17.4  Salt-affected land: utilisation and reclamation

Printer-friendly version

figure

 

Figure 17.27 Seasonally dry salt pans in southwest Western Australia support extensive collections of halophytes where samphire (Halosarcia spp.; see inset) form a fringing community just above the salt line. Saltbushes (Atriplex spp.) occur upslope together with grasses and sedges. Trees are least tolerant of these saline and sometimes waterlogged soils, and are furthest removed from the salt pan (Photograph courtesy Time Colmer and Digby Short)

Salinised landscapes are not necessarily unproductive (see Case study 17.1) and even dryland pastoralists across Australia exploit extensive areas of halophytes. Such communities fringing seasonally dry salt pans (Figure 17.27) develop a clear zonation according to soil salt concentration with samphires commonly forming a fringing community immediately above the salt line, followed by saltbushes and grasses, with trees furthest removed.

figure

 

Figure 17.28 Dry tailings from numerous old goldmines in southwest Western Australia are a potential source of dust, while modern mines generate a highly saline slurry from processing based on hypersaline groundwater (50000 mg soluble solids L-1, predominantly NaCl). Revegetation of such degraded areas poses major challenges for establishment and maintenance of stress-tolerant species where drought, heat and salt tolerance have to be combined with exclusion of heavy-metal residues. Halosarcia pergranulata occurs naturally in this area and together with several species of Atriplex has these attributes. Established plants of both genera are shown here along contour banks on a highly saline mining residue (Photograph taken by Greg Barrett and provided by Tim Colmer and Digby Short)

Samphire is a collective name for succulent articulate chenopods (inset, Figure 17.27; see Wilson (1980) for derivation and taxonomy) and refers to native species of Halosarcia that occur widely in pastoral and agricultural regions of Australia. They are especially common on drainage lines and margins of saline lakes. Samphires tolerate waterlogged and saline soil, survive moderate grazing, and are being trialled for revegetation of saline mine soils (Figure 17.28). Halosarcia pergranulata has been successfully trialled for revegetation of mined areas in Western Australian goldfields where stabilisation of disturbed areas against erosion was an immediate priority.

Establishing pastures on saltland improves site productiv-ity, and in so doing provides ground cover that forestalls erosion and returns organic matter to salinised soils. In addition, a vegetative cover of halophytic shrubs and herbs/grasses must transpire in order to grow, and by drawing moisture from saturated profiles air-filled pores are formed which are in turn conducive to further root growth. Overall, soil structure is progressively improved via positive interactions between physico-chemical properties, plant growth response and heightened activity of soil biota. Furthermore, when combined with deep-rooted perennials, revegetation can help restore hydrologic balance on disturbed sites and lead eventually to their reclamation.

A net reduction in water table via revegetation is only achievable if overall transpirational loss exceeds recharge to subsurface water. Moreover, this net loss needs to be sustained at a minicatchment level and commonly requires new plantings in both recharge areas (up slope) and adjacent to saline discharge areas (down slope). Surface discharge of saline groundwater restricts planting options down slope, but selections of salt-tolerant trees and shrubs have been trialled in combination with upslope establishment of fast-growing plantation species to produce a net reduction in water tables on a number of sites in eastern Australia (Marcar et al. 1995).

figure

 

Figure 17.29 Agroforestry plantings of eucalypts (including Eucalyptus occidentalis in more saline areas) lower groundwater by 75 cm relative to annual pasture. The trial site (25 km east of Narrogin in southwest Western Australia) was underlain by a shallow highly saline water table, and was planted with trees spaced at either 25 or 12.5 m with 5 m between rows, producing stocking rates of either 80 or 160 trees per hectare, with alleys of pasture between rows (Based on Scott and Crossley 1996)

By way of illustration, 4.7 ha of marginal land near Narrogin in southwest Western Australia (Figure 17.29) was planted with rows of trees in 1981 and spaced to produce the equivalent of either 80 or 160 trees per hectare. Several species were established including salt-tolerant ones such as Eucalyptus occidentalis on the more saline and waterlogged parts of this trial site. Seventy observation bores were drilled for ground-water measurement and monitored regularly from May 1981 to May 1994. Annual rainfall over this period ranged from about 200 to 500 mm (long-term annual average = 390 mm, with potential evaporation about 1900 mm).

Observations over 13 years (Figure 17.29) showed that trees began to impact on minimum groundwater levels (taken at the end of summer) by 1985 (four years after establish-ment), eventually reducing groundwater to below 200 cm compared with 140 cm under a pasture control by 1988. Notwithstanding year to year fluctuations in minimum levels across the entire site due to seasonal variation in rainfall and evapotranspiration, a net reduction in minimum levels of groundwater has been achieved by reforestation. Annual maximum levels (in winter) were also recorded (see Figure 1b in Scott and Crossley 1996). The continuing impact of trees was confirmed for both stocking rates (either 80 or 160 trees per hectare). Groundwater frequently fell to a depth of 150–200 cm compared with 100–125 cm under pasture. Gravimetric measurements of surface soil moisture (0–10 cm) at the start of the 1995 growing season showed a curvilinear increase in percentage moisture with distance from a tree row up to 50 m, implying that there was a substantial lateral zone of influence around each tree.

Widely spaced trees are known to draw upon more than their ‘area share’ of soil moisture in meeting transpirational needs (see pasture and tree comparisons by Greenwood et al. 1985). Moreover, isolated trees transpire faster (on a leaf area basis) due to a lower boundary-layer resistance compared to a closed-canopy forest, so that strategic plantings could at least match or might exceed evapotranspiration by former native forests and thus restore hydrologic balance to a new equilibrium. Leuning et al. (1991) explored this issue for a forested catchment 200 km south of Perth by simulating evapotranspiration from selected species of eucalypts as a function of canopy surface area and profile moisture reserves where stomatal sensitivity to vapour pressure deficit had been documented. Assuming a wet profile at planting (800 mm plant-available moisture) and provided established trees survived successive hot dry summers, a net reduction in profile water would be achieved after just one year, and the hydrologic balance for a given site would come to a new equilibrium after two to five years (faster where nutrient addition promoted faster growth of trees).

In practical terms, lowered water tables will alleviate saline seeps and are conducive to profile leaching, but do not constitute site reclamation. Deep-rooted salt-tolerant trees exclude salt (Section 17.2), and if groundwater is brackish further concentration via transpiration could render root zones so saline that trees will die. Moreover, in a reforestation scenario, substantial areas of both recharge areas and discharge zones need to be planted, so that cropping income lost by establishing trees on upslope arable land has to be taken into account. A question then arises as to whether long-term plantation forest is the most appropriate solution to episodes of salinisation in cultivated areas that tend to follow a run of wet years.

An alternative discussed by Passioura (1996) is to rely on other more valuable deep-rooted perennials such as lucerne that ‘operate intermittently, but nonetheless effectively’ in stabilising hydrology. In this scenario, tactical use of lucerne would scavenge water and nutrients that escape a succession of annual crop plants and which tend to accumulate at 2–3 m. Once profiles were dewatered, a return to cropping would be in order because the perennials would have exhausted their supply of deep water. Prospects for an effective alternation between perennials and annuals are greatly enhanced where subsoils retain a continuous collection of macropores (Section 3.1) which represent preferred pathways for deep roots. Afforestation could then prove helpful as an initial phase during which tree roots initiate formation of such pores.

Envisaging an alternation between annual and perennial cropping, Passioura (1996) suggests measurement of soil water suction at 2 m as an environmental cue for deployment of a deep-rooted perennial crop. Site characterisation will vary, but, generally, at suctions greater than about 20 kPa hydraulic conductivity of soil is so low that drainage rates are acceptably low. However, moist soil offers higher values for hydraulic conductivity and once suction falls below about 20 kPa it would be a sign that a deep-rooted perennial is called for.

Cropping according to subsurface moisture is not intended as a recipe for site reclamation, but as a means of containing further salinisation while generating a product with higher unit value than timber from arable areas up slope. Whether containment or reclamation is used, downslope migration of saline groundwater must be arrested. Complementary plantings of salt-tolerant plants in discharge areas, and especially up slope of saline seeps, might then survive long enough to be of lasting benefit.

Secondary salinisation is an outcome of interactions between plants, soils, salt and water where disturbance of hydrologic cycles in lands cleared for agriculture has occurred over decades. A return to site conditions prior to removal of native vegetation will be impossible in many instances, or could take centuries in more tractable cases. None the less, cropping will continue so that land management must address this hydologic issue and take remedial steps if secondary salinisation is to be contained.

 

»