Effect of soil salinity on growth of irrigated plantation Eucalyptus in south-eastern Australia

Agricultural Water Management 98 (2011) 1180–1188

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Effect of soil salinity on growth of irrigated plantation Eucalyptus in south-eastern Australia P.M. Feikema a,b,c,∗ , T.G. Baker a,c a b c

Department of Forest and Ecosystem Science, The University of Melbourne, Victoria, Australia Cooperative Research Centre for Plant-Based Management of Dryland Salinity, Perth, Western Australia, Australia Cooperative Research Centre for Forestry, Hobart, Tasmania, Australia

a r t i c l e

i n f o

Article history: Received 7 October 2010 Accepted 10 March 2011 Available online 6 April 2011 Keywords: Eucalyptus camaldulensis Eucalyptus grandis Eucalyptus globulus Electromagnetic induction Irrigation EM38 Soil salinity

a b s t r a c t Management of salinity may include establishing trees in saline areas to enhance discharge and may enable productive use of saline land. Field studies of the performance of trees in saline conditions are generally confined to the initial years after planting, and little quantitative data are available on the relationship between the growth rates of eucalypt species to soil salinity in field conditions at later ages (e.g. 10 years). In this study, the growth of irrigated Eucalyptus globulus, E. grandis and E. camaldulensis is examined in relation to soil salinity measured using an electromagnetic induction device (EM38). The EM38 was found to be an effective tool in determining survival and growth responses of three Eucalyptus species to levels of soil salinity. Differences in measured tree survival, stand volume and leaf area index were correlated with soil salinity. Of the three species, E. globulus performed best in terms of survival and volume growth to age 10 years under slight to moderate salinity conditions, while E. camaldulensis performed best under moderate to severe soil salinity. The ranking of these species for salinity tolerance is consistent with pot trials and younger field trials. This study highlighted the high spatial variability associated with soil salinity, and studies relating the growth of trees in the field should best be analysed on an areal or stand basis, thereby accounting for variability of salt stored in the soil, and reducing the influence of inter-tree competition on growth–salinity relationships. These results have implications for site selection and management of eucalypts in saline areas. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Soil salinisation is a significant and increasing problem in many areas of Australia. The estimated area of agricultural land in southern Australia affected to some extent by saline scalds, saline seeps, irrigated saline soil and shallow saline groundwater is over 5.2 million ha, and is expected to reach nearly 11 million ha by 2050 (National Land and Water Resources Audit, 2001). Revegetation of recharge areas with deep rooted perennials, including trees, is recognised as the preferred way to reduce secondary salinity induced by land clearing for agriculture. However, if trees can be sustainably grown in saline (discharge) areas, they offer a means of productively using degraded land of little alternative value, for economic benefit (Marcar and Crawford, 2004), as well as contributing to site rehabilitation and reducing salt loads to

∗ Corresponding author. Current address: Department of Forest and Ecosystem Science, The University of Melbourne, 221 Bouverie St, Carlton, VIC 3053, Australia. Tel.: +61 3 8344 0715. E-mail address: [email protected] (P.M. Feikema). 0378-3774/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2011.03.005

streams (Schofield et al., 1989). The low opportunity cost of agriculture on salt affected land may make tree growing more economic than on non-saline land, even if growth rates are reduced somewhat by salinity. The salt tolerance of particular species is a crucial factor in its success in providing environmental and economic benefits when grown under saline conditions (Dale and Dieters, 2007). A challenge in using vegetation for salinity control lies in the development of plantations with sufficiently tolerant species that provide commercial products as well as environmental benefits (Bartle et al., 2007). Prospective species are often studied as juveniles (up to 1 year old) in glasshouse conditions. However, results from controlled glasshouse experiments may not be directly transferrable to the field, where there may be substantial confounding temporal and spatial variation in salinity (Niknam and McComb, 2000). Zohar et al. (2010) argue that, when screening eucalypt seed sources for use on saline lands, it is critical to quantify the relationship between salinity and a measure of tree productivity under field conditions. Field studies of the performance of trees in saline conditions are generally confined to the first few years after planting. Inferences

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drawn from such trials with young trees should be applied to field growing trees with caution (Arndt et al., 2000; Callister et al., 2008) and performance and characteristics of trees in the field must be assessed. It cannot be assumed that growth rates observed in the first few years will continue as root zone water and salt conditions change as the trees develop. There are few data to quantify the relationship between the growth rates of eucalypt species to soil salinity in field conditions at later ages (e.g. 10 years). Compared with annual crops, it is difficult to evaluate the growth response of eucalypts to soil salinity because these may be affected by dormant periods, changes in climate from year to year, and temporal and spatial changes in the soil salinity profile. This study examined the growth of Eucalyptus camaldulensis, E. globulus and E. grandis at Timmering, in northern Victoria, as affected by soil salinity resulting from differences in irrigation water salinity. We investigated whether growth rates can be related to the distribution of salinity in the soil, and quantified the effect of soil salinity on the growth of each species. 2. Methods The study was undertaken in a field experiment comparing growth of Eucalyptus spp. irrigated with low salinity or high salinity water. The experiment was established under the Trees for Profit program for investigating potentially commercial options for tree growing to achieve land and water care benefits (TFPRC, 1992; Bren et al., 1993; Baker et al., 1994, 2005; Stackpole et al., 1995). 2.1. Site description The study site (36.31◦ S, 144.93◦ E; 100 m asl) is located near Timmering, in the northern irrigation region of Victoria, Australia. Average annual rainfall is approximately 445 mm and Class A pan evaporation is approximately 1550 mm. Average daily minimum and maximum temperatures for January (mid-summer) are 14.0 and 30.0 ◦ C and for July (mid-winter) are 2.6 and 12.7 ◦ C, respectively. The district is characterised by shallow groundwater, generally between 1 and 2 m from the surface, with EC ca. 10 dS m−1 . The soil at the site has a red brown duplex (i.e. texture contrast) primary profile form (Northcote, 1979) or Solonetz (FAO-Unesco, 1990), within the Wanalta loam or Alta clay association described by Skene and Poutsma (1962). The soil texture is a clay loam to about 15 cm depth, heavy clay 15–140 cm and medium clay below this. Soil pH increases from 5.6–7.9 (A horizon) to 9.0–9.4 (B horizon) (Bren et al., 1993). There is a layer of sand under the site at 4–8 m depth, with a hydraulic conductivity in the order of 10 m day−1 . 2.2. Experiment description 2.2.1. Treatments and design The experiment compared three species (E. camaldulensis, E globulus and E. grandis) in factorial combination with low and high irrigation water salinity. The species treatments were split within the irrigation treatments in a complete randomised block design with three replicates. 2.2.2. Establishment Planting lines were ripped once in March 1993 to 0.8 m depth with a V shaped winged tyne 0.6 m wide. The site was then graded in May 1993 and planting lines bedded immediately afterwards. Trees were planted in July 1993 in 12 tree × 7 row (30 m × 21 m) species treatment subplots, at 2.5 m between trees and 3 m between rows (1333 trees ha−1 ). Weeds were controlled pre- and post-planting with knockdown and residual herbicides. Fertiliser (250 g per tree

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of NPK 8:11:10) was applied in November 1993. Irrigation commenced in March 1994. 2.2.3. Irrigation water quantity and quality The experiment was flood irrigated with a total of ca. 800 mm year−1 applied in 10–12 irrigation events between November and April. The average electrical conductivity of the high salinity irrigation water (pumped groundwater) varied from 2.9 to 8.8 dS m−1 . Electrical conductivity of the low salinity irrigation water (river water supplied from the regional irrigation supply channel system) averaged 0.1–0.5 dS m−1 . 2.3. Tree measurements Trees were measured at ages 1.7, 4.0, 6.6, 8.4 and 10.4 years. All trees in the plots were measured for diameter over bark at 1.3 m above ground level (DOB ) and a subset for total height (H). Survival, mean stand height, basal area and stem volume (under bark) were calculated from individual tree measurements in each plot. Tree volume was calculated using a generalised individual tree volume function developed for fast growing eucalypts (Wong et al., 1999). 2.4. Leaf area index The leaf area index (LAI) of each plot was estimated at age 10.4 years in December 2003 using the Accupar Ceptometer (Decagon Devices Inc.). The device is a linear ceptometer that measures photosynthetically active radiation (PAR) in the 400–900 nm waveband. In each plot, 24 measurements of PAR were made systematically on a grid (12 in the interrows, and 12 between trees in the rows). Simultaneous measurements of PAR were logged every 30 s outside the plantation and the two series of PAR measurements were combined to derive an estimate of plant area index (PAI) according to methodology described in Decagon Devices Inc. (2001). The 24 estimates of PAI were averaged for each plot. The ceptometer measures all light blocking objects (i.e. leaves, stems, branches) and conversion is required to estimate LAI from PAI. In a separate study the ceptometer was calibrated using LAI data from destructive sampling of E. camaluldensis × globulus and E. camaldulensis × grandis hybrids with LAI values between 1 and 2 (Feikema, unpublished data). This calibration (n = 6; r2 = 0.62; p < 0.1) was used to estimate LAI from PAI in the present study. 2.5. Soil salinity Soil salinity in all plots was estimated at age 10.4 years (December 2003) using a Geonics EM38 (electromagnetic induction meter). The EM38 allows cost-effective in situ assessment of soil salinity (McKenzie et al., 1997). It is a portable instrument with a transmitter and receiver coil 1 m apart, and measures the apparent soil electrical conductivity (ECa ). When it is used in the horizontal mode (EM38H ) on a uniform soil, 75% of the signal response is estimated to come from the top 1.0 m of soil; and in the vertical mode (EM38V ), 75% of the signal is estimated to come from the top 1.8 m. Soil porosity, soil water content, clay content, soil temperature and soil salinity influence ECa (McNeill, 1980) and therefore calibration of the instrument is required with salinity measurements on soil samples taken from the profile. 2.5.1. EM38 calibration Soil profile samples were taken at 0.3 m depth intervals to 1.8 m from 13 auger-bored holes located to represent the observed range of ECa measurements across the experimental site. At each profile, 5–6 EM38H and EM38V measurements were made and averaged to provide a single reading of EM38H and EM38V respectively. Soil

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Fig. 1. Soil salinity as ECe by depth for the EM38 calibration profiles (n = 13) representing the low salinity (grey circles), and high salinity (black triangles) water irrigation treatments after 10.4 years.

temperature at 30 cm depth was measured with a soil thermometer over the same period EM38 measurements were made, to allow correction of ECa to a reference temperature of 25 ◦ C. Soil samples were air dried (<30 ◦ C), ground and sieved (<2 mm), and a 1:5 soil:water extract analysed for electrical conductivity at 20 ◦ C (EC1:5 ). Values of EC1:5 were converted to electrical conductivity of a saturated paste soil extract (ECe ) using correction factors based on soil texture (measured in the field) as given by Slavich and Petterson (1993). Linear regressions between average ECa (EM38H or EM38V ) and ECe were fitted for each depth interval. These allowed determination of the relative contribution of ECe at different depths to ECa , and to identify the depth from which most of the induced field is being transmitted. Linear regressions were developed between the ECa and average ECe for greater depth intervals (e.g. 0–150 cm depth). These allowed a depth-averaged soil salinity to be estimated from measurements of ECa at each measurement location. 2.5.2. EM38 measurements Measurements of ECa (EM38H and EM38V ) were made on a systematic grid within each tree measurement plot. Measurements were taken in the interbed area between each set of 4 trees (2 rows × 2 trees). A total of 36 measurements of ECa were made in each plot at a nominal grid spacing of 4 m and 2.5 m. Values of ECa were converted to ECe , and were averaged to obtain a single value of ECe for each plot. For a subset of trees in plots of E. camaldulensis and E. globulus, additional EM38H and EM38V measurements were made at two positions approximately 0.5 m adjacent to the base of each tree, and the two EM38 measurements were averaged to provide a representative value of EM38H and EM38V at each tree location. 2.6. Statistical analyses Differences in volume and LAI with soil ECe were tested using standardised major axis regression performed in SMATR (Warton et al., 2006) to test for differences (p < 0.05) in the gradient and elevation of different regressions. 3. Results 3.1. ECe and calibration of EM38 Soil salinity (ECe ) varied with depth in the 13 calibration profiles from 1 to 12 dS m−1 (Fig. 1) and the average to 150 cm depth varied

Fig. 2. Comparison of the relationship between soil salinity as ECe (0–150 cm depth) and ECa (EM38V ) measured in the clay loam to medium clay soil in the present study (Timmering) with those for several textural groups (after Halvorson et al., 1977).

from 2.0 to 8.3 dS m−1 . Linear regressions for EM38H and EM38V ECa measurements with ECe are presented (for individual depth intervals and averaged across more than one depth interval) in Table 1. EM38H and EM38V were similar in their ability to predict ECe for depth intervals to 60–90 cm, while EM38V was a better predictor for depth intervals 90–120 cm and below. The strongest relationships between ECa and ECe were observed at the 90–120 cm depth interval. Measurements of EM38H and EM38V were highly correlated and the relationships were not improved by combining both EM38H and EM38V . The strongest relationship was for EM38V averaged to 0–150 cm depth (Table 1). This depth interval includes the zone of highest ECe (80–140 cm) (Fig. 1) which is likely to correlate with previous or current high root length density and tree water uptake (Vertessy et al., 2000). Consequently, the zone between 0 and 150 cm provides the most useful indicator of salinity in the root zone given the depth limitation of the EM38. This relationship between EM38V averaged to 0–150 cm depth (Table 1) was used to convert measurements of ECa (EM38V ) to average ECe (dS cm−1 ) across the same depth. When compared with regressions developed by Halvorson et al. (1977) for sandy loam, clay loam and clay soil texture groups, the Timmering ECe 0–150 cm derived from EM38V was consistent with that expected for a clay loam topsoil and light clay subsoil (Fig. 2).

3.2. Soil salinity Treatment means of soil salinity (ECe , 0–150 cm depth) in the present study (Table 2), together with data from earlier sampling (Bandara et al., 2002), show a clear difference in trends over time between the low and high salinity water treatments (Fig. 3). Soil salinity increased between 2 and at 4 years after planting, and then remained fairly constant (ca. 4 dS m−1 ) afterwards in the low salinity treatment. Soil salinity in the high salinity treatment continued to rise to ca. 10 dS m−1 at approximately 6 years after planting, and then decreased to ca. 8 dS m−1 at 10 years after planting. Both the low and high salinity water treatment soil profiles had peaks in salinity around 90–150 cm depth at 10.4 years (Fig. 1). Average soil ECe in the low salinity irrigation treatments was 4.3–5.0 dS m−1 and 7.6–8.2 dS m−1 in the high salinity irrigation treatments (Table 2). Differences between treatments were significant for each species, and species had no significant effect on soil salinity. A frequency distribution (Fig. 4) provides information on the range and variation of soil ECe across the treatment plots.

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Table 1 Linear regressions of ECe (dS m−1 ) on measurements of ECa in horizontal (EMH ) and vertical (EMV ) orientations for individual soil layers and for averages across layers. All regressions are significant (p < 0.05, n = 13). Standard errors (s.e.) of the estimates are shown. Numerals in bold indicate the regression selected to calculate soil salinity (ECe ) in each plot from ECa values. Soil layer (cm)

0–30 30–60 60–90 90–120 120–150 150–180 0–60 0–90 0–120 0–150 0–180

ECe = a + b EMH

ECe = a + bEMV

a

b

r2

s.e.

a

b

r2

s.e.

−1.7181 −0.4384 0.5945 −0.1724 −0.7696 −1.4496 −1.0782 −0.5207 −0.4336 −0.5008 −0.6589

4.5201 5.1730 6.8735 8.0225 8.2694 8.7753 4.8466 5.5222 6.1473 6.5717 6.9390

0.446 0.705 0.814 0.836 0.631 0.483 0.608 0.742 0.808 0.800 0.743

1.51 1.00 0.98 1.06 1.89 2.72 1.17 0.98 0.90 0.98 1.22

−1.8550 −0.4467 0.5424 −0.3909 −1.2281 −2.1456 −1.1508 −0.5864 −0.5375 −0.6757 −0.9207

3.1979 3.5548 4.7524 5.6583 5.9974 6.5123 3.3763 3.8350 4.2909 4.6322 4.9455

0.472 0.704 0.823 0.879 0.702 0.563 0.624 0.756 0.832 0.840 0.799

1.47 1.00 0.96 0.91 1.70 2.50 1.14 0.95 0.84 0.88 1.08

Fig. 3. Average soil salinity as ECe (0–150 cm depth) with age since planting in the low and high salinity water irrigation treatments. Error bars indicate one standard deviation (n = 3 for years 2, 4, 6, and 8; n = 6 and 7 respectively for year 10).

3.3. Tree survival, growth and leaf area index Tree survival in all 3 species to age 6.6 years was little affected by the salinity of irrigation water (Fig. 5). However, by ages 8.4 and 10.4 years, survival of E. globulus and E. grandis was significantly less in the high salinity irrigation water treatment, whereas survival for E. camaldulensis remained unaffected. From age 4 years, stand volume growth (Fig. 6) in all species was lower in the high salinity irrigation water treatments, and markedly so by age 10.4 years for E. globulus and E. grandis in part because of lower survival. In the low salinity treatment, the species were ranked (mean annual increment at 10.4 years; m3 ha year−1 ): E. globulus (18.4) > E. grandis (14.7) > E. camaldulensis (10.8), whereas in the high salinity treatment, the ranking was E. camaldulensis (8.2) > E. globulus (6.3) > E. grandis (4.6). Linear regressions between stand growth measurements and average soil salinity (ECe , 0–150 cm) at age 10.4 years based on indiTable 2 Mean soil salinity as ECe predicted from ECa (0–150 cm depth, dS m−1 ) in Eucalyptus species and irrigation water salinity treatments 10.4 years after planting and commencement of irrigation. Numbers in brackets are one standard deviation. Values with the same superscript letters are not significantly different (p < 0.05). Species

Low salinity (channel water)

High salinity (groundwater)

E. camaldulensis E. globulus E. grandis

4.3a (1.1) 5.0a (1.4) 4.7a (0.5)

7.6b (0.4) 8.2b (0.5) 7.8b (0.6)

Fig. 4. Frequency distribution of soil ECe (0–150 cm depth) by treatment (low salinity, high salinity) and species (E. camaldulensis, E. globulus, E. grandis).

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Table 3 Relationships between stand growth measurements (y) at age 10.4 years and soil salinity as ECe (0–150 cm, dS m−1 , predicted from ECa measurements) of the form y = a + bECe . Species E. camaldulensis (n = 6)

E. globulus (n = 5)

E. grandis (n = 6)

#

Measurement (y) 3

−1

Live volume (m ha ) Total volume (m3 ha−1 ) Live basal area (m2 ha−1 ) Total basal area (m2 ha−1 ) Mean stand height (m) Survival (%) LAI Live volume (m3 ha−1 ) Total volume (m3 ha−1 ) Live basal area (m2 ha−1 ) Total basal area (m2 ha−1 ) Mean stand height (m) Survival (%) LAI Live volume (m3 ha−1 ) Total volume (m3 ha−1 ) Live basal area (m2 ha−1 ) Total basal area (m2 ha−1 ) Mean stand height (m) Survival (%) LAI

a

b

Significance#

r2

144 144 26.3 26.3 12.5 101 1.96 339 354 42.0 42.9 23.6 88.9 2.04 285 265 37.7 33.9 23.5 98.7 1.70

−7.51 −7.36 −1.16 −1.12 −0.17 −1.51 −0.06 −31.1 −30.2 −3.50 −3.08 −1.08 −5.25 −0.07 −29.6 −24.7 −3.63 −2.68 −1.28 −6.18 −0.05

p < 0.05 p < 0.05 p < 0.05 p < 0.05 n.s. n.s. p < 0.05 n.s. n.s. n.s. n.s. n.s. n.s. n.s. p < 0.05 p < 0.05 p < 0.05 p < 0.05 p < 0.05 n.s. n.s.

0.72 0.69 0.72 0.68 0.10 0.41 0.76 0.61 0.67 0.52 0.60 0.60 0.26 0.36 0.77 0.80 0.75 0.81 0.73 0.34 0.29

n.s. = not significant.

vidual plot measurements are given in Table 3. Generally, for each species, the strongest relationships were between soil salinity and stem volume and basal area. Survival, although negatively related, provided the weakest (and non significant) relationship with soil salinity. Relationships between increments (between ages 8.4 and 10.4 years) of volume, basal area or height were much weaker than those for volume, basal area and height at age 10.4 years and are not presented here. The decline of live volume at age 10.4 years per unit increase in soil salinity (ECe , 0–150 cm) was similar for E. globulus and E. grandis (p > 0.05), and nearly four times (p < 0.05) greater than that for E. camaldulensis (Fig. 7a). Estimated LAI at age 10.4 years respectively for the low and high salinity treatments averaged 1.7 and 1.5 for

E. camaldulensis, 1.8 and 1.4 for E. globulus and 1.5 and 1.3 for E. grandis. The decline of LAI per unit of increasing soil ECe was similar (p > 0.05) for all three species (Fig. 7b) Although, for the same range of soil salinity, E. grandis had nearly 30% lower LAI (p < 0.05) for a given soil salinity than E. globulus or E. camaldulensis. The effect of soil salinity on live volume at age 10.4 years, expressed as a percentage of potential growth unaffected by salinity, is shown in (Fig. 8). Zones representing non-saline, slight, moderate and severe salinity tolerance classes were adopted from Marcar and Crawford (2004). Marcar and Crawford (2004) suggest that up to a 25% reduction may occur if the given species is grown under conditions represented by their respective salinity tolerance

Fig. 5. Effect of irrigation water salinity on survival of E. camaldulensis, E. globulus and E. grandis over time (a) high salinity (groundwater) treatment relative to low salinity (channel water) treatment, and (b) average survival. Bars indicate ± 1 standard error of the mean (n = 3).

Fig. 6. Effect of irrigation water salinity on stand volume of E. camaldulensis, E. globulus and E. grandis over time (a) high salinity (groundwater) treatment relative to low salinity (channel water) treatment, and (b) average stand volume. Bars indicate ±1 standard error of the mean (n = 3).

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Fig. 7. Relationships between (a) live stem volume, and (b) leaf area index at 10 years of age, and soil salinity as ECe (0–150 cm depth). Slopes and elevation of linear regressions of volume and ECe were the same (p > 0.05) for E. globulus and E. grandis and lower (p < 0.05) for E. camaldulensis. Slopes of linear regressions for LAI and ECe were the same (p > 0.05) but the regression for E. grandis was significantly lower (p < 0.05) in elevation than for the other two species.

class for E. camaldulensis (ECe 4–8 dS m−1 ), and for E. globulus and E. grandis (ECe 2–4 dS m−1 ). It is assumed in Fig. 8 that growth of E. camaldulensis starts to decline when soil salinity exceeds 1.0 dS m−1 and that growth of E. globulus and E. grandis starts to decline when salinity exceeds 0.5 dS m−1 ECe . The species-specific relationships in Fig. 8 were derived from the linear regressions for volume as a function of soil salinity (Table 3).

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Fig. 9. Relationships between tree diameter (DOB ) at age 10.4 years and soil salinity as ECe (0–150 cm depth) for (a) E. camaldulensis (r2 = 0.041; p = 0.006; n = 180) and (b) E. globulus (r2 = 0.091; p = 0.0004; n = 132). High salinity and low salinity treatments are differentiated.

In this case, however, growth is expressed relative to expected growth under non-saline conditions. Using these relationships, for a soil salinity of 8 dS m−1 ECe for example, predicted volumes for E. camaldulensis, E. globulus and E. grandis would be 61%, 28% and 18% respectively of the expected growth in the absence of salinity effects. 3.4. Individual tree and EM38 measurements There were statistically significant but weak relationships for tree DOB and soil salinity (ECe , 0–150 cm) measured near the base of the tree for E. camaldulensis and E. globulus (Fig. 9). As for the stand, the rate of decrease in DOB was greater for E. globulus than for E. camaldulensis. There was no relationship between DOB increment between 8 and 10 years and soil salinity. 4. Discussion 4.1. Effect of salinity on tree survival and growth

Fig. 8. Decline in relative volume with increasing soil salinity (ECe , 0–150 cm depth).

Salt tolerance of plants in field conditions may vary significantly with many environmental factors such as soil fertility, soil physical conditions, irrigation method, distribution of salt in the root zone, and climate; with plant factors such as growth stage and variety (Kozlowski, 1997); and with management. In the present study, the range in soil salinity and in tree growth were sufficient to allow comparisons between species. While the range of salinity was large enough across the plots to allow the detection of significant regressions, the precision of the study was not sufficient

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to allow precise determination of a threshold value below which growth is not affected by salinity. For the purposes of evaluating tree growth, Marcar and Crawford (2004) describe root zone soil salinity classes: non-saline (ECe < 2 dS m−1 ), slight (ECe 2–4 dS m−1 ), moderate (ECe 4–8 dS m−1 ), high (ECe 8–16 dS m−1 ), and extreme (ECe > 16 dS m−1 ). The low salinity treatments may be described as being in the lower part of the moderate salinity class, and the highly saline treatments can be described as having moderate to high salinity. Survival is an important criterion for selection of species suitable for highly saline soils (Marcar and Crawford, 2004). Survival of E. camaldulensis was virtually unaffected by increased soil salinity, and reductions in growth performance were mainly due to a reduction in basal area rather than in tree height (Table 3), an observation also made by Sun and Dickinson (1995) and Akhtar et al. (2008). Akhtar et al. (2008) also observed that survival of E. camaldulensis in Pakistan was not affected by soil salinity (8–31 dS m−1 ECe) or planting density (818–2500 trees ha−1 ). Height growth of all species, and particularly that of E. camaldulensis, was less affected by soil salinity than basal area growth. Akhtar et al. (2008) noted, for E. camaldulensis at age 5 years, that the effect of soil salinity was more pronounced on diameter (and basal area) that on height. Height growth is not a strongly turgordependent process (Myers et al., 1998) and this may explain the smaller response compared to the reductions measured in basal area and LAI, which are turgor-dependent processes. While survival of E. globulus and E. grandis in the high salinity treatment started to decrease after 4 years, and more markedly after 6 years (Fig. 5b) growth in the high salinity treatment relative to low salinity treatment started to decrease from between 2 and 4 years (Fig. 5a). Increasing soil salinity over time to age 6 years led to a reduction in tree growth before it led to a reduction in survival, suggesting that the growth of trees was reduced by salt at concentrations lower than that required to induce significant mortality. The ranking of species for volume growth in this study is consistent with that for other irrigated plantations in the region (Baker et al., 2005), and with recommendations by Marcar and Crawford (2004), that E. camaldulensis is considered suitable for moderately saline (ECe 4–8 dS m−1 ) soils, and E. globulus and E. grandis are considered suitable for slightly saline (ECe 2–4 dS m−1 ) soils. The ranking of species salt tolerance also agrees with the ranking of these species both in pot trials (Marcar, 1989; Marcar et al., 1999) and in younger field trials (Marcar et al., 2003; Dunn et al., 1994). The study by Marcar et al. (1999) detected a slightly greater reduction in height of E. grandis than E. globulus seedlings (at 36 days). Interestingly, this relationship is consistent with our observations at 10 years of age; the difference in salt tolerance of E. globulus and E. grandis was not as great as the difference between these two species and E. camaldulensis. While woody plants become progressively more tolerant to salt with age (Kozlowski, 1997), the cumulative effect in the long term on physiological and morphological processes can lead to substantial reductions in stand volume. Munns (1993) proposed a two-stage model to describe plant response to salinity: growth is first reduced by a decrease in soil water potential (a water stress effect), and later by the effect of salt injury on the older leaves, which die owing to an increase of salt in the cell components. Munns (1993) also proposed that the accelerated death of older leaves led to a decrease in the supply of carbohydrates or growth hormones to meristematic regions, thereby inhibiting growth. The decline in stand volume with increasing soil salinity over time became more pronounced with tree age, in part because the difference in soil salinity between treatments increased in the first 6 years, and possibly because the cumulative effects of salinity on

tree growth become more pronounced with time. Effects of short term exposure to saline soil are probably limited to the reduction of the roots to take up water, and hormonal signals from the roots regulate leaf expansion (Munns and Termaat, 1986). These authors suggested that the main effect of salt to limit growth under longer term exposure to salt is the maximum salt concentration tolerated by the fully expanded leaves in the shoot. If the rate of leaf death (and abscission) approaches the rate of leaf production, the photosynthetic area will eventually become too low to support continued growth. In an earlier study at Timmering, Hamlet and Morris (1996) reported no significant reduction in the height growth of E. camaldulensis and E. grandis at age 1.5 years, and the growth of E. globulus, while mildly affected by salinity, was restricted by other site factors at this time. Relationships between stand volume and salinity are more pronounced in older stands, as other growth-limiting factors (such as weed competition) are reduced in relative importance (Morris et al., 1994). The soil salinity level at which the growth rate of a given species declines is influenced by tree age and site conditions (Bennett and George, 1995), and the effect of salinity on growth rate continues to be influenced by tree age and site conditions. Donaldson et al. (1983) suggested that at least between 6 and 10 years growth is required in order to screen for salt tolerance in the field. There are economic considerations associated with the reduction in growth rates due to salinity. Eucalyptus globulus showed a decline in volume growth, decreasing from a mean annual increment (MAI) of 18.4 m3 ha−1 year−1 at age 10 years in the low salinity treatment (soil ECe 5.0 dS m−1 ) to an MAI of 6.3 m3 ha−1 year−1 in the saline treatment (soil ECe 8.2 dS m−1 ). The cost of irrigating with water of lower salinity needs to be offset against increased growth rates. Benyon et al. (1999) observed that the average leaf area of E. camaldulensis trees under moderately saline conditions (4.5 ECe dS m−1 ) was less than 40% of that for trees growing under nonsaline conditions (0.5 ECe dS m−1 ). Using the relationship between LAI and soil salinity developed in this study (Table 3), the LAI of trees at 4.5 dS m−1 was 88% of that of trees growing at 0.5 dS m−1 soil salinity. The less severe reduction in LAI in our study may result from adequate water availability with irrigation (ca. 450 mm year−1 rainfall with 800 mm year irrigation) compared with ca. 660 mm year−1 rainfall and no irrigation in the study by Benyon et al. (1999). The reduction in leaf area due to soil salinity means that photosynthesis per plant is reduced. However, it is difficult to identify whether the reduction in photosynthetic rate is the cause of reduced growth, or the result of it (Munns and Tester, 2008). Changes in the leaf area/sapwood area ratio can act to maintain a similar water potential gradient in tree stems of the same species at different sites (Mencuccini and Grace, 1994). The relative differences in the ratio of LAI to basal area (LAI/BA) approximate the relative differences in leaf area to sapwood area ratio between species and between treatments. Values of LAI/BA were only slightly higher for E. camaldulensis in the high salinity treatment (0.089) than in the low salinity treatment (0.080). The differences between treatments for the other two species was much greater, with values of LAI/BA of 0.146 and 0.063 for E. globulus and 0.152 and 0.070 for E. grandis in the high and low salinity treatments, respectively. All three species had similar values of LAI/BA in the low salinity treatments. Increases in LAI/BA may be an adaptive response by E. globulus and E. grandis to increase water potential gradients in stems under saline conditions. 4.2. Individual tree measurements There was greater variability in the data for DOB and soil salinity on an individual tree basis than on a plot basis. The observed

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variability in tree growth–soil salinity relationships (Fig. 9) in individual trees may result from processes that limit growth other than salinity, and in particular, the effects of competition for resources with neighbouring trees. Assuming that the stand of trees is fully occupying the site, analysing data on a plot basis removes the effects of competition on individuals. Additionally, feedback processes may result in higher salinity levels associated with larger trees. Larger trees generally have greater leaf areas, and use, or have used, relatively more water. The uptake of saline water, in the absence of any leaching processes, will lead to an accumulation of salts in the root zone below critical thresholds. Therefore, larger trees that use more water may increase the root zone salinity to a greater extent than might be expected under a smaller tree using less water. This process confounds the growth–salinity relationship until a threshold salinity level (at which growth is severely affected by salinity) is reached, and this threshold level is species dependent. In addition, roots of larger trees may exist in the soil beyond the area of influence of our EM38 measurements. While the ECe may seem relatively high when measured near the base of a large tree, roots from that tree may be exploring soil of much lower salinity. Similarly, roots from neighbouring trees may also extend into the root zone of the particular tree area being measured with the EM38, and therefore effect the root zone salt concentration attributed to the measured tree. Therefore, the salinity as measured in the soil may not be representative of the salinity experienced by the tree. Correlations between ECa measurements at a tree location, and those taken at the two adjacent neighbouring locations were strong (r2 = 0.99; p < 0.001), irrespective of whether there was still a tree in that location, or the tree was missing or had died several years prior. Variation in DOB is much larger than the variation in soil salinity, and is an underlying reason for the poor relationship between tree size and soil salinity on an individual tree basis. This may not always be the case. Aragüés et al. (2010) detected stronger relationships for tree height (r2 = 0.52–0.65) and for trunk growth (r2 = 0.49–0.83) and mean soil salinity for olive trees at 5 years of age. However, trees in the study by Aragüés et al. (2010) were planted at 727 trees ha−1 , and were individually irrigated with the same amount of water, and so competition between individuals was low and the effects on growth were predominantly due to soil salinity. In the assessment of salt tolerance of tree species, it is important to analyse growth indices (e.g. DOB , height) at the plot scale rather than at the individual tree scale. This is due to high spatial variability of salt in the soil, and the difficulty in measuring the soil salinity experienced by an individual tree, and because stand scale factors (e.g. competition between individuals) will affect growth rates and also soil salinity. For example, stand density (or spacing) affects DOB , tree height, stand volume and soil salinity (Akhtar et al., 2008).

5. Conclusions Differences in tree survival and stand volume measured for E. camaldulensis, E. globulus and E. grandis were correlated with soil salinity measured using an EM38. Of the three species, E. globulus performed best in term of survival and volume growth to age 10 years under slight to moderate salinity conditions, and E. camaldulensis performed best under moderate to severe salinity conditions. Soil salinity is spatially highly variable, and long term growth effects are best analysed on a stand basis, thereby reducing the influence of other factors on the growth of individual trees, such as inter-tree competition. The electromagnetic induction technique (EM38) provided a rapid approach to estimate soil salinity in the field, and was an effective tool in relating the survival and growth of Eucalyptus species to levels of soil salinity.

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Acknowledgements Funding for this study was provided by the Victorian Department of Sustainability and Environment, the Victorian Department of Primary Industries, and the Cooperative Research Centre for Plant-Based Management of Dryland Salinity. The authors are grateful to D. Chessum who kindly allowed access to the plantation and maintained and applied water to the plantation. A number of people contributed to the design, establishment, maintenance and monitoring of the field study. The authors particularly thank J. Collopy, J. Morris, D. Stackpole and R. Stokes. They also thank K. Broadfoot and D. Cornwall from the Victorian Department of Primary Industries (Tatura) for instruction in the use of the EM38, and S. Yang for assistance with EM38 measurements.

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