Bleby, T.M., Aucote, M., et al. 1997, 'Seasonal Water Use Characteristics of Tall Wheatgrass [Agropyron elongatum (Host) Beauv.] in a Saline Environment', Plant Cell & Environment, vol. 11, pp. 1361-1371.

It is essential to characterize the water use of plants that have potential for the stabilization of rising saline groundwater which could lead to increases in soil salinity, In this study, several techniques were used to determine the seasonal water use characteristics of the perennial grass Agropyron elongatum (tall wheatgrass) growing in a moderately saline, dryland environment with a fluctuating shallow groundwater table varying in electrical conductivity between 0 and 10 dS m^-1. Soil conditions were examined in terms of water potential measurements, plant water sources were identified using a stable isotope of water (deuterium, ²H) and evapotranspiration was estimated using hydrological and ventilated chamber methods, Seasonal changes in soil water potential were caused by salt accumulation and soil moisture leading to changes in plant water availability, particularly in the surface soil region over summer and autumn, Evapotranspiration in A. elongatum was high over summer during the peak period of growth (4 mm d^-1), with evidence of water use from groundwater and from specific regions of the soil profile, Evapotranspiration was low during the period that A. elongatum was partially senescent in autumn (< 0.5 mm d^-1) and the lowest leaf water potential of -3 MPa that was measured occurred during this period of moderate water stress, Intermediate levels of water use (1.5 mm d^-1) were measured during winter when the entire soil profile was available for water uptake, Based on physiological characteristics, including aspects of summer water use, root morphology and salt tolerance in A. elongatum, we conclude that this species is suitable for stabilizing the level of moderately saline groundwater in parts of southern Australia, which could reduce the potential for soil salinization.

Bleby, T.M., Burgess, S.S.O., et al. 2004, 'A Validation, Comparison and Error Analysis of two Heat-pulse Methods for Measuring Sap Flow in Eucalyptus marginata Saplings', Functional Plant Biology, vol. 36, no. 6, pp. 645-658.

We validated and compared two heat-pulse methods for measuring sap flow in potted Eucalyptus marginata Donn ex. Smith (jarrah) saplings. During daylight hours and under well-watered conditions, rates of sap flow (0.1-0.5 kg h^-1) measured by the established compensation heat-pulse method (CHPM) and the newly developed heat-ratio method (HRM) were similar to rates measured with a weighing lysimeter, and most of the time there was no significant difference (P<0.001) between methods. The HRM accurately described sap flow at night when rates of flow were low (< 0.1 kg h^-1) or near zero, but the CHPM was unable to measure low rates of sap flow due to its inability to distinguish heat-pulse velocities below a threshold velocity of 0.1 kg h^-1 (3-4 cm h^-1). The greatest potential for error in the calculation of daily sap flow was associated with the misalignment of temperature sensors, the estimation of sapwood area and the method used to acquire total sap flow from point measurements of sap velocity. A direct comparison of the two heat-pulse methods (applied synchronously) revealed that the HRM had a more convincing mechanism for correcting spacing errors and was more resistant to random fluctuation in measurements than the CHPM. While we view the HRM more favourably than the CHPM in some key areas, both methods are valid and useful, within their constraints, for measuring transpiration in jarrah and other woody species.

Bucci, S.J., Scholz, F.G., et al. 2004, 'Processes Preventing Nocturnal Equilibration between Leaf and Soil Water Potential in Tropical Savanna Woody Species', Tree Physiology, vol. 24, no. 10, pp. 1119-1127.

The impact of nocturnal water loss and recharge of stem water storage on predawn disequilibrium between leaf (Ψ_L) and soil (ψ_S) water potentials was studied in three dominant tropical savanna woody species in central Brazil (Cerrado). Sap flow continued throughout the night during the dry season and contributed from 13 to 28% of total daily transpiration. During the dry season, Ψ_L was substantially less negative in covered transpiring leaves, throughout the day and night, than in exposed leaves. Before dawn, differences in Ψ_L between covered and exposed leaves were about 0.4 MPa. When relationships between sap flow and Ψ_L of exposed leaves were extrapolated to zero flow, the resulting values of Ψ_L (a proxy of weighted mean soil water potential) in two of the species were similar to predawn values of covered leaves. Consistent with substantial nocturnal sap flow, stomatal conductance (g_s) never dropped below 40 mmol m^-2 s^-1 at night, and in some cases, rose to as much as 100 mmol m^-2 s^-1 before the end of the dark period. Nocturnal g_s, decreased linearly with increasing air saturation deficit (D), but there were species-specific differences in the slopes of the relationships between nocturnal g_s and D. Withdrawal and recharge of water from stem storage compartments were assessed by monitoring diel fluctuations of stem diameter with electronic dendrometers. Stem water storage compartments tended to recharge faster when nocturnal transpiration was reduced by covering the entire plant. Water potential of covered leaves did not stabilize in any of the plants before the end of the dark period, suggesting that, even in covered plants, water storage tissues were not fully rehydrated by dawn. Patterns of sap flow and expansion and contraction of stems reflected the dynamics of water movement during utilization and recharge of stem water storage tissues. This study showed that nighttime transpiration and recharge of internal water storage contribute to predawn disequilibrium in water potential between leaves and soil in neotropical savanna woody plants.

Burgess, S.S.O., Adams, M.A., et al. 2000, 'Measurement of Sap Flow in Roots of Woody Plants: A Commentary', Tree Physiology, vol. 20, no. 13, pp. 909-913.

Measurements of sap flow in roots have recently been used to study patterns of resource acquisition by woody plants; however, the various thermometric methods employed have yielded disparate findings. These findings may be harmonized by accounting for the phenomenon of reverse sap flow in roots. We suggest that only methods capable of measuring slow and reverse rates of flow and that do not require assumptions of zero flow during the night are applicable to studies with roots. The heat ratio method and the constant power heat balance method fit these criteria, whereas the constant temperature heat balance, compensation heat pulse and thermal dissipation methods do not.

Burgess, S.S.O., Adams, M.A., et al. 2001, 'An Improved Heat Pulse Method to Measure Low and Reverse Rates of Sap Flow in Woody Plants', Tree Physiology, vol. 21, pp. 589-598.

The compensation heat pulse method (CHPM) is of limited value for measuring low rates of sap flow in woody plants. Recent application of the CHPM to woody roots has further illustrated some of the constraints of this technique. Here we present an improved heat pulse method, termed the heat ratio method (HRM), to measure low and reverse rates of sap flow in woody plants. The HRM has several important advantages over the CHPM, including improved measurement range and resolution, protocols to correct for physical and thermal errors in sensor deployment, and a simple linear function to describe wound effects. We describe the theory and methodological protocols of the HRM, provide wound correction coefficients, and validate the reliability and accuracy of the technique against gravimetric measurements of transpiration.

Burgess, S.S.O., Adams, M.A., et al. 1998, 'The Redistribution of Soil Water by Tree Root Systems', Oecologia, vol. 115, no. 3, pp. 306-311.

Plant roots transfer water between soil layers of different water potential thereby significantly affecting the distribution and availability of water in the soil profile. We used a modification of the heat pulse method to measure sap flow in roots of Grevillea robusta and Eucalyptus camaldulensis and demonstrated a redistribution of soil water from deeper in the profile to dry surface horizons by the root system. This phenomenon, termed "hydraulic lift" has been reported previously. However, we also demonstrated that after the surface soils were rewetted at the break of season, water was transported by roots from the surface to deeper soil horizons - the reverse of the "hydraulic lift" behaviour described for other woody species. We suggest that "hydraulic redistribution" of water in tree roots is significant in maintaining root viability, facilitating root growth in dry soils and modifying resource availability.

Burgess, S.S.O., Adams, M.A., et al. 2001, 'Correction: An Improved Heat Pulse Method to Measure Low and Reverse Rates of Sap Flow in Woody Plants', Tree Physiology, vol. 21, no. 15, pp. 1157.

Burgess, S.S.O., Adams, M.A., et al. 2001, 'Tree Roots: Conduits for Deep Recharge of Soil Water', Oecologia, vol. 126, no. 2, pp. 158-165.

In previous work, we provided evidence from sap flow measurements that when root systems span soil layers of different moisture content, water is redistributed by roots in the direction of the difference in water potential. In addition to the phenomenon termed "hydraulic lift", where water is redistributed from depth to dry topsoil, the process of "hydraulic redistribution" includes downward transfer of water when the surface layers of soils with low permeability become wet after rainfall. In this paper, we support our previous findings with evidence from measurements of soil water and estimate the quantities of water transferred to depth following rain. Amounts of water stored at depth are not likely to be significant for drought avoidance by plants. However, downward transfer of water may be important to plant establishment and the reduction of waterlogging in certain soil types.

Burgess, S.S.O. and Bleby, T.M. 2006, 'Redistribution of Soil Water by Lateral Roots Mediated by Stem Tissues', Journal of Experimental Botany, vol. 57, no. 12, pp. 3283-3291.

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.

Burgess, S.S.O. and Dawson, T.E. 2004, 'The Contribution of Fog to the Water Relations of Sequoia sempervirens (D. Don): Foliar Uptake and Prevention of Dehydration', Plant Cell & Environment, vol. 27, pp. 1023-1034.

Fog is a defining feature of the coastal California redwood forest and fog inputs via canopy drip in summer can constitute 30% or more of the total water input each year. A great deal of occult precipitation (fog and light rain) is retained in redwood canopies, which have some of the largest leaf area indices known (Westman & Whittaker, Journal of Ecology 63, 493-520, 1975). An investigation was carried out to determine whether some fraction of intercepted fog water might be directly absorbed through leaf surfaces and if so, the importance of this to the water relations physiology of coast redwood, Sequoia sempervirens. An array of complimentary techniques were adopted to demonstrate that fog is absorbed directly by S. sempervirens foliage. Xylem sap transport reversed direction during heavy fog, with instantaneous flow rates in the direction of the soil peaking at approximately 5-7% of maximum transpiration rate. Isotopic analyses showed that up to 6% of a leaf's water content could be traced to a previous night's fog deposition, but this amount varied considerably depending on the age and water status of the leaves. Old leaves, which appear most able to absorb fog water were able to absorb distilled water when fully submersed at an average rate of 0.90 mmol m² s^-1, or about 80% of transpiration rates measured at the leaf level in the field. Sequoia sempervirens has poor stomatal control in response to a drying atmosphere, with rates of water loss on very dry nights up to 40% of midday summer values and rates above 10% being extremely common. Owing to this profligate water use behaviour of S. sempervirens, it appears that fog has a greater role in suppressing water loss from leaves, and thereby ameliorating daily water stress, than in providing supplemental water to foliar tissues per se. Although direct foliar absorption from fog inputs represents only a small fraction of the water used each day, fog's in reducing transpiration and rehydrating leaf tissues during the most active growth periods in summer may allow for greater seasonal carbon fixation and thus contribute to the very fast growth rates and great size of this species.

Burgess, S.S.O. and Dawson, T.E. 2008, 'Using Branch and Basal Trunk Sap Flow Measurements to Estimate Whole-plant Water Capacitance: A Caution', Plant and Soil, vol. 305, pp. 5-13.

Thermometric sap flow sensors are widely used to measure water flow in roots, stems and branches of plants. Comparison of the timing of flow in branches and stems has been used to estimate water capacitance of large trees. We review studies of sap flow in branches and present our own data to show that there is wide variation in the patterns and timing of sap flow of branches in different parts of the crown, owing to the course of daily solar illuminance. In contiguous forest, east-facing and upper branches are illuminated earlier than west-facing and lower branches and most capacitance studies do not include adequate information about branch sampling regimes relative to the overall pattern of crown illuminance, raising questions about the accuracy of capacitance estimates. Measuring only upper branches and normalising these results to represent the entire crown is dangerous because flows at the stem base likely peak in response to maximum crown illuminance (and transpiration) and this will differ compared to the timing of peak flows in upper branches. We suggest that the magnitude of flow lags between branches and stems needs further study, with careful attention to branch position and method application before a robust understanding of capacitance, particularly in woody tissues of large trees, can be formed. We did not detect flow lags in the world's tallest and largest tree species Sequoia sempervirens and Sequoiadendron giganteum, despite measurement along large pathlengths (~57 and 85 m), which raises questions as to why large flow lags are often recorded for much smaller species. One conspicuous possibility is the different methods used among studies. Constant heating methods such as the thermal dissipation probe (and also heat balance methods) include heat capacitance behaviour due to warming of wood tissues, which delays the response of the sensors to changing sap flow conditions. We argue that methods with intrinsic heat-capacitance present dangers when trying to measure water-capacitance in trees. In this respect heat pulse methods hold an advantage.

Burgess, S.S.O., Pate, J.S., et al. 2000, 'Seasonal Water Acquisition and Redistribution in the Australian Woody Phreatophyte, Banksia prionotes', Annals of Botany, vol. 85, no. 2, pp. 215-224.

Sap flows in the xylem of plant roots in response to gradients in water potential, either between soil and atmosphere (transpiration) or soil layers of different moisture content (termed hydraulic redistribution). The latter has the potential to influence water budgets and species interactions, but we lack information for all but a Few plant communities. We combined heat pulse measurements of sap flow with dye and isotope tracing techniques to gauge the movement of xylem sap within, and exudation from, roots of Banksia prionotes (Lindley). We demonstrated 'hydraulic lift' during the dry season and provide some evidence that extremely dry soils limit hydraulic lift. In addition we report difficulties posed by spiralled xylem tissue in roots for the application of heat pulse techniques.

Hultine, K.R., Cable, W.L., et al. 2003, 'Hydraulic Redistribution by Deep Roots of a Chihuahuan Desert Phreatophyte', Tree Physiology, vol. 23, no. 5, pp. 353-360.

Downward redistribution of soil water through plant roots has important consequences for water and nutrient balance of and and semi-arid ecosystems. Nevertheless, information on the seasonal patterns and magnitudes of redistribution is lacking for all but a few plant species. We measured sap flow in the taproot and three main lateral roots of a 10-year-old Juglans major Torr. tree, on an ephemeral catchment in southeastern Arizona, to determine how patterns of redistribution respond to pulses of summer precipitation. Groundwater was beyond rooting depth and a hardpan prevented recharge of surface water to deep soil layers. Reverse flow (hydraulic descent) commenced in the taproot and deep lateral roots in early August after a series of moderate precipitation events, and abruptly ceased after all shallow roots were experimentally severed in mid-August. On some days, hydraulic descent continued in the deep lateral roots during periods of daytime transpiration, and the daily volume of hydraulic descent (deep lateral roots plus taproot) ranged from 10 to nearly 60% of daily transpiration. The persistent pattern of reverse flow demonstrates that, in some plants, water potential gradients from soil to leaf during transpiration are often smaller than those between soil layers within the rooting zone. Hydraulic descent may be an important component of the water balance of phreatophytic trees by facilitating root growth in deep soil layers and by transferring water away from shallow-rooted competitors.

Hultine, K.R., Scott, R.L., et al. 2004, 'Hydraulic Redistribution by a Dominant, Warm-desert Phreatophyte: Seasonal Patterns and Response to Precipitation Pulses', Functional Ecology, vol. 18, no. 4, pp. 530-538.

1. Hydraulic redistribution may have important consequences for ecosystem water balance where plant root systems span large gradients in soil water potential. To assess seasonal patterns of hydraulic redistribution, we measured the direction and rate of sap flow in tap-roots, lateral roots and main stems of three mature Prosopis velutina Woot. trees occurring on a floodplain terrace in semiarid south-eastern Arizona, USA. Sap-flow measurements on two of the trees were initiated before the end of the winter dormancy period, prior to leaf flush.

Hultine, K.R., Williams, D.G., et al. 2003, 'Contrasting Patterns of Hydraulic Redistribution in Three Desert Phreatophytes', Oecologia, vol. 135, no. 2, pp. 167-175.

We measured sap flow in taproots, lateral roots and stems within a single individual in each of three co-occurring tree species in a Chihuahuan desert arroyo to assess the seasonality and magnitude of hydraulic redistribution. Nocturnal reverse flow (hydraulic redistribution) was detected in shallow lateral roots of Fraxinus velutina and Juglans major during periods when surface soils were dry. Reverse flow in the Fraxinus lateral root ranged from near zero to 120 g h^-1, and was inversely correlated with nighttime vapor pressure deficit (D), suggesting that nighttime transpiration may have inhibited hydraulic redistribution. Reverse flow in the Juglans lateral root ranged from near zero to 18 g h^-1. There was no relationship between reverse flow and nighttime D in the Juglans lateral root, despite a weak positive relationship between nighttime D and rates of basipetal flow (flow towards the stem) in the taproot. Reverse flow in Fraxinus and Juglans ceased when surface soils were wetted by monsoon rains and flooding. We found no reverse flow or seasonal variation in root sap flow in Celtis reticulata. However, basipetal sap flow in Celtis roots continued throughout most of the evening, even during periods when D was near zero, and commenced in the morning more than two hours after the onset of sap flow in the main stem. Patterns of nocturnal root sap flow in Celtis may have been facilitated by the diurnal withdrawal from, and refilling of above ground storage compartments (i.e. above ground diurnal storage capacity), which may have prevented hydraulic redistribution. Species differences in nocturnal root function may have significant impacts on ecosystem hydrological fluxes, and should be considered when scaling fluxes to catchment, landscape, and regional levels.

Lee, J.-E., Oliveira, R.S., et al. 2005, 'Root Fuctioning Modifies Seasonal Climate', Proceedings of the National Academy of Sciences of the United States of America, vol. 102, no. 49, pp. 17576-17581.

Hydraulic redistribution (HR), the nocternal vertical transfer of soil water from moister to drier regions in the soil profile by roots, has now been observed in Amazonian trees. We have incorporated HR into an atmospheric general circulation model (the National Center for Atmospheric Research Community Atmospheric Model Version 2) to estimate its impact on climate over the Amazon and other parts of the globe where plants displaying HR occur. Model results show that photosynthesis and evapotranspiration increase significantly in the Amazon during the dry season when plants are allowed to redistribute soil water. Plants draw water up and deposit it into the surface layers, and this water subsidy sustains transpiration rates that deep roots alone cannot accomplish. The water used for dry season transpiration is from the deep storage layes in the soil, recharged during the previous wet season. We estimate that HR increases dry season (July to November) transpiration by about 40% over the Amazon. Our model also indicates that such an increase in transpiration over the Amazon and other drought-stressed regions affects the seasonal cycles of temperature through changes in latent heat, thereby establishing a direct link between plant root functioning and climate.

Oliveira, R.S., Dawson, T.E., et al. 2005, 'Evidence for Direct Water Absorption by the Shoot of the Desiccation-tolerant Plant Vellozia flavicans in the Savannas of Central Brazil', Journal of Tropical Ecology, vol. 21, pp. 585-588.

Oliveira, R.S., Dawson, T.E., et al. 2005, 'Hydraulic Redistribution in Three Amazonian Trees', Oecologia, vol. 145, pp. 354-363.

About half of the Amazon rainforest is subject to seasonal draughts of 3 months or more. Despite this drought, several studies have shown that these forests, under a strong seasonal climate, do not exhibit significant water stress during the season. In addition to deep soil water uptake, another contributing explanation for the absence of plant water stress during drought is the process of hydraulic redistribution; the nocturnal transfer of water by roots from moist to dry regions of the soil profile. Here, we present data on patterns of soil moisture and sap flow in roots of three dimorphic-rooted species in the Tapajós Forest, Amazônia, which demonstrate both upward (hydraulic lift) and downward hydraulic redistribution. We measured sap flow in lateral and tap roots of our three study species over a 2-year period using the heat ratio method, a sap-flow technique that allows bi-directional measurement of water flow. On certain nights during the dry season, reverse or acropetal flow (i.e., in the direction of the soil) in the lateral roots and positive or basipetal sap flow (toward the plant) in the tap roots of Coussarea racemosa (caferana), Manilkara huberi (maçaranduba) and Protium robustum (breu) were observed, a pattern consistent with upward hydraulic redistribution (hydraulic lift). With the onset of heavy rains, this pattern reversed, with continuous night-time acropetal sap flow in the tap root and basipetal sap flow in lateral roots, indicating water movement from wet top soil to dry deeper soils (downward hydraulic redistribution). Both patterns were present in trees within a rainfall exclusion plot (Seca Floresta) and to a more limited extent in the control plot. Although hydraulic redistribution has traditionally been associated with arid or strongly seasonal environments, our findings now suggest that it is important in ameliorating water stress and improving rain infiltration in Amazonian rainforests. This has broad implications for understanding and modeling ecosystem process and forest function in this important biome.

Scholz, F.G., Bucci, S.J., et al. 2002, 'Hydraulic Redistribution of Soil Water by Neotropical Savanna Trees', Tree Physiology, vol. 22, no. 9, pp. 603-612.

The magnitude and direction of water transport by the roots of eight dominant Brazilian savanna (Cerrado) woody species were determined with a heat pulse system that allowed bidirectional measurements of sap flow. The patterns of sap flow observed during the dry season in species with dimorphic root systems were consistent with the occurrence of hydraulic redistribution of soil water, the movement of water from moist to drier regions of the soil profile via plant roots. In these species, shallow roots exhibited positive sap flow (from the soil into the plant) during the day and negative sap flow (from the plant into the soil) during the night. Sap flow in the taproots was positive throughout the 24-h period. Diel fluctuations in soil water potential, with maximum values occurring at night, provided evidence for partial rewetting of upper soil layers by water released from shallow roots. In other species, shallow roots exhibited negative sap flow during both the day and night, indicating that hydraulic redistribution was occurring continuously. A third sap flow pattern was observed at the end of the dry season after a heavy rainfall event when sap flow became negative in the taproot, and positive in the small roots, indicating movement of water from upper soil layers into shallow roots, and then into taproots and deeper soil layers. Experimental manipulations employed to evaluate the response of hydraulic redistribution to changes in plant and environmental conditions included watering the soil surface above shallow roots, decreasing transpiration by covering the plant and cutting roots where probes were inserted. Natural and manipulated patterns of sap flow in roots and stems were consistent with passive movement of water toward competing sinks in the soil and plant. Because dry shallow soil layers were often a stronger sink than the shoot, we suggest that the presence of a dimorphic root system in deciduous species may play a role in facilitating leaf expansion near the end of the dry season when the soil surrounding shallow lateral roots is still dry.

Williams, D.G., Cable, W., et al. 2004, 'Evapotranspiration Components Determined by Stable Isotope, Sap Flow and Eddy Covariance Techniques', Agricultural and Forest Meteorology, vol. 125, no. 3-4, pp. 241-258.

Understanding and modeling water exchange in and and semiarid ecosystems is complicated by the very heterogeneous distribution of vegetation and moisture inputs, and the difficulty of measuring and validating component fluxes at a common scale. We combined eddy covariance (EC), sap flow, and stable isotope techniques to investigate the responses of transpiration and soil evaporation to an irrigation event in an olive (Olea europaea L.) orchard in Marrakech, Morocco. The primary goal was to evaluate the usefulness of stable isotope measurements of water vapor in the turbulent boundary layer for partitioning evapotranspiration under such dynamic conditions. The concentration and deuterium isotope composition (δ²H) of water vapor was collected from different heights within the ecosystem boundary layer of the olive canopy before and over several days following a 100mm surface irrigation. 'Keeling plots' (isotope turbulent mixing relationships) were generated from these data to estimate the fractions of evaporation and transpiration contributing to the total evapotranspiration (ET) flux. Transpiration accounted for 100% of total ET prior to irrigation, but only 69-36% of ET during peak midday fluxes over the 5-day period following irrigation. The rate of soil evaporation and plant transpiration at the stand level was calculated from eddy covariance measurements and the evaporation and transpiration fractions from isotope measurements. Soil evaporation rate was positively correlated with daily atmospheric vapor pressure deficit (D), but transpiration was not. Component fluxes estimated from the isotope technique were then compared to those obtained from scaled sap flow measurements. Sap flow in multiple-stemmed trees increased following the irrigation, but large single-stemmed trees did not. We matched the source area for eddy covariance estimates of total ET fluxes with scaled sap flow estimates developed for the different tree types. Soil evaporation was determined from the difference between total ET and the scaled sap flow. Ecosystem-level transpiration and soil evaporation estimated by the isotope approach were within 4 and 15% of those estimated by scaled sap flow, respectively, for periods of peak fluxes at midday. Our data illustrate the utility of the isotope 'Keeling plot' approach for partitioning ET at the ecosystem scale on short time steps and the importance of accurate spatial representation of scaled sap flow for comparison with eddy covariance measurements of ET.

Zeng, F., Bleby, T.M., et al. 2006, 'Water and Nutrient Dynamics in Surface Roots and Soils are not Modified by Short-term Flooding of Phreatophytic Plants in Hyperarid Desert', Plant and Soil, vol. 279, pp. 129-139.

Little is known of the mechanisms employed by woody plants to acquire key resources such as water and nutrients in hyperarid environments. For phreatophytic plants, deep roots are necessary to access the water table, but given that most nutrients in many desert ecosystems are stored in the upper soil layers, viable shallow roots may be equally necessary for nutrient uptake. We sought to better understand the interaction between water and nutrient uptake from soil horizons differing in the relative abundance of these resources. To this end, we monitored plant water and nutrient status before and after applying flood irrigation to four phreatophytic perennial plant species in the remote hyperarid Taklamakan desert in western China. Sap flow in the roots of five plants of the perennial desert species Alhagi sparsifolia Shap., Karelina caspica (Pall.) Less., Calligonum caput medusea Schrenk, and Eleagnus angustifolia Hill. was monitored using the heat ratio method (HRM). Additionally we measured predawn and midday water potential, foliar nitrate reductase activity (NRA), xylem sap nutrient concentration and the concentration of total solutes in the leaves before, 12 and 96 h after flooding to investigate possible short-term physiological effects on water and nutrient status. Rates of sap flow measured during the day and at night in the absence of transpiration did not change after flooding. Moderately high rates of sap flow (HRM heat pulse velocity, 5-25 cm h^-1) detected during the day in soils that had a near zero water content at the surface indicated that all species had contact to groundwater. There was no evidence from sap flow data that plants had utilised flood water to increase maximum rates of transpiration under similar climatic conditions, and there was no evidence of a process to improve the efficiency of water or nutrient uptake, such as hydraulic redistribution (i.e. the passive movement of water from moist soil to very dry soil via roots). Measurements of plant water status, xylem sap nutrient status, foliar NRA and the concentration of osmotically active substances were also unaffected by flood irrigation. Our results clearly show that groundwater acts as the major source of water and nutrients for these plants. The inability of plants to utilise abundant surface soil–water or newly available nutrients following irrigation was attributed to the absence of fine roots in the topsoil layer.