Site Rehabilitation the Clearance Vegetation in South Western Australia - Literature review Example

SALINITY PROBLEMS Name Course Institution Instructor Date Salinity problems Literature review As edaphic factors play a prominent role in structuring plant communities in saline environments, vegetation has been used in Queensland and globally to aid in mapping saline soils (AAS, 2004). Although many different types of salts occur naturally in the Australian landscape and many are essential for plant growth, sodium chloride is the focus of the majority of salinity research. This can be justified, owing to its abundance in Australian soils, its solubility, and its direct and indirect effects on plants (Wolf and Wagner, 2005). Direct effects of elevated salinity levels include altered osmotic gradients and a multitude of toxicities which can impact biochemical, molecular and physiological processes, different parts of the plant such as roots and leaves, in addition to different life-stages of the plant, such as germination, growth, reproduction and senescence (Acworth and Jankowski, 2001). Indirect effects include degraded soil structure and other toxicities. All of these factors are heterogeneous spatially and temporally. Impacts include reduced seed germination, the inhibition of nitrogen fixing ability of symbiotic organisms (Woinarski et al, 2002); reduced growth rates (Anderies, 2005) and root development (BoM, 2010); increased susceptibility to insect and herbivore attack; nutritional depletion, such as reduced essential ions and trace elements (Wagner 2001); and apparently death of endemic species whilst favouring exotic species, or weeds (Briggs and Taws 2003). Plant salt sensitivity and tolerance varies between species and with life history, and adult plants are often more capable of tolerating elevated salinity levels than juveniles or seedlings (Wagner 2001). Cramer and Hobbs (2002) indicate that it is essential to understand salt sensitivity and tolerance levels for individual species, including growth history variations, however, to mitigate and remediate disturbed ecosystems, also requires an understanding of the ecosystem response and the thresholds to increased salinity levels. Watson, Novelly, Thomas (2007) suggest glycophytes (non-halophytes) can adapt to high levels of salinity, provided that stress imposition occurs in moderate increments (as may be expected to occur in natural systems). The highest levels of soluble salts in salinised soils are generally found nearest the surface due to evaporation (Taws 2003; Barnett 2000; Wagner 2001; Semple et al. 2006), where the dominant soil processes include cation exchange and subsequent pH changes, aggregate dispersion, surface sealing, loss or deterioration of soil structure, and shrinkage and swelling (Wagner 2001). Additionally, salinity levels are highly variable in space and time (e.g. Barnett 2000; Odeh and Onus 2008), hence, all of these factors are likely to affect plant and soil biota and must be considered when investigating the effects of elevated salinity levels on plants. Salinity management for Australia requires a reappraisal of the current techniques. The Surface Water Model (also known as the Transient Salinity and NAS model) needs to be recognized as the main conceptual model for dryland salinity formation across Australian uplands. The present management techniques focusing on the rising groundwater model with large scale tree plantings to decrease recharge amounts and thereby decrease discharge, is not general and does not apply to the small, localised sites Australia. The results from this study concur with previous research describing in situ degradation as the main cause of the problem, as described in Chapter 2. It are the causes and the symptoms of the degradation that need the management. The predominant cause of upland dryland salinity is due to increased evaporation at the soil surface from periodic salty water flowing laterally across the surface as runoff and through the A horizon. The increased evaporative surface is generally induced by activities at the immediate soil surface which impact the ecological potential and the function of the soil. Physical, chemical, biological and hydrological factors are adversely affected, and the cumulative effects of many years of generally unsustainable land management practices causing soil and vegetation degradation, particularly clearing and stock grazing, become apparent. The implications of the management actions that are based on the rising groundwater model for natural resource management and remediation activities can be considerable. The universal acceptance of this model has inhibited effective management solutions, as the majority of remediation practices are presently focused on revegetation activities to manipulate (soak up) hypothetical excess water balances in the landscape. Most soil parameters measured showed extreme localized variability, laterally, vertically and temporally, which can rapidly change during small time periods and across small distances, especially the abrupt boundaries between scalds and adjacent vegetation. This heterogeneity has been previously documented from degraded salt-affected sites in eastern Australia (e.g. Semple et al., 2006; Barnett 2000; Wagner 2001; Bann and Field 2007; Rogers et al. 2005) and indeed other parts of the world (Johnson et al. 2001). The in situ heterogeneity of most attributes induced from the degradation suggests that some areas require more focused site specific management and attention than others, or alternatively, some areas require little or no attention. This must be a consideration when designing a management plan rather than treating sites generally. Additionally, the fact that considerable changes occur temporally, such as EC levels, and the amounts of certain actions and anions, suggest that management activities should address, or prevent, the extreme conditions, as it is likely that the conditions when levels are not extreme, do not drastically impact the biota. Plant et al. (2000) and Horney et al. (2005) discuss an applicable system for salinity management termed ‘Site Specific Management’ to address the soil heterogeneity at each affected site, which is basically what farmers and land managers already practice to some degree in Australia (David Hilhorst pers. comm. 2005). Field and Little, (2008) suggests that site-specific management has the potential to maximize agricultural production and economic return whilst conserving soil and water resources and enhancing soil quality. The methodology involves quantifying the soil properties using various surveys and analyses, to identify the site locations that require targeted management and the recommended techniques to achieve it. Due to the heterogeneity shown between bare degraded scalds and less degraded surrounding acacia woodlands, the application of this management concept is essential for these sites. Additionally, as salinity is a consequence of, or a symptom of vegetation and soil degradation, it is necessary to address the degradation holistically when attempting to manage elevated salinity levels. The remediation of the disturbed/degraded ecosystem to either a marginally or drastically improved state, needs to begin with the soil architecture, that is, its structure (Please et al, 2001). This is primarily achieved by the activities of microbes acting on organic materials produced either in situ or introduced from elsewhere. The soils with elevated salinity levels generally have high levels of sodium with low levels of calcium and carbonate, hence, the most preferred method to ameliorate or remediate the soil chemical problems would be to incorporate gypsum into the soil (Field and Little, 2008). Lime could also be used on the acid soils. Sulfuric acid and elemental sulfur are inapplicable, mainly due to the fact that carbonates are rare or absent (hence no CaSO4 will form). This can only be achieved with remediation activities that involve the addition of organic matter to improve the low SOM levels. SOM management practices are considered to be the most important factors in agricultural landscapes and the situation particularly applies at these sites. The added SOM provides plant nutrients, increases soil aggregation and structure (architecture), limits soil erosion and increases cation exchange and water holding capacities, making management of SOM of paramount importance, especially in these acacia woodland ecosystems, as they are elsewhere (Conant et al. 2004). As the soil has lost its porosity due to cumulative structural changes from the many years of unsustainable land management practices, much of the water that falls as rain flows across the surface as run-off, taking soil and nutrients with it, which not only reduces the (LFA) ‘nutrient cycling index’, but can effectively cause both recharge and discharge problems in lower landscape positions. There is more water to pond in the lower areas, often being inherently saline from flowing across the salty surfaces during its travel (Hughes et al. 2006), which subsequently evaporates or flows into nearby drainages. Sometimes these flow paths are artificially ponded due to roads, tracks and dams terminating the flow path (Bann and Field 2006a, b). Excess water that makes it to the local creeks is also likely to be salty. Managing dryland salinity will therefore first and foremost require that water be retained where it falls, with the emphasis on rapid infiltration and increased water and nutrient storage in the topsoil. It is also noted that the evaporation potential requires water within the soil profile that is within the evaporative potential range for the particular requisite circumstances for the process to progress. It is this water that transports the majority of salts and is closest to the surface, hence is the surface hydrology that requires the management. As the majority of the surface water in these landscapes moves through the A horizon, above the B horizon as interflow, it is surface hydrology that is prevalent in the process, not groundwater rising vertically through the semi- impermeable B horizon and A horizons to the surface. It is therefore this shallow water that needs to be addressed in remediation activities, or at least associated with those activities that are used to address any presumed rising groundwater. Additionally, reducing possible capillary rise is also a priority, which involves improving the soil structure, especially with the addition of organic material. Use of Acacia species for rehabilitation A number of Acacia species discussed by Bann and Field (2006c, 2010b) exhibit an increased tolerance of the degraded conditions. Their abundance and use at salinised sites is encouraging for both agricultural management and conservation purposes. Desirable outcomes can be achieved using large, native, perennial Acacia species which can effectively capture organic matter and nutrients moving over the soil surface, as they incorporate greater biomass (roots and litter) into the soil than small annual species. To summarize, a number of species are particularly promising for rehabilitating degraded salinized areas, including Acacia adunca and Acacia acinacea. Acacia acinacea appears to be the best performer, having specialized adaptations which allow it to grow onto and across the degraded scalded area, in addition to being tolerant of the adverse conditions associated with the degradation, including the extreme alkaline conditions. Wagner (2001) noted the persistence of couch at salinized sites Australia, Semple et al. (2006) and Mitchell (2009) have also recorded the efficacy of this species for salinity remediation activities at other sites across Australia. Williams (2002) reported the same from Queensland and it has also been documented from around the world. Indeed, Semple et al. (2003) found that Acacia acinacea offered advantages with its better growth form as warm season species are often difficult to establish from seed on scalded sites with unreliable warm season rainfall. Furthermore, the dense prostrate cover protects the soil from rainsplash protection and reduces the surface evaporation. Further trials should thus be performed using this Species and perhaps a mixture of some of the other endemic species mentioned in this research to combine the benefits of the mat forming habit of Acacia acinacea and the thick stem forming habits of the other species. As the native Acacia species require less fertilizer, Rogers et al (2005) indicate that they are economically sensible and may assist with controlling, or offsetting soil degradation. In many cases they are also self-seeding and just require the appropriate management practices, such as allowing summer growing species such as Acacia acinacea to set seed in summer. Acacia acinacea also needs to be managed to prevent its high rate germination (Langford et al. 2004). A number of the species are also shade tolerant and provide good green feed during winter and spring. Furthermore, these acacia species provide the additional benefit of reducing the evaporation rates from the soil surface and consequently, the evaporite deposition and salinity levels. The lack of biota is thus considered to be a requisite condition for elevated salinity levels. Andrews (2006) described similar on his Hunter Valley farm. Interestingly, the NSW Commissioner for Natural Resources in 2010, and ex-chief of CSIRO Land and Water, Professor John Williams, also recently stated that “To address dryland salinity in eastern Australia biodiversity conservation outcomes should be the focus for management” (Williams 2010). Due to the landscape heterogeneity and possible ecotype variations seed for remediation and revegetation activities should be sourced from the localized mature plants (i.e. local provenance). Clearly, more work is required that investigates the use of acacia species at degraded sites across Australia, particularly those commonly associated with soils with elevated salinity levels. Due to high occurrence in Australia, including the salinised sites investigated for this research, management activities utilizing acacia species is very viable, as they have been very successful at remediating other salinised sites in similar landscapes Australia (e.g. Malcolm 2005 and Barrett-Lennard 2003). They serve a number of purposes, including reducing soil surface evaporation and waterlogging, improving soil structure with deep root systems, providing habitat for a myriad of animals and supplying valuable fodder for stock. Therefore, to summarize the important considerations for most salinised site management activities on Australian upland sites affected by dry land salinity, both localised water and soil management is crucial. A site review is undertaken investigating the history, soils, hydrology and vegetation so as to develop a ‘Holistic Site Specific Management Plan’. This involves; 1) Correctly evaluate the site as being primary and/or secondary. This will have management implications, namely, managing a natural situation (which may involve returning the site to the original state) and remediating a degraded site to a less degraded one. Both cases involve consideration of future use. 2) Soil ‘above ground’ remediation activities, generally involving soil hydrology works with the objective to reduce and direct overland flows (runoff) and retain water and nutrients, including organic matter, such as litter, on site 3) Soil ‘below ground’ remediation activities involving soil amelioration works to address the physical, chemical, hydrological and biological parameters, with the objectives of recovering the soil structure and pedality, thereby improving infiltration, porosity and aeration, hence leaching of accumulated surface salts; reducing the surficial and subsurface interflows through the A horizon, hence retaining water and nutrients; increasing soil organic matter and microbial activity (litter and humus); increasing calcium levels with gypsum application and reducing sodium levels, in addition to chlorine and possibly bromine; improving (neutralising) the generally alkaline pH levels, commonly with lime. 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