Radial variation in sap flow in five laurel forest tree ... - Tree Physiology

Tree Physiology 20, 1149–1156 © 2000 Heron Publishing—Victoria, Canada

Radial variation in sap flow in five laurel forest tree species in Tenerife, Canary Islands M. SOLEDAD JIMÉNEZ,1 NADEZHDA NADEZHDINA,2 JAN ÈERMÁK2 and DOMINGO MORALES1 1

Dpto. de Biología Vegetal, Universidad de La Laguna, E-38207 La Laguna, Tenerife, Spain

2

Institute of Forest Ecology, Mendel University of Agriculture and Forestry, 61300 Brno, Zemedelska 3, Czech Republic

Received July 30, 1999

Summary Variations in radial patterns of xylem water content and sap flow rate were measured in five laurel forest tree species (Laurus azorica (Seub.) Franco, Persea indica (L.) Spreng., Myrica faya Ait., Erica arborea L. and Ilex perado Ait. ssp. platyphylla (Webb & Berth.) Tutin) growing in an experimental plot at Agua García, Tenerife, Canary Islands. Measurements were performed around midday during warm and sunny days by the heat field deformation method. In all species, water content was almost constant (around 35% by volume) over the whole xylem cross-sectional area. There were no differences in wood color over the whole cross-sectional area of the stem in most species with the exception of E. arborea, whose wood became darker in the inner layers. Radial patterns of sap flow were highly variable and did not show clear relationships with tree diameter or species. Sap flow occurred over the whole xylem cross-sectional area in some species, whereas it was limited to the outer xylem layers in others. Sap flow rate was either similar along the xylem radius or exhibited a peak in the outer part of the xylem area. Low sap flow rates with little variation in radial pattern were typical for shaded suppressed trees, whereas dominant trees exhibited high sap flow rates with a peak in the radial pattern. Stem damage resulted in a significant decrease in sap flow rate in the outer xylem layers. The outer xylem is more important for whole tree water supply than the inner xylem because of its larger size. We conclude that measurement of radial flow pattern provides a reliable method of integrating sap flow from individual measuring points to the whole tree. Keywords: conducting xylem, Erica arborea, Ilex perado, Laurus azorica, Myrica faya, Persea indica, radial pattern, sapwood, scaling, xylem water content.

Introduction A comprehensive knowledge of the conducting system within stems or trunks of woody species is important for scaling sap flow data obtained from point measurements to whole trees or shrubs and forests (Èermák and Kuèera 1990). Such knowledge is also needed to evaluate the behavior of different parts

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of crowns and roots (Nadezhdina and Èermák 1999). For example, the information would be useful in studies of xylem anatomy, variation of sap flow around stems and at different heights above ground (Èermák and Kuèera 1990, Gartner 1995, Goldstein et al. 1998), flow from internal storage within stems to the main stream of sap (Borchert 1994a, 1994b, Andrade et al. 1998) and flow at different depths below the cambium along the stem radius (Swanson 1971, Mark and Crews 1973, Edwards and Booker 1984, Èermák et al. 1992, Granier et al. 1994, Phillips et al. 1996). The conducting system of most forest woody species growing in humid and subhumid warm Mediterranean regions comprises several large vessels that remain functional for many years, unlike species growing in northern parts of Europe, which have vessels that are embolized by freeze–thaw events during winter (Zimmermann 1983). Consequently, those species (among them, laurel forest species) typically develop sapwood over the whole cross-sectional area of the stem (Schweingruber 1990, Èermák et al. 2000, Morales et al. 2000). It has generally been assumed that the whole sapwood cross-sectional area is uniformly conducting, although early researchers recognized differences in sapwood conductivity based on staining methods (MacDougal 1925, Arcikhovskiy 1931). Unfortunately, staining methods are destructive (Edwards and Jarvis 1981, Èermák et al. 1984, 1992), and their application is associated with unnatural changes in pressure. More accurate information about sap flow requires the use of methods that minimize damage to conducting tissues. Radial pattern of sap flow has rarely been characterized by nondestructive methods (Swanson 1971, Mark and Crews 1973), although several studies based on nondestructive methods have recently been reported (Phillips et al. 1996, Clearwater et al. 1999, Lu et al. 2000). We used the nondestructive heat field deformation method to evaluate radial patterns of sap flow in sapwood of tree species growing in the humid Mediterranean. This paper extends our survey of radial patterns of sap flow in Central European and dry Mediterranean species (Èermák and Nadezhdina 1998) to five laurel forest tree species growing in the Canary Islands. These laurel forest species were selected because their morphology and

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physiology has been studied in depth (Köhl et al. 1996, Morales et al. 1996a, 1996b, 1996c, 1997, Jiménez et al. 1996, 1999, González-Rodríguez et al. 1999).

Materials and methods Experimental site The experimental laurel forest (laurisilva) site is located in the Agua García mountains (28°27′32″ N, 16°24′20″ W), northeast of Tenerife, Canary Islands. It is on a slight slope (12°) facing NNE, at an altitude of 820–830 m. The bedrock is characterized as a mixture of olivinic basalt and volcanic ash with high permeability and drainage. Soil type is a colluvial Andisol covered with a 2–3-cm-thick layer of litter. The climate is humid Mediterranean with a mean annual temperature of 13.6 °C, relative humidity of 82% and precipitation of 756 mm. Vegetation at the site represents a laurel forest type with Viburnum rigidum Vent., Hedera canariensis Willd., Smilax canariensis Willd., Ranunculus cortusifolius Willd., Asplenium onopteris L. and six tree species: Laurus azorica (Seub.) Franco, Persea indica (L.) Spreng., Myrica faya Ait., Erica arborea L. and two species of Ilex, I. perado Ait. ssp. platyphylla (Webb & Berth.) Tutin and I. canariensis Poivet. All are evergreen broadleaf tree species except E. arborea, which has needle-like leaves. Details about the site and forest stand are described in our previous papers (Morales et al. 1996a, 1996b, 1996c). Sample trees Biometric data of the sample trees are given in Table 1. Three to 12 specimens of five tree species were evaluated. The stand was regrown from sprouts after cutting the original forest. Individual trees range in age from 30 to 50 years. Tree height ranges from 12 to 22 m. Xylem water content The volume of water (Vw, expressed as a percentage of the fresh volume of samples, V) and the volume of solid matter, Vd, were estimated on wood cores sampled with a Pressler’s borer (Suunto, Espoo, Finland), to distinguish sapwood and heartwood. Samples were always taken from the opposite sides of stems. Wood cores of 5.2 mm in diameter were placed in aluminum foil immediately after sampling and analyzed gravimetrically after cutting them into small pieces of measured length (dL). Dry mass, Md, was determined after drying for 48 h at 80 °C. Volume of solid matter (Vd) was calculated by dividing sample dry mass by specific mass of dry substance (1.54 g cm –3). The fractions of solid matter and water were then calculated by dividing by sample volume (Vd /V and Vw /V). Sap flow rate Sap flow was measured by the heat field deformation (HFD) method with linear radial heating of tissues and sensing of

temperature (Nadezhdina and Èermák 1998, Nadezhdina et al. 1998). Each sensor consisted of a needle-like heater and a series of differentially connected thermocouples inserted in two pairs of stainless steel hypodermic needles installed around the heater in axial (symmetrical above and below the heater) and tangential (asymmetrical, on the side of the heater) positions. Symmetrical pair placement increases the sensitivity of the sensor (Nadezhdina 1999). Each hypodermic needle had an outer diameter of 1.2 mm. Each series consisted of six thermocouples (copper-constantan) arranged at certain distances inside the hypodermic needles (with a distance of 5 to 15 mm between sensors), allowing simultaneous measurement of sap flow along the stem radius (Figure 1). The radial pattern of sap flow measured in this way is partially smoothed as a result of heat transfer through the metal of the needle. However, this heat transfer is dependent on the ratio of the longitudinal transfer through the thin walls of the needle to the tangential and radial transfer of heat from the massive surrounding wood, which has a much larger heat capacity than the needle. Earlier, it was found experimentally that there were very small differences between values measured at the same places with thermocouples inserted in the needles and with uncovered thermocouples, if they were more than 3 mm apart. Although heat transfer along the needle slightly averages the data, it has no significant impact on the results. Nevertheless, additional measurement points of sap flow along a radius were obtained by shifting the hypodermic needles radially in the stems during short-term measurements under stable weather conditions. The method is based on physical principles, including our previously validated stem heat balance term, and does not require calibration. Calculation of conducting xylem and sap flow Depth and area of conducting wood were estimated from the radial profiles of sap flow, taking into account the point where the sap flow reached zero (in cases where the sensors did not reach this point it was approximated on the basis of results obtained on similar trees). If the depth of conducting xylem reached the pith, the whole stem xylem cross-sectional area was assumed to be conducting. Sap flow data characterizing individual points expressed as flow per area (g cm –2 h –1 or as a percentage of maximum flow) were plotted by an exact fitting curve with a step of 2% of the radius. Sap flow rate for the whole tree (actual values for a given moment or mean over a certain period) was obtained by first multiplying rates at individual points characterizing sap flow along a radius, i.e., mean sap flow for individual narrow annuli, by the cross-sectional area of the corresponding annuli and then summing. Measurements Sample trees were measured around midday during warm (20 to 25 °C and 35 to 60% RH) sunny days in late February 1998. To analyze the influence of the social position of a tree on the radial pattern of sap flow, four trees growing nearby were measured simultaneously (two dominant and two sup-

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Table 1. Radial patterns of sap flow per area (expressed as a percentage of its maximum versus distance along the xylem radius expressed as a percentage) under conditions of high flow during fine weather around midday in February 1998 in the laurel forest in Agua García. Abbreviations: DBH = diameter at breast height with bark; BARK = bark depth; and CP = orientation of measuring point in the trunk (cardinal points). Types of curves: 1 = whole xylem area is conducting; 2 = outer part of xylem area is conducting and inner part is non-conducting; A = similar flow along the xylem radius; B = narrow peak in the outer part of xylem area; and C = wide peak in the outer part of xylem area. An example of each type of curve is shown in Figure 3. DBH (cm)

BARK (cm)

CP

Species and notes

Radial pattern of sap flow

Laurus azorica 12.4 15.9 16.4 18.5 19.4 19.6 19.9 20.7 23.4 25.5 27.9 31.4

0.2 0.3 0.4 0.4 0.3 0.4 0.4 0.5 0.3 0.4 0.5 0.6

E S SW S E S E SE NE SE S SW

Poor shaded crown, curved trunk Very small crown Small crown Dense crown, partially shaded Vital dense crown, partially shaded Medium crown, partially shaded Small crown (larger from E) curved trunk Dense crown, not much shaded Vital dense crown, not much shaded Small narrow crown Large open crown Dense large crown, partially shaded from S

1A 1B 1B 1B 1B 2B 2B 1A 2B 2B 2B 2B

Persea indica 11.1 15.0 15.3 24.2

0.5 0.6 0.7 0.9

NE E E E

Small sparse crown Sparse crown but open and vital Medium open growing crown Medium crown two main branches facing E and W

2C 1C 1C 2B

Ilex perado 10.6 17.2 19.4 22.2

0.3 0.3 0.3 0.5

W S E SW

Dense but shaded crown, more open from W Dense long crown, more shaded from SEW Dense long crown, shaded from E Sparse crown

1C 1C 1C 1A

Myrica faya 19.5 21.3 24.4 27.2 37.6

0.5 0.9 0.7 0.9 1.1

SW SE S E S

Extremely sparse crown, many dead branches Asymmetric medium size crown, shaded from E Small sparse crown Large crown, not shaded Large open crown

2B 2B 2C 1C 2B

Erica arborea 17.5 19.3 22.6

0.5 0.3 0.6

SW SE N

Sparse open crown Sparse asymmetric crown, shaded from S, curved trunk Very small crown, asymmetrical, curved trunk

2B 2B 2B

pressed specimens of P. indica and two similar specimens of L. azorica). To compare healthy and damaged trees, two neighboring specimens of P. indica were measured simultaneously. Results and discussion Radial pattern of xylem water content The pattern of xylem water content was fairly uniform along the xylem radius (about 35% by volume) in all broadleaf species and species with needle-like leaves, irrespective of tree size, although slight trends and some variation occurred (Figure 2). This pattern corresponds to that found in our previous, more detailed study of wood anatomy and vessel distribution

in L. azorica trees (Morales et al. 2000). Sapwood can be distinguished from heartwood according to the xylem water content, especially in coniferous species (Kozlowski and Pallardy 1997, Kravka et al. 1999). However, as we observed in other broadleaf tree species (see Èermák and Nadezhdina 1998), it cannot be distinguished in laurel forest species in this way. Furthermore, in these species, sapwood cannot be distinguished from heartwood by a change in color (cf. Stewart 1966, Wagenfuhr 1989), except in the case of large older trees of E. arborea, whose inner xylem became darker after drying. Microscopic analysis confirmed that the dark color was caused by inclusions in parenchymatic cells (mostly in pith rays), indicating that heartwood is differentiated at least in some specimens of this species.

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Figure 1. (A) Frontal view of thermocouple placements around the radial heater (heated needle) in the sensor based on the heat field deformation method (HFD). Symbols Ts and Ts′ denote series of symmetrical thermocouples placed in the axial direction above and below the heater. Symbols Ta and Ta′ denote series of asymmetrical thermocouples placed in the tangential direction on the side of the heater. (B) Diagram of the longitudinal section of a hypodermic needle (1.2 mm in diameter) containing six thermocouples (additional details have been described by Nadezhdina and Èermák 1998, Nadezhdina et al. 1998).

Variation in radial pattern of sap flow In contrast to the mostly uniform water content, radial patterns of sap flow in all of the experimental trees were highly variable (Table 1). Although no specific groups could be distinguished based on quantitative differences, we identified two main patterns: (1) the whole xylem area was conducting; or (2) the outer part of the xylem area was conducting, whereas the inner part was nonconducting. Within both of these types there were radial patterns with almost uniform flow along the xylem radius (A), with a pronounced and narrow peak in the outer part of the xylem area (B) and with a wide peak in the outer part of the xylem area (C) (see Table 1 and Figure 3). This high variation was within the wide range reported for other broadleaf and coniferous species (Swanson 1971, Èermák et al. 1992, 2000, Èermák and Nadezhdina 1998). No distinctive sap flow patterns were related to species or tree diameter (within the range of DBH = 12 to 32 cm). However, we noted that the number of sample trees was small in relation to the magnitude of variation in the described patterns. Such variation could arise for several reasons including wood development in some species (Mark and Crews 1973), social position of trees, their history and associated architecture, and state of health. In some large trees, sap flow was measured only in the outer xylem layers because the sensors were not of sufficient length to reach the inner layers (e.g., in most M. faya experimental trees; P. indica tree of DBH = 24.2; I. perado trees of DBH = 19.4 and 22.2; and L. azorica tree of DBH = 31.4 cm). In these trees, the sap flow pattern in the deeper xylem layers was approximated based on similar patterns measured in other trees.

Figure 2. Radial patterns of volumetric fractions of water (䊉) and solid matter (䉱) in the xylem of experimental trees of the five main laurel forest species in the experimental plot at Agua García. Mean values (and standard deviations) were calculated for 20 points along a xylem radius.

Impacts of tree social position and leaf distribution on radial pattern of sap flow Because the variation in sap flow along the radius could be a result of the impact of social position of trees, we analyzed two dominant and suppressed specimens of P. indica and two similar specimens of L. azorica simultaneously (Figure 4). Rates of total sap flow were much higher in dominant trees than in suppressed trees, and dominant trees exhibited a typical peak flow that always occurred in the outer xylem layers (corresponding with pattern B or C). In contrast, no peaks were visible in the radial pattern of sap flow in suppressed trees, which had a low rate of total sap flow that was almost uniform over the whole sapwood depth (corresponding with pattern A). Based on leaf distributions within crowns determined in previous studies (Morales et al. 1996a, 1996c), we conclude that leaf distribution within the crowns influenced radial sap flow patterns of dominant trees. For example, in the dominant L. azorica tree with a dense crown, a high and narrow peak of sap flow occurred in the outer xylem layers (pattern B),

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Figure 3. Examples of different types of curves described in Table 1. Each curve is the radial pattern of sap flow per area (expressed as a percentage of its maximum versus distance along the xylem radius expressed as a percentage) under conditions of high flow during fine weather around midday in February 1998 in the laurel forest in Agua García.

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Figure 4. Example of radial patterns of sap flow in suppressed and dominant trees of Laurus azorica (DBH = 12.4 and 19.4 cm, respectively) and Persea indica (DBH = 11.1 and 15.0 cm, respectively) in a closed-canopy forest stand in the experimental plot at Agua García.

dial flow patterns in suppressed trees reflected the almost uniform transpiration of foliage throughout their crowns. whereas in the dominant P. indica tree with a sparse crown, a wide peak of sap flow occurred deep in the xylem (pattern C). The social position of trees in closed forest stands is characterized by differences in resource availability, including soil water supply and illumination of tree canopies (Stewart et al. 1985, Baldocchi et al. 1986). In the laurel forest experimental plot, there is strong light attenuation (2% of incident photosynthetic active radiation or 4% of global radiation penetrates to the forest floor (Aschan et al. 1994, 1997)) as a result of the high leaf area index (LAI = 7.8, see Morales et al. 1996b). Illumination of dominant trees whose crowns reach the upper canopy changes dramatically with canopy depth, whereas illumination of shaded suppressed trees, which have crowns below the main canopy, differs little along the canopy depth (Èermák 1989, 1998). We obtained evidence that sap flow in the inner xylem layers is connected with the lower, deeper parts of crowns and increases when these crown parts are exposed to the high irradiances that characterize more open or sparse crowns. Sap flow in the outer sapwood layers reflected the high transpiration rates of the well-illuminated upper canopy, whereas sap flow in the inner sapwood layers reflected the low transpiration rates of the partially shaded lower canopy. Mutual shading of foliage is less important in sparse crowns where light can penetrate deeply to illuminate the foliage of the lower canopy. Thus, the negligible differences in xylem ra-

Impact of stem damage on radial pattern of sap flow Stem damage was observed in only one of the experimental trees. In one P. indica tree (DBH = 31.0 cm) with a large and well-exposed crown, the stem base (and probably also part of the corresponding root system) was partially decayed. A branch had decayed, leaving an open hole located 10 cm to the left and 50 cm below the sap flow measuring point. The radial pattern of sap flow in the damaged tree differed from that measured simultaneously in a nearby healthy P. indica tree of similar DBH. Sap flow per xylem area was about 80% lower in the damaged tree than in the healthy tree along 70 to 90% of the xylem radius. However, in the deeper xylem layers, sap flow was similar in the damaged and healthy trees (Figure 5). There were no visible differences in sapwood between the damaged and healthy trees in cores taken at the measuring points. Similar results have been reported by Èermák and Kuèera (1990), who observed that decomposition of woody stem or root tissues by fungi results in decreased absorption of water or its conduction in the xylem. In pine trees, infection of sapwood by Ceratocystis and Amylostereum spp. fungi, which produce toxins, caused significant restrictions in sap flow shortly after the attack (Coutts 1969, Yamaoka et al. 1990). Although we examined only two trees, our data provide strong evidence of changed conductive features in response to stem

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Figure 5. Radial pattern of sap flow in the Persea indica tree (DBH = 31.0 cm) with damaged stem base (open hole left by the decay of a branch located 10 cm to the left and 50 cm below the sap flow measuring point) compared with the pattern in a healthy tree of similar size in the experimental plot at Agua García. The small difference in tree size was corrected for based on xylem basal area.

tissue damage. We conclude that the radial pattern of sap flow is a useful method for pathological studies aimed at elucidating the importance of fungal infections on tree water supply and the consequences for its physiology. Sapwood and scaling from measuring points to whole tree Sap flow expressed as a percentage of its maximum along a xylem radius allowed generalization of radial patterns, and integration of sap flow was made possible by expressing flow in absolute values per area or per annulus (Figure 6). Compared with the radial pattern of sap flow per area, a somewhat different pattern of flow was obtained when the data were integrated over the whole tree by multiplying sap flow per area at different depths by the corresponding area of conducting annuli. Sap flow always approached zero close to the pith and increased significantly with increasing xylem radius in the direction of the cambium. Differences in radial patterns of flow were smaller when expressed per annulus than per xylem cross-sectional area. The large area of conducting wood close to the cambium reflects the greater importance of the outer xylem for supplying water to the whole tree compared with the inner xylem. Inner and outer xylem are often discussed in terms of heartwood and sapwood. The sapwood is generally considered hydroactive, whereas the heartwood is the permanently nonfunctioning, or inactive, part of the xylem. In all study species except E. arborea, we found no visible differences in wood color or in water content along the xylem radius. However, in some specimens the entire xylem area was fully conducting, whereas in other specimens only a part of the xylem area was conductive. Based on these observations and our previous findings (Èermák and Nadezhdina 1998), we conclude that the sapwood itself can be differentiated into conducting and non-

Figure 6. Scheme of the radial pattern of sap flow in trees with fully (e.g., Ilex perado) and partially (e.g., Laurus azorica) conducting sapwood and with fully conducting sapwood but with heartwood (Erica arborea). Sap flow was expressed per area of conducting wood and per cross-sectional area of annulus of conducting wood (taking into account that the measurement interval was 2% of xylem radius) along that radius. Sap flow in all of the annuli was summed to give the total flow for the whole tree.

conducting sapwood. In E. arborea, the only study species in which we could distinguish differences in wood color, the heartwood border, estimated according to a change in wood color, corresponded to the point where sap flow was zero (Figure 6), indicating that the entire sapwood area was functional in this species. We note that sapwood that does not conduct water may be only temporarily blocked, e.g., by embolisms

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RADIAL VARIATION IN SAP FLOW IN LAUREL FOREST TREES

(see Figure 6). Recovery of embolized vessels has been observed in some tree species including sugar maple (Acer saccharum Marsh.) under slight positive pressure (Tyree and Yang 1992) and Scotch pine (Pinus sylvestris L.) (Borghetti et al. 1991, Sobrado et al. 1992, Edwards et al. 1994). Sapwood cross-sectional area is a simple biometric parameter widely used for scaling transpiration data between measurement points and trees and between trees and forest stands. However, given the large variation in the radial pattern of sap flow that we observed, even within the sapwood area, we conclude that the successful use of sapwood cross-sectional area as a scaling parameter will depend on a detailed knowledge of sap flow radial patterns in all sample trees selected for longterm measurements (cf. Swanson 1971, Mark and Crews 1973, Edwards and Booker 1984, Èermák and Nadezhdina 1998). The radial pattern of sap flow has a critical impact on the integration of data from individual measuring points to whole trees and on the scaling up of data from individual trees to entire forest stands. This is true irrespective of the method of sap flow measurement (see reviews by Swanson 1994, Smith and Allen 1996 and Köstner et al. 1998), but particularly if a method is based on measurements of sap flow at one point within the sapwood or integrates sap flow over a part of the stem radius. Sap flow is usually overestimated when sensors are placed only in the outer parts of the sapwood and underestimated when sensors are placed only in the inner parts of sapwood (Èermák and Nadezhdina 1998). The possibility of significant errors associated with the radial position of sensors has also been noted (Clearwater et al. 1999). Conclusions

Broadleaf species and species with needle-like leaves that make up the laurel forest had an almost constant water content along the xylem radius (from the pith to cambium), irrespective of changes in wood color that were apparent in some species (e.g., Erica arborea) at least up to the age of about 50 years. Within a species, the radial pattern of sap flow was variable. Sap flow can take place over the whole cross-sectional xylem area or over only its outer part. Within the conductive part of the xylem, the radial pattern may be almost uniform or may exhibit a peak flow in the outer part of the xylem. Low sap flow rate with a more or less uniform radial pattern was typical for shaded suppressed trees, where a high sap flow rate with a peak was typical for dominant trees. Dense crowns with well illuminated foliage close to the top were associated with a narrow peak in sap flow in the outer part of the xylem. Sparse crowns with more evenly illuminated foliage at different canopy depths were associated with a wide peak in sap flow in the inner part of the xylem. Damage to the conductive part of the stem was reflected in a marked decrease in sap flow and a changed radial pattern. Measurement of the radial flow pattern seems to provide a robust method for proper integration of flow from individual measuring points to the whole tree.

Acknowledgments This work was undertaken on the basis of an agreement between the Universities of La Laguna (Spain) and Brno (Czech Republic). It was supported by DGICYT (Spanish Government) Project No. PB940580 and “Viceconsejería de Educación” (Canarian Government). The stay of J. Èermák and N. Nadezhdina in Tenerife was financed through the agreement made between the University of La Laguna and the “Banco Santander.” We thank Neil Abrey for his help with the English.

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TREE PHYSIOLOGY VOLUME 20, 2000

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