. Le pourcentage de bois d’aubier sur un rayon varie avec le dia-mètre et l’âge de l’arbre. Le bois d’aubier change aussi substantiellement avec la sécheresse(Picea abies, dans une proportion de 1 à 3 en l’espace de quelques mois). Le bois d’aubierdevrait être utilisé pour le changement d’échelle des flux de sève en mesurant à l’échelle del’arbre entier et à l’échelle des peuplements, seulement pour la période pendant laquelle il a étéde fait mesuré, ou bien le profil radial de densité de flux devrait être mesuré en continue pour évi-ter des possibles erreurs de changement d’échelle. (© Inra/Elsevier, Paris)

bois d’aubier / profil radial de flux de sève / teneur en eau du xylème / changement d’échelle

1. INTRODUCTION

In rigorous anatomical studies, the sap-wood ’splint’ is considered as xylem con-taining living cells and the heartwood’duramen’ is that with dead cells, often

impregnated with xylochromes, oleoresins,tannins and mineral compounds [2, 12].According to usual physiological termi-nology, the sapwood or hydroactive xylemis the outer part of the xylem conductingsap and the heartwood or inactive xylem isthe inner non-conducting xylem [4, 25,29]. The fraction of water remaining inthe heartwood (with a similar one also inthe sapwood) is bound and cannot be usedfor tree metabolism; available water is thatfraction of water which is found in tissuesabove the heartwood limit [34]. It can par-ticipate in the sap flow or serve as stor-age.

Sapwood cross-sectional area is a sim-ple biometric parameter widely used forscaling the transpiration data between treesand forest stands. It is known that theextent of the conducting role of sapwoodarea is different according to species, onto-genetic phases and environmental condi-tions [16, 32]. There are many studies con-firming strong allometric relations betweensapwood area and other biometric param-eters such as leaf area, e.g. [10, 15, 24,33]; however, the functional role of sap-wood area as a tissue supplying foliage

with water is not always easy to evaluate,especially when comparing differentspecies.

Sapwood area is principally large inconiferous and diffuse porous species withnarrow tracheids or vessels (diameterabout 0.05-0.1 mm) but small in ring-porous species with wide (diameter about0.2-0.3 mm) and hydraulically very effi-cient vessels [3, 7, 35]. This fact makes itsometimes difficult to compare behaviourof different species especially in mixedforest stands when using only this param-eter for scaling. Theoretical calculation ofthe sap flow, e.g. according to the Hagen-Poiseuille law, allows comparison of suchspecies, but this is usually far too compli-cated (especially when considering thatconducting elements are non-ideal capil-laries, water flows through pits, etc.). Thatis why this approach is usually not usedfor scaling in routine studies.

This study was focused on evaluation ofrelations of sapwood depth and area andassociated problems of upscaling sap flowdata obtained in measuring points (whichcharacterize radial sections of stems ofdifferent width given by the constructionof sensors) to the whole trees. Several treespecies contrasting in the conductive prop-erties of their xylem and growing in distantsites were examined in order to cover largerange of environmental conditions.

2. MATERIAL AND METHODS

2.1. Experimental sites

Altogether seven trees of Norway spruce(Picea abies (L.) Karst.) with diameters atbreast height (DBH) ranging between 17 and38 cm were studied in the plantation near thetown of Rajec, southern Moravia at an altitudeof 620 m (latitude 49°30’E and longitude17°20’N). The stand was characterizedas Fagetum quercino-abietinum with thepresence of Carex pilulifera and a negligiblenumber of herbal species connected witholigotrophic soils and raw humus. Oligotrophicbrown forest loamy soil with decreased poros-ity in some places and high nutrient concen-tration in the humus layer and in the A-horizonwas found. Depth of rhizosphere was around60 cm, and in some places 120 cm. Long-termmean annual air temperature was 6.6 °C; meanannual precipitation was 683 mm (400 mm pergrowing period).

Scots pine, Pinus sylvestris L.

(DBH = 28.6 cm) and three poplars Populusinteramericana, cv. Beaupre (DBH =

46.2-48.7 cm) were sampled in Brasschaat,see [8] and in Balegem, Belgium, respectively[22]. In Brasschaat, the original climax vege-tation (natural forest) was a Querceto-Betule-tum [30]. The experimental plot was a pineplantation, 1.5 % slope oriented N.N.E, alti-tude 16 m. (51°18’33"E and 4°31’ 14"). Soilcharacteristics were moderately wet sandy soil with a distinct humus and/or iron B-horizon,umbric regosol or haplic podzol in the F.A.O.classification [1]. The groundwater depth nor-mally ranged between 1.2 and 1.5 m and mightbe lower due to non-edaphic circumstances.In Balegem (coordinates: 50°55’7"E and3°47’39"N) the experimental site was also flat(altitude 50 m) and located on the originalorchard combined with meadow: moderatelygleyic loamy soil with a degraded texture B-horizon, coarser with depth; an Ap-horizon of30 cm FAO soil classification: glossaqualf [22].The climate was moist subhumid (C1), rainyand mesothermal (B’1). Mean (over 28 years)annual and growing season temperatures forthe region were 9.76 and 13.72 °C, precipitationwas 767 and 433 mm, respectively.

Olea europaea L. (DBH = 19 cm), Ficuscarica L. (DBH = 15.9 cm), Cupressus ari-zonica Green. (DBH = 20.7 cm), Cupressus

sempervirens L.D. (DBH = 28.3 cm), Pinuspinea L. (31.5 cm) and Quercus pubescensWilld. (DBH = 8.9; 19.7 and 34.4 cm) werestudied in central Tuscany, Italy, near the townof Radicondoli (latitude 43°15’3"N and lon-gitude 1 1°03’29"E, altitude 550 m). The sitewas typical with loamy soil containing high tovery high percentage of stones, mean annualand seasonal temperatures were 11.3 and15.6 °C, precipitation was 621 and 540 mm,respectively.

2.2. Methods of measurementand data evaluation

The sap flow rate in spruce was measured

using the tree trunk heat balance techniqueapplying bulk internal (direct electric) heating[4, 5, 18]. Five stainless steel electrodes andfour pairs of compensating thermocouplesarranged in different depths within sapwood[6] were used. In all other species we used theheat balance method based on linear radial

heating of tissues and sensing of temperature[23], applying dataloggers made by Environ-mental Measuring Systems & UNILOG, Brno,Czech Republic. A series of six thermocou-ples arranged in different distances (from 5 to15 mm) were placed in stainless steel hypo-dermic needles 1.2 mm in outer diameter. More

points of sap flow along the radius wereobtained under stable conditions, when the nee-dles were radially shifted during measurements.

Depth of conducting wood and corre-sponding area was estimated from the radialprofiles of sap flow, taking into account thepoint where the sap flow approached zero. Sapflow rate for the whole tree was obtained, whenindividual points of radial pattern of sap flowper area (splained by the exactly fitting curve)were multiplied by the corresponding areas ofannuli and summarized. For spruce, only sapflow data integrated over the sapwood by themeasuring system were at our disposal. Thatis why the radial pattern of flow was approxi-mately calculated using these totals and thepreviously estimated form of radial pattern inthis species [7]. In general, the sap flow rateintegrated for the whole trees according todirectly measured radial pattern of flow perarea was compared with the mean flow datacharacterizing individual sapwood layers (as

if using only one thermocouple within a sensorplaced at a different depth characterizing a cer-tain layer) when multiplied by correspondingsapwood area. Each layer was measured1) over 20 % of sapwood depth and 2) sepa-rately over 50 %. For this purpose, sapwoodwas distinguished from heartwood the classicalway, i.e. according to xylem water content.

The volumetric fraction of water (water vol-ume, Vw expressed in percentage of fresh vol-ume of samples, V) and specific dry mass (drymass, Md estimated after drying for 48 h at80 °C, divided by sample volume, Md/V) wasestimated on the wood cores sampled by thePressler’s borer (Suunto, Finland) from twoopposite sides of stems at breast height(1.3 m). Cores were placed in aluminium foilimmediately after sampling and analysed gravi-metrically, after being cut into small pieces,within a few hours. The volumetric fraction ofwater was applied to estimate the depth of sap-wood (and corresponding areas), here taken asxylem tissues, which differ in their hydrationfrom heartwood.

3. RESULTS AND DISCUSSION

3.1. Radial pattern of xylem watercontent

Sapwood and heartwood are woody tis-sues usually containing higher and loweramounts of water, respectively, but this isnot always the case. We found in sprucealmost 60 %vol in saturated xylem tissues(during early spring) and about 10-11 %volin heartwood (figure 1), which corre-sponds to our previous results [17]. Sap-wood was relatively deeper in larger trees(up to 60 % of xylem radius, rxyl) and shal-lower in smaller trees (up to 20 % of rxy1)of even age. Sapwood was slightly deeperon the southern side (as shown by its rela-tion to stem diameter at breast height:y = 0.175x; r2 = 0.92; SE = 0.45) and moreshallow on the northern side of stems

(y = 0.187x-0.94; r2 = 0.78; SE = 0.93).The radial pattern of water content dif-fered completely in fast growing and vig-

orous poplars, where we found less waterin the sapwood (25-30 %vol), whereasmuch more water was found in the heart-wood (60-80 %vol) (figure 1B).

3.2. Radial pattern of water contentand sap flow in different species

We found a variable radial pattern of

sap flow in species with very differentradial pattern of xylem water content (fig-ure 2). In all given figures, splaining curves fitted measured points withr2 > 0.99, thus exactly characterizing thepatterns. Sapwood water content was verylow in poplars (about 20 %vol) comparedto that in the heartwood (almost 80 %vol),but sap flow took place over the wholesapwood (peaking at about 70-90 % ofstem radius). There were almost no dif-ferences in xylem water content betweensapwood and heartwood in Olea europaea(mean value of about 40 %vol); however,higher sap flow rates were limited to sap-wood (peaking close to cambium) and

lower rates were observed in a wide tran-

sition area towards heartwood (below40 % of stem radius). The fraction of avail-able water in Ficus carica increased more

than two-fold from pith towards cambium(40-70 %vol) and no distinctive heartwoodwas identified here this way. This roughlycorresponds to sap flow, which demon-strated a peak in the outer part of thexylem, corresponding to sapwood, but ata lower level remained also in the inner

part of the xylem (also below 40 % of stemradius). The heartwood border identifiedfrom sapwood water content was almostthe same as that identified on the basis ofradial sap flow rate in Scots pine trees.However, water remained almost at thesame level (about 25 %vol) through sap-wood, while the sap flow pattern showed

peak values at about 90 % of the stemradius.

Different pattern of sap flow rates werealso found in other conifer species whichall have distinctive differences in xylemwater content between heartwood (15-20%vol) and sapwood (around 50 %vol).Cupressus arizonica is an example of atree with a radial pattern of sap flow veryclosely related to that of xylem water con-tent (although it is not so close on the otherside of the same stem). But even undersuch conditions, the sapwood does notconduct water uniformly across its wholearea. Differences between sapwood areasestimated by both the methods mentionedare still more pronounced in other trees in

the study, as shown by the example ofCupressus sempervirens and Pinus pinea(figure 3).

The radial pattern of sap flow per areadiffers from that calculated for corre-

sponding annuli. The importance of outerxylem layers for sap flow rate is increasingowing to increasing area of the annuli fromthe pith to cambium (if an equal width ofannuli is considered). The differencesbetween both totals are rather small in

species with shallow sapwood, but aresubstantial in species with deep sapwood(figure 4).

It is clear from the above results that

sapwood area estimated on the basis of

changes in xylem water content is par-tially related to conducting area, whichshould be applied for scaling the sap flowrate from measuring points (usually rep-resenting certain sections of sapwood) tothe whole trees. However, the relationsare not always straightforward. A veryvariable pattern of sap flow rate in differ-ent species indicates that for scaling pur-poses it is necessary to integrate properlythe actual radial profile of sap flow mea-sured per area and consider accordinglythe conducting areas of correspondingannuli. Rather small differences in theradial pattern of sap flow per area and perannuli in shallow sapwood species make ittechnically easier to integrate the flowcompared to that in deep sapwood species.Specific dry mass as a parameter some-times used to indicate conducting proper-ties of woody tissues and xylem watercontent can sometimes be used as an indi-cator of conductivity, but this is also notalways reliable, if large differencesbetween xylem tissues are not considered.

3.3. Changes in radial pattern of sapflow with tree diameter and age

The radial pattern of sap flow ratechanges with tree size and age irrespec-tively of the specific dry mass and xylemwater content (figure 5). Practically thewhole cross-sectional area of xylem wasconductive in young oak (Quercuspubescens) trees, even when high flowrates per area occurred only close to thecambium. However, sapwood area

decreased dramatically in older trees,reaching up to only 30 % of the xylemradius in adulthood. Similar and lower

percentages of conducting xylem in dif-ferent oak species were reported byPhillips et al. [27]. In pedunculate oak(Quercus robur) growing in floodplainforests we found the sapwood depth to beabout 60 % of the xylem radius in youngtrees (DBH = 8 cm) with the most impor-

tant flows up to 16 % [7]. In adult trees(DBH = 30 cm) the visible sapwoodreached about 19 % of the xylem radiusthere and the conductive sapwood about15 %, with the most important flows upto only 4 %. As demonstrated in ourrelated unpublished results, the larger partof the deeper layers in sapwood was activeonly in suppressed Q. robur trees, evenwhen they were relatively large (thosewith little summer growth, which pro-duced only low density earlywood com-posed of medium-sized vessels). How-ever, one or two annual rings with verylarge vessels were usually most active andeventually another one or two showed very

little activity in the main canopy trees,which was also confirmed by other studies[18].

3.4. Changes in radial water contentand total sap flow under drought

Saturated xylem water content com-pared to that under drought was shownonly on one large spruce (figure 6),

although the situation was similar in theother six sample trees already presentedin the above (see figure 1A). There wereno significant differences in specific drymass of xylem along stem radius. Undersaturated conditions, water content reachedmaximum (around 60 %vol) approximatelyat the centre of the sapwood, slightly closerto the cambium (at 20-30 mm). Watercontent was lower by about 5 %vol near

the cambium as well as at the same dis-

tance to the heartwood, where it decreased

abruptly to the heartwood, which wascharacterized by an almost constant watercontent of about 10-11 %vol down to the

pith. (Phloem water content was about65 %vol at the same time.) Under droughtin late summer the sapwood depthdecreased down to about 1/3 of that in sat-urated tissues; sapwood area in largelydehydrated tissues decreased to about 38% of that in saturated tissues (see figure 2).The fraction of xylem water decreasedunder drought to about 40 %vol in the

uppermost layers (at a depth of 0-1.2 cmbeneath the cambium, thus down to only 8% of the xylem radius). Mean fraction ofxylem water when calculated over theentire depth of sapwood reached only19 %vol. Phloem water decreased to about53 %vol. There was no change in the heart-wood water.

Since no radial pattern of sap flow wasmeasured in the experimental spruce, weassumed that it had an approximatelyGaussian-like pattern under good watersupply as shown previously [7, 21, 30].But it is clear that there must be a corre-

sponding dramatic change in the radialpattern under drought compared to that insaturated conditions, if the sapwood areadecreased 2.6 times (see figure 6). Con-sidering total sap flow per tree, or relativetranspiration (daily total of sap flowdivided by PET), its seasonal courseincreased by about 20 % during May andJune indicating development of foliageand reached about 75 % of PET at its sea-sonal maximum. However, this trend wasreversed from June to August under theimpact of continuous severe drought, whenthe relative transpiration decreased byabout half (figure 7). Considering adecreasing area of sapwood, this indicatesthat the outer part of the sapwood wasabout one third more efficient in con-

ducting water compared to its inner part.Similar results were obtained for Pinustaeda during drought by Phillips et al. [27],

who reported that the ratio of the dailyintegrated flux density in the inner to outerxylem decreased with soil moisture from0.44 to 0.36.

Our results on xylem water content inspruce generally correspond to the datafound for this species in other sites [17].The radial profile of xylem water contentis not directly related to the radial profileof sap flow and the outer xylem - sap-wood with higher water content representsthe potential conducting area only. How-ever, it is clear that the flow cannot take

place in the xylem where there is no freewater (i.e. in the xylem containing onlybound water - see figure 6) and thusdecreasing sapwood area must lead todecreasing sap flow. A similar situationindicating the importance of changes inthe soil water supply for stem hydraulics

has already been confirmed for broad-leafspecies [9]. Under high evaporationdemand, water is of course extracted fromall stem tissues, although our results showthat under long-term drought, water isextracted presumably from deeper layersof the sapwood. In contrast, dendrometerrecords reflect extraction of water fromthe outermost part of the last annual ringand phloem [11, 13, 26]. This means thatonly part of the water extracted fromxylem is associated with volume changesof the tissues. Older xylem located deeperin the stems is rigid and does not signifi-cantly change in volume under physio-logical conditions, although it containsand provides a significant amount of waterwhen necessary. The volume of the sprucestem can return almost to its original valueafter drought [14] and reverse embolismmay occur by refilling tracheids in theabsence of positive pressure [28]. Waterstorage in outer tissues is more readilyreplaced by rehydrating (night) flow, whiledeeper layers of sapwood remain mostlyempty in the long-term (and eventuallyrehydrate more slowly) owing to higherradial xylem resistances.

3.5. Scaling errors caused byneglecting the radial patternof flow

Rather large scaling errors may occur ifthe thermocouple applied in a sap flowsensor represents only one point along thexylem radius (one depth within the sap-wood) and the calculated value of sap flowis upscaled for the whole tree supposingthat equal sap flow rate occurs over theentire sapwood area. The actual situationdepends on the intergrating depth coveredby the sap flow sensor and the position ofthe sensor along the radius. Comparingall sample trees under study showed themagnitude of possible scaling errors (tableI). Sensors placed, for example, in theouter half of the sapwood mostly over-

estimated total tree sap flow (by about10-40 %) and those placed in deep innerlayers of sapwood always underestimatedit (by about 40-80 %). Such errors can bemuch larger under drought.

3.6. Assumed effect of climate

changes on radial patterns

Decreased sap flow rates occurred at asmall distance towards the pith from thepeak value in almost all trees under studyirrespectively of their species, size, ageand location (see figures 2-4). Such adecrease corresponds to about five annualrings, which indicates that some

unfavourable change in growing condi-tions occurred approximately betweenyears 1987 and 1991 over Europe. Thesmall number of sampled trees analysedhere does not allow general conclusions,but it seems that detailed measurementsof the radial pattern of sap flow can beapplied as an alternative field method forestimating the impact of climatic changeon woody vegetation.

4. CONCLUSIONS

1) Sapwood may contain a higher per-centage of available (free) water thanheartwood or the same percentage or heart-wood may contain a higher percentagethen sapwood (within the approximaterange 10-60 %vol). For some species it isimpossible to distinguish between sap-wood and heartwood only according towater content in woody tissues.

2) Sapwood cross-sectional area is asomewhat problematic parameter whenused alone for upscaling sap flow datafrom measuring points to whole trees.Depth of the actually conducting sapwood(estimated according to the radial patternof sap flow) may approach the depth ofsapwood. Sapwood estimated according

to xylem water content or a change inwood colour only is not reliable enoughfor scaling purposes, because the sapwooddoes not conduct water uniformly acrossits whole area.

3) The radial pattern of sap flow shouldbe considered when upscaling data frommeasuring points (usually representingcertain stem sections of different size) tothe whole trees. It is best to measure the

radial pattern (using more sensors alongxylem radius) continuously or at least todetermine the radial position of a smallernumber of representative thermocouplesapplied for routine studies on such a basis.

4) We confirm that fraction of sapwoodarea in xylem cross-sectional area is large(up to 100 %) in young trees and decreaseswith tree age.

5) High seasonal dynamics of tissuewater content and the associated radial

profile of sap flow during drought maylead to significant scaling errors if the sap-wood area is estimated, e.g. under condi-tions of good soil water supply and appliedalso to the possible period of drought.

ACKNOWLEDGEMENTS

The authors thank Dipl. Ing. J. Kucera fromthe Environmental Measuring Systems, Inc.,

Brno for his valuable help during field studieson spruce, Prof. Dr. R. Ceulemans, from theUniversity of Antwerpen (Wilrijk, Belgium),Dr. J. van Slyken from the Institute for Forestryand Game Management (Geraardsbergen, Bel-gium) and Dr. A. Raschi from the Institute ofAgrometeorology and Environmental Analysisfor Agriculture (Florence, Italy), for their valu-able support of this study when working onrelated joint studies.

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