Seasonal wetlands occur on the clay flats as the clay impedes water movement horizontally and vertically. Rainfall therefore collects on the surface and soils become waterlogged. These wetlands tend to dry soon after the rain stops in the late spring and early summer.
The hydrology is the main driver of the ecological functions of the assemblages that occur in clay pans. Variations in depth and timing of inundation have a major influence over the suites of flora that occur in a particular location and this explains some of the variation in the community’s composition across its extent. Changes in hydrological status will significantly alter the assemblages in the communities. More than 90% of the clay pan communities have been lost through clearing and drainage of their habitat since European settlement.
Much of the high species richness arises from geophytes and annual flora that flower sequentially as the clay pans dry. Perennial shrubs and herbs contribute less to total species richness (Gibson et al. 2005). The shrubs in clay pans may appear stressed or dead over summer with leaves yellowing, but can recover when water is again added to the system.
The clay pans contain a rich and variable flora including a series of wetland genera that are widespread such as Isoetes, Myriophyllum, Cotula and Eryngium and a suite that are characteristic of southern Australia including Stylidium, Tribonanthes, Drosera and Centrolepis. The clay pans also include a series of terrestrial taxa (Gibson et al. 2005).
There is a high variability in composition in the clay pan wetlands and this may be related to their naturally highly fragmented distribution, and a highly randomised recruitment of flora taxa (Gibson et al. 2005).
Gibson et al. (1994) defined four clay pan and clay flat communities based on different species composition, with a significant increase in species richness with rainfall, and higher species richness in flat clay pans as compared to clay basins. This probably relates to a shorter period of inundation and longer time when soils are wet but when there is no surface water present (‘wet terrestrial phase’). Both clay pan types are typically completely dry by mid-summer (Gibson et al. 2005).
Habitat and Floristics The clay substrate that occurs in these communities is a fine-grained material that combines one or more clay minerals with traces of metal oxides and organic matter. Clays are plastic due to their water content and become hard, brittle and non–plastic when dry. The clays that occur in this community can appear in various colors, from dull grey to brown.
Clays are distinguished from other fine-grained soils by differences in size and mineralogy. Silts, which are fine-grained soils that do not include clay minerals, tend to have larger particle sizes than clays. There is some overlap between clays and silts in particle size and other physical properties, and many naturally occurring deposits include both silts and clay. The proportion of silt and clay will influence the properties of the substrate, and both light clays with a higher proportion of silt, and heavy clay substrates with low levels of silt can occur in the clay pan communities. The composition, properties and consistency of the substrate can also vary within any one occurrence of the community.
The five clay pan communities that are considered in this plan are described below (see Appendix 2 for lists and characteristics of common taxa):
Herb rich saline shrublands in clay pans (Community Type 7 (SCP07)) The community occurs on heavy clay soils that are generally wet, and may have surface water present, from winter to mid-summer. Many locations hold water up to 30cm deep in early spring, and early flowering aquatic species are common. A succession of species including Centrolepis spp. and Stylidium spp. flower as the clay pans dry over a period of up to three months.
The community can occur under a shrub layer comprising Melaleuca viminea, M. osullivanii, M. cuticularis or Casuarina obesa or other shrubs but can also occur as woodlands or herblands. Some areas such as where Melaleuca cuticularis or Casuarina obesa occur as an overstorey may be saline for part of the year due to evaporation resulting in increased salinity.
A suite of herbs such as Philydrella pygmaea, Brachyscome bellidioides, Centrolepis aristata,Centrolepis polygyna, Pogonolepis stricta and Cotula coronopifolia; frequently occur in the community. Species such as Angianthusdrummondii, Eryngium pinnatifidum subsp. Palustre (G.J.Keighery 13459) and Blennospora drummondii occur in low frequency and were not recorded in community types 8 to 10 (Gibson et al. 1994).
Two Department of Water (DoW) bores occur within about 50m of this clay pan community and groundwater level data are available over extended periods (Yoongarillup and Bambun reserves (Occurrences 1, 5 – see Appendix 1) (DoW 2014)). For Bambun reserve, the groundwater has varied seasonally from 4-2.5m below ground surface since 2012. The Yongarillup bore level varied from two metres below ground to the ground surface between 2008 and 2013. The soils in the latter occurrence have a higher proportion of sand and a lower clay component than most other locations. This may result in greater interaction of groundwater and surface water at this location.
Herb rich shrublands in clay pans (Community Type 8 (SCP08))
The surface pools in this community do not generally contain water to the same depth or for as long as in community type 7, but aquatic annuals are still common. In the most recent analysis of a more comprehensive dataset of clay pan data by Gibson et al. (2005) that included areas outside of the Swan Coastal Plain, however, sites in these deeper basin clay pans grouped separately into the community ‘Clay pans with shrubs over herbs’, described below. This includes clay pans in the Brixton St wetlands (occurrences 35, 53), Bandicoot Brook (occurrence 37), Pursers (occurrences 102, 103, 106, 107), Julimar (occurrence 101), and Drummond (occurrences 99, 100).
Viminaria juncea, Melaleuca viminea, M. lateritia or M. osullivanii and occasionally Eucalyptus wandoo generally dominate this community. Hypocalymmaangustifolium, Acacia lasiocarpa var. bracteolata (long peduncle form P1) and Verticordiahuegelii can also occur. Typical herbs include Centrolepis aristata, Chorizandra enodis, Drosera menziesii subsp. menziesii, Drosera rosulata and Hyalosperma cotula. This community included a relatively high proportion of weeds due to historical disturbance (Gibson et al. 1994).
A DoW bore occurs in Hay Park in Bunbury bushland in this clay pan type (occurrence 48 – see Appendix 1), and is screened in the superficial aquifer. Water levels at this location varied seasonally in a range from 1.7 and 5.8m below ground between 2009 and 2014 (DoW 2014). A private bore occurs in this community in Kenwick, and had a static water level of 3.5m below ground when drilled in November 2011.
Dense shrublands on clay flats (Community Type 9 (SCP09)) The shrublands or open woodlands of this community are inundated for longer periods and have lower species richness and numbers of weed taxa than the other clay pan types. Sedges including Chorizandra enodis, Cyathochaeta avenacea, Lepidosperma longitudinaleand Leptocarpus coangustatus (formerly Meeboldina coangustata) are more common in this community. Shrubs including Hakea varia,Melaleuca viminea and Eutaxia virgata are common.
There are no high quality bore data available for locations near this clay pan type. Only one bore had publicly available data, near Wellard Nature Reserve (occurrence 69 – see Appendix 1). The readings were taken during winter 1962 and summer 1974, with the level varying from about 1.9m to 2.1m below the surface.
Shrublands on dry clay flats (Community Type 10a (SCP10a)) The community occurs on skeletal soils that have shallow microtopography and the habitat is the most rapidly drying of the four clay pans identified in Gibson et al. (1994). Shrubs in the community include Hakea sulcata, Hakea varia, Pericalymma ellipticum and Verticordia densiflora. Herbs and sedges that are also common include Schoenus rigens, Aphelia cyperoides, Centrolepis aristata, Schoenolaena juncea, Drosera gigantea subsp. gigantea, and Drosera menziesii subsp. menziesii. There are three bores which occur within 20m of the community. These are railway reserves Capel (BY25B, BY25A occurrence 95) and a private bore in Capel (occurrence 87). The range of the static water level is between 2.01m to 9.35m from top of casing with the recording taken at the end of autumn (27/05/2009); with one recording showing levels dropping to 1.22 m from ground level but with no date provided.
Clay pans with shrubs over herbs (See also Appendix 3 for list of common taxa)
These clay pans are usually dominated by a shrubland of Melaleuca lateritia (robin red breast bush) with dense herbs. This community is known from the Swan Coastal Plain and Jarrah Forest IBRA regions.
The clay pans are characterised by taxa that are adapted to presence of surface water such as Hydrocotyle lemnoides or to a combination of terrestrial and wet phases such as Glossostigma diandrum, Liparophyllum capitatum (formerly Villarsia capitate), and Eleocharis keigheryi (Gibson et al. 2005).
Forbes and Vogwill (2012) studied water relations in a clay pan of this type in Drummond Nature Reserve. They found evidence that there is little connection between the surface and groundwater systems. There is one private bore within or in the 20m buffer zone of the community (SWAMP 117).
Hydrology There are few data available on surface water characteristics of the clay pan communities, however V & C Semeniuk Research Group (2001) completed detailed hydrological studies of the Brixton Street wetlands in Kenwick that are dominated by clay pans (including occurrences 35, 53-56, 92). They noted that discharge of ponded water can occur through slow infiltration or evapo-transpiration, with slow rainfall runoff from former natural channels and constructed tracks occurring at 0.002-0.006 m3/second for several days to weeks after rain. Sumplands (primarily clay pans) are inundated at the Brixton Street site for three to five months a year and reach a maximum depth of 40-50cm, with tracks holding water for longer periods (V & C Semeniuk Research Group 2001).
V & C Semeniuk Research Group (2001) note that groundwater in the muds (clays) and muddy sands was hyposaline and increased with depth. Water in adjacent drains that were probably mainly collecting surface water was fresh. Moisture contents were highest during maximum rainfall with wetlands that contained muds (clays) retaining moisture for longest in the Brixton St wetlands (V & C Semeniuk Research Group 2001). The amount of moisture in the top 15cm of soil was low across the site, but greatest in muds (clays), then muddy sands, then sands. The sands therefore had the lowest moisture retention rate and highest porosity. Differences in soil moisture retention have implications for drought tolerance of the associated vegetation, with greater impacts of drought occurring in vegetation on sandy clay soils than on clays due to high porosity and lower moisture retention. Lower rainfall periods can cause reduced flowering seasons and stunted growth, decreased recruitment of less drought tolerant taxa such as some annuals and increases in more drought tolerant flora, and death of more drought susceptible taxa (V & C Semeniuk Research Group 2001).
The hydrographs for a drain adjacent to the Brixton St wetlands during below average rainfall were below groundwater level, so this drain primarily affects surface water (V & C Semeniuk Research Group 2001). In periods of below average rainfall, regional groundwater therefore has minor influence on the Brixton St wetlands with the major influences being direct rainfall, ponding, infiltration and sub-surface perching. In wetter periods, groundwater levels may reach the drains. The effect of drains is, however, to shorten periods of inundation and waterlogging of surface sediments and the potential for infiltration.
Studies of linkages between groundwater and surface water in the south west Australian clay pans are very limited, but where completed generally indicate a lack of connection between the two systems. For example Forbes and Vogwill (2012) studied two clay pans in Drummond Nature Reserve, including an occurrence of the Clay pans with shrubs over herbs. They found evidence that there is little connection between the surface and groundwater systems. V & C Semeniuk Research Group (2001) also concluded that there is no relationship between groundwater and wetlands at some of the Greater Brixton Street sites, that includes several clay pan types.
In addition there are data for a few bores that occur close to or within the clay pan communities, and the bore data for these have been extracted from Department of Water (2014) Water INformation (WIN) database. The figures below provide data about changes in groundwater depth over time beneath examples of the clay pan communities. In each case, zero metres represents ground surface. The figures indicate the seasonal nature of the superficial watertable, and the lack of connection of groundwater to surface in each case.
Figure 1: Bore data from 2003 to 2014 for occurrences 99, 100 of clay pans with shrubs over herbs in Drummond Nature Reserve. Water depth below ground, adapted from Forbes and Vogwill 2011 and Department of Parks and Wildlife unpublished data. The base of the bore is at 5.4m, and the bore is dry when level is -5.4m.
Figure 2: Bore data 2012-2013 for a clay pan type SCP07 in Bambun reserve (depth below ground, occurrence 5; data from DoW 2014)
Figure 3: Bore data for Hay Park SCP08 clay pan for 2009-2014 (depth below ground; occurrence 48; data from DoW 2014)
Figure 4: Bore data for 2009-2013 for Yoongarillup clay pan community type SCP07 (occurrence 1; WIN site reference 23023199, depth below ground; data from DoW 2014)
Indirect evidence suggests that evaporation of surface waters can result in increases in salinity in clay pan substrates. For example, samphires and Casuarina obesa, that are adapted to saline soils, are recorded from a suite of locations, in particular, of the Herb rich saline shrublands in clay pans. Locations that have samphires in the clay pans include Bullsbrook Nature Reserve (occurrence 8) and Brixton St wetlands (occurrences 35, 53).
Related biodiversity impacts and benefits Other TECs, declared rare and priority flora, and threatened fauna, either occur in the remnant vegetation containing the clay pan communities or within the communities themselves, and will benefit from recovery actions implemented to improve the quality or security of the community.
Other TECs co-occurring within remnant vegetation that contains the clay pan communities are:
Southern wet shrublands, Swan Coastal Plain (community type 2 endangered)
Shrublands on southern Swan Coastal Plain Ironstones (Busselton area) (community type 10b, critically endangered)
Corymbia calophylla woodlands on heavy soils of the southern Swan Coastal Plain (community type 1b, vulnerable)
Corymbia calophylla - Kingia australis woodlands on heavy soils, Swan Coastal Plain (community type 3a critically endangered)
Corymbia calophylla – Xanthorrhoea preissii woodlands and shrublands (community type 3c, critically endangered)
Corymbia calophylla - Eucalyptus marginata woodlands on sandy clay soils of the southern Swan Coastal Plain (community type 3b vulnerable)
Forests and woodlands of deep seasonal wetlands of the Swan Coastal Plain (Plain (community type 15 vulnerable)
Shrublands on calcareous silts of the Swan Coastal Plain (community type 18 vulnerable)
Banksia attenuata woodlands over species rich dense shrublands (community type 20a, endangered)
There are three critically endangered fauna known to be dependent on clay pans and the surrounding communities for a portion of their life/breeding cycle. These are Pseudemydura umbrina (Western Swamp Tortoise) and two native bees: Leioproctus douglasiellus and Neopasiphae simplicior.
Leioproctus muelleri is a recently discovered native bee that is only known from Keane Rd (Occurrences 88 and 89) but is not listed as threatened fauna.