Groundwater-dependent ecosystems (GDEs) are those that rely on the surface or subsurface expression of groundwater to meet all or some of their life cycle requirements (Eamus et al., 2006). The dependence of GDEs on groundwater varies both spatially and temporally (Eamus et al., 2006). Ecosystems may be obligate GDEs, with a continuous or entire dependence on groundwater, or facultative GDEs, with an infrequent or partial dependence on groundwater (Zencich et. al., 2002). Plants that depend solely on moisture held within the soil profile are known as vadophytes and are not groundwater dependent (Sommer and Froend, 2010). In the Gloucester subregion, as in much of Australia, there is considerable uncertainty as to the nature of groundwater dependency for much terrestrial vegetation. The hydroclimatic environment of the Gloucester subregion is subtropical. Average annual rainfall is reasonably high (960 to 1400 mm/year) and is summer dominated, when potential evaporation is also highest (McVicar et al., 2014). Nonetheless, the region is still classified as being water limited inasmuch as potential evaporation (1400 to 1700 mm/year) exceeds rainfall in most months of the year. Rainfall is also highest along the margins of the subregion because this area is associated with higher elevation, whereas the deficit of rainfall, relative to potential evaporation, is greater throughout much of the lowland areas of the subregion. The Gloucester Basin underlies the Gloucester subregion and is characterised as a closed hydrogeological system. Thus, water entering the system must leave as either surface water or groundwater discharge (Dawes et al., 2018). Groundwater recharge is estimated to be up to 17% of rainfall under steady-state conditions and up to 28% of rainfall under transient conditions, with high values associated with alluvial aquifers (McVicar et al., 2014). This combination of rainfall deficit and surface water and groundwater recharge create the potential for GDEs to exist within the Gloucester subregion.
The subregion has three main hydrogeological units (McVicar et al., 2014) relevant to sustaining GDE structure and function, which provide a useful conceptual framework for examining landscape classes dependent on groundwater:
- alluvial aquifers along major creek lines
- relatively shallow weathered/fractured rock aquifers
- impermeable Alum Mountain Volcanics that underlie these hydrogeological units.
The geomorphology of the Gloucester subregion has been described in detail elsewhere (McVicar et al., 2014; Dawes et al., 2018), and only a brief summary is presented here as context (Figure 23). The Quaternary alluvial aquifers are developed close to the rivers. Soils in these alluvial deposits are dominated by Tenosols and are composed of clay layers and highly permeable sediments with high hydraulic conductivities (up to 500 m/day). The thickness of the alluvia varies from 9 to 15 m and the watertable is shallow and responsive to rainfall and flood events close to the river.
The Permian fractured rock and weathered zone is up to 150 m thick. It underlies the alluvial system and extends to the edges of the subregion. These shallow rock hydrogeological units are composed of interbedded sandstone, silt and claystone. Soils of the fractured rock and weathered zone tend to be dominated by Kurosols. Typically, these soils have a sharp, abrupt boundary between the upper coarser-textured A horizon and the finer-textured B horizon. This boundary may provide a pathway for subsurface lateral flows of water. Generally, hydraulic conductivities of these aquifers are low with a sluggish response to rainfall. However, these hydraulic conductivities are highly variable as a result of fracturing and fault zones within the formation.
The outcropping Alum Mountain Volcanics are generally considered to be impermeable but localised fractures may provide pathways for localised groundwater flow paths (McVicar et al., 2014). These flow paths may be expressed as springs along the margins of the basin, driven by localised circulation of meteoric water.
Figure 23 Conceptual model of the major groundwater processes in the Gloucester subregion
GDE = groundwater-dependent ecosystem
The water requirements of GDEs are poorly understood and there is large uncertainty as to the frequency, timing and duration of groundwater use by GDEs within the Gloucester subregion. In general, transpiration of groundwater is expected to decline as the depth to groundwater increases, but there is very limited evidence to support this assumption within Australia. O’Grady et al. (2010) reviewed estimates of groundwater discharge in Australia and concluded that there is considerable variation in the relationship between transpiration of groundwater and depth to groundwater. Factors such as the rooting depth of a particular species (which is usually not known), hydroclimatic environment and groundwater salinity all impact on groundwater use by vegetation. Zolfaghar et al. (2014) examined the structure and productivity of eucalypt forest across a depth-to-watertable gradient in the upper Nepean catchment in NSW. They found that where groundwater was shallow, vegetation had significantly higher biomass and productivity than sites where groundwater was deeper than approximately 10 m. The relationships between depth to groundwater and the structural and functional attributes of the vegetation communities were highly non-linear, with steep declines in leaf area index and biomass over a range of 5 to 10 m depth to groundwater. However, it is important to note that the study was largely correlative and did not quantify the groundwater requirements of the vegetation.
Specific studies of GDEs within the Gloucester subregion are limited. Existing mapping of GDEs is based on a multiple-lines-of-evidence approach that incorporated existing vegetation mapping, modelled groundwater levels and remote sensing (Kuginis et al., 2012). Modelled depths to groundwater (Summerell and Mitchell, 2011) for the subregion are generally shallow (within 16 m of the ground surface).
Of the five GDE landscape classes that were identified as likely to be groundwater dependent in the Gloucester subregion, qualitative models were developed for three landscape classes: ‘Forested wetlands’, ‘Wet sclerophyll forests’ and ‘Dry sclerophyll forests’. A qualitative model for rainforests was not developed owing to its small area within the subregion and lack of proximity to coal resource development. GDEs occur within each of the three hydrogeological units described above but they are predominantly associated the weathered/fractured rock zone and alluvial aquifers (Table 22). Few GDEs are present above the Alum Mountain Volcanics. The distribution of GDEs within the assessment extent (AE) of the Gloucester subregion is illustrated in Figure 6 of Section 2.7.2 . The structure and composition of the forested wetland landscape has been described in detail in the ‘Riverine’ landscape group (see Section 2.7.3.1).
Table 22 Area (ha) of groundwater-dependent ecosystem landscape classes within each of three hydrogeological units within the entire assessment extent (AE) of the Gloucester subregion
The wet sclerophyll forests of NSW occur on moderately fertile soils in high rainfall areas, and are characterised by a tall, open, sclerophyllous tree canopy and a luxuriant understorey of soft-leaved, mesophyllous, shrubs, fern and herbs (Keith, 2004). Many understorey plants are rainforest species or have close rainforest relatives. Rainforests may be embedded within a matrix of wet sclerophyll forest and the two often blend together as intermediate forms. More than 30% crown cover of emergent, non-rainforest species (including eucalypts, brushbox and turpentine) results in a classification of wet sclerophyll forest rather than rainforest (DECC, 2007). The main vegetation communities are described in Table 23.
Table 23 Main vegetation communities within the ‘Wet sclerophyll forests’, ‘Rainforests’ and ‘Dry sclerophyll forests’ landscape classes in the Gloucester subregion
|
Vegetation community |
Source |
|
North coast wet sclerophyll forests have a subdominant stratum of mesophyllous small trees or tall shrubs up to 15 m tall and a second understorey layer of mesophyllous shrubs above a continuous ground stratum of ferns and herbs. Vines are also present on shrubs and smaller trees. They occur both in coastal ranges and foothills, and on alluvium in sheltered creek flats. They grade into both northern hinterland forests (with decreasing shelter or moisture) and subtropical rainforests (with increasing shelter, moisture or fertility). Dominant canopy species include Eucalyptus acmenioides (white mahogany), E. microcorys (tallowwood), E. pilularis (blackbutt), E. saligna (Sydney blue gum), Lophostemon confertus (brush box) and Syncarpia glomulifera (turpentine) which occur in various combinations. |
NSW Office of Environment and Heritage (n.d. a) |
|
Northern warm temperate rainforests consist of closed forest up to 30 m tall, generally lacking emergents. The canopy is comprised of 4–15 species but is dominated by Acmena smithii (lilly pilly), Ceratopetalum apetalum (coachwood) and Doryphora sassafras (sassafras). It occurs in sheltered gullies and slopes in the hilly to steep terrain of the coast and escarpment on moderately fertile soils in high rainfall areas, extending above 1000 m in elevation, on granites, rhyolites, syenites or sedimentary substrates that yield acid soils with moderate levels of nutrients. Occasional lianas and epiphytes, open shrub/sapling stratum and variable fern/herb groundcover occur amongst copious leaf litter. Mosses, liverworts and lichens may be conspicuous on tree trunks or the forest floor. |
NSW Office of Environment and Heritage (n.d. b) |
|
Hunter-Macleay dry sclerophyll forests are dry open eucalypt forests to 30 m tall that are associated with the major coastal river valleys along the NSW coast. They have a mixed sclerophyll and mesophyll shrub stratum, and grassy ground layer. They occur below 400 m elevation in foothills and undulating terrain in rain-shadow valleys, on well-drained loams derived from shales. Main overstorey species include Corymbia maculata (spotted gum), Eucalyptus crebra (narrow-leaved ironbark), E. moluccana (grey box), E. propinqua (grey gum), E. siderophloia (grey ironbark) and Syncarpia glomulifera (turpentine). |
NSW Office of Environment and Heritage (n.d. c) |
Wet sclerophyll forests are tall, dense forests (30 to 60 m height) dominated by Eucalyptus trees. There are only small amounts of the ‘Wet sclerophyll forests’ landscape class within the AE for the Gloucester subregion and these are restricted to the weathered/fractured rock and Alum Mountain Volcanics land zones. Wet sclerophyll forests are divided into two subgroups (subformations) depending on whether their understorey is shrubby or grassy (NSW Office of Environment and Heritage, n.d. d). Both have a tall, straight-trunked eucalypt canopy and a mesophyllous understorey; however, the grassy subformation has a more-open form with fewer shrubs and small trees, and occurs in slightly drier habitats. Within the AE, wet sclerophyll forests are dominated by the Keith vegetation class ‘North Coast wet [shrubby subformation] sclerophyll forests’ (Dawes et al., 2018). Rainforests within the AE are primarily Keith vegetation class ‘Northern warm temperate rainforests’ (Keith, 2004). Note that the NSW-listed (Threatened Species Conservation Act 1995 (TSC Act)) ‘Lowland Rainforest in the NSW North Coast and Sydney Basin Bioregions’ and Commonwealth-listed (Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act)) ‘Lowland Rainforests of Subtropical Australia’, are associated with both north coast wet sclerophyll forests and northern warm temperate rainforests.
The ‘Wet sclerophyll forests’ and ‘Rainforests’ landscape classes provide habitat for a diverse range of plant and animal species. A number of EPBC Act-listed species included in the water-dependent asset register for the Gloucester subregion (McVicar et al., 2015) are known to occur in both landscape classes including the swift parrot (Lathamus discolor), giant barred frog (Mixophyes iteratus), giant burrowing frog (Heleioporus australiacus), stuttering frog (Mixophyes balbus), grey-headed flying fox (Pteropus poliocephalus), koala (Phascolarctos cinereus) and trailing woodruff (Asperula asthenes). Others, such as Charmhaven apple (Angophora inopina), regent honeyeater (Anthochaera phrygia), Guthrie’s grevillia (Grevillia guthrieana), Hastings River mouse (Pseudomys oralis), leafless tongue orchid (Cryptostylis hunteriana), red goshawk (Erythrotriorchis radiatus), and slaty red gum (Eucalyptus glaucina) occur only in wet sclerophyll forest.
The ‘Dry sclerophyll forests’ landscape class is predominantly confined to the weathered/fractured rock zone in the south of the AE. The majority of the landscape class is dominated by dry sclerophyll forests of the Keith vegetation class ‘Hunter-Macleay dry sclerophyll forests’ (Table 23), with smaller areas of ‘Sydney coastal dry sclerophyll forests’ and ‘Coastal dune dry sclerophyll forests’ (Dawes et al., 2018). Structurally these community types are classified as open forests, occurring on soils of lower fertility and characterised by an overstorey to 30 m dominated by Corymbia maculata and a range of Eucalyptus species including E. crebra, E. fibrosa and E. umbra. Understories are mixed and contain a range of shrubs with a more or less continuous ground layer. Typical shrub species include Acacia parvipinnula, Allocasuarina torulosa along with smaller shrubs such as Breynia oblongifolia, Daviesia ulicifolia, Lissanthe strigosa, Notelaea longifolia, Persoonia linearis, Pultenaea villosa and Rapanea variabilis. The dry sclerophyll forests may provide habitat for a range of EPBC Act-listed plant species identified in the asset register for the Gloucester subregion (McVicar et al., 2015) including Charmhaven apple (Angophora inopina), leafless tongue orchid (Cryptostylis hunteriana), and slaty red gum (E. glaucina). EPBC Act-listed animal species that might use this forest type as habitat include the grey-headed flying fox (Pteropus poliocephalus), Hastings River mouse (Pseudomys oralis), koala (Phascolarctos cinereus), regent honeyeater (Anthochaera phrygia) and the swift parrot (Lathamus discolor).
