JXB Advance Access originally published online on August 22, 2006
Journal of Experimental Botany 2006 57(12):3283-3291; doi:10.1093/jxb/erl085
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RESEARCH PAPER |
Redistribution of soil water by lateral roots mediated by stem tissues
1School of Plant Biology, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009 Australia
2Cooperative Research Centre for Plant-Based Management of Dryland Salinity, 35 Stirling Highway, Crawley, WA 6009, Australia
*To whom correspondence should be addressed. E-mail: ssb{at}cyllene.uwa.edu.au
Received 23 February 2006; Accepted 20 June 2006
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Abstract |
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Evidence is increasing to suggest that a major activity of roots is to redistribute soil water. Roots in hydraulic contact with soil generally either absorb or lose water, depending on the direction of the gradient in water potential between root and soil. This leads to phenomena such as ‘hydraulic lift’ where dry upper soil layers drive water transfer from deep moist layers to the shallow rhizosphere and, after rain or surface irrigation, an opposite, downward water transfer. These transport processes appear important in environments where rainfall is strongly seasonal (e.g. Mediterranean-type climates). Irrigation can also induce horizontal transfers of water between lateral roots. Compared with transpiration, the magnitudes, pathways, and resistances of these redistribution processes are poorly understood. Field evidence from semi-arid eucalyptus woodlands is presented to show: (i) water is rapidly exchanged among lateral roots following rain events, at rates much faster than previously described for other types of hydraulic redistribution using sap flow methods; (ii) large axial flows moving vertically up or down the stem are associated with the horizontal transfer of water between roots on opposite sides of the stem. It appears that considerable portions of the stem axis become involved in the redistribution of water between lateral roots because of partial sectoring of the xylem around the circumference of these trees.
Key words: Hydraulic redistribution, root systems, sap flow, soil water, xylem
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Introduction |
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The idea that plant root systems can act as ‘equalisers’ of soil moisture dates back more than 70 years (Breazeale, 1930; Caldwell et al., 1998). Early evidence that roots redistribute water within the rhizosphere came from greenhouse and laboratory measurements of water transfer across root systems that spanned physically separated soil compartments, or so-called ‘split-root’ experiments (Baker and Van Bavel, 1986; Sakuratani et al., 1999; and see Caldwell et al., 1998, for a review).
The first observations of roots acting as conduits for water transfer within the rhizosphere of natural ecosystems were made by Richards and Caldwell in 1987 (Richards and Caldwell, 1987). These authors used thermocouple psychrometers to measure increases in the water potential of shallow soil layers during the night (i.e. in the absence of transpiration to drive water absorption and soil moisture depletion). They concluded that water was being absorbed by deep roots with access to moist soil and then lost from shallow roots into dry surface soil. This phenomenon, which they termed ‘hydraulic lift’, substantially improved our understanding of the ecological importance of water transfer by roots. Since then, a succession of field studies, using an ever-broadening range of techniques, has added considerable detail to our understanding of hydraulic redistribution, including the role of species (Espeleta et al., 2004), soil type (Yoder and Nowak, 1999), root morphology (Hultine et al., 2003b), and weather (Williams et al., 1993), to name just a few examples.
Stable isotope tracing (Dawson, 1993) and sap flow measurements in roots (Burgess et al., 1998) have also brought important advances to our understanding of the dynamics of water transfer by root systems. Burgess et al. (1998) used sap flow measurements in roots and Schulze et al. (1998) used stable isotope labelling techniques to demonstrate that hydraulic lift reverses direction if surface soil layers become wetter than deep soil layers following rain or irrigation (Smith et al., 1999). To reflect the multidirectional nature of water transfer by root systems, the term hydraulic redistribution was coined to include both hydraulic lift and downward transfer of water following rain (Burgess et al., 1998).
Underscoring the multidirectional nature of water transfer by roots, Brooks et al. (2002) provided evidence of both vertical and horizontal transfer of water by roots of two conifer trees. Deuterium-labelled water was detected horizontally from a partial irrigation of the root system and provided a field-based example to indicate horizontal redistribution. Similarly, a recent study by Smart et al. (2004) demonstrated both downward and horizontal (or ‘transverse’) redistribution following partial irrigation of grapevines with deuterated water. The results of this study and that of Brooks et al. (2002) suggest that horizontal redistribution of water may be more limited than vertical redistribution owing to lateral roots on opposing sides of stems being connected by pathways of relatively high resistance at the stem base.
Little is know about the horizontal redistribution of water under field conditions and, more generally, how water moves within plants in the presence of multiple sources and sinks joined by pathways of varying resistances. For example, it is not clear whether roots growing in dry soil are equal competitors for water with above-ground organs, given that the axial orientation of xylem vessels favours water movement from root to shoot, rather than from root to root. Smart et al. (2004) recently provided further information on this topic by using a stable isotope tracer to show that water moves ‘around’ or ‘across’ the stem from roots on an irrigated side of a grape vine to roots on a non-irrigated side. Although deuterium-labelled irrigation water did not appear in soils on the non-irrigated side of the vine until one week after application, deuterium-labelled water injected directly into the stem of the vine moved rapidly around/across the stem and into roots on all sides of the stem.
Taken as a whole, the broad suite of experimental data clearly indicates that water can be redistributed by root systems in any direction, but always according to the gradient in water potential. An important caveat is that the hydraulic connectivity and conductivity between various roots at the stem base will probably determine the rates of transfer.
A clear understanding of the root–stem and root–root pathways requires information on a number of aspects of wood anatomy. For example, in angiosperms, long vessels and perforated end-plates of comparatively low resistance yield hydraulic conductivities that are much greater along vessels than between vessels (where water must move via pits). Since xylem vessels are typically aligned axially, flow along vessels usually means flow along the axis of a plant, whereas flow between vessels is ‘radial’ or ‘circumferential’. The relative importance of these flow vectors will vary according to different vessel lengths and diameters (e.g. as found in roots versus stems; Pate et al., 1995), different pit and end-plate resistances, and a range of other anatomical factors (Orians et al., 2005). Spiralling of xylem vessels (Howard, 1932) will also alter flow distributions (Brooks et al., 2002). Flow paths may be further altered by the presence of heartwood, latewood, or rays, which may partly or completely interrupt flow in radial or circumferential directions. Zwieniecki et al. (2003) also shows that flows via pits will be strongly ion-mediated.
In the present study, field measurements of xylem sap flow were made to gain an insight into how water may be exchanged between lateral roots connected to different sides of a tree. Given that wood is essentially a porous material, the guiding hypothesis in this study was that water might flow in practically any direction within wood tissues, but the dominant directions will be dictated by the orientation and physical properties of xylem vessels, including the location of ‘non-conductive’ tissues such as heartwood. The aim was to elucidate patterns of water uptake and redistribution in lateral roots by carefully measuring flow vectors through roots and stems.
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Materials and methods |
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Site description
The research was conducted at the Corrigin Water Reserve approximately 2 km west of Corrigin, Western Australia (32°19' S, 117°52' E). This reserve consists of 1096 ha of remnant vegetation containing a range of plant communities typical to Western Australia's wheat-belt region. Low-lying areas have heavy soils and are sparsely wooded. The study site was chiefly a mixture of wandoo (Eucalyptus wandoo Blakely) and salmon gum (Eucalyptus salmonophloia F. Muell.) trees ranging in height from 10 m to 30 m, with a scattered dwarf scrub understorey (e.g. Olearia, Grevillea spp.). This type of woodland is widespread throughout the semi-arid region of southwestern Australia (Beard, 1990), but little information is available on rooting habit (Hobbs and O'Connor, 1999). Lamont (1985) recorded that E. wandoo saplings have root systems dominated by a taproot, whereas mature trees are dominated by an extensive system of lateral roots extending out approximately 10 m at 20–40 cm depth. These findings agree with our excavations of both wandoo and salmon gum saplings and observations of wind-thrown mature specimens.
Surface soils at the site were a light brown sandy loam, which altered rapidly to a heavy clay–loam subsoil. The long-term (since 1910) average annual rainfall for Corrigin is 376 mm, with an annual pan evaporation of 1817 mm (Bureau of Meteorology, Australia). A Mediterranean-type climate prevails, with most rain falling in the cooler winter months.
Sap flow measurements
The heat ratio method (Burgess et al., 2001a, b) was used to measure sap flow in stems and roots of three wandoo and three salmon gum trees of similar size (
40–60 cm diameter at breast height). In December 2003, 16 probe sets were installed to instrument the six trees in a fashion typical for transpiration estimates. Three probes were inserted 120° apart at 1.3 m height around the circumference of each tree. Only two probes 180° apart were available for the third individual of each species. In all cases, probes were sent via AM 16/32 multiplexers to CR10X dataloggers (Campbell Scientific Inc. Logan, Utah, USA). Burgess and Dawson (2004) provide a detailed description of a similar system.
In July 2004 a further 40 probe sets were progressively installed to monitor specific patterns of sap flow in individual trees; 35 of the 40 probes were installed in two salmon gums. Soil was excavated around the base of these two trees, exposing 7–8 major lateral roots fairly evenly distributed around the circumference of each tree. Although all roots could be considered ‘lateral’, some roots had a steeper angle of descent than others. Due to the size of the trees and the extremely hard soil, it was only possible to excavate to a depth of approximately 60 cm. As a result, information on the existence of a taproot or sinker roots could not be obtained for these specimens, but the findings of Lamont (1985) for the closely related E. wandoo and our own observation of wind-thrown E. salmonophloia suggest that a large taproot was unlikely. A single sap flow sensor was placed in each lateral root and a ring of eight further probes was installed in the stem of each tree (1 m above the roots), evenly spaced at 45° intervals around the circumference.
Overall, a total of 56 probe sets were used during this experiment, connected to five separate and electrically isolated dataloggers. For reference purposes, spot checks of the behaviour of the instruments were also made using commercially available devices, including a HRM sensor (HRM-30, ICT International, Armidale, Australia) and compensation heat pulse sensor (SF-100, Greenspan Technology, Warwick, Australia). The latter instrument, which can only measure flow unidirectionally (Burgess et al., 2000a), was installed upside-down with the aim of confirming possible flow reversals measured with the HRM devices.
During the course of the sap flow measurements, a number of experimental treatments were initiated to test their effects on data collected using the HRM. Each treatment was generally made on one probe for each of the salmon gum trees being monitored (n=3) and included (i) completely removing all bark down to the xylem from a few cm around the site of probe insertion, (ii) completely removing all bark and then sealing around the probe with waterproof putty, (iii) affixing a plastic rain shield to protect probes from rainfall and stem flow, and (iv) completely severing the xylem around individual probes. This last treatment was done by making a U-shaped cut 15 cm wide, 15 cm high, and 10 cm deep into the wood of the trunk around the probe.
Aside from five probe sets subject to the xylem severing treatment above, the majority of sap flow data were not corrected for zero bias using the methods outlined by (Burgess et al., 2001a), which again would involve cutting the xylem. This decision was made to permit further collection of data from most probes beyond the course of the present investigation. Our method of calculating a zero offset for calibration purposes was to use night-time values at least 3 weeks after any rain event, when the analysis of flow trajectories indicated rain-induced redistribution processes had attenuated completely. Since night-time transpiration can affect night-time flow rates (Benyon, 1999; Snyder et al., 2003, note two spikes at approximately day 11 and day 31 of Fig. 4 due to nights of very low humidity), only night-time values from periods of high humidity were used.
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Meteorological measurements
A tipping bucket rain gauge (CS701), leaf wetness sensor (model 237), and temperature/relative humidity sensor (CS500) were also attached to a CR10X datalogger (all devices Campbell Scientific Inc.) to monitor weather conditions.
Soil moisture measurements
On one occasion in June 2005 following a period of considerable rainfall, soil moisture samples were collected to a depth of 40 cm depth using a hand auger. Three replicate samples were collected 5 m from the base of the tree depicted in Fig. 3 at all four cardinal positions. Gravimetric soil water content was calculated by weighing and then drying samples to a constant weight at 70 °C.
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Results |
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The measurements of sap flow in roots showed that immediately following rain, the whole course of flow in certain lateral roots would rapidly elevate, whilst in other roots, flows would rapidly decrease and even reverse. These contrasting patterns were most obvious at night: uptake of water continued in some roots despite the absence of transpiration, whilst flows in other roots completely reversed. Figure 1A shows average night-time flow rates in seven different lateral roots around the circumference of a single tree. After each rain event, this pattern of elevated flows in some roots and reverse flows in other roots would slowly attenuate over a number of days until ‘normal’ (i.e. flows corresponding only to the typical course of transpiration, i.e. no reverse or elevated flows at night) patterns of flow resumed. The amount of time taken for this to occur scaled approximately with the size of each rain event (see Fig. 4, which indicates recovery times ranging from >10 d for large rain events and
2 d for small events). The locations of roots with elevated positive flows and roots with reverse flows were more or less restricted to two distinct ‘hemispheres’ around the circumference of the tree. Roots in northern and eastern sectors of the tree tended to have positive flows in response to rain, whilst roots in southern and western sectors showed reverse flows (see Fig. 3). Between these two ‘hemispheres’ were transitional regions where flows were not altered by rain.
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Measurements of sap flow using eight probe sets spaced around the circumference of stems showed an almost identical pattern to that measured in roots (Fig. 1B). For all sensors, there was broad correspondence between the flow patterns in the various sectors of the stem and those of the individual roots radiating out, not directly beneath, but adjacent to each stem sector. For example, flows in the N sector of the stem corresponded well with flows measured in a lateral root in the NE sector (compare Fig. 1A, B). Similarly, flows in the W sector of the stem flows showed good agreement with flows in a lateral root in the NW sector (Fig.1A, B). Data for the other salmon gum tree, that had a similar arrangement of instruments, followed a broadly similar pattern and for simplicity are not discussed in further detail (Fig. 2A, B). Probe locations, and the approximate magnitude and direction of flows measured by each probe in the first tree are summarized diagrammatically in Fig. 3.
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A long-term record of night-time sap flow in the stem of the first salmon gum tree (S1) is shown in Fig. 4. Throughout the year, rainfall always produced the same sap flow dynamics as documented in Figs 1–3. Again, the magnitude of changes to sap flow patterns in stems scaled with the size of the rain event. Consecutive rain events tended to produce an additive effect. The large, isolated summer rainfall event seen in the early part of the record in Fig. 4 gives an indication of the maximum duration an individual rain event altered sap flow patterns in stems (10–12 d).
Experiments to exclude the possibility that flow changes were due to factors other than xylem sap flow showed that neither complete removal of all bark tissues, nor waterproofing around probes with putty, nor adding plastic rain shields altered the phenomenon described above (data not shown). The final treatment completely severed the active xylem surrounding four probe sets. Cutting the xylem immediately stopped all flow dynamics, including the changes caused by rain (Fig. 5; days 177 onwards). Spot measurements with both the HRM-30 and SF-100 devices also broadly confirmed the magnitude and direction of flows (including large reverse flows) measured with the HRM devices used here (data not shown).
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Soil moisture measurements
Soil moisture was significantly greater at the north and east sampling positions compared with the south and west positions around the tree depicted in Fig. 3. Values were 11.6% for N, 11.6% for E, 8.8% for S, and 8.5% for W. Samples at the N position differed significantly from S and W at the 0.1 level (Student's t test), whereas samples at the E position differed significantly from S and W at the 0.01 level.
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Discussion |
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Patterns of sap flow among lateral roots on opposing sides of the salmon gum trees following rain showed a donor–recipient pattern similar to hydraulic redistribution between taproots and lateral roots as extensively documented in the literature using a range of thermometric techniques (Burgess et al., 1998; Smith et al., 1999; Scholz et al., 2002; Hultine et al., 2003a). Although hydraulic redistribution between lateral roots has been demonstrated in greenhouse-based split-root experiments (Sakuratani et al., 1999) or induced by irrigation in field-based studies (Brooks et al., 2002), the authors are not aware of similar reports of lateral redistribution resulting from rain.
What drives water movement between lateral roots?
Hydraulic redistribution in any form requires that a root system span soil layers (or compartments/patches) of different water potential. Our simple survey of soil moisture contents on different sides of a single tree demonstrated that considerable variability in water contents (and by inference soil water potentials) of the upper soil layers is possible, and this variability was in agreement with the direction of hydraulic redistribution observed here. The reasons for soil moisture variability around single trees remain unclear, but may include rain shadows, patches of non-wetting soil, or varying soil textures. Soils in this region typically exhibit strong texture contrasts from the A to B horizon and variable subsurface relief (Tennant et al., 1992) can alter at what depth a wetting front encounters heavy subsoils. During excavations, pockets of extremely dry soil were often encountered even after months of heavy winter rain.
In addition to soil variability, lateral rooting depth may vary in the vertical plane. It is possible that some lateral roots may support sinker roots that have penetrated the heavier subsoils by following macropores. Such lateral roots would then function partly as a taproot, and ‘similar-looking’ laterals on different sides of a tree may be functionally quite different. Given the case where the direction of flow in the SW-oriented root (Fig. 3) is at odds with roots on immediately adjacent sides, it could be surmised that patterns of redistribution in each individual tree would be a complex interaction of soil moisture distribution and root distribution. Despite the exception of the SW-oriented root of the tree depicted in Fig. 3, the distinct ‘sidedness’ seen for this and the second salmon gum measured similarly suggests hypotheses such as rain shadow effects are worth further investigation.
Sap flow patterns in stems
Installation of eight probes around the circumference of two salmon gum trees demonstrated substantial spatial variation in night-time water flows in response to rain, which followed a distinct temporal progression. The distribution of different flow vectors followed a regular pattern around the stem. Nearly every 45° sector of the stem had a night-time rate of sap flow that was slightly different to adjacent sectors. Although considered beyond the scope of this present investigation, it was noted that the direction and magnitude of flows measured deeper in the xylem (data not shown) did not always match those measured in the outer xylem, adding a complexity which suggests radial sectoring in addition to circumferential sectoring. Both circumferential and radial sectoring of flow pathways have previously been demonstrated using sap flow techniques (Nadezhdina, 1999).
The overall pattern of night-time sap flow detected in stems following rain consisted of (at least for the outer xylem) large positive flows in sectors on one side of the tree grading smoothly via sectors of neutral-flow to maximum negative flows in sectors on the other side of the tree. To our knowledge, such night-time flow dynamics have not been recorded for stems before. Possible disturbance to the measurements from such influences as stem flow, changes to bark moisture content, water entry into probe drill-holes, or datalogger grounding problems was ruled out experimentally. Critically, xylem-severing experiments demonstrated that flow patterns were solely dependent on the continuity of xylem tissues surrounding the probes (bark removal had no effect), confirming that xylem water flux was responsible for the measured flow patterns. Further confirmation of the magnitudes and directions of flow measured by these sap flow systems by two commercial sap flow devices is significant, since the SF-100 uses a different principle of measurement (see a description of the compensation heat pulse method by Burgess et al., 2000a), while the HRM-30 has the same measurement principle but an entirely different electronic platform.
The tight coupling of the simultaneous positive and negative flows in stems during the night strongly suggests a donor–recipient relationship between flows of the same type measured in the roots of these trees. In fact, the direction and magnitude of flows in each stem sector closely corresponded to those of roots, not immediately below each sector, but at approximately 45° rotation to the stem. Given the similarity of flow patterns in the stem with those of the roots, and allowing for some spiralling of stem tissues (Howard, 1932), it is argued that the flows measured in the stem were the result of exchange of water between roots, as described below.
What is the mechanism for transfer of water among poorly-connected lateral roots?
Under the assumption that a specialized ‘junction zone’ (Burgess et al., 2000b) does not exist at the stem base to facilitate water exchange among roots, a number of other pathways might be considered. Firstly, the possibility of a radial exchange of water between different sides of these mature trees can be excluded: tests with dye infiltration readily demonstrate that heartwood blocks the majority of this pathway. Secondly, flows around the circumference of the tree may be possible via intervessel pits. If this pathway were extremely conductive, one might expect rapid flows in a narrow circumferential band immediately connecting the insertion points of each root. However, compared with axial flow, it is more difficult for water to move sideways through stem tissues, since it will encounter a dense succession of vessel walls (and fibre cells) and must in each case traverse pit membranes of comparatively high resistance (Sperry and Hacke, 2004).
A third possibility is thus raised: if circumferential water movement is largely limited by pit membrane resistance, one means of increasing conductivity in this direction is to increase the cross-sectional area over which water can flow through pits. Given the comparatively high axial conductivity of xylem, this can readily be achieved via the axial movement of water. For example, assume one lateral root acts as a sink for water, exerting, for example, a hydrostatic tension of 3 MPa on the rest of the tree. Because of low axial resistance, there is only a small gradient in the water potential along the paths connecting this root to the rest of the tree. Hence a tension of 3 MPa exerted at the base of the stem that might induce circumferential water movement will produce a comparable driving force for further circumferential water movement a number of metres higher up the stem. The same rules apply to the other side of a tree, where a lateral root acting as a source of water (e.g. a tension of only 1 MPa) is connected. Thus, circumferential flow will be driven across a plane that extends a number of metres above soil level. Ultimately water that moves circumferentially must also move axially from the ‘source’ to the ‘sink’, both of which are at the base of the tree. In other words, a net circumferential transport of water ought to induce a considerable accompanying axial flow. Figure 6 is a simple cartoon of this concept of circumferential transport of water between poorly-connected roots on opposite sides of a stem as mediated by substantial axial flows.
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As support for this idea that circumferential transport must occur over a considerable axial plane, it is worth remembering that the survival of trees in double saw cut experiments depends on the cuts being made a considerable vertical distance apart (Orians et al., 2005). Further support comes from the findings of Schulte (2006), who was only able to draw water from one branch of a root to another if a considerable length of the parent root was retained to facilitate the water transfer. Just how large the axial plane along which circumferential transport occurs is in E. salmonophloia and E. wandoo remains to be measured; preliminary sap flow measurements at 4 m height suggest tissues at this height are still partly involved. The amount of axial flow relative to circumferential transport should reflect the ratio of axial to circumferential hydraulic conductivity of xylem, which depends on a range of anatomical factors including vessel diameter, frequency of contact between vessels and the number and conductivity of intervessel pits (Domec et al., 2005; Orians et al., 2005). The fact that E. salmonophloia demonstrates distinct flow paths over a number of metres of stem tissue suggests xylem of this species is highly sectored rather than integrated.
How widespread is this phenomenon?
To date, stem-mediated hydraulic redistribution has been observed in five species: Eucalyptus salmonophloia, E. wandoo, E. astringens Maiden, E. horistes L. Johnson & K. Hill (K Brooksbank, unpublished data), and E. polybractea RT. Baker (J Carter, unpublished data). E. astringens has a ‘mallet’ growth form (slender-stemmed tree with steeply angled branches), while E. horistes and E. polybractea have the ‘mallee’ growth form (woody species with multiple stems arising from a lignotuber). These three species and the two tree-form eucalypts described in the present study are all endemic to semi-arid areas of Western Australia, and all grow on heavy soils.
In terms of this study's work, sap flow has been measured in a number of other species using the HRM, including other eucalypts, but this phenomenon has not been detected before. The absence of this phenomenon in gymnosperm species such as Sequoia sempervirens D. Don (S Burgess, unpublished data) might be expected, since tracheid structures are unlikely to give axial flow such a strong advantage over flow in other directions. Its absence in other Eucalyptus species will need to be explained in terms of rooting architecture, soil properties, and xylem structure.
In terms of the broader context of research on sap flow using a range of techniques in a range of species and ecosystems, it is not certain as to whether this phenomenon is more widespread, but in some cases has not been detected with the methods employed. Many of the previous studies on Eucalyptus species (Barrett et al., 1995) have used commercially available compensation heat pulse techniques that have poor resolution at the lower end of the measurement range (Becker, 1998) and cannot detect flow reversals (Burgess et al., 2000a). These techniques have been used previously in E. salmonophloia by Farrington et al. (1994) who noted rapid responses to rain, but provided no information on night-time sap flow rates. Similarly, the thermal dissipation probe or ‘Granier method’ (Granier, 1985), which has been popular in many North American studies, is sometimes used in a mode that resets minimum night-time flows to zero every night. This approach would obscure any night-time dynamics as reported in the present study. Thus, the question remains as to how widespread this phenomenon is, and whether species or soil-type is the most important factor generating this type of water flow in tree stems.
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Conclusions |
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Although Brooks et al. (2002) and Smart et al. (2004) have demonstrated the possibility of lateral redistribution of water under field conditions following irrigation, the findings presented here are, as far as is known, the first example of the transfer of water between lateral roots as a natural phenomenon following rain. Furthermore, the magnitudes of sap flow resulting from the exchange of water between roots in these large trees were considerably greater than those in any other studies the authors are presently aware of that used comparable techniques. Speeds of water flow in roots at night approached rates more typical of those found in stems during peak transpiration. (Here we mean peak flows in the absence of this phenomenon. Since flows caused by transpiration are additive to flows caused by hydraulic redistribution, peak rates under such conditions can be higher than is ‘typical’ and will always be greater than any night-time flow rates.) These rapid flows suggest fairly large volumes of soil water are being redistributed, with potentially large ecological and hydrological implications for these water-limited ecosystems.
Secondly, it is believed that this is the first time that large axial flows involving many metres of stem tissue have been implicated as part of the transport pathway that permits water to be transferred between roots. Night-time flow rates in the stems were not much slower than those measured in roots; clearly, exchange of water between roots is a non-trivial transport role for stem tissues in some species. Further work is needed to determine its prevalence in woody plants, i.e. does it reflect certain root architectures, growth forms, soil types or climatic factors? Subsequent research should focus on the relative hydraulic conductivities of xylem in the radial, circumferential, and axial planes. Such knowledge should provide further insights into flow paths and source–sink relationships within a plant.
Finally, a broader survey of species/ecosystems where this process may be relevant would be beneficial to assess its implications for typical transpiration studies that utilize sap-flow gauges installed in stems. Where axial water movement that is not the result of transpiration is present, either very careful circumferential sampling strategies, or protocols to separate the two processes, will be needed in order to estimate total plant water use.
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Acknowledgements |
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We wish to thank the Cooperative Research Centre for Plant-Based Management of Dryland Salinity and the Australian Research Council for financial assistance. We thank Perry Swanborough and Patrick Mitchell for assistance in the field, Hans Lambers, Erik Veneklaas, Mark Adams, Jenny Carter, and Kim Brooksbank for helpful discussions. Thanks also to Robin Campbell at the Corrigin Land Care Office and the Ecosystems Research Group (UWA School of Plant Biology) for general assistance.
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Footnotes |
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Present address: Department of Biology, Duke University, Durham, NC 27708, USA. 
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References |
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