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Species differences in transpiration on a saline discharge site
Agricultural Water Management 50 (2001) 65±81
Species differences in transpiration on a saline discharge site Richard G. Benyona,*, Nico E. Marcarb, Swaminathan Theiveyanathanb, W. Mark Tunningleyb, Alan T. Nicholsonc a
CSIRO Forestry and Forest Products, Plantation Forest Research Centre, P.O. Box 946, Airport Road, Mount Gambier, SA 5290, Australia b CSIRO Forestry and Forest Products, P.O. Box E4008, Kingston, ACT 2604, Australia c NSW Department of Land and Water Conservation, P.O. Box 207, Wellington, NSW 2820, Australia Accepted 5 July 2000
Abstract Growth, sap velocity, tree water use and transpiration rates per unit of leaf area were compared between Eucalyptus occidentalis Endl., Eucalyptus spathulata Hook., Eucalyptus leucoxylon F. Muell., and Eucalyptus cladocalyx F. Muell. on a moderately-saline discharge site near Wellington, NSW, Australia. These were four of the best performed species in a 7-year old trial of 36 species and provenances. Even though all trees were the same age and had grown under identical conditions, water use per tree was four±®ve times greater in E. spathulata than in the other three species. This difference was due to a large difference in tree size. E. spathulata had grown faster than the other species and had a mean tree leaf area four±®ve times greater than the other species. Species differences in water use per unit of leaf area were smaller, but sometimes statistically signi®cant. During a period of cool dry weather in late winter, there were no signi®cant differences between species in transpiration per unit of leaf area. In early summer, however, when the maximum vapour pressure de®cit reached 6±7.5 kPa on some days, E. leucoxylon had a 22% lower rate of transpiration per unit of leaf area than the other three species. This difference was presumably due to a stronger stomatal response to high vapour pressure de®cit in E. leucoxylon than the other species. During a period of warm humid weather in late summer, transpiration per unit of leaf area was 75% higher in E. cladocalyx compared with the other three species. The reason for this difference is not known, but it may indicate a species difference in root architecture, and hence a difference in access to ground water or soil water. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Transpiration; Sap ¯ow; Leaf area; Salinity; Eucalyptus occidentalis; Eucalyptus spathulata; Eucalyptus leucoxylon; Eucalyptus cladocalyx * Corresponding author. Tel.: 61-8-8721-8112/8100; fax: 61-8-8723-9058. E-mail address: [email protected] (R.G. Benyon).
0378-3774/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 7 7 4 ( 0 0 ) 0 0 1 2 1 - 9
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1. Introduction Dryland seepage salinity is widely acknowledged as a significant and worsening problem in many areas of Australia, with over 21 000 km2 affected Australia-wide (Robertson, 1996), and has largely resulted from rises in local and regional groundwater in response to wide spread clearing of native vegetation for agriculture. Although reestablishment of deep-rooted perennial vegetation, such as trees, into recharge areas, is widely recognised as the best way to slow this rising groundwater, large areas will require revegetation and the full benefits are only likely to be realised in the long term. Large scale revegetation of recharge areas will involve considerable refocussing of agricultural practices. Planting of trees in discharge locations may provide environmental benefits in the short term (e.g. assisting with reducing soil erosion and the opportunity for water use from locally high water tables) and may enable productive use of already salinised land. Trees planted in saline areas will need to cope with the prevailing soil salinity and water regimes as well as any increase in root-zone salinity that may result from use of saline groundwater by the trees. Thus, the salt tolerance of a particular species is crucial to its sustainable culture in a plantation or agroforestry design (Marcar et al., 1995). Faster growth rates on saline sites will provide better prospects for commercial returns as long as tree products have sufficient economic uses. Planting of fast-growing species will also allow more rapid attainment of maximum leaf area index (LAI) than slow growing species for a given planting density. In saline areas, more salt tolerant species may be able to sustain a higher LAI, and hence higher rates of growth and water use than less tolerant trees. Provided that increases in root-zone salinity can be contained below species-specific salinity thresholds, fast growing species have the potential to use more water on saline discharge sites and hence, at some sites, slow the hydrologically-driven salinisation process. Many catchment scale experiments show that vegetation change can change the hydrological balance of a catchment. Such changes are often related to changes in rates of transpiration (Bosch and Hewlett, 1982). Whether high or low rates of transpiration are desired, it is important to understand how transpiration rates differ between tree species. There is only limited information on the relationship between water use rates of different species and their growth rates under saline and non-saline conditions. Some recent studies indicate that transpiration rates per unit of leaf area generally do not differ substantially between species growing under common environmental conditions (Hatton et al., 1998; Khanzada et al., 1998; Benyon et al., 1999). However, more information on whether the `constant leaf water efficiency' principle (Hatton et al., 1998) holds in a variety of soils and climates would give modellers more confidence in excluding species considerations (and, therefore, simplifying their approach) in predicting the effects of revegetation with trees. In this paper, we compare stem basal area growth, leaf area, sap velocities, daily water use rates and water use per unit of leaf area among four eucalypt species planted in a species evaluation trial on a saline discharge site in centralwest NSW, Australia. Our null hypothesis is that the principle of constant leaf water efficiency holds during different times of the year even on a site where trees may have access to shallow, saline groundwater, and that for trees grown under common conditions, species differences in water use are largely dependent on species differences in leaf area.
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2. Methods 2.1. Site description The trial site is located 19 km southeast of Wellington in centralwest NSW (latitude 328420 S, longitude 1498020 E; 425 m above mean sea level). The climate in the Wellington region is temperate, with cool to cold winters and warm to hot summers. Long-term mean monthly maximum/minimum temperatures vary from about 158C/28C (July) to 328C/178C (January). The mean annual rainfall of 656 mm is evenly distributed throughout the year. Mean annual pan evaporation is 1715 mm, with maximum mean monthly pan evaporation of about 250 mm in December and January (summer), and minimums of about 50 mm in June and July (winter). The soils at the site are yellow soloths and solodics (fine sandy loam topsoils overlying moderately structured light clay to medium clay subsoils). Salinity has developed on the site as a result of rising groundwater, pushed closer to the soil surface by an upslope dyke, with the result that perched groundwater flows within 0±4 m of the soil surface, depending on position (up slope or down slope of the dyke), recent rainfall and time of year. The most saline areas are sometimes waterlogged in winter and spring. 2.2. Trial information and tree selection A species evaluation trial was planted in late October 1990 after the site was rotary hoed, ripped, mounded and treated with Roundup#. Seedlings were fertilised with 50 g `Starter 15', watered and mulched with hay at planting. The trial consisted of 8 replicates, in a randomised complete block design with 36 seedlots in five tree (row) plots, or 180 trees per replicate. The trial was thinned at age 3, leaving only two trees per plot. Additional details are provided by Benyon et al. (1999). Trees in three adjacent replicates of the trial, on the moderately saline part of the site, were used for this study. At age 7, Eucalyptus occidentalis Endl., Eucalyptus spathulata Hook., Eucalyptus leucoxylon F. Muell., and Eucalyptus cladocalyx F. Muell. were the four best performing species in these replicates, in terms of survival, mean height and mean basal area per tree. Each replicate contained one or two trees of each of these species, giving five trees per species, or a total of 20 sample trees. Due to the earlier thinning, and poor survival and growth of many of the other species, the 20 sample trees probably suffered relatively small competition from neighbouring trees, and hence their growth rates were likely to be somewhat representative of pre-closed-canopy conditions. In these circumstances it was not possible to obtain reliable estimates of maximum plantation LAI for these species at this site. 2.3. Measurement of tree leaf area Tree leaf area was estimated in August 1997, December 1997 and February 1998. For each tree, the diameters of all the first-order branches were measured. Three to five of these were selected at random, with the probability of selection being proportional to branch cross-sectional area. Diameters of all second-order branches on each of the
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Table 1 Number of trees sampled during each measurement period Period of sap flow measurement
26 July±12 August 22 November±10 December 5±18 February
E. occidentalis
E. spathulata
E. leucoxylon
E. cladocalyx
Sap flow
Leaf area
Sap flow
Leaf area
Sap flow
Leaf area
Sap flow
Leaf area
4 3 4
3 3 4
5 3 4
3 3 5
5 3 4
3 3 4
5 5 4
5 5 4
three±five first-order sample branches were measured. Four or five second-order branches were then randomly selected on each first-order branch, i.e. a total of between 12 and 25 second-order branches per tree. For each second-order branch, the number of leaves was counted. One small branch (6±10 leaves), usually of third or fourth order, was randomly selected from each second-order branch and sealed in a plastic bag for laboratory determination of leaf area and subsequent calculation of the average area per leaf. A regression was fitted relating the natural log of branch leaf number to the natural log of branch cross-sectional area. This relationship was applied to the first-order branches on which the second-order branch diameters had been measured. The estimated leaf numbers and measured cross-sectional areas for these first-order branches were added to the regression. The final regression equation was applied to all first-order branches to estimate the total number of leaves on the tree. This was multiplied by the average area per leaf, determined from the 12±25 small sample branches, to give tree leaf area. The number of sample trees on each occasion is shown in Table 1. At the end of the study, in February 1998, one tree of each species was felled, and leaf area determined destructively. All leaves were harvested, and leaf total fresh weight measured in the field. A sub sample of fresh leaves was also weighed in the field at the same time, sealed in a plastic bag and stored at 58C for laboratory determination of the leaf area:fresh weight ratio. This ratio was applied to the total fresh weight of leaves to give tree leaf area. These destructive estimates were compared with estimates based on leaf counts made before the trees were felled. 2.4. Measurement of tree water use The unified nomenclature for sap flow measurements proposed by Edwards et al. (1997) is used throughout this paper. The term `sap velocity' refers to the speed of sap moving through the sapwood, expressed on a total sapwood area basis (denoted vs by Edwards et al. (1997)), in mm s 1. Sap velocities in the 20 sample trees were measured for periods of 2±3 weeks in winter (July/August 1997), early summer (November/December 1997) and late summer (February 1998). Due to the amount of time required to non-destructively estimate leaf area, and due to occasional failure of some of the sap flow sensors, it was not possible to collect measurements from all 20 trees in all three sampling periods. A minimum of three trees per species was sampled in each period (Table 1).
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Sap velocities were measured at four points within the sapwood of each tree every 30 min using sap flow sensors employing the compensation heat pulse technique (SF-300 Sapflow Sensor, Greenspan Technology, Warwick, Qld, Australia). Wounding around a sub sample of the sap flow sensors was measured in wood samples collected in February 1998. Heat pulse velocities were corrected for wounding using the coefficients of Swanson and Whitfield (1981). Wood and water volume fractions were measured in 5 mm diameter cores collected from each tree. Corrected heat pulse velocities were converted to sap velocities using equations described by Edwards and Warwick (1984). At the beginning of each measurement period, the sap flow sensors were used to measure the thickness of the conducting wood at breast height (1.3 m) on the north, east, south and west side of each tree. The method was the same as that described by Hatton et al. (1995). The E. spathulata trees were all multi-stemmed, with stem diameters ranging from 20 to 120 mm. For each of these trees, the cross-sectional area of each stem was measured and the total stem cross-sectional area of the tree calculated. Within a tree, the four sap flow sensors were then allocated randomly among the stems, with the probability of a stem being selected for sampling being proportional to its cross-sectional area. Some large diameter stems (>100 mm), for example, were allocated two sensor sets, while most of the small diameter stems (<40 mm) were not sampled at all. Stratified random sampling, similar to that recommended by Dye et al. (1991) and Hatton et al. (1995) was used to determine the sensor positions within each tree. The conducting wood was divided into four concentric rings of equal cross-sectional area. One sensor was randomly located within each ring. In the case of the multi-stemmed trees, the sensors were located randomly within each sample stem. The average sap velocity was calculated as the arithmetic mean of the four sample points. 2.5. Measurement of stomatal conductance and leaf water potential On a day during the December 1997 and February 1998 measurement periods, stomatal conductance of abaxial and adaxial surfaces was determined for three fully expanded leaves per tree in lower or mid canopy positions for two or three trees per species, between 10.00 and 12.00 h using a LiCor 1600 (LiCor, Lincoln, NE, USA). Pre-dawn water potential was measured on the same days on at least three leaves per species by inserting cut petioles into the chamber of a Schlolander pressure bomb apparatus and obtaining the balancing pressure. Water potential was usually measured within 15 min of cutting the petioles and individual leaves were placed into plastic bags and kept on ice until measured. 2.6. Measurement of soil water, salinity and pH Surface soil volumetric water content to 0.8 m depth was estimated next to each tree at the beginning and end of each sap flow measurement period using a neutron probe (503 Hydroprobe, Campbell Pacific, Pachero, CA, USA). One access tube was located 1 m to the east of each tree. Soil water was measured at 0.1 m depth intervals to 0.8 m. Salinity,
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expressed as the electrical conductivity of a 1:5 soil:water solution (EC1:5), was determined in a laboratory on soil samples collected in December 1997 and February 1998. A soil auger was used to collect samples from 0±0.15, 0.15±0.30, 0.30±0.45 and 0.45±0.60 m depths at a point close to each tree on which sap flow was measured. For each soil sample, gravimetric water content and pH1:5 were also determined. 2.7. Weather measurements Net radiation, air temperature, relative humidity and rainfall were recorded in an open field about 100 m from the site using an automatic weather station (Weather Master 2000, Environdata Australia Pty. Ltd., Warwick, QLD, Australia). Rainfall was recorded as hourly totals, while other variables were sensed at 1 min intervals and recorded as hourly averages. Vapour pressure deficit was calculated from air temperature and relative humidity. 2.8. Calculation of daily mean sap velocity, tree water use and water use per unit of leaf area The Greenspan Sapflow Sensor cannot distinguish low sap flows from no sap flow below a nominal threshold velocity of about 0.015 mm s 1 (Vertessy et al., 1997). In reality, the threshold value varies markedly between sensors (Becker, 1998). For each sensor set, a threshold value was determined from the night-time measurements. A visual appraisal of the raw sap flow data was used to identify four nights during the measurement period when apparent flows were at their lowest. For these nights, the mean and standard deviation of flows between midnight and 05.00 h were calculated, and the upper 1% confidence limit estimated. Any values higher than this were deemed to represent real flow. For each tree, the mean sap velocity on each day (06.00±06.00 h) was calculated as the mean of all corrected flows (actual flow if above the threshold value and zero flow if below) recorded every 30 min during that 24 h period. For each day, tree water use, in litres, was calculated as the product of mean sap velocity and conducting wood crosssectional area. For each of the three measurement periods, tree mean daily water use was divided by tree leaf area, giving average water use per unit of leaf area. The statistical significance of species effects were determined by one-way analysis of variance (ANOVA). The significance of differences between individual species were analysed using t-tests. 3. Results 3.1. Accuracy of estimates of tree leaf area based on leaf counts Averaged over the four sample trees the difference between the two methods of estimating tree leaf area was about 9% (2.5 m 2 per tree). For the E. spathulata and
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E. cladocalyx trees there was no difference between the two methods. For the other two trees, the leaf count method under-estimated tree leaf area compared with the felling and weighing method (3 m2, or 10%, for E. leucoxylon and 7 m2, or 24%, for E. occidentalis). The size of the error was unrelated to tree leaf area. With only one tree felled per species, it was not possible to determine whether the level of accuracy of the leaf count method differed between species. The sample size was not large enough to determine whether the two under-estimates represented a consistent pattern of bias associated with the leaf count method. 3.2. Species comparisons of tree size Means and standard errors of basal area, conducting wood area and leaf area for each species are shown in Fig. 1. Data are only shown for the three trees of each species that were sampled on all three occasions. There was large variation in tree size between species. Eucalyptus spathulata had the largest leaf area per tree, being about three times greater than E. cladocalyx, four times greater than E. occidentalis and five times greater than E. leucoxylon. Species differences in basal area and conducting wood area were of a similar magnitude (Fig. 1). 3.3. Species comparisons of sap velocity and response to VPD Differences in sap velocities between species were smaller than for growth (Fig. 2). In July/August, species differences were not statistically significant. During November/December, sap velocities were higher in all species compared with July/August. There was greater variation between species, but again the differences were not statistically significant (Fig. 2). This period included several hot days with low humidity, when day-time vapour pressure deficits (VPD) sometimes peaked at between 5 and 7.5 kPa. On these days, sap velocities in E. leucoxylon dropped markedly during the middle of the day compared with the other three species (Fig. 3). On 22 November, in mild conditions, the diurnal course of sap velocities was almost identical in the four species (Fig. 3). From 24 to 26 November, day-time maximum VPDs of 6±7.5 kPa were recorded (Fig. 3). On these days, sap velocities from late morning to early evening each day, averaged only about 0.08 mm s 1 in E. leucoxylon compared with about 0.14 mm s 1 in the other species. On 25 and 26 November, the differences in daily means between E. leucoxylon and the other three species were statistically significant (P < 0:05). Day-time hourly mean sap velocities were plotted against day-time hourly mean VPD for the period shown in Fig. 3 (see also Fig. 4). Sap velocities were similar for E. leucoxylon and the other three species up to a VPD of about 3 kPa. As VPD increased between 3 and 5.5 kPa, there was a corresponding decrease in sap velocity. This decrease was more marked in E. leucoxylon. There was no further decline in sap velocity above a VPD of about 5.5 kPa. For any VPD between 5.5 and 7.5 kPa, mean sap velocity remained constant at an average 0.08 mm s 1 in E. leucoxylon and 0.16 mm s 1 in the other three species. Differences between E. occidentalis, E. spathulata and E. cladocalyx during this period were small and not statistically significant (data not shown).
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Fig. 1. Species comparisons of: (a) tree mean basal area; (b) conducting wood area and (c) leaf area in a 7-year old species trial. The same three sample trees of each species were measured on each occasion. Bars indicate the standard errors of the estimates.
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Fig. 2. Species comparisons of daily mean sap velocities in a 7-year old species trial in winter, early summer and late summer. The same three sample trees of each species were measured on each occasion. Bars indicate the standard errors of the estimates.
The response to high VPD in E. leucoxylon was observed on 8 of the 18 measurement days during November/December. It did not vary markedly between the three E. leucoxylon trees sampled during this period (data not shown). The greatest variation in sap velocities between species occurred in February (Fig. 2). E. cladocalyx had the highest mean (0.090 mm s 1) and E. spathulata and E. occidentalis the lowest (0.064 and 0.061 mm s 1). The difference between E. cladocalyx and E.
Fig. 3. Comparison of mean sap velocities in E. leucoxylon (mean of three trees) and three other eucalypt species (mean of 11 trees) during a 6-day period in early summer. The diurnal course of vapour pressure deficit is also shown.
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Fig. 4. Day-time hourly mean sap velocity vs. day-time hourly mean vapour pressure deficit. Filled circles represent the mean of three E. leucoxylon trees. Open diamonds represent the mean of 11 trees of three other eucalypt species. The data were collected between 21 and 26 November.
spathulata was significant (P < 0:05). The difference between E. cladocalyx and the other two species was not significant. Day-time vapour pressure deficits during February were not high enough to elicit the reduction in sap velocity observed in E. leucoxylon in November/December. The mean sap velocity in E. leucoxylon during February was actually 15±20% higher than in E. occidentalis and E. spathulata (Fig. 2), although the differences between these three species were not statistically significant. 3.4. Species comparisons of water use per tree E. spathulata had daily water use four±six times greater per tree than the other three species (Fig. 5). This difference was largely due to differences in tree size. Conducting wood area per tree was several times greater in E. spathulata than in the other three species (Fig. 1), whereas differences in sap velocity were comparatively small (Fig. 2). Thus, on this site, differences in water use per tree were largely related to differences in growth rates and the associated differences in conducting wood area and leaf area. 3.5. Species comparisons of water use per unit of conducting surface Microscopic analysis of leaves and porometer measurements of stomatal conductance (data not shown) indicated that E. cladocalyx was hypostomatous, while the other species were amphistomatous. To account for this difference, the projected leaf areas of E. occidentalis, E. spathulata, and E. leucoxylon were doubled and transpiration was expressed per unit of conducting surface area (conducting surface).
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Fig. 5. Species comparisons of daily mean tree water use in a 7-year old species trial in winter, early summer and late summer. The same three sample trees of each species were measured on each occasion. Bars indicate the standard errors of the estimates.
Average water use per unit of conducting surface for each species is shown in Fig. 6. Variation between species in July/August was only minor, and not statistically significant. In November/December, E. leucoxylon had a lower water use per unit of conducting surface than the other three species (P < 0:05). The greatest difference in transpiration per unit of conducting surface between species occurred in February. During this period, transpiration was significantly higher (P < 0:001) for E. cladocalyx than for the other three species (Fig. 6). Compared to a
Fig. 6. Species comparisons of average transpiration per unit of conducting surface in a 7-year old species trial in winter, early summer and late summer. The same three sample trees of each species were measured on each occasion. Bars indicate the standard errors of the estimates.
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mean transpiration rate during February of 1.18 l m 2 per day in E. cladocalyx, rates in the other three species during this period were 0.57 l m 2 per day in E. leucoxylon, 0.70 l m 2 per day in E. spathulata and 0.74 l m 2 per day in E. occidentalis. Between the November/December and February measurement periods, transpiration per unit of conducting surface increased in E. cladocalyx, but decreased in the other species (Fig. 6). 3.6. Relationship between tree water use and soil water, salinity and pH Differences in soil water content of the top 0.8 m, both between species and over time, were small, and hence are unlikely to have had much influence on tree water use. Measurements of soil water at depths below 0.8 m were not made. Since the lateral extent of the trees' root systems were not known it is unlikely that measurements of soil water at greater depths would have assisted in determining from what soil depths the different species were obtaining their water. Mean pre-dawn water potential did not differ significantly between the December 1997 and February 1998 measurements. Values were similar between E. cladocalyx ( 1.76 MPa), E. occidentalis ( 1.71 MPa) and E. leucoxylon ( 1.62 MPa), but significantly lower (P < 0:05) in E. spathulata ( 2.48 MPa). This difference appeared to be unrelated to species differences in water use per unit of conducting surface. Rainfall was below average through most of the study. In 8 months from July 1997 to February 1998, only about 200 mm of rain was recorded at the site. The only statistically significant change in surface soil water content occurred between 4 and 16 February when, for all four species, soil water content of the top 0.8 m of soil increased by an average 4 mm. About 35 mm of rain fell between 7 and 9 February. The mean EC1:5 of the top 0.6 m of soil for each species varied from 1.0 to 1.34 dS m 1 in December 1997 and February 1998, and soil pH1:5 varied from 8.03 to 8.5. There were no significant differences in salinity or pH between species or between the December and February measurements. Salinity and pH were not measured in July/ August 1997 and they were not measured below 0.6 m. 4. Discussion In two instances, we rejected the null hypothesis, that the principle of constant leaf water efficiency holds during different times of the year in a plantation growing over a shallow water table. During the November/December measurement period, E. leucoxylon had lower transpiration per unit of conducting surface than the other three species (Fig. 6). In February, E. cladocalyx had a higher transpiration rate per unit of conducting surface than the other species (Fig. 6). As we discuss later, however, the effect of these differences on average water use per tree is minor compared to differences in water use per tree resulting from differences between species in tree leaf area. 4.1. Species differences in transpiration per unit of conducting surface On days of high VPD, such as 24±26 November, sap velocity, and hence transpiration per unit of conducting surface, in E. leucoxylon declined markedly compared with the
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other three species (Figs. 3 and 4). As a result, E. leucoxylon had lower average transpiration per unit of conducting surface than the other three species during this period. We presume this difference was due to a species difference in stomatal response to VPD. However, since stomatal conductance during this period was only measured directly for one very brief period (only one measurement on each of three leaves per tree in the lower or mid canopy on one morning at a time when VPD was relatively low), we cannot conclude with certainty that this was the reason for the difference. To indicate the likely magnitude of species differences in stomatal conductance (and resistance) in response to VPD, the tree transpiration, tree leaf area and hourly weather data (solar radiation, air temperature and air relative humidity) collected at the site during November were used in the Penman±Montieth equation, as described by Landsberg (1999), to back-calculate stomatal conductance and stomatal resistance for each species. The relationship between estimated resistance and VPD is illustrated in Fig. 7. We show resistance in the figure, rather than conductance, as this most clearly highlights the difference between the species at high VPDs. The weather data were measured in an open field about 100 m from the trees. Microclimatic conditions within the canopy may have differed from those at the weather station. These conditions (e.g. leaf surface VPD) may have differed between species, which could also have resulted in a difference in transpiration without there being a difference in stomatal conductance. Species differences in the relationship between stomatal conductance and VPD have been reported in laboratory studies (e.g. Sheriff, 1977) and field studies (e.g. Hookey et al., 1987). The latter identified differences in the relationship between stomatal conductance and VPD among 23 eucalypt species and provenances. These relationships also varied seasonally (Hookey et al., 1987). There is still debate on whether stomata respond to the rate of transpiration or somehow `sense' humidity or VPD directly (Whitehead, 1998). Many studies indicate that stomatal closure is a feedback response to
Fig. 7. Comparison between two species of the relationship between inferred stomatal resistance and vapour pressure deficit. Stomatal resistance was calculated from tree water use, leaf area, radiation, air temperature and relative humidity, using the Penman±Monteith equation. Water use and climate data were day-time hourly averages for the period between 21 and 26 November.
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the rate of transpiration (e.g. Mott and Parkhurst, 1991; Monteith, 1995), although in some studies evidence exists for a feedforward response controlled by metabolic signals (e.g. Grantz, 1990; Bunce, 1996). A decrease in transpiration rate with increasing VPD is not consistent with a feedback stomatal response (Franks et al., 1997). The halving of transpiration in E. leucoxylon as VPD increased from 3 to 5.5 kPa was an apparent feed forward response to increasing VPD. Such a large reduction in transpiration with increasing VPD is unusual (Monteith, 1995). Differences in stomatal conductance measured on individual leaves do not necessarily translate into differences at the tree or stand level. If conditions at the leaf surface are strongly decoupled from conditions in the air outside the leaf boundary layer, the transpiration rate will be determined mainly by net radiation rather than stomatal conductance (Jarvis and McNaughton, 1986). Our measurements of transpiration were collected at the tree level, rather than the leaf level. Despite this, a species difference in the relationship between transpiration rate per unit of conducting surface and VPD (measured at a nearby weather station) was observed. It is likely the canopies of the study trees were strongly coupled with regional VPD, as the study site was surrounded by open grazing land and the canopy of the plantation itself was discontinuous and aerodynamically rough. Under these conditions, species differences in the inferred relationship between stomatal conductance and VPD did result in species differences in transpiration. In large, continuous stands, where conditions inside the canopy boundary layer may be poorly coupled with regional VPD, species differences in the relationship between stomatal conductance and VPD may not translate into species differences in plantation water use (Jarvis and McNaughton, 1986). It is unlikely that the stronger response to VPD observed in E. leucoxylon would translate to large differences in transpiration between species at this site over an entire year. The November/December measurement period was the hottest period of the summer, with the highest VPDs. On most days of the year, the VPD would not be high enough to elicit the reduction in transpiration observed in E. leucoxylon. Hence, averaged over a whole year, differences in response to high VPD between these four species would be expected to make little difference to their respective rates of transpiration at locations with a similar climate to Wellington. Species differences may, however, be important at sites with drier, hotter summers than Wellington, and limited water availability. In such conditions, over summer, E. leucoxylon would deplete soil water reserves more slowly than the other three species, perhaps allowing it to survive better during drought periods. On the other hand, if planted to lower a shallow water table, lower water use of E. leucoxylon during summer may be undesirable. The difference in transpiration per unit of conducting surface that occurred in February between E. cladocalyx and the other species is more difficult to explain. It is not likely to have been a stomatal response to VPD because the weather in February was cooler and more humid than in November/December. The higher transpiration in E. cladocalyx was observed at all VPDs, not just at high VPDs (data not shown). Lower mean transpiration per unit of conducting surface of the other three species in February compared to November/December (Fig. 6) may have been due to lower potential evapotranspiration. Net radiation and day-time VPDs were both lower in February than in November/ December (data not shown). In E. cladocalyx, however, transpiration per unit of
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conducting surface was higher in February than in November/December (Fig. 6). The difference between species in February may have been indicative of differences in root architecture. If E. cladocalyx had more roots near the soil surface than the other species, it may have been able to make greater use of 35 mm of rain that fell between 7 and 9 February. Alternatively, E. cladocalyx may have had a deeper root system for accessing ground water, or a more extensive root system for exploiting a larger volume of soil (i.e. a lower shoot to root ratio), or may have been better adapted to extracting water at low soil water potential. Given the small variation in surface soil water during the study, it is likely all trees in the study were using water from deeper in the soil profile and possibly from the water table. Two piezometers were installed to 3 m depth within the study area before the November/December measurement period, but these remained dry throughout the study. Piezometers further up-slope, to depths of 6 m, however, showed that there was ground water present between 3 and 4 m depth during the period of study. In an adjoining trial about 100 m away, 8-year old E. camaldulensis Dehnh. trees were shown to be transpiring ground water at 2 mm per day during the 1997/1998 summer. Previously Hatton et al. (1998) found no differences in leaf water efficiency between species in several eucalypt stands subject to seasonal water limitations. Stand LAI were all in equilibrium with long-term rainfall (Hatton et al., 1998). Our site differed in that it was a mixed species plantation growing over a shallow water table, with spacings between trees imposed by initially uniform planting densities and subsequent thinning. With large spaces between trees resulting from poor survival of some species and removal of trees by thinning, the leaf area of individual trees was probably still increasing, and a long-term equilibrium LAI had probably not yet been attained. In this study, when comparing transpiration between species, we found it was better to express transpiration per unit of conducting surface than per unit of projected leaf area. In the August and November/December measurement periods, if transpiration was expressed per unit of projected leaf area, the amphistomatous E. cladocalyx had half the rate of transpiration compared with the other three species, which were all hypostomatous. Hence, if selecting species based on tree leaf areas measured in field trials, it is important to determine whether each species is hypostomatous or amphistomatous. 4.2. Implications of growth differences for management of saline discharge sites For managing saline discharge sites, the species differences in growth rates are more important than the species differences in transpiration per unit of conducting surface area. A sixfold variation in average water use per tree between the four species (Fig. 5), was associated mainly with variation in tree size (Fig. 1). At 7 years of age, E. spathulata had by far the largest leaf area per tree of any species in the saline parts of this site. Consequently, it also had the largest water use per tree. This large difference was made possible by the large spacing between trees created by poor survival or growth of many of the species in the trial and the subsequent thinning. The large spacing between trees meant that individual trees were able to expand their canopies at a rate uninhibited by competition for space or light from neighbouring trees. With the small plots (five trees, single row) it was not possible to determine the potential maximum LAI attainable by each species at this site.
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It could be argued that, from a hydrological viewpoint, species differences in growth rates, especially in leaf area growth per tree, do not matter. When planting blocks of trees to use up water, many species will eventually attain the maximum LAI supportable by that site. This argument ignores the effects of planting density on the costs of plantation establishment and on the time taken for hydrological benefits to begin to accrue. For a given planting density, use of a slower growing species would delay canopy closure. Alternatively, reducing the time to plantation canopy closure by increasing the planting density would increase the costs of establishing the plantation. Hence, irrespective of whether the trees are planted for commercial or hydrological benefits, identification of fast growing genotypes for the intended planting sites is important. 5. Conclusions To maximise water uptake in plantations grown over shallow water tables, careful selection of species to identify those best adapted for fast growth at the site is important to ensure rapid development of large, leafy crowns. While tree water use is largely related to tree leaf area, significant differences in water use per unit of leaf area can also occur for different species growing under common conditions. In this study, significant differences in transpiration rates occurred as a result of differences in the response of transpiration to high VPD. In one species, transpiration per unit of leaf area was reduced by about 50% at very high VPDs, while at the same time, in three other species only a slight reduction occurred. Species differences in transpiration per unit of leaf area can also occur at times when VPDs are not high. The reasons for this could not be determined in this study, but may have been a reflection of species differences in root architecture, giving some species the ability to exploit a greater volume of soil, or extract more ground water. Acknowledgements Funding of this study by the Joint Venture Agroforestry Program (CSF-46A, Agroforestry Design Guidelines for Balancing Catchment Health with Primary Production) is gratefully acknowledged. The assistance of Russell Millard (NSW Land and Water), and Debbie Crawford and Afzal Hossain (CSIRO Forestry and Forest Products) with field data collection is also gratefully acknowledged. We thank the Campbell family, owners of the `Bonada' and `Easterfield' properties, for providing the land for, and access to, the study site. We also thank Dr. Tom Hatton and Dr. Don White for their valuable comments on the manuscript.
References Becker, P., 1998. Limitations of a compensation heat pulse velocity system at low sap flow: implications for measurements at night and in shaded trees. Tree Physiol. 18, 177±184. Benyon, R.G., Marcar, N.E., Crawford, D.F., Nicholson, A.T., 1999. Growth and water use of Eucalyptus
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camaldulensis and E. occidentalis on a saline discharge site near Wellington, NSW. Aust. Agric. Water Manage. 39, 229±244. Bosch, J.M., Hewlett, J.D., 1982. A review of catchment experiments to determine the effect of vegetation changes on water yield and evapotranspiration. J. Hyrdol. 55, 3±23. Bunce, J.A., 1996. Does transpiration control stomatal responses to water vapour pressure deficit? Plant Cell Environ. 19, 131±135. Dye, P.J., Olbrich, B.W., Poulter, A.G., 1991. The influence of growth rings in Pinus patula on heat pulse velocity and sap flow measurement. J. Exp. Bot. 42, 867±870. Edwards, W.R.N., Warwick, N.W.M., 1984. Transpiration from a kiwifruit vine as estimated by the heat pulse technique and the Penman±Montieth equation. New Zealand J. Agric. Res. 27, 537±543. Edwards, W.R.N., Becker, P., CermaÂk, J., 1997. A unified nomenclature for sap flow measurements. Tree Physiol. 17, 65±67. Franks, P.J., Cowan, I.R., Farquhar, G.D., 1997. The apparent feedforward response of stomata to air vapour pressure deficit: information revealed by different experimental procedures with two rainforest trees. Plant Cell Environ. 20, 142±145. Grantz, D.A., 1990. Plant response to atmospheric humidity. Plant Cell Environ. 13, 667±679. Hatton, T.J., Moore, S.J., Reece, P.H., 1995. Estimating stand transpiration in a Eucalyptus populnea woodland with the heat pulse method: measurement error and sampling strategies. Tree Physiol. 15, 219±227. Hatton, T.J., McEwan, K., Taylor, P., 1998. Does leaf water efficiency vary among eucalypts in water limited environments? Tree Physiol. 18, 529±536. Hookey, G.R., Bartle, J.R., Loh, I.C., 1987. Water use of eucalypts above saline groundwater. Australian Water Resources Council Research Project 84/166. Unpublished work. Water Authority of Western Australia, 96 pp. Jarvis, P.G., McNaughton, K.G., 1986. Stomatal control of transpiration: scalling up from leaf to region. Adv. Ecol. Res. 15, 1±49. Khanzada, A.N., Morris, J.D., Ansari, R., Slavich, P.G., Collopy, J.J., 1998. Groundwater uptake and sustainability of Acacia and Prosopis plantations in southern Pakistan. Agric. Water Manage. 36, 121±139. Landsberg, J.J., 1999. Tree water use and its implications in relation to agroforestry systems. In: Landsberg, J.J. (Ed.), The Ways Trees Use Water. Rural Industries Research and Development Corporation Publication No. 99/37. RIRDC, Canberra, 102 pp. Marcar, N.E., Crawford, D.F., Leppert, P.M., Jovanovic, T., Floyd, R., Farrow, R. (Eds.), 1995. Trees for Saltland: A Guide to Selecting Native Species for Australia. CSIRO Publications, Melbourne, 72 pp. Monteith, J.L., 1995. A reinterpretation of stomatal responses to humidity. Plant Cell Environ. 18, 357±364. Mott, K.A., Parkhurst, D.F., 1991. Stomatal responses to humidity in air and helox. Plant Cell Environ. 14, 509±515. Robertson, G., 1996. Saline land in Australia, its extent and predicted trends. In: Proceedings of the 4th National Conference and Workshop on the `Productive Use and Rehabilitation of Saline Lands', Albany, Western Australia, Promaco Conventions Pty. Ltd., 25±30 March 1996, Canning Bridge, WA, Australia, pp. 345±358. Sheriff, D.W., 1977. The effect of humidity on water uptake by, and viscous flow resistance of, excised leaves of a number of species: physiological and anatomical observations. J. Exp. Bot. 28, 1399±1407. Swanson, R.H., Whitfield, W.A., 1981. A numerical analysis of heat pulse velocity theory and practice. J. Exp. Bot. 32, 221±239. Vertessy, R.A., Hatton, T.J., Reece, P., O'Sullivan, S.K., Benyon, R.G., 1997. Estimating stand water use of large mountain ash trees and validation of the sap flow measurement technique. Tree Physiol. 17, 747±756. Whitehead, D., 1998. Regulation of stomatal conductance and transpiration in forest canopies. Tree Physiol. 18, 633±644.
Species differences in transpiration on a saline discharge site Richard G. Benyona,*, Nico E. Marcarb, Swaminathan Theiveyanathanb, W. Mark Tunningleyb, Alan T. Nicholsonc a
CSIRO Forestry and Forest Products, Plantation Forest Research Centre, P.O. Box 946, Airport Road, Mount Gambier, SA 5290, Australia b CSIRO Forestry and Forest Products, P.O. Box E4008, Kingston, ACT 2604, Australia c NSW Department of Land and Water Conservation, P.O. Box 207, Wellington, NSW 2820, Australia Accepted 5 July 2000
Abstract Growth, sap velocity, tree water use and transpiration rates per unit of leaf area were compared between Eucalyptus occidentalis Endl., Eucalyptus spathulata Hook., Eucalyptus leucoxylon F. Muell., and Eucalyptus cladocalyx F. Muell. on a moderately-saline discharge site near Wellington, NSW, Australia. These were four of the best performed species in a 7-year old trial of 36 species and provenances. Even though all trees were the same age and had grown under identical conditions, water use per tree was four±®ve times greater in E. spathulata than in the other three species. This difference was due to a large difference in tree size. E. spathulata had grown faster than the other species and had a mean tree leaf area four±®ve times greater than the other species. Species differences in water use per unit of leaf area were smaller, but sometimes statistically signi®cant. During a period of cool dry weather in late winter, there were no signi®cant differences between species in transpiration per unit of leaf area. In early summer, however, when the maximum vapour pressure de®cit reached 6±7.5 kPa on some days, E. leucoxylon had a 22% lower rate of transpiration per unit of leaf area than the other three species. This difference was presumably due to a stronger stomatal response to high vapour pressure de®cit in E. leucoxylon than the other species. During a period of warm humid weather in late summer, transpiration per unit of leaf area was 75% higher in E. cladocalyx compared with the other three species. The reason for this difference is not known, but it may indicate a species difference in root architecture, and hence a difference in access to ground water or soil water. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Transpiration; Sap ¯ow; Leaf area; Salinity; Eucalyptus occidentalis; Eucalyptus spathulata; Eucalyptus leucoxylon; Eucalyptus cladocalyx * Corresponding author. Tel.: 61-8-8721-8112/8100; fax: 61-8-8723-9058. E-mail address: [email protected] (R.G. Benyon).
0378-3774/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 7 7 4 ( 0 0 ) 0 0 1 2 1 - 9
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1. Introduction Dryland seepage salinity is widely acknowledged as a significant and worsening problem in many areas of Australia, with over 21 000 km2 affected Australia-wide (Robertson, 1996), and has largely resulted from rises in local and regional groundwater in response to wide spread clearing of native vegetation for agriculture. Although reestablishment of deep-rooted perennial vegetation, such as trees, into recharge areas, is widely recognised as the best way to slow this rising groundwater, large areas will require revegetation and the full benefits are only likely to be realised in the long term. Large scale revegetation of recharge areas will involve considerable refocussing of agricultural practices. Planting of trees in discharge locations may provide environmental benefits in the short term (e.g. assisting with reducing soil erosion and the opportunity for water use from locally high water tables) and may enable productive use of already salinised land. Trees planted in saline areas will need to cope with the prevailing soil salinity and water regimes as well as any increase in root-zone salinity that may result from use of saline groundwater by the trees. Thus, the salt tolerance of a particular species is crucial to its sustainable culture in a plantation or agroforestry design (Marcar et al., 1995). Faster growth rates on saline sites will provide better prospects for commercial returns as long as tree products have sufficient economic uses. Planting of fast-growing species will also allow more rapid attainment of maximum leaf area index (LAI) than slow growing species for a given planting density. In saline areas, more salt tolerant species may be able to sustain a higher LAI, and hence higher rates of growth and water use than less tolerant trees. Provided that increases in root-zone salinity can be contained below species-specific salinity thresholds, fast growing species have the potential to use more water on saline discharge sites and hence, at some sites, slow the hydrologically-driven salinisation process. Many catchment scale experiments show that vegetation change can change the hydrological balance of a catchment. Such changes are often related to changes in rates of transpiration (Bosch and Hewlett, 1982). Whether high or low rates of transpiration are desired, it is important to understand how transpiration rates differ between tree species. There is only limited information on the relationship between water use rates of different species and their growth rates under saline and non-saline conditions. Some recent studies indicate that transpiration rates per unit of leaf area generally do not differ substantially between species growing under common environmental conditions (Hatton et al., 1998; Khanzada et al., 1998; Benyon et al., 1999). However, more information on whether the `constant leaf water efficiency' principle (Hatton et al., 1998) holds in a variety of soils and climates would give modellers more confidence in excluding species considerations (and, therefore, simplifying their approach) in predicting the effects of revegetation with trees. In this paper, we compare stem basal area growth, leaf area, sap velocities, daily water use rates and water use per unit of leaf area among four eucalypt species planted in a species evaluation trial on a saline discharge site in centralwest NSW, Australia. Our null hypothesis is that the principle of constant leaf water efficiency holds during different times of the year even on a site where trees may have access to shallow, saline groundwater, and that for trees grown under common conditions, species differences in water use are largely dependent on species differences in leaf area.
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2. Methods 2.1. Site description The trial site is located 19 km southeast of Wellington in centralwest NSW (latitude 328420 S, longitude 1498020 E; 425 m above mean sea level). The climate in the Wellington region is temperate, with cool to cold winters and warm to hot summers. Long-term mean monthly maximum/minimum temperatures vary from about 158C/28C (July) to 328C/178C (January). The mean annual rainfall of 656 mm is evenly distributed throughout the year. Mean annual pan evaporation is 1715 mm, with maximum mean monthly pan evaporation of about 250 mm in December and January (summer), and minimums of about 50 mm in June and July (winter). The soils at the site are yellow soloths and solodics (fine sandy loam topsoils overlying moderately structured light clay to medium clay subsoils). Salinity has developed on the site as a result of rising groundwater, pushed closer to the soil surface by an upslope dyke, with the result that perched groundwater flows within 0±4 m of the soil surface, depending on position (up slope or down slope of the dyke), recent rainfall and time of year. The most saline areas are sometimes waterlogged in winter and spring. 2.2. Trial information and tree selection A species evaluation trial was planted in late October 1990 after the site was rotary hoed, ripped, mounded and treated with Roundup#. Seedlings were fertilised with 50 g `Starter 15', watered and mulched with hay at planting. The trial consisted of 8 replicates, in a randomised complete block design with 36 seedlots in five tree (row) plots, or 180 trees per replicate. The trial was thinned at age 3, leaving only two trees per plot. Additional details are provided by Benyon et al. (1999). Trees in three adjacent replicates of the trial, on the moderately saline part of the site, were used for this study. At age 7, Eucalyptus occidentalis Endl., Eucalyptus spathulata Hook., Eucalyptus leucoxylon F. Muell., and Eucalyptus cladocalyx F. Muell. were the four best performing species in these replicates, in terms of survival, mean height and mean basal area per tree. Each replicate contained one or two trees of each of these species, giving five trees per species, or a total of 20 sample trees. Due to the earlier thinning, and poor survival and growth of many of the other species, the 20 sample trees probably suffered relatively small competition from neighbouring trees, and hence their growth rates were likely to be somewhat representative of pre-closed-canopy conditions. In these circumstances it was not possible to obtain reliable estimates of maximum plantation LAI for these species at this site. 2.3. Measurement of tree leaf area Tree leaf area was estimated in August 1997, December 1997 and February 1998. For each tree, the diameters of all the first-order branches were measured. Three to five of these were selected at random, with the probability of selection being proportional to branch cross-sectional area. Diameters of all second-order branches on each of the
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Table 1 Number of trees sampled during each measurement period Period of sap flow measurement
26 July±12 August 22 November±10 December 5±18 February
E. occidentalis
E. spathulata
E. leucoxylon
E. cladocalyx
Sap flow
Leaf area
Sap flow
Leaf area
Sap flow
Leaf area
Sap flow
Leaf area
4 3 4
3 3 4
5 3 4
3 3 5
5 3 4
3 3 4
5 5 4
5 5 4
three±five first-order sample branches were measured. Four or five second-order branches were then randomly selected on each first-order branch, i.e. a total of between 12 and 25 second-order branches per tree. For each second-order branch, the number of leaves was counted. One small branch (6±10 leaves), usually of third or fourth order, was randomly selected from each second-order branch and sealed in a plastic bag for laboratory determination of leaf area and subsequent calculation of the average area per leaf. A regression was fitted relating the natural log of branch leaf number to the natural log of branch cross-sectional area. This relationship was applied to the first-order branches on which the second-order branch diameters had been measured. The estimated leaf numbers and measured cross-sectional areas for these first-order branches were added to the regression. The final regression equation was applied to all first-order branches to estimate the total number of leaves on the tree. This was multiplied by the average area per leaf, determined from the 12±25 small sample branches, to give tree leaf area. The number of sample trees on each occasion is shown in Table 1. At the end of the study, in February 1998, one tree of each species was felled, and leaf area determined destructively. All leaves were harvested, and leaf total fresh weight measured in the field. A sub sample of fresh leaves was also weighed in the field at the same time, sealed in a plastic bag and stored at 58C for laboratory determination of the leaf area:fresh weight ratio. This ratio was applied to the total fresh weight of leaves to give tree leaf area. These destructive estimates were compared with estimates based on leaf counts made before the trees were felled. 2.4. Measurement of tree water use The unified nomenclature for sap flow measurements proposed by Edwards et al. (1997) is used throughout this paper. The term `sap velocity' refers to the speed of sap moving through the sapwood, expressed on a total sapwood area basis (denoted vs by Edwards et al. (1997)), in mm s 1. Sap velocities in the 20 sample trees were measured for periods of 2±3 weeks in winter (July/August 1997), early summer (November/December 1997) and late summer (February 1998). Due to the amount of time required to non-destructively estimate leaf area, and due to occasional failure of some of the sap flow sensors, it was not possible to collect measurements from all 20 trees in all three sampling periods. A minimum of three trees per species was sampled in each period (Table 1).
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Sap velocities were measured at four points within the sapwood of each tree every 30 min using sap flow sensors employing the compensation heat pulse technique (SF-300 Sapflow Sensor, Greenspan Technology, Warwick, Qld, Australia). Wounding around a sub sample of the sap flow sensors was measured in wood samples collected in February 1998. Heat pulse velocities were corrected for wounding using the coefficients of Swanson and Whitfield (1981). Wood and water volume fractions were measured in 5 mm diameter cores collected from each tree. Corrected heat pulse velocities were converted to sap velocities using equations described by Edwards and Warwick (1984). At the beginning of each measurement period, the sap flow sensors were used to measure the thickness of the conducting wood at breast height (1.3 m) on the north, east, south and west side of each tree. The method was the same as that described by Hatton et al. (1995). The E. spathulata trees were all multi-stemmed, with stem diameters ranging from 20 to 120 mm. For each of these trees, the cross-sectional area of each stem was measured and the total stem cross-sectional area of the tree calculated. Within a tree, the four sap flow sensors were then allocated randomly among the stems, with the probability of a stem being selected for sampling being proportional to its cross-sectional area. Some large diameter stems (>100 mm), for example, were allocated two sensor sets, while most of the small diameter stems (<40 mm) were not sampled at all. Stratified random sampling, similar to that recommended by Dye et al. (1991) and Hatton et al. (1995) was used to determine the sensor positions within each tree. The conducting wood was divided into four concentric rings of equal cross-sectional area. One sensor was randomly located within each ring. In the case of the multi-stemmed trees, the sensors were located randomly within each sample stem. The average sap velocity was calculated as the arithmetic mean of the four sample points. 2.5. Measurement of stomatal conductance and leaf water potential On a day during the December 1997 and February 1998 measurement periods, stomatal conductance of abaxial and adaxial surfaces was determined for three fully expanded leaves per tree in lower or mid canopy positions for two or three trees per species, between 10.00 and 12.00 h using a LiCor 1600 (LiCor, Lincoln, NE, USA). Pre-dawn water potential was measured on the same days on at least three leaves per species by inserting cut petioles into the chamber of a Schlolander pressure bomb apparatus and obtaining the balancing pressure. Water potential was usually measured within 15 min of cutting the petioles and individual leaves were placed into plastic bags and kept on ice until measured. 2.6. Measurement of soil water, salinity and pH Surface soil volumetric water content to 0.8 m depth was estimated next to each tree at the beginning and end of each sap flow measurement period using a neutron probe (503 Hydroprobe, Campbell Pacific, Pachero, CA, USA). One access tube was located 1 m to the east of each tree. Soil water was measured at 0.1 m depth intervals to 0.8 m. Salinity,
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expressed as the electrical conductivity of a 1:5 soil:water solution (EC1:5), was determined in a laboratory on soil samples collected in December 1997 and February 1998. A soil auger was used to collect samples from 0±0.15, 0.15±0.30, 0.30±0.45 and 0.45±0.60 m depths at a point close to each tree on which sap flow was measured. For each soil sample, gravimetric water content and pH1:5 were also determined. 2.7. Weather measurements Net radiation, air temperature, relative humidity and rainfall were recorded in an open field about 100 m from the site using an automatic weather station (Weather Master 2000, Environdata Australia Pty. Ltd., Warwick, QLD, Australia). Rainfall was recorded as hourly totals, while other variables were sensed at 1 min intervals and recorded as hourly averages. Vapour pressure deficit was calculated from air temperature and relative humidity. 2.8. Calculation of daily mean sap velocity, tree water use and water use per unit of leaf area The Greenspan Sapflow Sensor cannot distinguish low sap flows from no sap flow below a nominal threshold velocity of about 0.015 mm s 1 (Vertessy et al., 1997). In reality, the threshold value varies markedly between sensors (Becker, 1998). For each sensor set, a threshold value was determined from the night-time measurements. A visual appraisal of the raw sap flow data was used to identify four nights during the measurement period when apparent flows were at their lowest. For these nights, the mean and standard deviation of flows between midnight and 05.00 h were calculated, and the upper 1% confidence limit estimated. Any values higher than this were deemed to represent real flow. For each tree, the mean sap velocity on each day (06.00±06.00 h) was calculated as the mean of all corrected flows (actual flow if above the threshold value and zero flow if below) recorded every 30 min during that 24 h period. For each day, tree water use, in litres, was calculated as the product of mean sap velocity and conducting wood crosssectional area. For each of the three measurement periods, tree mean daily water use was divided by tree leaf area, giving average water use per unit of leaf area. The statistical significance of species effects were determined by one-way analysis of variance (ANOVA). The significance of differences between individual species were analysed using t-tests. 3. Results 3.1. Accuracy of estimates of tree leaf area based on leaf counts Averaged over the four sample trees the difference between the two methods of estimating tree leaf area was about 9% (2.5 m 2 per tree). For the E. spathulata and
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E. cladocalyx trees there was no difference between the two methods. For the other two trees, the leaf count method under-estimated tree leaf area compared with the felling and weighing method (3 m2, or 10%, for E. leucoxylon and 7 m2, or 24%, for E. occidentalis). The size of the error was unrelated to tree leaf area. With only one tree felled per species, it was not possible to determine whether the level of accuracy of the leaf count method differed between species. The sample size was not large enough to determine whether the two under-estimates represented a consistent pattern of bias associated with the leaf count method. 3.2. Species comparisons of tree size Means and standard errors of basal area, conducting wood area and leaf area for each species are shown in Fig. 1. Data are only shown for the three trees of each species that were sampled on all three occasions. There was large variation in tree size between species. Eucalyptus spathulata had the largest leaf area per tree, being about three times greater than E. cladocalyx, four times greater than E. occidentalis and five times greater than E. leucoxylon. Species differences in basal area and conducting wood area were of a similar magnitude (Fig. 1). 3.3. Species comparisons of sap velocity and response to VPD Differences in sap velocities between species were smaller than for growth (Fig. 2). In July/August, species differences were not statistically significant. During November/December, sap velocities were higher in all species compared with July/August. There was greater variation between species, but again the differences were not statistically significant (Fig. 2). This period included several hot days with low humidity, when day-time vapour pressure deficits (VPD) sometimes peaked at between 5 and 7.5 kPa. On these days, sap velocities in E. leucoxylon dropped markedly during the middle of the day compared with the other three species (Fig. 3). On 22 November, in mild conditions, the diurnal course of sap velocities was almost identical in the four species (Fig. 3). From 24 to 26 November, day-time maximum VPDs of 6±7.5 kPa were recorded (Fig. 3). On these days, sap velocities from late morning to early evening each day, averaged only about 0.08 mm s 1 in E. leucoxylon compared with about 0.14 mm s 1 in the other species. On 25 and 26 November, the differences in daily means between E. leucoxylon and the other three species were statistically significant (P < 0:05). Day-time hourly mean sap velocities were plotted against day-time hourly mean VPD for the period shown in Fig. 3 (see also Fig. 4). Sap velocities were similar for E. leucoxylon and the other three species up to a VPD of about 3 kPa. As VPD increased between 3 and 5.5 kPa, there was a corresponding decrease in sap velocity. This decrease was more marked in E. leucoxylon. There was no further decline in sap velocity above a VPD of about 5.5 kPa. For any VPD between 5.5 and 7.5 kPa, mean sap velocity remained constant at an average 0.08 mm s 1 in E. leucoxylon and 0.16 mm s 1 in the other three species. Differences between E. occidentalis, E. spathulata and E. cladocalyx during this period were small and not statistically significant (data not shown).
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Fig. 1. Species comparisons of: (a) tree mean basal area; (b) conducting wood area and (c) leaf area in a 7-year old species trial. The same three sample trees of each species were measured on each occasion. Bars indicate the standard errors of the estimates.
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Fig. 2. Species comparisons of daily mean sap velocities in a 7-year old species trial in winter, early summer and late summer. The same three sample trees of each species were measured on each occasion. Bars indicate the standard errors of the estimates.
The response to high VPD in E. leucoxylon was observed on 8 of the 18 measurement days during November/December. It did not vary markedly between the three E. leucoxylon trees sampled during this period (data not shown). The greatest variation in sap velocities between species occurred in February (Fig. 2). E. cladocalyx had the highest mean (0.090 mm s 1) and E. spathulata and E. occidentalis the lowest (0.064 and 0.061 mm s 1). The difference between E. cladocalyx and E.
Fig. 3. Comparison of mean sap velocities in E. leucoxylon (mean of three trees) and three other eucalypt species (mean of 11 trees) during a 6-day period in early summer. The diurnal course of vapour pressure deficit is also shown.
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Fig. 4. Day-time hourly mean sap velocity vs. day-time hourly mean vapour pressure deficit. Filled circles represent the mean of three E. leucoxylon trees. Open diamonds represent the mean of 11 trees of three other eucalypt species. The data were collected between 21 and 26 November.
spathulata was significant (P < 0:05). The difference between E. cladocalyx and the other two species was not significant. Day-time vapour pressure deficits during February were not high enough to elicit the reduction in sap velocity observed in E. leucoxylon in November/December. The mean sap velocity in E. leucoxylon during February was actually 15±20% higher than in E. occidentalis and E. spathulata (Fig. 2), although the differences between these three species were not statistically significant. 3.4. Species comparisons of water use per tree E. spathulata had daily water use four±six times greater per tree than the other three species (Fig. 5). This difference was largely due to differences in tree size. Conducting wood area per tree was several times greater in E. spathulata than in the other three species (Fig. 1), whereas differences in sap velocity were comparatively small (Fig. 2). Thus, on this site, differences in water use per tree were largely related to differences in growth rates and the associated differences in conducting wood area and leaf area. 3.5. Species comparisons of water use per unit of conducting surface Microscopic analysis of leaves and porometer measurements of stomatal conductance (data not shown) indicated that E. cladocalyx was hypostomatous, while the other species were amphistomatous. To account for this difference, the projected leaf areas of E. occidentalis, E. spathulata, and E. leucoxylon were doubled and transpiration was expressed per unit of conducting surface area (conducting surface).
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Fig. 5. Species comparisons of daily mean tree water use in a 7-year old species trial in winter, early summer and late summer. The same three sample trees of each species were measured on each occasion. Bars indicate the standard errors of the estimates.
Average water use per unit of conducting surface for each species is shown in Fig. 6. Variation between species in July/August was only minor, and not statistically significant. In November/December, E. leucoxylon had a lower water use per unit of conducting surface than the other three species (P < 0:05). The greatest difference in transpiration per unit of conducting surface between species occurred in February. During this period, transpiration was significantly higher (P < 0:001) for E. cladocalyx than for the other three species (Fig. 6). Compared to a
Fig. 6. Species comparisons of average transpiration per unit of conducting surface in a 7-year old species trial in winter, early summer and late summer. The same three sample trees of each species were measured on each occasion. Bars indicate the standard errors of the estimates.
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mean transpiration rate during February of 1.18 l m 2 per day in E. cladocalyx, rates in the other three species during this period were 0.57 l m 2 per day in E. leucoxylon, 0.70 l m 2 per day in E. spathulata and 0.74 l m 2 per day in E. occidentalis. Between the November/December and February measurement periods, transpiration per unit of conducting surface increased in E. cladocalyx, but decreased in the other species (Fig. 6). 3.6. Relationship between tree water use and soil water, salinity and pH Differences in soil water content of the top 0.8 m, both between species and over time, were small, and hence are unlikely to have had much influence on tree water use. Measurements of soil water at depths below 0.8 m were not made. Since the lateral extent of the trees' root systems were not known it is unlikely that measurements of soil water at greater depths would have assisted in determining from what soil depths the different species were obtaining their water. Mean pre-dawn water potential did not differ significantly between the December 1997 and February 1998 measurements. Values were similar between E. cladocalyx ( 1.76 MPa), E. occidentalis ( 1.71 MPa) and E. leucoxylon ( 1.62 MPa), but significantly lower (P < 0:05) in E. spathulata ( 2.48 MPa). This difference appeared to be unrelated to species differences in water use per unit of conducting surface. Rainfall was below average through most of the study. In 8 months from July 1997 to February 1998, only about 200 mm of rain was recorded at the site. The only statistically significant change in surface soil water content occurred between 4 and 16 February when, for all four species, soil water content of the top 0.8 m of soil increased by an average 4 mm. About 35 mm of rain fell between 7 and 9 February. The mean EC1:5 of the top 0.6 m of soil for each species varied from 1.0 to 1.34 dS m 1 in December 1997 and February 1998, and soil pH1:5 varied from 8.03 to 8.5. There were no significant differences in salinity or pH between species or between the December and February measurements. Salinity and pH were not measured in July/ August 1997 and they were not measured below 0.6 m. 4. Discussion In two instances, we rejected the null hypothesis, that the principle of constant leaf water efficiency holds during different times of the year in a plantation growing over a shallow water table. During the November/December measurement period, E. leucoxylon had lower transpiration per unit of conducting surface than the other three species (Fig. 6). In February, E. cladocalyx had a higher transpiration rate per unit of conducting surface than the other species (Fig. 6). As we discuss later, however, the effect of these differences on average water use per tree is minor compared to differences in water use per tree resulting from differences between species in tree leaf area. 4.1. Species differences in transpiration per unit of conducting surface On days of high VPD, such as 24±26 November, sap velocity, and hence transpiration per unit of conducting surface, in E. leucoxylon declined markedly compared with the
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other three species (Figs. 3 and 4). As a result, E. leucoxylon had lower average transpiration per unit of conducting surface than the other three species during this period. We presume this difference was due to a species difference in stomatal response to VPD. However, since stomatal conductance during this period was only measured directly for one very brief period (only one measurement on each of three leaves per tree in the lower or mid canopy on one morning at a time when VPD was relatively low), we cannot conclude with certainty that this was the reason for the difference. To indicate the likely magnitude of species differences in stomatal conductance (and resistance) in response to VPD, the tree transpiration, tree leaf area and hourly weather data (solar radiation, air temperature and air relative humidity) collected at the site during November were used in the Penman±Montieth equation, as described by Landsberg (1999), to back-calculate stomatal conductance and stomatal resistance for each species. The relationship between estimated resistance and VPD is illustrated in Fig. 7. We show resistance in the figure, rather than conductance, as this most clearly highlights the difference between the species at high VPDs. The weather data were measured in an open field about 100 m from the trees. Microclimatic conditions within the canopy may have differed from those at the weather station. These conditions (e.g. leaf surface VPD) may have differed between species, which could also have resulted in a difference in transpiration without there being a difference in stomatal conductance. Species differences in the relationship between stomatal conductance and VPD have been reported in laboratory studies (e.g. Sheriff, 1977) and field studies (e.g. Hookey et al., 1987). The latter identified differences in the relationship between stomatal conductance and VPD among 23 eucalypt species and provenances. These relationships also varied seasonally (Hookey et al., 1987). There is still debate on whether stomata respond to the rate of transpiration or somehow `sense' humidity or VPD directly (Whitehead, 1998). Many studies indicate that stomatal closure is a feedback response to
Fig. 7. Comparison between two species of the relationship between inferred stomatal resistance and vapour pressure deficit. Stomatal resistance was calculated from tree water use, leaf area, radiation, air temperature and relative humidity, using the Penman±Monteith equation. Water use and climate data were day-time hourly averages for the period between 21 and 26 November.
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the rate of transpiration (e.g. Mott and Parkhurst, 1991; Monteith, 1995), although in some studies evidence exists for a feedforward response controlled by metabolic signals (e.g. Grantz, 1990; Bunce, 1996). A decrease in transpiration rate with increasing VPD is not consistent with a feedback stomatal response (Franks et al., 1997). The halving of transpiration in E. leucoxylon as VPD increased from 3 to 5.5 kPa was an apparent feed forward response to increasing VPD. Such a large reduction in transpiration with increasing VPD is unusual (Monteith, 1995). Differences in stomatal conductance measured on individual leaves do not necessarily translate into differences at the tree or stand level. If conditions at the leaf surface are strongly decoupled from conditions in the air outside the leaf boundary layer, the transpiration rate will be determined mainly by net radiation rather than stomatal conductance (Jarvis and McNaughton, 1986). Our measurements of transpiration were collected at the tree level, rather than the leaf level. Despite this, a species difference in the relationship between transpiration rate per unit of conducting surface and VPD (measured at a nearby weather station) was observed. It is likely the canopies of the study trees were strongly coupled with regional VPD, as the study site was surrounded by open grazing land and the canopy of the plantation itself was discontinuous and aerodynamically rough. Under these conditions, species differences in the inferred relationship between stomatal conductance and VPD did result in species differences in transpiration. In large, continuous stands, where conditions inside the canopy boundary layer may be poorly coupled with regional VPD, species differences in the relationship between stomatal conductance and VPD may not translate into species differences in plantation water use (Jarvis and McNaughton, 1986). It is unlikely that the stronger response to VPD observed in E. leucoxylon would translate to large differences in transpiration between species at this site over an entire year. The November/December measurement period was the hottest period of the summer, with the highest VPDs. On most days of the year, the VPD would not be high enough to elicit the reduction in transpiration observed in E. leucoxylon. Hence, averaged over a whole year, differences in response to high VPD between these four species would be expected to make little difference to their respective rates of transpiration at locations with a similar climate to Wellington. Species differences may, however, be important at sites with drier, hotter summers than Wellington, and limited water availability. In such conditions, over summer, E. leucoxylon would deplete soil water reserves more slowly than the other three species, perhaps allowing it to survive better during drought periods. On the other hand, if planted to lower a shallow water table, lower water use of E. leucoxylon during summer may be undesirable. The difference in transpiration per unit of conducting surface that occurred in February between E. cladocalyx and the other species is more difficult to explain. It is not likely to have been a stomatal response to VPD because the weather in February was cooler and more humid than in November/December. The higher transpiration in E. cladocalyx was observed at all VPDs, not just at high VPDs (data not shown). Lower mean transpiration per unit of conducting surface of the other three species in February compared to November/December (Fig. 6) may have been due to lower potential evapotranspiration. Net radiation and day-time VPDs were both lower in February than in November/ December (data not shown). In E. cladocalyx, however, transpiration per unit of
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conducting surface was higher in February than in November/December (Fig. 6). The difference between species in February may have been indicative of differences in root architecture. If E. cladocalyx had more roots near the soil surface than the other species, it may have been able to make greater use of 35 mm of rain that fell between 7 and 9 February. Alternatively, E. cladocalyx may have had a deeper root system for accessing ground water, or a more extensive root system for exploiting a larger volume of soil (i.e. a lower shoot to root ratio), or may have been better adapted to extracting water at low soil water potential. Given the small variation in surface soil water during the study, it is likely all trees in the study were using water from deeper in the soil profile and possibly from the water table. Two piezometers were installed to 3 m depth within the study area before the November/December measurement period, but these remained dry throughout the study. Piezometers further up-slope, to depths of 6 m, however, showed that there was ground water present between 3 and 4 m depth during the period of study. In an adjoining trial about 100 m away, 8-year old E. camaldulensis Dehnh. trees were shown to be transpiring ground water at 2 mm per day during the 1997/1998 summer. Previously Hatton et al. (1998) found no differences in leaf water efficiency between species in several eucalypt stands subject to seasonal water limitations. Stand LAI were all in equilibrium with long-term rainfall (Hatton et al., 1998). Our site differed in that it was a mixed species plantation growing over a shallow water table, with spacings between trees imposed by initially uniform planting densities and subsequent thinning. With large spaces between trees resulting from poor survival of some species and removal of trees by thinning, the leaf area of individual trees was probably still increasing, and a long-term equilibrium LAI had probably not yet been attained. In this study, when comparing transpiration between species, we found it was better to express transpiration per unit of conducting surface than per unit of projected leaf area. In the August and November/December measurement periods, if transpiration was expressed per unit of projected leaf area, the amphistomatous E. cladocalyx had half the rate of transpiration compared with the other three species, which were all hypostomatous. Hence, if selecting species based on tree leaf areas measured in field trials, it is important to determine whether each species is hypostomatous or amphistomatous. 4.2. Implications of growth differences for management of saline discharge sites For managing saline discharge sites, the species differences in growth rates are more important than the species differences in transpiration per unit of conducting surface area. A sixfold variation in average water use per tree between the four species (Fig. 5), was associated mainly with variation in tree size (Fig. 1). At 7 years of age, E. spathulata had by far the largest leaf area per tree of any species in the saline parts of this site. Consequently, it also had the largest water use per tree. This large difference was made possible by the large spacing between trees created by poor survival or growth of many of the species in the trial and the subsequent thinning. The large spacing between trees meant that individual trees were able to expand their canopies at a rate uninhibited by competition for space or light from neighbouring trees. With the small plots (five trees, single row) it was not possible to determine the potential maximum LAI attainable by each species at this site.
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It could be argued that, from a hydrological viewpoint, species differences in growth rates, especially in leaf area growth per tree, do not matter. When planting blocks of trees to use up water, many species will eventually attain the maximum LAI supportable by that site. This argument ignores the effects of planting density on the costs of plantation establishment and on the time taken for hydrological benefits to begin to accrue. For a given planting density, use of a slower growing species would delay canopy closure. Alternatively, reducing the time to plantation canopy closure by increasing the planting density would increase the costs of establishing the plantation. Hence, irrespective of whether the trees are planted for commercial or hydrological benefits, identification of fast growing genotypes for the intended planting sites is important. 5. Conclusions To maximise water uptake in plantations grown over shallow water tables, careful selection of species to identify those best adapted for fast growth at the site is important to ensure rapid development of large, leafy crowns. While tree water use is largely related to tree leaf area, significant differences in water use per unit of leaf area can also occur for different species growing under common conditions. In this study, significant differences in transpiration rates occurred as a result of differences in the response of transpiration to high VPD. In one species, transpiration per unit of leaf area was reduced by about 50% at very high VPDs, while at the same time, in three other species only a slight reduction occurred. Species differences in transpiration per unit of leaf area can also occur at times when VPDs are not high. The reasons for this could not be determined in this study, but may have been a reflection of species differences in root architecture, giving some species the ability to exploit a greater volume of soil, or extract more ground water. Acknowledgements Funding of this study by the Joint Venture Agroforestry Program (CSF-46A, Agroforestry Design Guidelines for Balancing Catchment Health with Primary Production) is gratefully acknowledged. The assistance of Russell Millard (NSW Land and Water), and Debbie Crawford and Afzal Hossain (CSIRO Forestry and Forest Products) with field data collection is also gratefully acknowledged. We thank the Campbell family, owners of the `Bonada' and `Easterfield' properties, for providing the land for, and access to, the study site. We also thank Dr. Tom Hatton and Dr. Don White for their valuable comments on the manuscript.
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