Using the EM38 to measure the effect of soil salinity on Eucalyptus globulus in south-western Australia

Agricultural water management Agricultural

Water Management

27 ( 1995) 69-86

Using the EM38 to measure the effect of soil salinity on Eucalyptus globulus in south-western Australia D.L. Bennett*, R.J. George Western Australian Department ofAgriculture. P.O. Box 1231, Bunbury, W.A. 6230, Ausrralia Accepted 26 April 1994

Abstract Landholders in the > 600 mm p.a. rainfall zone of south-western Australia are establishing small ( < 10 ha) plantations of Eucalyptus glob&s near saline seeps to lower groundwater levels and obtain additional income from timber and pulp. Soil salinity (as measured by Geonics electromagnetic induction meters) reduced the survival and growth rate of different-aged E. globulus at five sites. E. glob&us can survive moderate soil salinities [apparent electrical conductivity (EC,) as measured by a Geonics EM38 in the horizontal position (EM38H) up to 150 mS/m]. However growth rates declined at 50 to 75 mS/m. A combination of soil salinity and waterlogging caused a reduction in growth rate at 2.5 mS/m. Plantings up to 14 years old were adversely affected by continuing rises of the watertable and increased salt concentrations around the roots. Adequate site investigations (EM38, depth and salinity of watertable) will reduce the risk of establishing small plantations low in the landscape. Many existing plantations have been established too close to saline seeps and are now showing evidence of growth decline and stress associated with soil salinity. The Geonics EM38 is suitable for advisers and farmers to assess sites rapidly, before planting E. globulus. Ke.vwords: Eucafyprus globulus; Salinity; Electromagnetic

induction

1. Introduction

In south-western Australia in 1989, it was estimated that over 443000 ha or 2.83% of previously arable land was too saline for conventional agriculture (George, 1990). Schofield et al. ( 1989) showed that reforestation of lower slope and groundwater discharge areas reduced groundwater levels and slowed the rate of salinisation. Corporate investors and farmers have established many plantations of Eucafyptus globulus near saline seeps for the * Corresponding

author

0378-3774/95/$09.50 SSD10378-3774(95)001

0 1995 Elsevier Science B.V. 126-9

All rights reserved

production of wood pulp in areas receiving more than 600 mm/yr annual rainfall (Bartle, 199 1) The growth rates of the plantations, as measured by the Mean Annual Increment (M. A. I.) of merchantable wood fibre, varies from 10 to 50 cubic metres and is worth A$20 to A$35 per cubic metre (A$ = US$O.70 in 1993), when managed on a 1O-year rotation (Shea and Hewett, 1990). These plantings are attractive to landholders wanting to lower watertables while at the same time getting income from timber pulp. Many plantations are located in the lower slopes of catchments which are developing dryland salinity. The effect of soil salinity on the growth rate and survival of E. globulus is very important. Annual pasture species such as barley grass (Hordeum spp.), traditionally used as visual indicators of the severity of soil salinity, are poorly correlated with soil salinity but well correlated with the depth to saline groundwater (N&en, 198 1). Indicator plants are greatly affected by both agricultural management and waterlogging intensity. Accurate site assessment using soil samples is time consuming, expensive and can be highly variable (Slavich and Read, 1984), making comparisons between measured soil salinity and tree species performance difficult (N. Pettit, pers. comm., 1992). By contrast, the Geonics EM38 is a portable instrument designed to take in situ field measurements of apparent soil conductivity. Cameron et al. ( 198 I ) and Norman and Heslop ( 1990) successfully used the EM38 for large soil salinity surveys, while Slavich and Read ( 1984) used the EM38 to show the effect of soil salinity on barley (Hordeum lxlgare) yield. They found that the EM38 provided reliable estimates of soil salinity without intensive soil sampling. We were interested to see the effect of soil salinity on the survival and growth rates of E. globulus and whether the EM38 could be used as a field tool to help assess the suitability of cleared farmland for growing E. globulus.

2. Methods 2. I. Site selection We measured tree performance and soil salinity on established plantations within the 600 to 900 mm rainfall zone (Fig. 1). Cleared farmland within this rainfall zone will support most of the future commercially viable plantations of E. globulus (Shea and Hewett, 1990). The tree density within each plantation was approximately 1000 stems per hectare at establishment. The sites chosen had developing soil salinity problems. We chose plantings which were between 0.5 and 14.5 years old, located in different catchments and on soil types regularly used for plantings throughout the region (Table 1). 2.2. EM38 function and calibrutiorl The electromagnetic induction meter used in this study was the Geonics EM38, which has a transmitter and repeater coil separated I m apart. This instrument measures apparent soil electrical conductivity (EC,) in units of millisiemens per metre (mS/m). When the EM38 is used in the horizontal mode (EM38H) on a uniform soil, 75% of the signal response is estimated to come from the top 1.O m of soil. In the vertical mode (EM38V), 75% of the signal is estimated to come from the top 1.8 m. However, for heterogeneous

D.L. Bennett, R.J. George /Agricultural

Water Management 27 (1995) 69-86

71

A

600

lsohyet (mm)

q

Nunnenburg’s

\

\

ALBANY+--‘

Fig. 1. Location of the study sites.

Table I Summary of location, tree age and soil descriptions

for the sites where measurements

Site name

Location

Catchment

English’s

Mt. Barker

Hay River

6 months

Wunnenburg’s

Bowelling

18 months

Hilder’s

Boyup Brook

Stene’s Valley Plantings Stene’s Strip Plantings

Collie

East Collie River Blackwood River Bingham River Bingham River

Collie

Tree age

22 months 4.5 years 14.5 years

were obtained Soil description Deep gravelly yellow loam (over clay at > 0.50 m) Gravelly brown loam (over clay at > 0.50 m) Deep grey sand (over clay at > 2.00 m) Brown sandy loam (over clay at < 0.20 m) Brown sandy loam (over clay at > 0.30 m)

72

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Water Mnnngernent 27 (I 995) 69-86

soils the proportion that comes from different soil layers depends on the conductivity of those layers (Corwin and Rhoades, 1982). The principles of electromagnetic induction and soil conductivity measurements are described in detail in McNeil1 ( 1980a, b). Soil electrical conductivity is influenced by soil porosity, water content, clay content, soil temperature and soil salinity (McNeill, 1980a). To calibrate the EM38 with measured soil salinity, we used the electrical conductivity of a soil-water extract (EC,). EC, is the standard laboratory method of measuring soil salinity for plant response purposes (Richards, 1954). By calibrating the EM38 (EC,) with EC, at various depth intervals, it is possible to determine the relative contribution of EC, to EC, over the other factors, as well as the depth from which most of the signal was being transmitted. Similar calibration methods were used by Lesch et al. ( 1992) and Slavich and Petterson ( 1990). We calibrated the EM38 at two sites which reflected the extremes of a range of soil textures found in the region. The sites were on a waterlogged, deep sand near Boyup Brook (Hilder’s) and a saline heavy-clay soil at Waterloo (Gelmi’s). At each sample site, EM38H and EM38V were measured and soil samples obtained at 0.25-m intervals for analysis of EC,. The 14 sandy sites were sampled down to the watertable (which had an EC of 500 mS/m) at 1 m below ground and the 10 clayey sites were sampled down to 2 m. Linear regressions were calculated between EM38 and EC, for each depth to obtain depth interval calibrations. Saturation percentages (SP) of the soil samples were also measured (as an estimate of clay content) and correlated with EM38 to determine the effect of clay content on EC,. At other sites in south-western Australia, McFarlane and Ryder ( 1990) obtained calibrations for sandy and gravelly duplex soils near Esperance, while Ferdowsian and Greenham ( 1992) used the electrical conductivity of 1:5 soil/water solutions (EC 1:5, 32 sites) to calibrate EC, readings on the Denmark Catchment. 2.3. EM38 and tree measurements Field measurements were carried out in 1992 during the late summer and early autumn. We chose this period since it is the period when most site assessment and preparation work for plantations is carried out. At all sites, rainfall and/or capillary rise from the near-surface watertables ensured electrical conductance was not limited by inadequate soil moisture. The EM38 was used to delineate areas within established plantations containing more than 100 trees with EC,‘s of less than 2.5 mS/m to more than 175 mS/m. These areas were pegged before detailed measurements were made. At all sites EC, values decreased upslope away from saline seeps. Measurements of EC, by both EM38V and EM38H were made next to the base of every tree within the selected area. Where trees were planted on single ridge mounds up to 0.3 m high, the measurements were made adjacent to the mound. Ridge mounding is used on waterlogging-prone sites in south-western Australia to improve tree survival and growth (Pettit and Froend, 1992). Measurements of tree growth and survival were made at every site where EC, was measured. Trees aged up to I 8 months old had measurements of tree height (h) and crown diameter (d) recorded. These were used to calculate a Crown Volume Index (CVI) for each tree using the formula, CVI = h. (d)’ (Biddiscombe et al., 1985). For older trees, measurements of height (h) and trunk diameter over-bark at breast height (dbhob) were

D.L. Bennett, R.J. Georgr/Agriculturctl

Water Mmcrgement

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27 (199.5) 69-86

recorded (Wood, 1990). The Merchantable Volume of Wood (MVW) within each tree was calculated using an equation developed by the Department of Conservation and Land Management, Western Australia, for E. glob&s grown in south-western Australia (G. Ellis, pers. comm., 1993). Both CVI and MVW were used as comparative indices of growth between trees within the same plantations. For each site, the correlation coefficient (R*) between the growth index of the surviving trees and both EM38H and EM38V was calculated. The EM38H and tree measurements were then divided into groups based on 25 mS/ m increments of EM38H, and the means of the growth measurements and survival calculated for each group. Leaf samples were taken from the trees within the measured plot at Hilder’s and analysed for the concentration of chloride (mg/g of dry matter) using end-point titration with silver nitrate on a water extract of the ground leaf material (Piper, 1947).

3. Results 3.1. Calibration

of EM38 with EC,

Monthly soil temperatures at 0.5 and 1 .O m depths at Albany and Perth, together with the temperature correction coefficients for the electrical conductivity of soil solutions (Richards, 1954), are given in Table 2. The coefficients for the two depths vary only slightly and we averaged them to correct EC, to 25°C. The coefficients vary widely between the seasons (31% at Perth and 25% at Albany), however have smaller (maximum of 15%) variation within seasons. Salinity measured with the EM38 was well calibrated with EC, at specific depths and soil types (Table 3a). EC, was well correlated with EM38V and EM38H at 0.75 to 1.OOm at the sandy site. There was a good correlation for EM38H at 0.25 to 0.75 m at the clayey site. For EM38V, the best correlation was at 0.50 to 0.75 m depth The best calibrations for Table 2 Average monthly soil temperatures (T) at 0.5 and 1 m depths for Albany and Perth and the temperature coefficient (F) required to correct EC, to 25°C Month

Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec.

correction

Perth:

Albany: T(0.5)

F(0.5)

T( I .O)

F( 1.0)

T(0.5)

F( 0.5)

T( 1.O)

F( I .O)

22 23 22 20 16 14 13 13 15 15 18 20

1.06 1.04 1.06 1.11 1.22 1.28 1.31 1.31 1.25 1.25 1.16 1.11

21 22 22 20 18 16 14 13 15 15 17 19

1.09 1.06 1.06 1.11 1.16 1.22 1.28 1.31 1.25 1.25 1.19 1.14

28 28 26 23 18 15 14 15 17 20 23 26

0.94 0.94 0.98 1.04 1.16 1.25 1.28 1.25 1.19 1.11 1.04 0.98

21 21 21 24 20 17 15 15 17 19 22 25

0.96 0.96 0.96 1.02 1.11 1.19 1.25 1.25 1.19 1.14 1.06 1.oo

Table 3 Calibration relationships for the EM38 for soils in south-west Australia: (a) calculated by the authors at different depths for deep sand and heavy clay soils: (b) from McFarlane and Ryder ( 1990); (c) from Ferdowsian and Greenham ( 1992) Depth (m) (a) Deep Sand: EM38H 0.75-I .OO EM38V 0.75-I .oo Heavy Clay: EM38H 0.25-0.75 EM38V 0.5fSo.75

Equation

R’

*(=p
0.77

70

*

EC, = 4.76 EC, + 78.36

0.87

53

*

EC,=3.31

190

0.85

71

i%

EC, = 2.97 EC, + 162

0.92

58

*

EC, = 8.40 EC, - 200

0.95

*

EC, = 6.67 EC, - 239

0.93

*

ECI:5=0.38EC,-

1.04

0.90

*

EC I :5 = 0.42 EC, - 1.74

0.90

*

EC,=4.72

EC,+

EC,+

I Il.98

(b) EM38H 0.60-0.90 EM38V 0.60-0.90 Cc) EM38V 0.20-0.40 EM38V 0.20-0.80

both sandy and clayey soils give very similar EC, values derived from EM38H (EC,) (Fig. 2). Small ( < 20%) differences arise at high EM38 values ( 175 mS/m). The ratios between EC, and EC I:5 were 7 for the clayey soils and 2 1 for the deep sands. Correlation’s between SP and EC, were positive, but poor, at all depths. The mean SP of the clay and the sand was 54% and 23%, respectively. 3.2. Tree survival and growth Tree growth was slightly more highly correlated with EM38H than with EM38V at all sites except English’s (where EM38H was much more highly correlated). For example, for 1&month-old trees at Wunnenburg’s the R2 values for tree growth were 0.324 for EM38V and 0.325 for EM38H (not significantly different). There were poorer correlations between tree growth and EM38 at English’s (6-month-old trees), with EM38V not being significantly correlated. At all sites and for all tree ages, there was a trend of decreasing survival and tree growth with increasing soil EC, (Fig. 3). Within the measured area of Hilder’s, soil EM38H values were all less than 75 mS/m. At Stene’s strip plantation, there were no areas with EM38H values less than 50 mS/m.

D.L. Bennett, R.J. George /Agriculturd

OJ

J

0

25

Water Management

27 (1995)

75

69-86

-I 50

75

100

125

150

175

2Txl

ECa (mSh)

Fig. 2. Comparison

of three EM38H calibration

equations from South-West

Australia.

The survival of the 6-month-old trees at English’s was reduced to 75% at EC,‘s between 100 and 150 mS/m and to below 20% at salinities above 150 mS/m. The mean growth rates of the trees declined steadily with increasing soil salinity, however, the variation about the mean increased with higher EC,. At Wunnenburg’s, where the trees were 18 months old, the survival of E. glob&s was reduced from 80% to less than 50% on soils with EC, greater than 125 mS/m. There is a rapid growth decline when EC, is above 75 mS/m. There is also a decline in CVI for salinities between the 25 to 50 mS/m range and the 50 to 75 mS/m range. Leaf margin necrosis (Fig. 4) was present on trees when EM38H exceeded 50 mS/m. Rogers (1985) found that in seven Eucalyptus species (including E. globulus), leaf margin necrosis was generally associated with higher leaf chloride concentrations and suggested that it is a sign that the tree is close to its salinity tolerance limit. Hilder’s site (tree age 22 months) had lower salinity levels (EC, < 75 mS/m) within the measured area. The site was severely affected by winter waterlogging due to the presence of both a deep semi-confined saline aquifer ( > 1000 mS/m) and a perched aquifer with a shallow watertable. Monthly monitoring since 199 1 has shown that the perched watertable (EC of 500 mS/m) remained within 0.5 m of the soil surface for most of the year. Survival of E. globulus was reduced from 100% in the 0 to 25 mS/m range to below 80% in the 50 to 75 mS/m range. Tree growth (MVW) in the 50 to 75 mS/m range was one third that of growth in the 0 to 25 mS/m range. Leaf inter-vein chlorosis, leaf-margin necrosis (Fig. 5) and subsequent leaf drop was prevalent within the trees in the 25 to 75 mS/m range. The concentration of chloride in the leaf tissue of these trees was variable, with levels ranging from 10 to 25 mg/g of dry matter. Leaf chloride concentrations from trees within the 0 to 25 mS/m range were lower and between 5 and 7.5 mg/g. Dryland salinity at the site has

76

D.L. Bennett, R.J. George /Agricultural

B”gttshb

- SUNhd

at Age

6 Monms

Water Mnnu~ement

27 (1995) 69-86

Engltrh’s - Growth

Wunnenburg’s

Hl!deh

Fig. 3. Survival

- Swvival

at Age

6 Mmths

- Growth et Age 18 Months

at Age 22 Months

and growth response (CVI

and MVW)

of E &hulr~.v10soil salinity

(as measured by the EM%)

D.L. Bennett. R.J. George /Agricultural

Water Management

Fig. 4. Margin necrosis on adult and juvenile leaves from 18-month-old

27 (1995) 69-86

E. globulus

at

77

Wunnenburg’s.

continued to spread, and developed above the plantation in February 1993. Watertables were rising as a result of clearing since 1970. At the Stene’s valley planting site (tree age 4.5 years), the survival of E. gEobulus was less than 65% where soil EC, was above 150 mS/m, compared to over 85% in less saline areas. For soil salinities higher than 50 mS/m, the growth rate was approximately halved. Here, the watertable (EC 900 mS/m) rises to within 1.5 m of the soil surface in winter and falls to 4.0 m in summer. The site is well drained and the groundwater fluctuations have remained constant since the site was planted. The survival of E. globulus at the Stene’s strip-planting site (tree age 14.5 years) was greater than 75% in the 50 to 125 mS/m range and below 65% when EC, exceeded 125 mS /m. Survival steadily decreased to less than 20% for EC, above 175 mS/m. The growth measurements show that peak rates occurred in the 50 to 75 mS/m range, and steadily declines at higher salinities. Since 1977, the minimum summer groundwater levels recorded in a piezometer placed in the middle of the plantation have risen from 1.O m below ground to the ground surface. During the same period, the EC of the groundwater remained at 2000 mS/m. The soil salinity within this same plantation has risen sharply between 1981 and 1990 (Fig. 6). For example the salt storage at 0.76 m depth increased sharply from 0.1 I to 2.25 g/kg, and from 0.69 to 2.38 g/kg at 3.04 m depth.

D.L. Bennett. R.J. Grorgr /Agriculturd

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Water Mana~rtnrttt 27 (1995) 69-86

Fig. 5. Margin necrosis and inter-vein chlorosis on juvenile leaves from 22.month-old E. glohulus at Hilder’\. Soilsallntty(g/kg) 0 0.48

0.76

J -

1

2

3

_-,

1.52

f

I.90

7

6

10

9

I

1 ,

p” 4.10

-

$

r

4.54

6

3

??1981

I

E 5.32

5

1

_.

1.10

4

1

\

0

1

EI

19w

(not sampled deeper than 4.10

m in 1990)

9 6.08 6.13 6.64 7.60 8.3‘5 9.12 9.48

Fig. 6. Changes in soil salinity at Stew‘s wip plantation of 14.5.year-old

E. ~lohulus

79

4. Discussion 4.1. Tree growth and survival in relation to EC, There are slightly better correlations between EM38H and tree growth than EM38V and tree growth. The theoretical depth of penetration of the EM38 signal is deeper in the vertical than in the horizontal mode, however the calibration equations (Table 3) suggest that the actual depth of penetration does not differ greatly between modes. We consider the results to suggest that the growth and survival of E. globulus is primarily influenced by soil salinity within a metre of the surface. Inions ( 199 1) showed that the growth of E. globulus in south-western Australia is related to soil nitrogen and phosphorous concentrations at 0.10 m depth, silt percentage, clay percentage and bulk density at 0.50 m depth, and meteorological variables (rainfall and solar radiation). This suggests that the physical and chemical soil conditions near the surface are most important for the growth of E. globulus. It follows that high salt levels within this zone will also have a major impact on tree performance. Inions ( 1991) did not measure salinity in his work. For all sites and all tree ages, the survival of E. globulus was reduced to below 65% for EM38H values in excess of 150 mS/m (EC,= 700-800 mS/m). At low salinities, the variation in survival rates observed between sites may be explained by both site and age variations. As a consequence, our results cannot be viewed as a time series analysis of response to salinity. To do this would require measurement of a particular plantation at regular intervals over many years which is beyond the scope of this study. Donaldson et al. ( 1983) suggests that a minimum of between 6 and 10 years growth is required to screen Eucalyptus species for salt tolerance in the field. Eucalyprus glob&s appears able to survive at high soil salinity levels. However the tree’s growth rate begins to be affected at much lower salinities. We believe that effects are caused by stress due to high NaCl concentrations in the root zone. In glasshouse trials, Macar ( 1989) reported growth rate reductions and stress symptoms in 22 Eucalyptus species (including E. globulus) with increasing concentrations of NaCI. Of the 22 species tested, E. globulus was ranked the tenth most salt sensitive and had poor survival ( < 50%) when the NaCl concentration of their irrigation water was 300 mol/m3 (2600 mS/m) There is a relatively large amount of information available on the tolerance of trees to salinity from pot studies, however there is limited knowledge of field soil salinities tolerated by Australian tree species (Macar et al., 1991). Studies that have been reported (Pepper and Craig, 1986 and Macar et al., 1991) have concentrated on reporting species that have high salt tolerance. Pepper and Craig ( 1986) considered that E. camaldulensis and E. rudis were salt sensitive because they had low survival where soil EC, exceeded 1000 mS/m. Macar et. al. ( 1991) considers that trees with moderate to high salinity should be tolerant of soils with EC, values of 15004000 mS/m. Macar ( 1991) did not include E. globulus in this category. Stewart ( 1988) reported that plantations of E. globulus (and other low salt tolerance species) would suffer significant reductions in growth and survial if watered with drainage water that had salt concentrations in excess of 1000 mS/m. We found that the soil salinity level at which the growth rate of E. globulus declines is influenced by tree age and site conditions. Compared to older trees, the very young trees at

English’s site did not exhibit large growth reductions until high EC, values were reached. On these more saline areas, there was a large variation in growth rates. This variability was clearly visible at the site, and has been observed in other very young Eucalyptus trees at other saline sites (Bennett and George, 1992; G. Ellis, pers. comm. 1992). It is possible that during the first winter the trees have a large proportion of their roots confined to the salt-leached mounds. As salts re-accumulate in the mounds during periods of high evaporation over summer, growth rate reductions at lower EC, may occur. Also, as the trees grow older and their roots explore a larger soil volume, the influence of the mounds will diminish. This trend is evident in the growth of the 18-month-old trees at Wunnenburg’s. There, the mean CVI of the trees growing in soil with EC, levels greater than 75 mS/m (EC, = 450 mS/m), is less than half that of trees growing on less saline ground. 4.2. Soil salinity and waterlogging The combined effects of soil salinity and waterlogging on tree growth rates can be seen at the Hilder’s site. At this location, stress symptoms occur at much lower soil salinity levels ( > 25 mS/m EC, or 250 mS/m EC,, compared to >50 mS/m EC, or 350 mS/m EC,) than at Wunnenburg’s and Stene’s Valley, which are non-waterlogged. Excavation of one of the affected trees revealed that some roots had died, and others had become thickened and spongy, possibly due to the development of aerenchyma. At this and other sites, the combination of waterlogging and salinity have proven more difficult for tree establishment in the field than salinity alone (Biddescombe er al., 1985). Craig et al. ( 1990) reported reduced Acacia seedling growth rates under a combination of saline and waterlogged conditions. Similarly, Van Der Moezel and Bell ( 1990) noted that for several Eucalyptus species the combination of salinity and waterlogging greatly increased plant tissue levels of Na and Cl compared to salinity alone. This growth reduction was related to lower stomata1 conductance, transpiration and net photosynthesis. We also found very high Cl concentrations in salt-affected trees at Hilder’s. These results suggest that in addition to measuring EC,, the likelihood of waterlogging needs to be estimated, as this may affect salinity tolerance and growth rates. 4.3. Impact ofsalinisation

on tree survival and growth

The growth measurements from the Stene’s valley planting provide the clearest indication of the impact of soil salinity on E. globulus growth. This plantation is salt affected, but not waterlogged (watertable below 1.5 m in winter) and it is within the age range at which the majority of the site effects on tree performance have been reached (Donaldson et al., 1983). Here, the growth rates of trees growing in soil with salinity (EM38) levels above 50 mS/ m (EC, = 350 mS/m) are approximately half of those in less saline areas. The economic implications of the growth rate reductions are evident at Stene’s strip planting site (age 14.5 years). Trees affected by salinity at this site have a mean MVW of 0.29 m3 per tree, and the plantation has a mean survival rate of 50%. This means that the plantation has a total wood volume of 145 m’/ha, which gives a M. A. I. of 10 m’/ha/year. This is the lower limit of commercial productivity for E. glob&s plantations in southwestern Australia and would make plantings uneconomic where distances to processing

D.L.

Bennett,

R.J. George

/Agrimltunrl

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Mcrt~trgrtnent

27 (lYY5)

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centres are greater than 50 km. Salinity risk and transport distance are both greatest in the eastern areas (Fig. 1) and make accurate site assessment during plantation establishment very important. 4.4. Risk of salinisation The results from the Stene’s strip plantation (tree age 14 years) also show that watertables and soil salinity levels have the capacity to increase beyond critical limits within the life span of a commercial E. glob&s plantation. The large increase since 1981 in near-surface soil salinity (Fig. 6) as a result of the watertable rising by 0.1 m/yr, suggests that plantations established on sites with saline watertables have a very high risk of failure within the first IO-year rotation. These measurements support the visual evidence of the watertable rising under the plantation after it was established, concentrating dissolved salts into the root-zone of the trees. The trees on the lower edge of Stene’s strip plantation have died and fallen over. Trees near the centre of the plantation were dying at the time of measurement while trees furthest upslope were exhibiting leaf necrosis and leaf drop. In the Stene’s catchment example, approximately 30% of the original forest was cleared in the late 196Os, and 14% of this cleared area was reforested in 1976 and 1977 (Schofield et al., 1989). Compared to most catchments that have been cleared for agriculture in southwestern Australia this catchment has a small proportion cleared and a large proportion reforested. These results have important implications for plantation managers contemplating the establishment of small E. globulus plantations low in the landscape, and in particular, near saline seeps. It demonstrates that assessment of the risk of continued groundwater rise is required. 4.5. The efSect of clay content, water content, soil temperature and soil mounds on EC, For each soil type, the EM38 response was highly correlated with soil EC, for particular depth intervals (Table 3a). These intervals were within the theoretical depth of penetration of the instrument and were similar to those obtained by other authors who have calibrated the EM38 over a range of soils in south-western Australia. The best depth-interval calibrations (for sandy and gravelly duplex soils) of McFarlane and Ryder ( 1990) are presented in Table 3b. They found that over 93% of the variance in EC, could be explained by the profile salinity at 0.60 to 0.90 m depth. The other local calibrations of Ferdowsian and Greenham ( 1992), are shown in Table 3c. The soils of this catchment had EC,:EC1:5 ratios ranging from 7.3 to 16.2 and ranged from clays to sandy loams. They found that 90% of the variance in EM38H and EM38V could be explained by soil salinity at 0.20 to 0.40 m and 0.20 to 0.80 m, respectively. Small ( < 20%) differences between EC, values derived from EM38H (EC,) for clay and sand at high EC, occur because the regression for clay has less slope than for sand (Fig. 2). This is consistent with conclusions by Slavich and Petterson ( 1990) who found that the slope of their regressions between EC, and EC, decreased with increasing SP. In our study, 77 and 85% of the variation in soil EC, (for the clay and sand, respectively) could be described by EM38H alone. Our positive (but poor) correlations between SP and EM38H indicate that clay content may have a small effect on EC, ( as derived from EC,) by changing

the slope of the regression. Also, Slavich and Petterson ( 1990) found that including SP in the regression only reduced the standard deviation by 8%. They concluded that the omission of SP would still enable useful estimates of EC, to be determined by using the EM38 for studies on the tolerance of crops and pasture to salinity. The best-fit EM38H calibration derived by McFarlane and Ryder ( 1990) from duplex soils, gives similar derived soil EC, values to those that we obtained (EM38H) for the midsalinity ranges (Fig. 2). However at very low and very high salinity’s, the differences become larger. We consider that this is due to the high slope of their calibration equation. Their soils had SP levels which fell well within the range of the SP levels we measured for clay and sand, so this difference in slope is unlikely to be explained by clay content. The slope of their regression is influenced (increased) by the inclusion of a single reading from a highly saline area (EC, = 250 mS/m, compared to all other points which were less than 150 mS/m). McFarlane and Ryder ( 1990) proposed that the low water content of the surface soil caused poorer correlations with EC, at shallow ( < 0.60 m) depths. However, by including gravimetric water content in their correlations, only a further 5 and 3 percent (in the vertical and the horizontal respectively) of the variance could be explained. We also found poor correlations at shallow depths. However, the soils at our sites were moist because rainfall preceded the surveys and the sites were in discharge areas which remain moist throughout the year. The latter factor is likely to control the EM38 response at most E. globulus plantation sites where it is to be used to define the extent of areas suitable for planting around saline seeps. However, the effect of variations in soil moisture on EC, is not adequately described in the literature and warrants further investigation. The soil temperature correction coefficients (Table 2) vary widely between the seasons and it would be important to consider this variability and correct the EM38 measurements accordingly, if seasonal changes in soil salinity were being measured. Fortunately there is a much smaller variation within seasons. For example, during late summer and autumn when this study was undertaken, the variation in EC, which could be due to temperature was estimated to be a maximum of 15%. We observed almost no difference between EM38 readings taken on, or adjacent to soil mounds. Pettit and Froend ( 1992) reported that salt leaching only occurs from the soil of double-ridge mounds during winter and this salt re-accumulates in the mounds because of evaporation in summer and autumn. Our results, obtained during summer and autumn, support those findings. It would appear that mounding only provides a reduction in salt content (and waterlogging intensity) for the tree seedling during the months after establishment. In subsequent years, exploration of the surrounding soil by the tree’s roots controls the tree’s growth and survival.

5. Conclusions The results from our study indicate that electromagnetic induction (EM38) is an effective tool which can be used by plantation managers and farmers in the region as a method of determining the likely survival and growth of E. globulus to changes in soil salinity. Good correlations between apparent electrical conductance (EC, from EM38) and soil salinity

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(EC,), over a range of soils, were found. Soil salinity has the dominant influence on the response of the EM38 However, seasonal variations in soil temperature ( < 15Yo), soil texture ( < 20%) and soil moisture content (3%) can have a small effect on the calibrations and should be noted by intending users. The disadvantages caused by these variations are minor compared to the advantages of obtaining rapid, repeatable and reliable soil salinity estimates without intensive soil sampling and laboratory analysis. Soil salinity within 1.0 m of the surface has been shown to cause large reductions in survival and growth rates within established E. globulus plantations of several ages. Our results suggest that the trees can survive reasonably well up to moderate soil salinity levels ( < 150 mS /m). However their growth rates are affected at much lower levels. This decline in growth is important to plantation owners since it means lower timber yield and income as well as reduced groundwater use by the plantation. The EM38 level (EC,) at which the growth reductions become apparent is between 25 and 75 mS/m for trees older than 12 months. Waterlogging was observed to reduce the salinity level required to cause growth reductions. For this reason, if waterlogging is likely, E. globulus should not be planted if EM38H exceeds 25 mS/m. This maximum salinity level could be increased to 50 mS/m at drier sites if watertables are not likely to rise under the plantations. Our research has also shown that the area affected by salinity is expanding as a result of continued groundwater rise and may cause significant growth declines within plantations during a single timber rotation of 10 years. This may be a significant problem where; ( 1) ( 2) (3) (4)

there has been a large proportion of the catchment cleared within the past 20-30 years; where the proportion reforested is small (less than 25%) ; where the new plantings are located in the lower slopes of the catchment; and where the transport costs are high.

Under these conditions, we recommend drilling, salinity surveys (EM38, EM3 1 and airborne systems) and a continued monitoring programme. We would also actively encourage strip or alley plantings higher in the landscape to intercept soil-moisture and provide the benefits of shade and shelter within the farm enterprise.

Acknowledgements

The authors wish to thank Gavin Ellis for advice and help with tree measuring, and the Western Australian Department of Conservation and Land Management for allowing us to use their equation for the calculation of wood volumes for E. glob&s in south-west Australia. Brian Wren and his staff carried out the soil and leaf analysis. Kelvin Baldock from the Western Australian Water Authority, kindly provided the groundwater and soil salinity information for the Stene’s site. We also acknowledge the co-operation of Peter Hilder who provided us with his groundwater monitoring records. We acknowledge the helpful comments of reviewers Don McFarlane and Richard Moore.

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References Bartle, J.R., 1991. Tree crops for profit and land improvement. J. Agric. W. Au%., 3 I : I I-I 7. Bennett, D.L. and George. R.J., 1992. Early survival and growth of river red gum clones on salt-affected land in high rainfall areas of South-West Australia. Land Water Res. News, 12: 12-16. Biddescombe, E.F., Rogers, A.L., Greenwood, E.A.N. and De Boer, E.S.. 1985. Growth of tree species near salt seeps, as estimated by leaf area, crown volume and height. Aust. Forest Res., 15: 141-154. Cameron, D.R., De Jong, E.. Read, D.W.L. and Oosterveld, M. 1981. Mapping salinity using resistivity and electromagnetic inductive techniques. Can. J. Soil Sci., 61: 67-78. Corwin, D.L. and Rhoades, J.D.. 1982. An improved technique for determining soil electrical conductivity-depth relations from above ground measurements. Soil Sci. Sot. Am. J., 46: 517-520. Craig, G.F., Bell, D.T. and Atkins C.A., 1990. Response to salt and waterlogging stress of ten taxa of Acncio selected from naturally saline areas of Western Australia. Aust. J. Bat., 38: 619-630. Donaldson, D.R., Hasey, J.K. and Davies, W.B., 1983. Eucalypts out perform other species in salty flooded soils, Calif. Agric.. 37: 20-21, Ferdowsian, R. and Greenham, K.J., 1992. Integrated Catchment Management - Upper Denmark Catchment. Tech. Rep. No. 130, Western Australian Department of Agriculture Division of Resource Management. George, R., 1990. The 1989 saltland survey. J. Agric. W. Aust., 4: 159-166. Inions, G., 1991. Relationships between environmental attributes and the productivity of Eucdyptm globulus in South-West Western Australia. Proceedings, Third Australian Forest Soils and Nutrition Conference. pp. I 16 132. Lesch, SM., Rhoades. J.D.. Lund, L.J. and Corwin, D.L., 1992. Mapping soil salinity using calibrated electromagnetic measurements. Soil Sci. Sot. Am. J., 56: 540-548. Macar, NE, 1989. Salt tolerance of frost-resistant eucalypts. New Forests, 3: 141-149. Macar, N.E., Crawford, D.E. and Leppert, P.M., i 99 I. The potential of trees for utilisation and management of salt-affected land. In: N. Davidson and R. Galloway (Editors), Productive Use of Saline Land. Australian Centre for International Agricultural Research, Canberra, pp. 17-22. McFarlane, D.J. and Ryder, A.T., 1990. Salinity and Waterlogging on the Esperance Downs Research Station. Tech. Rep. No. 108, Western Australian Department of Agriculture Division of Resource Management. McNeil], J.D., 1980a. Electrical Conductivity of Soils and Rocks. Tech. Note TN-5, Geonics Pty. Ltd., Ontario. Canada. McNeill, J.D., 1980b. Survey Interpretation Techniques: EM38. Tech. Note TN-6, Geonics Pry. Ltd, Ontario, Canada. Norman, C.P. and Heslop, K., 1990. Soil Salinity Survey of the Boort/West of Loddon Salinity Management Plan Area. Res. Rep. No. 125, Department of Agriculture. Victoria. N&en, R.A., 1981. Salt-affected land in the shire of Wongan-Ballidu, Western Australia. Aust. J. Soil Res., 1: 87-91. Pepper, R.G. and Craig, G.F., 1986. Resistance of selected Eucalyptus species to soil salinity in Western Australia. J. Appl. Ecol., 23: 977-987. Pettit. NE and Froend, R.H., 1992. Research into Reforestation Techniques for Saline Groundwater Control. Rep. No. WS97, Water Authority of Western Australia. Piper, C.S., 1947. Soil and Plant Analysis. Hassellpress Publishing Company, Adelaide. Richards, L.A. (Editor), 1954. Diagnosis and Improvement of Saline and Alkali Soils. Handbook No. 60, United States Department of Agriculture. Rogers, A.L., 1985. Foliar salt in Euca/_vptus species. Aust. Forest Res., 15: 9-16. Schofield, N.J., Loh, I.C., Scott. P.R.. Bartle, J.R., Ritson, P., Bell, R.W.. Borg, H., Anson, B. and Moore, R., 1989. Vegetation Strategies to Reduce Stream Salinities of Water Resource Catchments in South-West Australia. Rep. No. WS33. Water Authority of Western Australia. Shea, S.R. and Hewett, P.N.. 1990. Plantation Forestry in Western Australia: Achievements and Prospects. Department of Conservation and Land Management. Western Australia. Slavich, P.G. and Petterson. G.H.. 1990. Estimating average rootzone salinity from electromagnetic induction (EM-381 measurements. Aust. J. Soil Re\.. 2X: 453-63. Slavich, P.G. and Read, B.J.. 1984. A\sessmcnt of electromagnetic induction measurements of soil salinity for

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indication of crop response. In: E. Humphries (Editor), Rootzone Limitations in Clay Soils. Soil Science Society, Riverina Branch, pp. 33-40. Stewart, H.T.L., 1988. A review of irrigated forestry with Australian tree species. Proceedings, International Forestry Conference for the Australian Bicentenary, Vol. 2. Van Der Moezel, P.G. and Bell, D.T.. 1990. Saltland reclamation: selection of superior Australian tree genotypes for discharge sites. Proc. Ecol. Sot. Aust., 16: 545-549. Wood, G.B., 1990. Measuring trees and logs. In: K.W. Kremer (Editor), Trees for Rural Australia. Inkata Press Pty. Ltd., Melbourne, pp. 271-283.