Heat Pulse Velocity Technique Research Articles

Calibration of compensation heat pulse velocity technique for measuring transpiration of selected indigenous trees using weighing lysimeters

The compensation heat pulse velocity (CHPV) is one of the most widely used methods to measure sap flow in woody plants. However, the accuracy of this method has not been fully explored especially for indigenous tree species of South Africa. The aim of this study was to evaluate the accuracy of the CHPV method in quantifying tree transpiration for selected tree species. Three indigenous trees sampled in a monolith form; black karee (Sersia lancea), buffalo thorn (Ziziphus mucronata) and wild olive (Olea africana) grown on weighing lysimeters (1 m × 1 m × 1.3 m) were installed with CHPV probes to measure sap flow on the stem half hourly, simultaneously with lysimeter measurements of transpiration. The surfaces of the lysimeters were covered with a 10 cm layer of Styrofoam, overlain by a 2 cm layer of gravel to minimize evaporation to a negligible level. Both the lysimeter and CHPV measurements were divided into two sets. The first set was used to develop tree specific calibration equations as well as an equation combining the three species used, here called a combination equation. The second set of data was used for validating the equations. Transpiration rates ranged from negligible at night to daily peaks of 3.5, 1.7 and 1.4 L h−1 for buffalo thorn, black karee and wild olive, respectively. Good agreement indices between CHPV and lysimeters were obtained when using both the tree specific equations and combination equation across species (D = 0.778–1.000, RMSE = 0.001–0.017 L h−1, MAE < 0.001 L h−1 and MBE = −0.0007 to 0.0008 L h−1). It was concluded that the CHPV method can accurately measure tree water use, and therefore can be useful for water resources management in forested areas.

Sap flow in Searsia pendulina and Searsia lancea trees established on gold mining sites in central South Africa

The Witwatersrand Basin Goldfields (WBG) have seen over a century of continuous mining that has generated extensive tailings storage facilities (TSF), together with “footprints” remaining after the residue has been removed for reprocessing or consolidation into larger TSFs. These are now believed to number several hundred and cover a total area of 400–500km2. Acid mine drainage (AMD) from these structures is widespread and has resulted in contamination of soils, groundwater and surface water systems. Sustainable and long-term control measures are required to limit environmental contamination. The Mine Woodlands Project, initiated by the University of the Witwatersrand and AngloGold Ashanti Ltd, aims to investigate the use of trees for hydraulic control of mine seepage, as well as contaminant immobilization. A variety of exotic and indigenous tree species was planted in high density stands within site species trials located close to TSFs in the Orkney and Carltonville districts. The aim is to evaluate their survival and growth, as well as water use and contaminant uptake or immobilization.This paper describes a study of the annual pattern of sap flow rates in two species of indigenous tree (Searsia lancea (L. F.) F.A. Barkley and S. pendulina (Jacq.) Moffett, comb. nov.) established in plantation form. These species occur naturally in central and western South Africa. Sap flow was monitored continuously over a full year in eight stems representing each species, using the heat ratio version of the heat pulse velocity technique. Plot sap flow was estimated by scaling up according to the number and size of stems, and utilizing functions relating leaf dry mass and leaf area to stem diameter. The deciduous species S. pendulina was found to use 591mm of water over a full growing season, while the evergreen species S. lancea was found to use 1044mm over a full year. Differences in sap flow patterns between these species are attributed largely to different leaf dynamics. We conclude that S. lancea has potential for the hydraulic control of mine seepage water in phytoremediation systems in the WBG.

Estimating Transpiration of Whole Grapevines under Field Conditions

yield, growth and quality. Diurnal whole-plant transpiration, to be used in combination with a soil evaporation model to estimate vineyard evapotranspiration, was quantified by measuring sap flow in grapevines. Sap flow was measured in grapevine trunks by means of the heat pulse velocity technique. Diurnal sap flow ranged from almost zero to c. 5 L/d per grapevine under various atmospheric, viticultural and soil conditions. Sap flow showed good linear correlation with leaf area per grapevine and reference evapotranspiration (ETo). Grapevines with similar leaf area trained onto horizontally orientated trellis systems transpired more than those on vertical trellises under the same atmospheric conditions. Approximately 90% of the variation in daily transpiration could be explained by means of multiple linear regression, with leaf area and daily ETo as the independent variables. However, grapevines with horizontal and vertical canopies required slightly different models.

Seasonal variation of night-time sap flow of a young olive orchard: the unconsidered process for evapotranspiration estimations

Night-time sap flow (Sn), with transpiration as an important proportion of it at moderate to high vapor pressure deficits (VPD), is an important unconsidered factor that contributes significantly to total evapotranspiration (ET) of horticultural and fruit tree crops. This nocturnal process will be likely increased in a climate change scenario, with increases in night-time temperatures at higher rates compared to diurnal temperatures. The aim of this study was to characterise night-time water consumption over a commercial drip-irrigated young olive orchard (Olea europaea L. 'Arbequina') located in Pencahue valley, Maule Region, Chile (35°23-LS; 71°44-LW; 96 m a.s.l.) and its dynamics within the 2010/11 season. Four olive trees were selected for sap flow measurements using the Compensated Heat Pulse Velocity technique (CHPV). The canopy conductance (Gc) was calculated by inverting a modified Penman- Monteith equation. The aerodynamic conductance (ga) was calculated using an algorithm of the two-layer model proposed by Shuttleworth and Wallace (1985). An eddy covariance system was installed in the orchard to measure real ET. Results showed that Sn varied between 1.79 and 3.09 L tree-1 night-1 depending mainly of the atmospheric demand. The diurnal sap flow (Sd) measured was from 7.1 to 18.2 L tree-1 day-1. Parabolic shape curves described the relationship between Sn and Gc. Furthermore, the Sn/Sd ratio changed between 16 and 25% depending on the weather conditions, which it is not currently considered in ET models. It is clear that there is a nocturnal flow of water from soil to plant and water movement within the plant, but it is not yet clear the partition between the transpiration process and hydraulic redistribution. However, the former could be more likely due to the highly significant correlations found between VPD and Sn.

WATER USE BY A KIWIFRUIT VINE: CALIBRATION, MEASUREMENTS AND A MODEL

The Heat Pulse Velocity technique (HPV) is a method for determining sap flow velocity through plant stems and, subsequently, plant water use. The ‘T max ’ HPV method measures the time taken for a maximum temperature rise to be recorded by a single sensor downstream from a pulsed heater. This method was used to measure rates of sap movement in stem sections of Actinidia chinensis ‘Hort16A’ vines. Water passing through these sections was collected and compared with the estimates obtained from the ‘Tmax ’ method. Sap flows were also measured, in a whole kiwifruit vine, using the ‘T max ’ technique and were in good agreement with calculated transpiration rates. Transpiration rates reached a maximum of about 60 L d -1 for kiwifruit vines in mid summer.

Apparatus for non-destructive measurement of grapevine trunk cross-sectional area

Measuring sap flow in grapevine trunks, using the heat pulse velocity technique, requires accurate, non-destructive measurement of trunk cross-sectional areas. A profile measuring apparatus (PMA) was constructed to improve the accuracy of these measurements, particularly for irregular cross-sections. The low-cost PMA provided accurate, non-destructive measurements of grapevine trunk cross-sectional areas when compared to actual areas. Trunk cross-sectional areas estimated from diameter or circumference measurements proved to be less accurate than those obtained using the PMA. Results showed that estimated areas could result in large errors in the case of thick (greater than 30 cm2), irregular trunks.

Sap flow of several olive trees estimated with the heat-pulse technique by continuous monitoring of a single gauge

The use of the compensation heat-pulse velocity (CHPV) technique to estimate sap flow in olive trees as a means of irrigation scheduling can be a difficult task because of the naturally heterogeneity of hydraulic functioning of sapwood area in this species. As a result, the monitoring of an unreasonably large number of both trees and CHPV gauges per tree is needed for accurate estimation of orchard transpiration. The approach used in this paper restricts the monitoring of a large number of both trees and gauges to a very short time (a day) and allows the long term estimation of transpiration of several olive trees by the continuous monitoring of a sole CHPV gauge installed in a single tree. In an experimental olive orchard in southern Italy, we monitored six CHPV gauges in three well irrigated trees (treatment T100) and six CHPV gauges in three rain-fed trees (treatment T0) at half hour intervals for 60 days during summer 1999. For each ith gauge at each day, we linearly regressed the sap flow ( Q i , l h −1) against the mean sap flow of the all other n−1 gauges of the same treatment (Qm i ). The regression parameters were used to obtain an estimation of Qm i (EQm i ) throughout the trial using the data of any single ith gauge. In order to test the prediction power of the model of ith gauge, the estimated mean sap flow (EQm i ) values were regressed day by day against the actually measured Qm i . The same procedure was also applied at the time scale of a day, that is to the daily cumulative value of Q i and Qm i . In all cases, we obtained high statistical significance of the models applied to data as confirmed by the high adjusted R 2 estimates. Our analyses show the feasibility of estimating the transpiration rate of many trees by monitoring sap flow in a single tree by the use of a sole CHPV gauge. The results of this work indicate that CHPV can be a useful and powerful method for irrigation scheduling for olive and other tree species.

Comparative water use of wattle thickets and indigenous plant communities at riparian sites in the Western Cape and KwaZulu-Natal

Large-scale funding by both the Government and the private sector continues in support of the Working-for-Water Programme, which is active in many regions of the country. One justification for this programme of alien tree removal is the streamflow enhancement that is believed to follow the replacement of dense stands of invasive trees by indigenous, largely herbaceous or shrub dominated plant communities. Often the densest stands of invader trees occur within riparian zones, where removal of trees in close proximity to stream channels is believed to strongly enhance streamflow. Few data are available, however, to support this assumption. Results from a number of research catchments have consistently shown that afforestation significantly decreases streamflow where the pre-afforestation vegetation was seasonally dormant mountain grassland or fynbos (Versfeld, 1994). The net difference in evapotranspiration (ET) between riparian thickets of alien trees and riparian fynbos may be quite different, due to the yearlong availability of soil water and enhanced plant growth in riparian zones. The water use of alien invasive trees in South Africa remains largely unknown, adding further uncertainty to the effect of alien removal on streamflow. This paper describes the results of a comparative study of annual ET between indigenous riparian plant communities and riparian wattle thickets (Acacia mearnsii() at four sites in the Western Cape and KwaZulu-Natal. The Bowen ratio energy balance (BREB) technique was used to record a 12-month record of 20 min evaporation rates from a fynbos riparian plant community in the Jonkershoek valley (Western Cape), and a grassland riparian community on the property Gilboa in the KwaZulu-Natal midlands. Closed-canopy, mature stands of self-established (A. mearnsii( in the Wellington and Groot Drakenstein areas of the Western Cape were selected to provide comparative transpiration data. The heat pulse velocity (HPV) technique was used to record hourly sap-flow rates in six sample trees representing the range of tree sizes at both wattle sites. Total daily sap flow in all sample trees experiencing adequate soil water availability was found to be very closely correlated to tree size and an index defined as the product of mean daily vapour pressure deficit (VPD) of the air and the number of daylight hours. These relationships were used to predict the water use of wattle thickets at Jonkershoek and Gilboa, using VPD and day-length data recorded at these sites. Published estimates of canopy rainfall interception were added to the sap flow (transpiration) component to yield a combined annual ET to compare to the BREB ET data. Table 1 summarises the annual evapotranspiration at each site. (table***) We conclude that the removal of riparian wattle and its replacement by indigenous herbaceous plants may indeed result in significant reductions in annual ET, and could very likely lead to streamflow enhancement. However, this study has clearly shown that annual ET varies considerably in different riparian plant communities, and that one must consider the structural and physiological characteristics of both the pre-clearing and post-clearing vegetation in order to predict the net change in ET. This conclusion supports an earlier view (Versfeld et al., 1998) that an improved methodology of general applicability is required to enhance the accuracy of water use predictions for a wide range of alien and indigenous plant communities. Such predictions are important to prioritise clearing operations in areas invaded by alien trees. WaterSA Vol.27(4) 2001: 529-538

Enhanced transpiration in response to wind effects at the edge of a blue gum (Eucalyptus globulus) plantation.

In Australia, tree planting has been widely promoted to alleviate dryland salinity and one proposed planting configuration is that of strategically placed interception belts. We conducted an experiment to determine the effect of tree position in a belt on transpiration rate. We also assessed how much the effect of tree position can be explained by advection and environmental conditions. Daily transpiration rates were determined by the heat pulse velocity technique for four edge and 12 inner trees in a 7-year-old Tasmanian blue gum (Eucalyptus globulus) plantation in South Australia. Various climatic variables were logged automatically at one edge of the plantation. The relationship between daily sap flow and sapwood area was strongly linear for the edge trees (r2 = 0.97), but only moderately correlated for the inner trees (r2 = 0.46), suggesting an edge effect. For all trees, sap flow normalized to sapwood area (Qs) increased with potential evaporation (PE) initially and then became independent as PE increased further. There was a fairly close correlation between transpiration of the edge and inner trees, implying that water availability was partially responsible for the difference between inner and edge trees. However, the ratio of edge tree to inner tree transpiration differed from unity, indicating differences in canopy conductance, which were estimated by an inverse form of the Penman-Monteith equation. When canopy conductances were less than a critical value, there was a strong linear relationship between Qs of the edge and inner trees. When canopy conductances of the edge trees were greater than the critical value, the slope of the linear relationship was steeper, indicating greater transpiration of the edge trees compared with the inner trees. This was interpreted as evidence for enhancement of transpiration of the edge trees by advection of wind energy.

Transpiration of cottonwood/willow forest estimated from sap flux

Cottonwood/willow forests in the American Southwest consist of discrete, even-aged vegetation patches arranged in narrow strips along active and abandoned stream channels of alluvial flood plains. We used the heat-pulse velocity technique in this study to estimate transpiration in 12 such forest patches along a perennially flowing reach of the San Pedro River in southeastern Arizona, USA during five periods from April to October 1997. Transpiration per unit sapwood area was consistently higher for the larger cottonwood trees found on outer secondary channels compared to that of smaller cottonwood trees along the active channel, but statistically significant differences were found only in August and October. Conversely, transpiration per unit sapwood area in willow was markedly higher for trees along the primary channel than for those few larger trees that were sampled on the outer margins of the forest. Average daily transpiration at the canopy scale among the patches in July was 4.8±0.7 mm per day and ranged from 5.7±0.6 mm per day in young forest patches adjacent to the primary stream channel to 3.1±0.6 mm per day in more successionally advanced patches on secondary channels. Differences in our estimates of transpiration between forest patches along primary and secondary stream channels were related to differences in the ratio of sapwood area to ground area of the forest patches, and leaf area index. Estimates of transpiration from this forest type, and projections of transpiration and groundwater flux over larger areas on the San Pedro River, should take into account structural variation in these forests that relate to population dynamics of dominant trees.

Radial variation in sap velocity as a function of stem diameter and sapwood thickness in yellow-poplar trees.

Canopy transpiration and forest water use are frequently estimated as the product of sap velocity and cross-sectional sapwood area. Few studies, however, have considered whether radial variation in sap velocity and the proportion of sapwood active in water transport are significant sources of uncertainty in the extrapolation process. Therefore, radial profiles of sap velocity were examined as a function of stem diameter and sapwood thickness for yellow-poplar (Liriodendron tulipifera L.) trees growing on two adjacent watersheds in eastern Tennessee. The compensation heat pulse velocity technique was used to quantify sap velocity at four equal-area depths in 20 trees that ranged in stem diameter from 15 to 69 cm, and in sapwood thickness from 2.1 to 14.8 cm. Sap velocity was highly dependent on the depth of probe insertion into the sapwood. Rates of sap velocity were greatest for probes located in the two outer sapwood annuli (P1 and P2) and lowest for probes in closest proximity to the heartwood (P3 and P4). Relative sap velocities averaged 0.98 at P1, 0.66 at P2, 0.41 at P3 and 0.35 at P4. Tree-specific sap velocities measured at each of the four probe positions, divided by the maximum sap velocity measured (usually at P1 or P2), indicated that the fraction of sapwood functional in water transport (f(S)) varied between 0.49 and 0.96. There was no relationship between f(S) and sapwood thickness, or between f(S) and stem diameter. The fraction of functional sapwood averaged 0.66 +/- 0.13 for trees on which radial profiles were determined. No significant depth-related differences were observed for sapwood density, which averaged 469 kg m(-3) across all four probe positions. There was, however, a significant decline in sapwood water content between the two outer probe positions (1.04 versus 0.89 kg kg(-1)). This difference was not sufficient to account for the observed radial variation in sap velocity. A Monte-Carlo analysis indicated that the standard error in estimated mean f(S) declined rapidly with increasing sample size. At n = 10, the coefficient of variation in mean f(S) was 7% and at n = 15 it was slightly less than 5%. These observations indicate that radial variation in sap velocity is an important, albeit often overlooked, source of uncertainty in the scaling process. Failure to recognize that not all sapwood is functional in water transport will introduce systematic bias into estimates of both tree and stand water use. Future studies should devise sampling strategies for assessing radial variation in sap velocity and such strategies should be used to identify the magnitude of this variation in a range of non-, diffuse- and ring-porous trees.

Estimating water use by sugar maple trees: considerations when using heat-pulse methods in trees with deep functional sapwood.

Accurate estimates of sapwood properties (including radial depth of functional xylem and wood water content) are critical when using the heat pulse velocity (HPV) technique to estimate tree water use. Errors in estimating the volumetric water content (V(h)) of the sapwood, especially in tree species with a large proportion of sapwood, can cause significant errors in the calculations ofsap velocity and sap flow through tree boles. Scaling to the whole-stand level greatly inflates these errors. We determined the effects of season, tree size and radial wood depth on V(h) of wood cores removed from Acer saccharum Marsh. trees throughout 3 years in upstate New York. We also determined the effects of variation in V(h) on sap velocity and sap flow calculations based on HPV data collected from sap flow gauges inserted at four depths. In addition, we compared two modifications of Hatton's weighted average technique, the zero-step and zero-average methods, for determining sap velocity and sap flow at depths beyond those penetrated by the sap flow gauges. Parameter V(h) varied significantly with time of year (DOY), tree size (S), and radial wood depth (RD), and there were significant DOY x S and DOY x RD interactions. Use of a mean whole-tree V(h) value resulted in differences ranging from -6 to +47% for both sap velocity and sap flow for individual sapwood annuli compared with use of the V(h) value determined at the specific depth where a probe was placed. Whole-tree sap flow was 7% higher when calculated on the basis of the individual V(h) value compared with the mean whole-tree V(h) value. Calculated total sap flow for a tree with a DBH of 48.8 cm was 13 and 19% less using the zero-step and the zero-average velocity techniques, respectively, than the value obtained with Hatton's weighted average technique. Smaller differences among the three methods were observed for a tree with a DBH of 24.4 cm. We conclude that, for Acer saccharum: (1) mean V(h) changes significantly during the year and can range from nearly 50% during winter and early spring, to 20% during the growing season;(2) large trees have a significantly greater V(h) than small trees; (3) overall, V(h) decreases and then increases significantly with radial wood depth, suggesting that radial water movement and storage are highly dynamic; and (4) V(h) estimates can vary greatly and influence subsequent water use calculations depending on whether an average or an individual V(h) value for a wood core is used. For large diameter trees in which sapwood comprises a large fraction of total stem cross-sectional area (where sap flow gauges cannot be inserted across the entire cross-sectional area), the zero-average modification of Hatton's weighted average method reduces the potential for large errors in whole-tree and landscape water balance estimates based on the HPV method.

Water relations, stomatal response and transpiration of Quercus pubescens trees during summer in a Mediterranean carbon dioxide spring.

Variations in water relations and stomatal response of Quercus pubescens Willd. were analyzed under Mediterranean field conditions during two consecutive summers (1993 and 1994) at two locations characterized by different atmospheric CO(2) concentrations because of the presence at one of them of a CO(2) spring. Trees at the CO(2) spring site have been growing for generations in elevated atmospheric CO(2) concentrations. The heat-pulse velocity technique was used to estimate water use of trees during a 5-month period from June to November 1994. At the end of the sap flow measurements, the trees were harvested and foliage and sapwood area measured. At both sites, maximum leaf conductance was related to predawn shoot water potential. Effects of summer drought on plant water relations, including whole-plant transpiration, were severe, but leaf conductance and water potential recovered to predrought values after major rainfall in September. Leaf conductance, predawn water potential, and sometimes sap flow, decreased in parallel with increases in hydraulic resistance, reaching a minimum in midsummer. Hydraulic resistance was higher in trees at the control site than in trees at the CO(2) spring site. The effect of elevated CO(2) concentration on leaf conductance was less at high leaf-to-air water vapor pressure difference than at low leaf-to-air water vapor pressure difference. Mean and diurnal sap fluxes were consistently higher in trees at the control site than in trees at the CO(2) spring site. During the summer period, plant water use varied between the two sites. Trees at the control site had consistently higher sap flow at corresponding values of sapwood cross-sectional area than trees at the CO(2) spring site. Because trees at the CO(2) spring site supported a smaller foliage area for a corresponding sapwood cross-sectional area than trees at the control site, the overall mean sap flux/foliage area ratio did not differ between sites. The results are discussed in terms of effects of elevated CO(2) concentration on plant water use at the organ and whole-tree scale.

Whole-tree transpiration and water-use partitioning between Eucalyptus nitens and Acacia dealbata weeds in a short-rotation plantation in northeastern Tasmania.

Whole-tree water use in 4- and 8-year-old plantations of Eucalyptus nitens Deane and Maiden (ex Maiden) in the presence and absence of Acacia dealbata Link. weeds was estimated by the heat pulse velocity technique during a six-week summer period. Maximum sap velocities were recorded between 5 and 15 mm under the cambium for both eucalypt and acacia trees, and marked radial and axial variations in sap velocity were observed. The latter source of variation was most pronounced in mixed stands where crowns were asymmetrical. Mean daily sap flux ranged from 1.4 to 103.6 l day(-1) for eucalypts and from < 0.1 to 8.4 l day(-1) for acacias. Stem diameter explained 98% of the variation in sapwood area for E. nitens and 89% for A. dealbata, and was determined to be a suitable parameter for scaling water use from the tree to stand level. Plot transpiration varied from 1.4 to 2.8 mm day(-1) in mixed 8-year-old plots and was 0.85 mm day(-1) in a mixed 4-year-old plot. The degree of A. dealbata infestation was associated with absolute plot water use and regression models predicted that, in the absence of acacia competition, plot water use for the 8-year-old stand would approach 5-6 mm day(-1) during the growing season.

Modelling the surface conductance of a broad‐leaf canopy: effects of partial decoupling from the atmosphere

The transpiration of a mature beech (Fagus sylvatica L.) forest was measured over a whole season by the heat pulse velocity technique and the results analysed in terms of a new analytical canopy conductance model, which takes into account the effects of partial decoupling from the atmosphere on the local humidity environment experienced by the canopy. Stand daily transpiration ranged from 0·62 to 2·97 mm d–1, with a seasonal mean value of 1·97 mm d–1. Maximum canopy conductance was 18·5 mm s–1, with a mean estimated value of 5·0 mm s–1; computed values were little affected by the assumption of neutral atmospheric conditions. The decoupling coefficient Ω varied greatly on a daily and seasonal basis, with an average value of 0·28. As a result of partial decoupling, the estimated vapour pressure deficit at the notional canopy surface exceeded the values measured above the canopy by 380 Pa on average. When correctly expressed in terms of humidity at the canopy surface, the model explained 80% of the variance in half‐hourly transpiration measurements. Upon cross‐validation it still explained 72% of the variance, as compared to only 40% when correction for partial decoupling was not introduced. A baseline canopy conductance of 0·7 mm s–1, not modulated by the environment, was estimated. The implications of the model are discussed for the representation of canopy conductance and transpiration of broad‐leaf forests.

Sap-flow velocities and distribution of wet-wood in trunks of healthy and unhealthy Quercus robur, Quercus petraea and Quercus cerris oak trees in Hungary

Sap-flow of Quercus robur, Quercus petraea and Quercus cerris oak trees were studied. 43K radioisotope tracing, the heat pulse velocity technique and the Granier-method were employed. Numerous intense pulses were observed in healthy Quercus petraea superposing onto the usual diurnal change. Only a few pulses were observed in unhealthy Quercus petraea, in healthy Quercus cerris and healthy and unhealthy Quercus robur trees. Proportion of wet-wood assessed by γ- and X-ray computer tomography and magnetic resonance imaging was significantly less in healthy Quercus petraea trees than in healthy Quercus cerris trees. Proportion of wet-wood was higher in healthy trees than unhealthy trees of both species.

Sap flow in Bornean heath and dipterocarp forest trees during wet and dry periods.

Daytime xylem sap flow was measured by the compensation heat pulse velocity technique in dipterocarp forest trees growing on sandy loam (haplic acrisol) and in heath forest trees growing on bleached sand (albic arenosol) in Brunei during wet and dry periods. In dipterocarp forest trees, daily sap flow was stable during the dry period but declined on rainy days presumably as a result of reduced transpiration. At the heath site, daily sap flow of the small trees was stable during the dry period, whereas it declined in the large trees as the dry season progressed. In some trees at both sites, the hydroactive area of xylem was less during the dry period than during the wet period. Daily sap flow was linearly related to projected crown area, and a common regression fitted trees of different species from both forest types. The dry/wet period ratio of sap flow (about 0.7) was statistically equivalent for heath and dipterocarp forest trees, whereas it had been expected that trees on the freely draining arenosol would suffer greater water stress during drought. For well-watered trees >/= 0.2 m dbh (diameter at 1.3 m above ground) in both the dipterocarp and heath forests, stand water use was estimated to be 40 mm month(-1).

Water use by Pinus radiata trees in a plantation.

We used the heat-pulse velocity technique to estimate transpirational water use of trees in an experimental 16-year-old Pinus radiata D. Don plantation in South Australia during a 4-month period from November 1993 to March 1994 (spring-summer). Fertilization and other silvicultural treatments during the first 8 years of the plantation produced trees ranging in diameter at a height of 1.3 m from 0.251 to 0.436 m, with leaf areas ranging from 83 to 337 m(2). Daily water use was greater for large trees than for small trees, but transpiration per unit leaf area was nearly identical. Daily transpiration was highly correlated with available soil water in the upper 1 m of soil and weakly correlated with irradiance and air temperature. For the stand (0.4 ha), estimated rates of transpiration ranged from 6.8 to 1.4 mm day(-1) in wet and dry soil conditions, respectively. Total water use by the plantation during the 4-month study period was 346 mm. Water transpired by the trees was about three times that extracted from the upper 1 m of soil. Large trees extracted water from the same soil volume as small trees and did not exhibit a greater potential to extract water from deeper soil when the upper horizons become dry.

Evaluation of the heat pulse velocity technique for measurement of sap flow in rainforest and eucalypt forest species of south‐eastern Australia

ABSTRACTSap flow in the stems of two cut saplings each of Eucalyptus maculata (a canopy eucalypt forest tree), Doryphora sassafras and Ceratopetalum apetalum (both canopy rainforest trees of south‐eastern coastal Australia) was measured by the heat pulse velocity technique and compared with water uptake from a potometer. Scanning electron micrographs of wounding caused by implantation of temperature sensor and heater probes into the sapwood showed that wounding was similar in rainforest and eucalypt species and was elliptical in shape. A circular wound has been implicitly assumed in previous studies. Accurate measurements of sapling water use were obtained using the smaller transverse wound dimension rather than the larger longitudinal dimension because maximum disruption of sap flow through the xylem vessels occurred in the transverse plane. Accurate measurements of sap flux were obtained above a minimum threshold sap velocity. These velocities were 15·7,10·9 and 9·4 cm h−1 for E. maculata, C. apetalum and D. sassafras, respectively. Below the threshold sap velocity, however, sap flow could not be accurately calculated from measurements of heat pulse velocity. The minimum threshold sap velocity appeared to be determined by probe construction and xylem anatomy. Despite the elliptical wounding and inaccurate measurement of sap flow below the threshold sap velocity, total sap flow over the experimental period for two saplings of each species was within 7% of water use measured by the potometer.

Estimating transpiration from 6‐year‐old Eucalyptus grandis trees: development of a canopy conductance model and comparison with independent sap flux measurements

ABSTRACTDaily patterns of stomatal conductance (gs), xylem pressure potential (P) and canopy microclimatic variables were recorded on 11 sample days as part of a one‐year study of the water use of Eucalyptus grandis Hill ex Maiden in the eastern Transvaal, South Africa. Measured gs was found to be largely controlled by quantum flux density (Q) and ambient vapour pressure deficit (D). Canopy conductance (gc) was determined for hourly intervals using gs measurements and leaf areas in four different canopy levels. A simple model was constructed to allow the prediction of gc and transpiration from Q, D and season of year. The model was used to estimate transpiration rates from 10 trees in a later study of similarly‐aged E. grandis trees, in which sap flow in each tree was measured using the heat pulse velocity (HPV) technique. Five of the trees were monitored on a summer day and five on a winter day. Correspondence between HPV sap flow and modelled transpiration was good for the summertime comparisons, but measured winter‐time sap flow rates were underestimated by the model, especially under conditions of high sap flow. The discrepancy is believed to result from having insufficient data from the conductance study to describe the response of gs to relatively high D in winter. Marked variation in transpiration per unit leaf area indicates that a relatively large number of trees must be sampled for the HPV technique to be used to obtain a mean rate for an entire stand in winter.