Draft banksia Woodlands of the Swan Coastal Plain – Draft description and threats



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1.3 Vegetation


The principal structural features of the ecological community are:

  • A distinctive upper sclerophyllous layer of low trees1 (occasionally large shrubs more than 2 m tall), dominated or co-dominated by one or more of the Banksia species identified below.

  • An emergent tree layer of medium or tall (>10 m) height Eucalyptus or Allocasuarina species may be present above the Banksia canopy.

  • A species-rich understory that consists of:

    • A mid-ground sclerophyllous shrub layer; and/or,

    • A herbaceous ground layer of cord rushes, sedges and perennial and ephemeral forbs, that sometimes includes grasses.

The canopy of the Banksia Woodlands is most commonly dominated or co-dominated by Banksia attenuata (candlestick banksia) and/or B. menziesii (firewood banksia). Other species may dominate some examples of the ecological community, for instance, B. prionotes (acorn banksia) or B.ilicifolia (holly-leaved banksia), with B. burdettii (Burdett’s banksia), more common on the Dandaragan Plateau.

Other trees of a medium height may be present and include Eucalyptus todtiana (coastal blackbutt, pricklybark), Nuytsia floribunda (Western Australian Christmas tree), Allocasuarina fraseriana (western sheoak), Callitris arenaria (sandplain cypress) and Xylomelum occidentale (woody pear).

Emergent taller trees can occur and may include Corymbia calophylla (marri), Eucalyptus gomphocephala (tuart) and E. marginata (jarrah).

Key species in the mid-ground sclerophyllous shrub layer of the ecological community include members of the families Asteraceae, Dilleniaceae, Droseraceae, Ericaceae, Fabaceae, Haemodoraceae, Myrtaceae, Orchidaceae, Proteaceae, Restionaceae and "lilies". Widespread species include Adenanthos cygnorum (woolly bush), Allocasuarina humilis (dwarf sheoak), Bossiaea eriocarpa (common brown pea), Conostephium pendulum (pearl flower), Daviesia spp., Eremaea pauciflora, Gompholobium tomentosum (hairy yellow pea), Hibbertia hypericoides (yellow buttercups), Jacksonia spp., Kunzea glabrescens, Petrophile linearis (pixie mops), Philotheca spicata (pepper and salt), Stirlingia latifolia (blueboy) and Xanthorrhoea preissii (balga).

Key species in the herbaceous ground layer include members of the families Cyperaceae, Haemodoraceae and Restionaceae. Widespread species include Amphipogon turbinatus (tufted beard grass), Burchardia congesta (milkmaids), Caladenia spp. (spider orchids), Dasypogon bromeliifolius (pineapple bush), Desmocladus flexuosus, Drosera erythrorhiza (red ink sun dew), Lepidosperma angustatum (sword sedge), Lomandra hermaphrodita, Lyginia barbata (southern rush), Lyginia imberbis, Mesomelaena pseudostygia (semaphore sedge), Patersonia occidentalis (purple flag), Podolepis spp., Stylidium brunonianum (pink fountain trigger plant), Stylidium piliferum (common butterfly trigger plant), Trachymene pilosa (dwarf parsnip), and Xanthosia huegelii (heath xanthosia).

Consistent with observations across most of the Southwest Australian Floristic Region (Hopper and Gioia, 2004; Hopper, 2009), Banksia Woodlands are characterized by a high species richness (α-diversity) and high species geographic turnover (ß-diversity) in the shrub and herbaceous layers. Despite the common structural features of the ecological community across the Swan Coastal Plain bioregion, which include a canopy dominated or co-dominated by Banksia species and a species-rich shrub and herbaceous understorey, only a small proportion of the understorey species are widespread (see above). Many understorey species are locally endemic.



The diversity in Banksia Woodlands is associated with the understorey, with only 15 native trees associated with the overstorey (Keighery and Keighery, 2016). Surveys have recorded more than 600 native plant taxa from the 233 sampled points on the Swan Coastal Plain that contain one or more of the four characteristic Banksia tree species – B. menziesii, B. attenuata, B. prionotes and/or B. ilicifolia. An average of 50 plant taxa occur within the sampled points (100 m2) of Banksia Woodlands in the Perth area.

1.3.1 Variability


The Banksia Woodland ecological community has north–south and east–west gradients in species distribution. The structure (height, cover, density) and composition of Banksia Woodlands varies in relation to three major environmental gradients.

  • Rainfall gradient. The composition and vegetation structure of the community changes as rainfall increases from north to south, and to a lesser extent, west to east. To the north of the Swan Coastal Plain, where rainfall is lower, Banksia Woodlands exhibit lower tree height and density, gradually intergrading with Kwongkan heath, which occupies upper slopes and ridges (sometimes on laterite without a sand mantle), while Banksia Woodlands are increasingly confined to lower slopes and deeper sands (Beard, 1989). To the south of the Swan Coastal Plain, where rainfall is higher, Banksia Woodlands include mixed assemblages of Eucalyptus, Allocasuarina and Banksia in the canopy or subcanopy. These mixed stands also occur on the eastern Swan Coastal Plain and Dandaragan Plateau where rainfall is higher due to orographic effects of the Gingin Scarp and Darling Scarp.

  • Edaphic gradient. As described under 1.2 – Location and physical environment, the Banksia Woodlands mainly occur on three coastal sand dune systems, particularly on the Bassendean and Spearwood systems. Species richness generally increases in an easterly direction, with the lowest found on the youngest sands (Quindalup) and the greatest on the oldest sands (Bassendean). There are some other floristic differences between the Bassendean and Spearwood dunes, for example, Eucalyptus gomphocephala (tuart) occurs only on Spearwood sands. The ecological community does not typically occur on alluvial, granite, limestone, laterite and other lithic substrates. Localised transitions occur where these substrates are juxtaposed with Bassendean and Spearwood sands. Unusual examples of Banksia Woodlands occur on sandflats at some locations, where overbank flows of rivers periodically enrich soil moisture and nutrient status. These woodlands have an understorey dominated by ephemeral forbs and a relatively low diversity and density of shrubs, unlike most other forms of the community.

  • Catenary gradient. Banksia Woodlands typically occur on the tops and slopes of sand dunes, but do not occur on clay flats. In the north, where rainfall is lower, they are largely confined to sheltered dune slopes and flats, with heathlands occupying the dune crests and upper slopes.

Groundwater levels, groundwater quality, and seasonal fluctuations and flows in groundwater interact with the above factors, to influence the structure and composition of the Banksia Woodlands.

The dominant Banksia species in the woodlands are opportunistic phreatophytes (deep-rooted species) and obtain at least part of their water needs from groundwater at the water table, but the depth at which groundwater is exploited varies greatly between species and depends on the depth of the water table. This leads to compositional changes in the dominant Banksia species in the dunal landscape depending on water table depth. Typically, Banksia littoralis and B. ilicifolia are confined to seasonal damplands in interdunal swales where the water table is less than five metres deep throughout the year, whereas other species occur at higher elevations on the dunes. The extent to which Banksia attenuata in particular is groundwater dependent decreases with increasing water table depth, and this species is generally unable to access groundwater in areas where the water table depth is more than about 30 metres (Zencich et al., 2002).

The composition of the Banksia Woodlands, particularly in the shrub and ground layers, can exhibit a high degree of variation across short distances (e.g. < 500m). Encompassing this variation, floristic sub-communities were described on the Swan Coastal Plain, reflecting similarities in geography and soil type (Gibson et al., 1994) (also see Table 1, adapted from Government of Western Australia, 2000).

In addition to variation due to environmental gradients, the structure and composition of the ecological community may vary from that described above due to natural or human-induced disturbance, including fire.

A number of vegetation communities or floristic types are encompassed within the Banksia Woodlands ecological community. Some of these are listed as threatened or priority ecological communities in WA. Further detail on each of these ‘sub-communities’ will be provided in the final conservation advice document, to provide information to assist with consideration of particular sites of the ecological community.

1.3.2 Fungi


The area is rich in fungi species. There has been no comprehensive survey of fungi in Banksia woodlands across the Swan Coastal Plain, though survey data for fungi are available for a some reserves and sites in the Perth region (Bougher, 2011; Perth Urban Bushland Fungi, 2011) see http://www.fungiperth.org.au/Reports-all/Fungi-Surveys.html)
Table 1: Floristic Community Types (identified in Gibson et al., 1994, and in the System 6 and Part 1 Update, DEP 1996) (adapted from Government of Western Australia, 2000; Urban Bushland Council, 2011).

Note: these Floristic Community Types generally are not mapped and may not describe all types of Banksia Woodlands that are included in the ecological community. Information is being sought on which FCTs occur within the Banksia Woodland ecological community.



Key follows the table.

FCT

FCT name

Distribution

ASR

WA TEC

Supergroup 3 – Uplands centred on Bassendean Dunes and Dandaragan Plateau

20a

Banksia attenuata woodlands over species rich dense shrublands

PMR+/S

64.5

EN (WA)

20b

Eastern Banksia attenuata and/or Eucalyptus marginata woodlands

PMR+/N

59.7

EN (WA)

20c

Eastern shrublands and woodlands

PMR

60.4

CR (WA);
EN (EPBC Act)

20d

Dandaragan Plateau shrublands and woodlands




67.6




21a

Central Banksia attenuata - Eucalyptus marginata woodlands

PMR/N

52.0




21b

Southern Banksia attenuata woodlands




57.5




21c

Low lying Banksia attenuata woodlands or shrublands

PMR+

38.5

P3

22

Banksia ilicifolia woodlands

>PMR/C

30.0

P2

23a

Central Banksia attenuata - Banksia menziesii woodlands

PMR

59.0




23b

Northern Banksia attenuata - Banksia menziesii woodlands

>PMR/S

47.0




23c

North-eastern Banksia attenuata - Banksia menziesii woodlands

(PMR)

53.0




S9

Banksia attenuata woodlands over dense low shrublands

(PMR)/S

38.9




Supergroup 4 – Uplands centred on Spearwood and Quindalup Dunes

24

Northern Spearwood shrublands and woodlands

PMR*

38.9




25

Southern Eucalyptus gomphocephalaAgonis flexuosa woodlands

>PMR/S

48.1

P3

28

Spearwood Banksia attenuata or Banksia attenuata - Eucalyptus woodlands

>PMR/S

55.1




Key

Column 1: FCT (Floristic Community Type) Codes

The numbers of the types additional to Gibson et al. (1994) are italicised if they are subsets of an existing group (in types 19, 20, 23 and 30) and italicised and preceded by an S if they are supplementary groups.



Column 2: FCT name and General Description

Descriptions are based on generalised information from all plots in the group. Structural units are categorised into forest, woodlands, shrublands, sedgelands and herblands after Gibson et al. (1994).


Column 3: Distribution in relation to the Perth Metropolitan Region

PMR

confined to PMR

N

Northernmost location in the PMR

PMR+

predominantly in PMR

S

Southernmost location in the PMR

(PMR)

rare in PMR

C

PMR central to distribution

blank

outside PMR







>PMR

distribution goes well beyond the PMR










* except for isolated occurrence outside normal range

Column 4: ASR (Average Species Richness) per Floristic Community Type

Average species richness per 10m x 10m plot, less those species only occurring in a single plot (single records). Some community types can have a high proportion of single records and these estimates of average species richness are underestimates in some cases.



Column 5: WA TEC (Threatened Ecological Communities and Priority Ecological Communities under WA legislation)

CR = Critically Endangered; EN = Endangered; P2, P3 = WA priority ecological community categories



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