Salinity.docx

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Geography
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GGRB28H3
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Stephanie Perpick

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Description
This report describes in detail the current field method employed to identify, assess and map the extents of dryland salinity in Victoria. Salinity has long been recognised as a feature of the Victorian landscape. Where hydrological balances have or had been maintained by retention of deep-rooted vegetation and natural water flow into sinks such as salt lakes continued unchanged, the distribution of salt in the landscape did not change significantly; apart from seasonal ebbs and flows. However, a significant increase in soil salinity levels (thought to result from changes in land use following European settlement), became evident in the 1940s. By the 1950s soil salinity was acknowledged as a major environmental problem that had significant impacts upon both natural and built assets. Through the 1970s and 1980s above average rainfall led to a significant rise in groundwater levels and an increase in the extent of salt affected soil. In order to manage an environmental threat such as soil salinity, it is necessary to understand the extent and severity of the problem, and then provide ongoing feedback to intervention programs by monitoring change over time. Early efforts to identify the size of the problem and combat it were piecemeal and fragmented and it was not until the late 1980s that a nationally coordinated approach led to the development of a set of criteria for identifying and assessing dryland soil salinity in Victoria. A Victorian field guide (Spotting Soil Salinity) was also produced to assist in mapping the extent of the problem and since the late 1980s almost all soil salinity mapping in Victoria has used both these standards and the field guide. The mapping standards were formally documented in 1996 (Allen 1996). Changes in community values and technology have altered the way salinity is viewed and the way it is mapped since the field mapping method was last documented in 1996. Both land managers and the broader community have come to take a broader view of salinity, and as well as viewing salinity as a threat to agricultural productivity and built infrastructure, the value of naturally saline areas for the unique ecosystems they support is now recognised. The advent of Global Positioning System (GPS) technology has revolutionised the way field data can be mapped. GPS technology is now commonly used for a variety of applications, and provides an integral tool for further dryland salinity mapping projects in Victoria. In addition, there have been advances in the way sites are attributed to take better advantage of the GIS environment in which the data is stored. Field data collected using the method described in this report is added to the soil salinity layer in the Corporate Spatial Data Library (CSDL). This dataset forms the primary source for almost all dryland salinity monitoring and reporting in Victoria. As such it represents a valuable public resource as land salinity has been accepted as one of the Matters for Targets indicators within the National Framework for Natural Resource Management Standards. Additionally, the extent and severity of land salinity has been listed by the Victorian government as one of the Victorian Catchment indicators. Its status will be reported in a statewide Catchment Condition Report in 2007 and a State of Environment Report for Victoria and a second National Land and Water Audit are both due in 2008. Method for field assessme nt of dry land salinity in Victoria using GPS technology 1 Introduction Dryland salinity is a major soil and water problem in Victoria and refers to the build-up of salt in the soil and groundwater in non-irrigated areas. Early mapping targeted secondary (human induced) salinity, but over time a number of primary (natural) sites have been mapped as their environmental value was recognised. To date, over 260 000 ha of land has been mapped and this estimate includes both primary and secondary salinity located across non-irrigated areas of Victoria and impacts upon both natural and built assets. Victorian government estimates put the direct cost of salinity in irrigated and dryland areas at $50 million annually and the Victorian Auditor General calculated that $1.8 billion of private and public funds had been spent on salinity management over the period 1990-2001 (Auditor General Victoria 2001). In order to manage an environmental threat such as soil salinity, it is first necessary to understand the extent and severity of the problem, and then provide ongoing feedback to intervention programs by monitoring change over time. In addition, the Victorian government is committed to a number of salinity monitoring requirements, particularly in the Murray Darling Basin, where end of valley targets (for salt-loads) have been set for northern catchments. Land salinity has been accepted as one of the Matters for Targets indicators within the National Framework for Natural Resource Management Standards. The extent and severity of land salinity has been listed by the Victorian government as one of the Victorian Catchment indicators. Its status will be reported in a statewide Catchment Condition Report in 2007. A State of Environment Report for Victoria and a second National Land and Water Audit are both due in 2008 (Clark 2006). To provide feedback to landholders, land managers and federal and state government funding bodies, all dryland salinity mapping conducted across Victoria was compiled onto a single database in the early 1990s. This database records the extent of all recorded salt affected sites in Victoria at a scale of 1:25 000 (Allan 1994). A number of other attributes, including severity, have been recorded at many of the sites. This database is currently held on the Corporate Spatial Data Library (CSDL) in a Geographic Information System (GIS) environment and is managed by the Department of Primary Industries (DPI) for the Department of Sustainability and Environment (DSE). It forms the primary source for almost all dryland salinity monitoring and reporting in Victoria (Figure 1). The extent and severity of soil salinity in irrigated areas has primarily been mapped via ground-based electromagnetic (EM38) surveys 1 rather than using the method described in this report which is based on an assessment of vegetation and other physical indicators . This information is held on a separate database. 1The EM38 surveys were conducted as part of a whole farm planning exercise for farmers in the irrigation district where the high value of the land and the farming enterprises justified the extra time and cost. 1 This report details the current field method employed to identify and assess dryland salinity in Victoria. It represents an updated version of the method documented by Allan (1996) and is heavily based on that text. There are a number of significant differences between the current method and that described by Allan (1996), primarily in the attributes recorded for each site and the technology and method used to map the spatial extents of the saline areas. However, much of the original approach remains and to save frequent repetition, the 1996 report is not always cited. Nevertheless the authors would like to acknowledge a debt to previous contributors and their documentation. 1.1 History of dryland salinity assessment Salinity has long been recognised as a feature of the Victorian landscape. Robertson (1898) referred to the appearance of saline springs on the Dundas Tablelands in Western Victoria in 1853 shortly after European settlement of the district commenced. While hydrological balances were maintained by retention of deep-rooted vegetation and natural water flow into sinks such as salt lakes continued unchanged, the distribution of salt in the landscape was unlikely to change significantly over long periods-apart from seasonal ebbs and flows. However, a significant increase in soil salinity levels (thought to result from changes in land use following European settlement), became evident in the 1940s. The 1950s saw dryland salinity acknowledged as a major environmental problem in Victoria and government agencies began to assess the size of the problem and monitor its progress. Cope(1958), estimated that over 5000 ha of land was affected by dryland salinity. Through the 1970s and 1980s above average rainfall led to a significant rise in groundwater levels and an increase in the extent of salt affected soil. By the late 1970s, the Soil Conservation Authority (1978) estimated that 85 000 ha was affected by dryland salinity. In the early 1900s it was recognised that certain plants were useful indicators for identifying both saline and alkaline soils. A body of research based on the interaction between vegetation and the chemical and physical characteristics of soil was developed in the USA (United States Salinity Laboratory Staff 1954). The first salinity surveys in Victoria were confined to specific regions and assessment criteria varied from region to region. Over time it was recognised that a piecemeal or uncoordinated approach to identifying the extent and severity of soil salinity would not be in the best interests of the community. As a consequence, a project was funded under the National Soil Conservation Project to develop a standard method for identifying, assessing and recording dryland salinity in Victoria (Matters 1987). Following this initiative, a statewide project was established in 1989 to compile all existing data on secondary discharge sites and to coordinate the collection of further data by Department of Conservation and Natural Resource (CNR) regional staff (Allan 1994). In addition, a strategy involving a network of monitoring sites was designed to identify how soil salinity changed over time. This work is continued through the Salinity Discharge Monitoring and Salinity GIS Project funded by the Department of Sustainability and Environment (DSE) and undertaken by the Department of Primary Industries (DPI), Bendigo (Clark 2005). Much of the field data was collected in the late 1980s and early 1990s and a database of dryland salinity in Victoria was created which recorded every mapped discharge site in the state (Allan 1994). With the advent of Geographic Information Systems (GIS), the soil salinity database was loaded onto the CSDL in the early 1990s. This database has been maintained on the CSDL since that time by DPI on behalf of DSE. The last coordinated, regional mapping programs were completed in North East Victoria in 1998 and the Glenelg Hopkins region in 1999. The extent of soil salinity in the Mallee has only recently been mapped using a combination of spatial modelling based on ortho-rectified airphotos from autumn 2003, Ecological Vegetation Condition (EVC) mapping, a digital elevation model (DEM) and limited ground truthing. This survey identified areas of moderate to extreme levels of soil salinity typically characterized by halophytic plant communities. It was not able to identify slightly saline areas or areas where salinity is transient (Grinter and Mock 2007). Mapping since the late 1990s has been ad hoc and limited in extent, although some small areas are being remapped to monitor change over time, notably in the North East and Corangamite CMA catchments. Almost a decade of below average rainfall has seen a general fall in watertables across Victoria and many saline areas shrank in size, although it is becoming clear that there may not always be a direct correlation between a change in the depth to watertable and change in the extent and severity of soil salinity at a site. Allan (1994) identified two types of dryland salinity, primary and secondary. Both of which are mapped using the same salinity severity class criteria:  Primary (or natural) salinity, which existed long before European settlement and is held to be the result of natural environmental processes.  Secondary (or induced) salinity, which has occurred since European settlement and is considered to be the result of changes in land and water management practices. When the salinity monitoring project commenced, the focus was on identifying secondary saline sites as they were considered as areas of productivity loss that were more amenable to reclamation. At such sites efforts were frequently made to rehabilitate the site or at least stop it degrading further and regain some productivity for the landholder. With productivity as the focus, it appeared there was little to be gained from investing in salinity treatments in areas that were saline prior to European settlement (Allen 2006) and primary sites tended to be ignored. However, in recent years the community’s understanding of salinity has become more sophisticated and primary saline sites have been recognised as natural assets from a biodiversity perspective and the salinity program aims to identify primary saline sites to support their management and preservation. 2 Primary salinity Natural or primary salinity existed for thousands of years in many parts of Victoria before European settlement and is considered to be a natural feature of the landscape. Many saline areas were permanently or seasonally inundated, or frequently became waterlogged. Over geological time periods, patterns of waterlogging, inundation and the salinity of soil, ground and surface waters has changed with climate (Allen 2006). Secondary salinity (as distinct from primary salinity), by definition is a direct consequence of post-European settlement activity. Primary salinity sites tend to be populated by a highly diverse and stable community of salt-tolerant vegetation where the vegetation community, climate and the environmental conditions at the site have achieved a state of equilibrium over many years. There may be relatively small seasonal fluctuations in the vegetation community as some species are either favoured or disadvantaged by short-term climatic variation (Matters 1987) or where annual species may dominate in years when conditions are most favourable. Longer-term change in climate may cause some species to disappear or enable other species to colonise a site (Allen 2006). The total area of primary salinity in Victoria is estimated to be 250 000 ha (Allan 1994) and includes semipermanent and permanent wetlands on the coast and inland as well as dryland areas such as salt flats and drainage lines. A significant number of these sites have not been included in the soil salinity database as its focus was primarily on secondary salinity. A classification of primary salinity may be as follows: Salt marshes and saline swamps (coastal and inland) Salt marshes are not restricted to the coast and may well occur inland. Typical examples are tidal flats or periodically inundated depressions inland. Salt marshes tend to be treeless and dominated by grasses. Saline swamps are similar to salt marshes, but tend to be dominated by trees e.g. mangrove or tea tree. Not many inland saline swamps remain. Although salt marshes are a distinct class of primary salinity from salt swamps, they tend to occur in similar parts of the landscape. Approximately 100 000 ha of coastal salt marshes and swamps exist in Victoria, with major occurrences at Discovery Bay (western Victoria), Westernport Bay, Andersons Inlet, north of Wilsons Promontory, and along the Gippsland Lakes 2 (Corrick pers.comm. ). To date, salt marshes and swamps within the tidal influence of the sea have generally not been included in soil salinity surveys and as a consequence are not represented in the soil salinity layer on the CSDL. This may change as local government authorities come under increased pressure to allow residential development closer to the coast. Salt lakes Salt lakes may be distinguished from salt flats and salt pans as they have permanent or near permanent water under ‘natural’ conditions and as a consequence only support vegetation around the edges. Typical examples are Lake Corangamite or Lake Albacutya, although alteration to water flows has changed their primary status to some extent. Salt flats (including playa) and salt pans Salt pans and flats are saline areas of low relief generally associated with arid or desert environments—pans are notably hard and flats are dry lake beds or adjacent to large water bodies. Playas are a special form of salt flat that are alkaline in nature and tend to be of uncertain surface strength and often overlie soft mud. Salt flats and salt pans are commonly found in the north-western Victoria. Natural salt springs, seepages and drainage lines These arise where natural upwelling or drainage of saline water occurs. Examples of this type of primary salinity can be found in south-western Victoria around Dundas and Linton. Identifying primary salinity in the field Primary salinity must be distinguished from secondary salinity in the field because secondary dryland salinity is regarded as a land degradation problem while naturally saline ecosystems are considered natural assets. However, in practice it can be difficult to distinguish between primary and secondary salinity in the field. Adding to the complexity is that primary sites may also have a secondary salinity component if changes in land management and land use lead to an increase in salinity severity at the site, an expansion of the original site, or both. Allen (2006) developed a field-based method based on composition of the vegetation community for discriminating between primary and secondary saline sites. The method is based on the premise that long established sites contain species that are adapted to saline environments that may have a relatively limited ability to disperse and colonise new sites. The following criteria result from this premise: • A site is likely to be a primary site if it has a high diversity of native salt tolerant species that have low capacities for dispersing and colonising newly salinised sites. This criterion may not hold true where a secondary saline site adjoins a primary site from which otherwise poor colonisers may more readily disperse.  A site with few species considered to be poor colonisers, may be a secondary site or it may be a highly disturbed primary saline site. In some instances, the grazing of primary sites may lead to the introduction of volunteer salt-tolerant species and other weeds, making the distinction between primary and secondary salinity difficult. Primary salinity tends to occur at certain positions in the landscape as a result of interaction between surface water, groundwater and the landscape. Some region specific examples of the links between primary salinity, landscape and process are:  The presence of clay lunettes, typically on the eastern side of lakes or depressions—is also an indicator of natural salinity in the north-west and west of the state. Clay lunettes are formed down-wind of a saline lake that periodically dries out. High levels of soil salinity impact on the soil structure of the exposed lake bed leaving it prone to wind erosion. Deposition of the lake bed sediment forms the distinctive crescent-shaped lunettes over time (e.g. Lake Tyrell). However, sand lunettes may also be formed adjacent to freshwater lakes by wave and wind action at times of high water level (Allen 2006).  Primary salinity may form within water bodies found at the terminal end of a surface water and/or groundwater drainage system within basalt terrain in southern Victoria. Over time salts may accumulate as a result of evaporation. An example of this process is Lake Corangamite.  Natural wetlands and depressions that act as windows to naturally saline groundwater bodies. An example of this is the salt lakes within the Douglass Depression in the Wimmera (Swane et al 2001). 3 Secondary salinity Secondary salinity is characterized by sites that have become affected by increasing levels of soil salinity since European settlement due to the impacts of changing land management practices and land use. Originally, mapping was restricted to secondary soil salinity in non-irrigated agricultural areas, however as community attitudes to salinity matured, the environmental value of primary sites was recognized and a number of primary salinity sites were also mapped. Over 260 000 ha of land has been identified and mapped as saline, including natural dryland salt flats and wetlands inland and some of the saline wetlands along the coast. These areas are recorded on the soil salinity layer held on the CSDL. Across Victoria approximately 25 000 ha of the mapped area has been positively identified as purely secondary salinity. Over 8000 ha of the mapped dryland salinity was positively identified as primary salinity, almost 12000 was considered to be a combination of primary and secondary salinity, while around 217 000 ha of land mapped as saline has not been identified as either primary or secondary salinity. It is not possible to state conclusively how much of the state is affected by secondary salinity, but if we accept Allan’s estimate (Allan 1994) of 15 000 ha of secondary salinity in the Mallee as reasonable, the mapped area affected by secondary dryland soil salinity in Victoria could range from 40 000 ha to as much as 160 000 ha. By implication, secondary salinity occurs at disturbed sites (created either by direct destruction of vegetation or loss of vegetation and topsoil caused by increased soil salinity levels) which may not yet have achieved a state of equilibrium between the vegetation community, climate and other environmental features of the site. The vegetation community at such sites may see an initial influx of species capable of dealing with the changing environment. The composition of the vegetation community may stabilize if the site attains some degree of equilibrium, but is open to change if this new found equilibrium is disrupted (Brown pers. comm.). Secondary dryland saline sites may be populated by both native and non-native plant species with differing tolerance of salinity from many plant groups e.g. grasses, legumes, native forbs and 3 Austin Brown (Statewide Leader, Pedology and Soil Physics, Future Farming Sytems Division, DPI) June 2007 herbs (Matters 1987). One characteristic common to most species at a secondary site is the ability to adapt to change and/or an ability to colonise disturbed areas. Species with this characteristic are known as ruderal plants, and they tend to be efficient seed producers and have relatively short life cycles and are. Fewer salt-tolerant native species have these characteristics than introduced species, and as a consequence vegetation communities at secondary saline sites are usually dominated by introduced species (Allen 2006). There are some exceptions to these principles. For example, in the Victorian Mallee, members of the Halosarcia and Sarcocornia species are not short lived or known as particularly efficient seeders and are often the only vegetation apart 4 from annuals to colonise secondary saline sites (Brown pers.comm.). 3.1 Drivers of secondary salinity Several causal mechanisms, all linked to changed land management practices and land use since European settlement, have been identified and are believed to lead to increased levels of secondary dryland salinity: Increased groundwater recharge The clearing of deep-rooted perennial native vegetation and its replacement with shallow-rooted low water-using annual plants common in pasture and cropping enterprises generates increased accessions to groundwater leading to raised groundwater levels and increased groundwater discharge (Cook et al 2001, Cope 1958, Jenkin 1981, Rowan 1971). Some examples of secondary dryland salinity resulting from increased accession to watertables and the geophysical features that control this expression of salinity within the landscape are:  The presence of aquitards (clay units) within sand dunes that cause perched saline watertables; typical example Victorian Mallee (Rowan 1971).  Where weak points in aquitards of regional groundwater systems allow for upward groundwater flow and discharge at the soil surface; typical example along the Riverine Plains of Victoria (Macumber 1978).  Groundwater discharge occurring at the break-of-slope caused by a change in hydraulic gradient; typical example Palaeozoic sediments, central Victoria (Jenkin 1981).  Increased base flow to streams, which leads to an increase in stream saltloads. Typical examples include Collie River basin Western Australia (Peck and Williamson 1987) and Moorabool River, Victoria (Evans 2006). Altered surface and sub surface environments While increased recharge following European settlement is considered the most common cause of secondary salinity, there exist examples where dryland salinity has been attributed to other processes, such as:  Transient salinity. This type of salinity is not related to shallow watertables. Transient salinity occurs in cropping and pasture areas overlaying sodic subsoils, and results from a combination of reduced leaching associated with sodic clays, low rainfall in dryland areas, transpiration by vegetation and high evaporation in summer. More than 60% of the 20 million ha of cropping soil in Australia has the potential to be affected by transient salinity as opposed to about 16% likely to be affected by salinity resulting from shallow watertables (Rengasamy 2002).  Alteration of shallow regolith flow systems and accumulation of salt caused by post clearing and geomorphological processes, such as landslide and debris flows. Typical examples include the Heytsbury region of Victoria (MacEwan et al 1996) and the southern Tablelands of NSW (Acworth et al 1997).  The removal/disturbance of vegetation communities in primary groundwater discharge zones, which leads to increased evaporation and alterations in soil chemistry resulting in increased salinity levels in soil and groundwater and decreased soil pH (Fawcett 2004; Dahlhaus and Cox 2005).  Salinity in and around wetlands and watercourses caused by disruption of the existing local salt and water budget; typical example Lake Wallace, Edenhope (Fawcett and Huggins 2004, Vinall 2000). 4Austin Brown, (Statewide Leader, Pedology and Soil Physics, Future Farming Sytems Division, DPI), June 2007 • Alteration of watercourses and shallow regolith groundwater systems due to 19th century mining activities, typical examples around the Ballarat goldfields (Dahlhaus and Cox 2005). 4 Major indicators of dryland salinity This methodology for dryland salinity assessment is based on a visual appraisal of locations as salinity affects the soil and vegetation at salinised sites. These effects frequently produce or initiate changes in the environment that form the distinctive characteristics common to salt-affected landscapes that are readily observed in the field. The major visual indicators of salinity are: vegetation species and community, surface moisture, bare soil, soil surface condition, salt accumulation and organic matter stains. The position in the landscape where these indications occur is contextually valuable and can significantly inform site evaluation. Excess salt levels in the soil can produce two major effects to visually alter a site: 1. Plant growth may be inhibited as a result of increased osmotic pressure of the soil solution making it difficult for plants to absorb water from the soil. At low to moderate salt concentrations, plants are able to manage the osmotic balance between the soil and their root system by accumulating internal salts. This process maintains an osmotic gradient that enables water influx to continue. However there is a penalty, for under such conditions energy used to accumulate solutes is no longer available for plant growth. As salinity increases, individual plants may be affected to a greater degree. Typical symptoms are: severe leaf tip burning, disturbance of cell membrane function, internal solute balances and even death (Matters 1987). Less commonly, toxic effects due to excessive accumulation of ions may directly affect the surface membranes of plant roots or tissues, or the uptake or metabolism of essential nutrients (United States Salinity Laboratory staff 1954). 2. Increased percentages of exchangeable sodium on the soil cation exchange process leading to a deterioration in the soil structure (Cope 1958). Loss of structure can lead to hard-set surfaces which decrease water infiltration, increase runoff and overland salt movement, decrease seed germination and emergence and increase the potential for surface soil erosion. In addition, poor subsurface soil structure will reduce both internal drainage during wet conditions and moisture holding capacity during dry conditions (significant factors contributing to transient salinity unrelated to shallow watertable processes). Reduced subsoil stability can lead to tunnel and gully erosion and increase the connectivity of saline groundwater to surface streams and rivers. 4.1 Vegetative indicators Dryland salinity assessment based on an assessment of the vegetation community largely relies on observation of ground and under-storey species of plants. As most salt- tolerant species will also grow in non-saline soil, it is variation in the composition of the plant community compared to the surrounding area rather than the presence of one or two salt-tolerant species that is the key to identifying a salt-affected site. When traversing a site from non-saline to saline soil, the number of salt-tolerant plants will tend to increase and the number of salt-sensitive plants will tend to decrease. Sites should only be classified as saline if there is a significant change in the plant community from a mix of both salt-tolerant and salt-sensitive species to one dominated by salt-tolerant species that cannot be plausibly explained by some other management or environmental factor. The presence of a vegetation community dominated by salt- tolerant species and with decreased numbers or total absence of salt-sensitive species in conjunction with any of the physical signs (presence of groundwater, bare soil, deteriorating soil surface structure, salt or organic stains or the position in the landscape where saline soil is commonly found) is a good indication of salinity without the need to take soil samples. Plant response to salt Excessive levels of soil salinity can effect vegetation in several ways: 1. Increased concentration of salts in the soil water may increase the osmotic gradient between plant and soil by reducing a plant’s ability to take up the moisture required to balance moisture loss via the leaf canopy. The total concentration of solute particles rather than their chemical nature is responsible for this effect. In such cases the plant tends to dehydrate and in severe cases die. A common symptom of plants affected in this manner is a cupping or rolling of the leaves, however water deficient plants in non-saline soil may show similar symptoms (United States Salinity Laboratory Staff, 1954). 2. In saline soils the presence of excessive concentrations of ions such as chloride, sulfate, bicarbonate, sodium, calcium, magnesium and less frequently potassium and nitrate may lead to injury such as firing of the leaf margins, chlorotic and necrotic areas on the foliage or simply depressed plant growth. These symptoms may be partly caused by a reduced ability to metabolise essential nutrients, although it is not always possible to clearly distinguish the mechanisms related to specific ions (United States Salinity Laboratory Staff, 1954). At the upper end of a plant’s salinity tolerance range morphological changes such as stunting, thickening of the leaf surface, reduction of leaf hairiness, semi-succulence in leaves and reddening of the plant may occur. These are common stress responses and may be brought on by any one of a number of environmental factors such as soil moisture deficit, poor cultivation practices, water logging, spray drift, root disease and insect attack as well as increasing soil and water salinity levels. Stress symptoms can be useful in the field to show that a plant may be at the upper end of its tolerance range and therefore helpful when assessing severity levels. However, it is important to remember that rising salinity levels are only one of a number of possible stress factors and field observation combined with knowledge of the likely management and environmental processes influencing a site should be considered when looking at stress response. Stress response alone is not sufficient grounds to classify an area as saline as many other processes can evoke a similar response. Not all of the relationships between plant and soil are fully understood and although it is generally acknowledged that while increasing salt concentrations in the soil profile generally have a negative effect on plant growth, vegetation sometimes shows little effect from increased salinity levels or moisture stress. Vegetation response to salinity may vary with climate and soil type and there are examples in both the Corangamite (Allan 1994) and Glenelg Hopkins regions where laboratory measurement of levels of soil salinity were higher than expected given the vegetation present and the general physical conditions at the site. It is not known exactly why this occurs, but it is presumably a function of the variation of the soil water conditions within the root zone and atmospheric conditions (particularly those conditions that affect evaporation) that allow species to establish and grow more successfully than predicted. At times of the year when surface salinity is leached by rainfall, seed often has a much better opportunity to establish and develop salt tolerant mechanisms before salt levels increase again with evaporation. In some regions the climatic factors may combine to reduce the osmotic pressure gradient between soil and leaf thereby limiting water utilisation and loss. Regional differences in vegetation response to salinity need to be known and understood to produce accurate maps of soil salinity. To do this adequately, it will be necessary to collect and analyse soil samples and correlate them with the vegetation community and condition for different regions. It is hoped that regional salinity field guides will be developed for all of Victoria. Salt-tolerant species Salt-tolerant species have physiological mechanisms that allow them to cope with high salinity levels and grow in saline environments, each species having a specific range of tolerable soil salinity in a particular climatic zone. In saline areas, this capability gives these plants a competitive advantage over salt-sensitive species, and as soil salinity levels increase, species with a higher tolerance tend to replace less tolerant species. However, these physiological mechanisms have a downside, as less energy is available for plant growth because much energy is used in accumulating solutes to maintain the osmotic gradient. They cannot be switched on and off and in non-saline environments, halophytic plants are at a competitive disadvantage to more vigorous, glycophytic species, which do not employ such mechanisms. Appendix 1 lists some of the salt-tolerant species found in Victoria, but it is by no means a comprehensive list of all Victorian salt-tolerant plants. Some species are rated in terms of their ability to tolerate soil salinity and others are only listed as tolerant. Salt-sensitive species Salt-sensitive species do not have the physiological mechanisms that allow them to cope with the effects of increased soil salinity levels and suffer reductions in yield/ growth when salinity levels increase beyond a tolerable threshold. The United States Department of Agriculture (USDA) accepted 2 dSm -1as a threshold above which yields of sensitive crops may be restricted (United States Salinity Laboratory Staff 1954). An important indicator of salinity is a noticeable reduction in the number of salt- sensitive species from a suspected discharge site compared to the adjacent non- saline area. This is a useful indicator, but taken on its own could be misleading and it should always be examined in conjunction with an assessment of change in the distribution of salt-tolerant species. Morphologic changes in salt-sensitive species may result from increased soil salinity. Matters (Matters 1987) states that such changes may take the form of decreased germination rate, slow growth rate, incomplete life cycle (e.g. plants do not flower), diminished abundance, depressed health (commonly apparent by characteristic yellowing and stunting of crop or pasture species), greater susceptibility to disease and decreased seed viability. These changes may also result from water- logging, nutrient deficiency or other limiting environmental factors, and there should be a thorough examination of the surrounding area to determine if the changes are limited to a particular location and if there is a corresponding increase in salt-tolerant species. In the absence of any other indicators, it is not good practice to classify such a site as saline based solely on morphological change. Appendix 2 lists some of the known salt-sensitive species found in Victoria, but it is by no means a comprehensive list of all Victorian salt-sensitive plants. Tree health Dieback or death of trees may be due to salinity, though other factors such as drought, flooding, insect attack, damage by stock or wildlife, soil compaction, cultivation or other forms of disturbance must be exhaustively ruled out before salinity is attributed as the cause. Limitations on assessment using vegetative indicators Given that the main assessment criterion is identification of vegetation, it is essential that field assessment be restricted to the time of year when the critical vegetation is present and can be easily and reliably identified. This is usually around flowering time in late spring through to early summer but will vary depending on the species, region and season. Failure to observe this restriction may reduce the accuracy of the field assessment. As an example, subterranean clover (Trifolium. subterraneum) is a salt-sensitive pasture species common across much of Victoria. It is an annual, highly palatable and frequently not visible after mid-summer. Sea barley grass (Hordeum marinum), like subterannean clover has an annual life cycle, however it is generally not palatable to stock or native wildlife and frequently persists in an identifiable form after setting seed for most, if not all the year. Sea barley grass is found across much of Victoria, but its concentration is generally higher in saline sites. It is not uncommon to find subterannean clover thriving in areas around but not in discharge sites. These same areas may also carry a relatively small population of sea barley grass, which increases within the saline area. An assessment in autumn, based solely on the area of sea barley grass may over-classify the salt affected area by mapping all of the sea barley grass cover as saline. A more accurate assessment at such a site would take place in late spring or early summer when both species are easily identified and the areas where subterannean clover dominated would be assessed as non-saline. Field assessment should not be based on the presence or absence of a single species, but should take into account the whole plant community present at a site and the non-saline areas adjacent and the physical characteristics of the whole site and surrounding area. If assessment is taking place in years of below average rainfall, there may be very little plant germination or growth and flowering may be brought forward or limited. This is likely to reduce the window of opportunity to conduct vegetation-based assessments. In years of severe water shortage and high temperatures, some annual species may not germinate at all and increased grazing pressure from stock and wildlife brought on by the general shortage of feed may further shorten any window of opportunity. In extreme years the option exists to delay the assessment until favourable climatic conditions return. To state the obvious; if the indicator species are not present then a vegetation-based assessment is not feasible at that time. Changes in land management and ground cover may also reduce the usefulness of this vegetation-based assessment. This is covered in more detail in the section titled ‘masking’. This field assessment method relies on a visual assessment of vegetation communities and other physical indicators by individual officers in the field. This involves subjective judgement and will vary from individual to individual. One of the aims of publishing this methodology is to set out clear guidelines that will assist field officers to make judgements consistent with other field officers. To further reduce the subjective nature of the field assessment, it is recommended that: • All staff members are trained in using the method prior to commencing field work. Training is currently funded by the Statewide Salinity Monitoring Program.  The number of staff working on a mapping project is kept to a minimum i.e. It is better to have two people working for two weeks each than four staff working for one week each.  All staff working on a mapping project should spend the first day or two mapping sites together. This will assist them to bring their assessments into alignment and will produce a more consistent assessment. 4.2 Physical indicators of salinity Groundwater Free water or dampness at the ground surface (particularly in summer) may be due to groundwater discharge. Recent rainfall events and waterlogging are other possible causes. Free water should be analysed for salt using a field electroconductivity (EC) meter. Any water registering above 0.5 dSm in winter may reasonably be assumed to be discharging saline groundwater and therefore the site is deemed to be salt affected. See the section titled ‘waterlogging’. Bare soil If soil salinity levels rise to critical levels, salt-sensitive vegetation may die. Salt- tolerant species may then establish, though this is dependent on the proximity of a suitable seed source, the amount of traffic, grazing and cultivation and susceptibility of the site to water and wind erosion. At extremely high levels of salinity no plant species can persist, bare soil is exposed and a damaging cycle commences, as bare soil is more prone to erosion by wind, water or traffic. Evaporation rates are likely to increase, as there is no barrier to water being drawn to the surface by capillary action leading to an accumulation of salt near the soil surface. This will accelerate the risk of erosion, as excess salt in a soil tends to break down soil structure. Stock frequently exacerbate baring as they like the cool and salty conditions found at a discharge site, often preferentially grazing the salt-affected vegetation and camping on these sites. The process of ‚baring‛ due to salinity usually begins in depressions, these being marginally but critically closer to the watertable. Not all bare sites will be associated with salinity. It is important to thoroughly examine a site in the field to determine if salinity is the prime cause of bare soil or some other agent such as stock or human traffic, wind or water erosion or cultivation is the underlying cause. Often several processes will be occurring simultaneously at a site and there may be some evidence of soil salinity nearby. Areas around gates and dams are often bare because of traffic, water erosion may be the primary cause on some slopes and in gullies or stock may wallow in a depression that is wet but not salty. Salt stain or crystals Salt is sometimes observable (more so on bare soil) as minerals (evaporites) that form a white stain, or actual salt crystals (encrustation). This is one of the more definitive indications of salinity. The evaporite stain or encrustation may be tasted for confirmation. Position in the landscape Regional groundwater aquifer systems have flat or gently sloping watertables that extend under a number of surface water catchments. Due to the evapo-transpiration process, discharge generally occurs where the watertable reaches to within 2 m of the ground surface. However, this may vary depending on soil type and climate and topography. Discharge tends to appear at topographically low sites, i.e. on flats, drainage lines, lake margins, depressions and stream banks. Local groundwater aquifer systems have watertables that generally follow surface topography. Perched watertables, caused by an aquifer overlying a confining (usually clay) layer, may discharge as seeps along slopes. Salinity may occur at the break-of-slope where a change in surface gradient leads to a corresponding decrease in hydraulic gradient and a consequent rise in groundwater. Local groundwater systems may overlie and interact with regional groundwater systems. Soil blackening Salt-affected soil that is also low in phosphorus and nitrogen can sometimes be recognised by a characteristic blackening due to the dispersion of dark coloured organic matter (Matters 1987). 4.3 Factors affecting recognition of dryland salinity There a number of factors affecting the recognition of dryland salinity. Two types of recognition errors can be produced, omission and commission errors. Omission error results when an area is classed as non-saline when it is in fact saline. Commission error results when an area is classed as saline when it is non-saline. Some of the factors affecting recognition of dryland salinity may produce both types of error. Masking Site disturbance or modification such as cultivation, slashing or harvesting can conceal the presence of salinity at a site. This is termed ‘masking’, and to determine if this effect is in operation it may be necessary to take a soil sample for analysis or make a return visit to assess the site when a crop, pasture or other vegetation regime has developed. Masking generally increases the likelihood of omission errors and may dampen assessments of severity levels at a site. Another form of masking occurs when the vegetation community within an area has been modified by external factors to favour saline tolerant species i.e. ‘salinity treatments’. Treatment of discharge sites usually incorporates non-saline areas bordering the saline site. Planting the treated area with salt-tolerant species such as tall wheat grass, strawberry clover or Puccinellia may make it difficult to identify the extent of the saline area and its severity given the ability of these species to grow on both saline and non-saline soil. This form of masking favors commission error. At such sites the salt- tolerant vegetation often follows paddock boundaries. If the site has been treated for a long period and has had time for the vegetation to reflect the underlying soil salinity levels, the non-saline areas may contain a number of salt-sensitive species that have been able to out-compete the salt-tolerant species. The areas of higher soil salinity may be showing degradation and reduced plant cover, or invasion by species with a higher level of tolerance if a source of seed exists nearby. As a rule of thumb, unless there is invasion by species with higher tolerance or bare ground to indicate increasing soil salinity, or invasion by salt-sensitive species, the mapping officer can assume that the salinity must have been at least moderate (class 2 severity) for the landholder to have bothered treating the site. Although the mapped extent of the salt-tolerant vegetation may well reflect paddock boundaries, mapping the edge of the salt-tolerant vegetation will place the site on the data base for future monitoring and contribute to a greater understanding of the distribution of soil salinity across a catchment. Another form of masking may arise during seasonal wet periods if salt-sensitive plants with a short life cycle can complete their growth and reproductive stages before salt levels in the soil rise again. These oppor
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