4. Análisis de información
4.1 Impacto a nivel neuronal
One distinctive site type makes up this group which is almost exclusive to basalt dominated upland or tableland terrain associated with the Chichester and other ranges. Soils are formed in situ and are clays that crack deeply seasonally with gilgai microrelief and many basalt cobbles and stones on the surface.
Vegetation is usually dominated by tussock grasses of the genera Astrebla, Chrysopogon, Eragrostis and Aristida. There may be occasional patches of Acacia xiphophylla (snakewood) shrubs but usually no tall or mid shrubs. The ground layer includes numerous perennial legume creepers and diverse annuals in season. The rare Ischaemum
albovillosum (tableland white grass) occurs on this site type.
The site type is preferred by grazing animals, both domestic and native, and is frequently heavily impacted.
Basaltic uplands of the Wona land system support a unique grassland site type (BUTG). Soils are very stony cracking clays supporting the perennial grasses Astrebla pectinata (barley Mitchell grass), A. elymoides (weeping Mitchell grass), Eragrostis xerophila (Roebourne Plains grass),
Aristida latifolia (feathertop threeawn) and Eriachne species
(wiregrass). This vegetation is highly preferred by livestock and kangaroos, and can be degraded to annual grasslands or herbfields if grazing is uncontrolled. Some priority species including Ischaemum albovillosum (tableland white grass) occur solely within this site type which is represented in the Chichester Range National Park.
4. Basaltic upland tussock grassland
(BUTG)
Sampling
5 inventory sites, 17 condition sites, 131 traverse points
General information
BUTG occurs on basaltic upland gilgaied stony plains and some lower stony plains. Soils are deep cracking clays with many to very abundant mantles (>20%) of basalt pebbles and cobbles (usually 2-20 cm) but also larger stones and boulders. Slopes are up to 3%, but commonly much less.
Physiognomy and composition
BUTG is a tussock grassland with isolated (<2.5% PFC) low shrubs and very occasionally isolated taller shrubs such as snakewood (Acacia xiphophylla). It is dominated by the Mitchell grasses Astrebla elymoides and A. pectinata or (rarely) by Ischaemum albovillosum. Basal cover of the perennial grasses rarely exceeds 5%. Perennial creepers
Desmodium campylocaulon, Glycine falcata, Rhynchosia minima and Vigna sp. are common on BUTG. Annual
grasses and herbs are very common in season.
22 perennial species were recorded at the 5 inventory sites, with an average of 8 species per site, considerably lower than the survey average of 16. 34 annual species were recorded, with an average of 12 species per site.
The following perennial species (by stratum) are dominant and/or common:
Tall shrubs: Dominant – not present as a recognisable stratum
Common – Acacia xiphophylla Mid shrubs: Dominant – not present as a
recognisable stratum
Common – Senna artemisioides subsp. x sturtii
Low shrubs: Dominant – occasionally present as a recognisable stratum; Senna
hamersleyensis or Sida fibulifera
Common – Neptunia dimorphantha,
Senna symonii, Sida fibulifera
Perennial grasses: Dominant – Astrebla elymoides, A.
pectinata or Ischaemum albovillosum
Common – Aristida latifolia,
Chrysopogon fallax, Eragrostis xerophila, Eriachne obtusa
Other plant forms: Common – Cajanus marmoratus,
Desmodium campylocaulon, Glycine falcata, Rhynchosia minima, Vigna sp.
(creepers)
Common annuals include Brachyachne convergens,
Calocephalus spp., Flaveria australasica, Heliotropium spp., Iseilema vaginiflorum, Ptilotus carinatus, P. exaltatus, P. gomphrenoides and Streptoglossa bubakii.
Patterns of grazing impact
Traverse condition summary (131 assessments): Vegetation - good 22%; fair 31%; poor 47%. Soil erosion - nil 100%.
BUTG is preferentially grazed by domestic stock and kangaroos. Overgrazing will remove the dominance of palatable perennial grasses, often leaving a tussock grassland dominated by Aristida latifolia, or an annual community of herbs and grasses. BUTG is not susceptible to erosion as the clay topsoil and stone mantles make it inherently stable. However, if stone mantles are removed, such as along tracks on long slopes, erosion can occur.
Nature conservation
BUTG provides a unique habitat with large pebbles and cobbles and wide seasonal cracks in the soil for small mammals and invertebrates. The following threatened flora were recorded on BUTG: Goodenia pascua (P3), Ischaemum
albovillosum (P2) and Tephrosia sp. Cathedral Gorge (F.H.
Mollemans 2420) PN (P3). Ischaemum albovillosum and
Tephrosia sp. Cathedral Gorge were exclusive to this site
type.
BUTG is fairly well represented in conservation reserves within the survey area; 11 of the 131 traverse assessments were recorded on conservation reserves. BUTG was recorded in the Millstream-Chichester National Park. It was not recorded on unallocated Crown land. BUTG is a unique site type that supports threatened flora and is preferentially grazed.
Table 2. Summary of attributes for BUTG variation recorded from condition (C) sites
Vegetation attributes Observations on natural variation and grazing impact
Spatial distinction of margin/edges Usually sharply bounded and ‘benched’ upper topographic site type (C229), with tight to the type of site stony/less stony soil patch pattern (usually at too fine a scale to be distinguishable on aerial
photography or satellite imagery).
Patchy microtopograhy of very Dominance or density of Eriachne obtusa was mainly on stony patches (C50) while Astrebla stony and less stony soil patches pectinata was mainly on less stony patches (C57).
Perennial grass species dominance Ubiquitously, except where degraded with perennial grass associations lost and replaced by annual herbfields.
Perennial grass species richness 1-6 species per site, from Astrebla pectinata, Astrebla elymoides, Aristida latifolia, Chrysopogon
fallax, Ischaemum albovillosum, Eragrostis xerophila, Eriachne obtusa.
Sites with only one or two species present and at high abundance (frequency) may be natural variation and not necessarily adversely impacted by prior usage.
Perennial grass frequency Astrebla pectinata up to 95% (C52), Aristida latifolia up to 90% (C74).
Perennial grasses grazed Astrebla pectinata (C51, C58).
preferentially with decreaser Astrebla pectinata as a remnant to increasing Aristida latifolia (C228).
response Eragrostis xerophila (C223).
Perennial grasses with Aristida latifolia evidently increased on some sites (C57), sometimes heavily grazed by
intermediate grazing response kangaroos (C228) and recorded as a remnant in a mixed annual herbfield (C223).
Presence of perennial shrubs Varied, some sites none, on others isolated to scattered cover with low species richness and and creepers beyond their valuable presence little indication of any indicator value. Species include Cajanus
marmoratus, Desmodium campylocaulon, Neptunia dimorphantha, Rhyncosia minima, Sida fibulifera, Vigna sp.
Shrubs probably increasers Glycine falcata, Sida fibulifera (C49).
Presence of Priority Species of Ischaemum albovillosum appears to be a fairly short-lived or weak perennial. It was found on
perennial grass 31% of sites, sometimes co-dominant with Aristida latifolia (C59, C73).
Perennial vegetation replacement All perennial grasses had been eliminated from some sites (C48). Annual herbfields develop by an annual herbfield dominated by Iseilema vaginiflorum and Streptoglossa bubakii.
As a result of this survey, CALM has added BUTG on the Wona land system to the list of Priority Ecological
Communities under Category 3 (iii). This site type should be considered for further reservation.
Gradational associations
BUTG is a distinctive community occurring in elevated parts of the landscape specifically on cracking clay soils and thus has clear and sharp boundaries with other site types such as Hill spinifex grassland (HSPG).
Land systems
BUTG is the most extensive site type on the Wona land system. It also occurs as a minor site type on Rocklea, White Springs and Bonney land systems.
D.
PLAIN HUMMOCK GRASSLAND
SITE TYPES
Site types in this group are widely distributed throughout the survey area. The sites are mostly near level to gently sloping erosional surfaces as pedeplains or exposed valley fill deposits (in the case of calcretes) below ranges and hill tracts and upslope from depositional surfaces such as hardpan wash plains and alluvial plains associated with the major rivers of the area. They are essentially transfer zones receiving water from adjacent uplands and shedding water to plains
downslope. The vegetation is dominated by Triodia species hummock grasses with variable shrub layers dominated by sclerophyllous and/or Eremaean genera including Acacia,
Senna, Eremophila and Ptilotus.
low rises. Slopes range from level to 6%, and relief is up to 15 m, although mostly 6 m or less. Soils are most commonly loams, calcareous loams, sandy earths, loamy earths and clays. They are generally shallow with variable cover of pebbles which are mostly 2-60 mm in size.
PHSG is the most common site type in the survey area, being recorded at 23% of the traverse points.
Physiognomy and composition
PHSG is hummock grassland of Triodia species, with isolated to scattered (<2.5-20% PFC) trees and shrubs. The dominant hummock grass species varies but is often Triodia wiseana (limestone spinifex). The grass layer may have up to 60% PFC (but most commonly 10-50%) depending on time since the last fire. For a short period after fire where the hummock grass PFC is <5%, the low shrub stratum may be dominant.
303 perennial species were recorded at the 103 inventory sites, at the survey average of 16 species per site. 163 annual species were recorded with an average of 6 species per site.
The following perennial species (by stratum) are dominant and/or common:
Trees: Dominant – occasionally recorded as a
stratum; commonly Corymbia
hamersleyana or Eucalyptus leucophloia
Common – Corymbia hamersleyana,
Hakea lorea subsp. suberea
Tall shrubs: Dominant – variable; most commonly acacias; often Acacia inaequilatera Common – A. ancistrocarpa, A.
inaequilatera, A. victoriae, Grevillea wickhamii
Mid shrubs: Dominant – variable; most commonly acacias and occasionally sennas Common – Acacia ancistrocarpa, A.
bivenosa, A. inaequilatera, Senna, glutinosa, Senna glutinosa subsp. x luerssenii, S. glutinosa subsp. pruinosa
Low shrubs: Dominant – very variable Common – Acacia stellaticeps,
Corchorus spp., Goodenia stobbsiana, Indigofera monophylla, Mollugo molluginis, Ptilotus astrolasius, P. calostachyus, Senna artemisioides
subsp. helmsii, Senna artemisioides subsp. oligophylla, Senna symonii, Sida
echinocarpa, Solanum lasiophyllum
Perennial grasses: Dominant – variable; often Triodia
wiseana
Common – Cenchrus ciliaris,
Paraneurachne muelleri, Triodia lanigera, T. longiceps, T. plurinervata, T. pungens, T. wiseana
Other plant forms: Common – Rhynchosia minima (creeper)
Common annuals include Aristida contorta, Cleome
viscosa, Dysphania rhadinostachya, Enneapogon caerulescens, Eriachne pulchella, Euphorbia australis, Goodenia spp., Ptilotus aervoides, P. exaltatus, Senna notabilis, Sporobolus australasicus and Yakirra australiensis. Hummock grassland (spinifex) site types are the most
widespread and common in the Pilbara and occur on many different landforms and soil types. This sandy plain supports
Triodia plurinervata (pincushion spinifex) (PHSG) which is of
little use for pastoralism but provides habitat for a variety of small ground dwelling fauna.
5. Plain hard spinifex grassland (PHSG)
Sampling
103 inventory sites, 2,790 traverse points
General information
PHSG occurs extensively on plains throughout the survey area, including stony plains, loamy plains, gravelly plains and gritty-surfaced plains. PHSG also occurs on footslopes and
Patterns of grazing impact
Traverse condition summary (2,790 assessments): Vegetation - good 98%; fair 2%.
Soil erosion - nil 100%.
The spinifexes and most of the shrubs on PHSG are unattractive to grazing animals and the site type is largely unaffected by grazing. Pastoral potential is very low. Ground cover and plant species composition is influenced far more by burning history and seasonal conditions post burning than by grazing. Soils associated with PHSG are usually stable.
Nature conservation
This extensive site type includes many plants ubiquitous to the Pilbara, however, it also contains species of particular conservation value. The following threatened flora were recorded on PHSG: Abutilon trudgenii ms (P3), Acacia
balsamea (P4), Bulbostylis burbidgeae (P3) and Sida sp.
Wittenoom PN (P3). It also provides habitat for various vertebrates and invertebrates.
PHSG is well represented in conservation reserves within the survey area. It was recorded on the Karijini and Millstream-Chichester National Parks, Cane River Nature Reserve and the Meentheena pastoral lease. It occurs extensively on unallocated Crown land.
Gradational associations
PHSG grades into Hill spinifex grassland (HSPG) on footslopes below hills. It also grades laterally into Plain soft
spinifex grassland (PSSG) where plains receive more water
and into Sandplain hard spinifex grassland (SHSG) or
Sandplain soft spinifex grassland (SSSG) on plains with deep
sands.
Land systems
PHSG is the most extensive site type on the following land systems Adrian, Billygoat, Boolgeeda, Coongimah, Egerton, Giralia, Kumina, Lochinvar, Nirran, Paraburdoo, Paterson, Peedamulla, Platform, Pyramid, Satirist, Spearhole, Stuart, Tanpool, Uaroo and White Springs. It is the co-dominant site type with Plain soft spinifex grassland (PSSG) on the Macroy and Taylor systems and with Plain snakewood
shrubland with chenopod low shrubs (PSCS) on the Sherlock
system. It is a major site type on Buckshot land system and occurs as a minor site type on another 30 land systems.
6. Plain soft spinifex grassland (PSSG)
Sampling
66 inventory sites, 21 condition sites, 1,799 traverse points
General information
PSSG occurs extensively on stony plains, loamy plains and gritty-surfaced plains throughout the survey area. PSSG also occurs on gently sloping footslopes. Soils are generally shallow and may have gravelly or pebbly surface mantles of variable density. They are most commonly sands, sandy
earths and loamy earths. Slopes range from level to 7% but are mostly <3%, and relief is up to 15 m, but most commonly less than 6 m. PSSG is the second most common site type in the survey area, being recorded at 15% of the traverse points.
Physiognomy and composition
PSSG is a hummock grassland of resinous Triodia species, with isolated to moderately close (<2.5-25% PFC) trees and shrubs. The shrubs may form prominent strata and are commonly acacias. The dominant grass is mostly Triodia
pungens (soft spinifex), but is occasionally T. epactia. The
grass layer may have 0-60% PFC (commonly 10-40%) partly depending on time elapsed (or the cumulative rainfall) since the last fire. For a year or two after fire when grass cover is very low the site type may take the form of a low or mid height shrubland with or without a seasonal herb understorey.
278 perennial species were recorded at the 66 inventory sites, with an average of 15 species per site, slightly below the survey average. 127 annual species were recorded, with an average of 6 species per site.
The following species (by stratum) are dominant and/or common:
Trees: Dominant – occasionally recorded as a
stratum; often Corymbia hamersleyana or Eucalyptus leucophloia
Common – Acacia pruinocarpa,
Corymbia hamersleyana, Grevillea wickhamii, Hakea lorea subsp. suberea
Tall shrubs: Dominant – variable; most commonly acacias
Common – Acacia ancistrocarpa, A.
atkinsiana, A. inaequilatera, Grevillea pyramidalis, Hakea lorea subsp. suberea
Mid shrubs: Dominant – variable; most commonly acacias, occasionally sennas
Common – Acacia ancistrocarpa, A.
bivenosa, A. inaequilatera, Senna glutinosa, Senna glutinosa subsp. x luerssenii
Low shrubs: Dominant – very variable; often acacias, corchorus or sennas Common – Acacia stellaticeps,
Bonamia rosea, Corchorus sidoides, C. walcottii, Euphorbia australis, Indigofera monophylla, Mollugo molluginis, Ptilotus australasicus, P. calostachyus, Senna artemisioides
subsp. oligophylla, Solanum
lasiophyllum, Tephrosia uniovulata
Perennial grasses: Dominant – Triodia pungens or T.
epactia
Common – Aristida holathera var.
holathera, A. latifolia, Cenchrus ciliaris, Chrysopogon fallax, Eragrostis eriopoda, Paraneurachne muelleri
Common annuals include Aristida contorta, Cleome
viscosa, Dysphania rhadinostachya, Eriachne aristidea, E. pulchella, Fimbristylis sp., Ptilotus exaltatus, Senna notabilis, Sporobolus australasicus, Trichodesma zeylanicum and Yakirra australiensis.
Patterns of burning and grazing impact
Traverse condition summary (1,799 assessments): Vegetation - good 93%; fair 6%; poor 1%. Soil erosion - nil 98%; minor 2%.
PSSG is susceptible to wind erosion after burning but this is not usually a major problem as stability is quickly restored by post-fire regrowth unless grazing pressure is excessive.
Soil surfaces supporting PSSG were usually recorded as having no crypogamic crusting or only as minor areas (up to 5%).
PSSG sites with young soft spinifex are moderately attractive to livestock but mature stands are unattractive and of little use for pastoralism.
Generally PSSG is resilient under grazing but it can become degraded when small patches are burnt and stock congregate heavily on these preventing the spinifex and other palatable plants from establishing. This can result in bare patches or changes in species composition towards unpalatable plants. Suijdendorp (1981) suggests that shrub invasions can occur after a winter burn. The unpalatable low shrub Acacia stellaticeps (poverty bush) may thrive and establish thick stands at the expense of the spinifex. A stable, less productive disclimax is reached. Winter burning may promote prolific growth of other unpalatable plants such as
Senna notabilis (cockroach bush) or Corchorus species.
About 33% of the condition sites had evidently been burnt within the previous 3 years. The high frequency of fires means that the presence or absence of particular plants or suites of plants may be more closely related to season of burning and post-fire stages rather than to grazing pressure. Suijdendorp (1967, 1981) suggests that summer burning coupled with deferred grazing for a few months after rain will facilitate the establishment of seral species (some of which are palatable to livestock) in competition with spinifex and delay its resumption of dominance. While in several cases the season of the fire was unknown, there was little conclusive evidence from the site vegetation record to distinguish summer-burnt from winter-burnt sites, in terms of their cover or frequency of spinifex seedlings, other perennial grasses, other perennials or annuals. All of these
characteristics were subject to high variation between sites, so a larger number of sites observed over more years would be needed to examine the possible qualitative and quantitative differences stressed by Suijendorp (1981).
In some cases, presumably following good seasons and where conditions were suitable, the germination and regrowth of soft spinifex seedlings into hummock grass cover was rapid - achieving frequency measurements of up to 94% (C90) within a year of fire.
Table 3. Summary of vegetation attributes for variation recorded from recently burnt (<3 years) PSSG condition sites
Vegetation attributes Observations on natural variation and grazing impact Frequency of soft spinifex grasses 26% to100%, mean 71%.
Projected Foliar Cover of soft spinifex grasses 2% to 57%, mean 30%. Other perennial grass species richness 0-4 species; mean 1.5. Woody and herbaceous perennial species richness 0-21 species; mean 9. Palatable (desirable) woody and herbaceous perennial
species richness 0-7 species; mean 2.0.
Main palatable (desirable) woody and herbaceous
perennial species frequency Boerhavia gardneri to 8% Bonamia alatissima to 12% Bonamia erecta to 6% Bonamia media to 20% Bonamia pannosa to 32% Bonamia rosea to 32% Cajanus marmoratus to 6% Cullen pogonocarpum to 8% Evolvulus alsinoides to 36% Goodenia microptera to 26% Ptilotus astrolasius to 76% Ptilotus axillaris to 4% Rhyncosia minima to 2% Sida pilbarensis to 34% Sida virgata to 6% Tephrosia stipuligera to 2% Tephrosia uniovulata to 36% Tribulus hirsutus to 10% Solanum diversifolium to 26%
Nature conservation
This common site type is habitat for a range of fauna dependent on spinifex for food or shelter.
The following threatened flora were recorded on PSSG:
Abutilon trudgenii ms (P3), Bulbostylis burbidgeae (P3), Eremophila pilosa ms (P1) and Sida sp. Wittenoom PN (P3). Eremophila pilosa was exclusive to this site type.
PSSG is well represented in conservation reserves within the survey area. It was recorded in the Karijini and Millstream-Chichester National Parks, Cane River Nature Reserve and the Meentheena pastoral lease. It occurs extensively on unallocated Crown land.
Gradational associations
PSSG grades into Hill spinifex grassland (HSPG) upslope. It also grades laterally into other spinifex site types such as
Alluvial plain soft spinifex grassland (ASSG) on alluvial
plains and drainage floors and Sandplain hard/soft spinifex
grassland site types (SHSG, SSSG) on plains with deep
sands. It has sharp boundaries with spinifex sites based on substrate changes over short distances, e.g. Calcrete hard
spinifex grassland (CASG).
Land systems
PSSG is the most extensive site type on Bonney, Dollar, Mallina and Urandy land systems. It is the co-dominant site type with Plain hard spinifex grassland (PHSG) on Macroy and Taylor land systems and with Stony acacia-eremophila
shrubland (SAES) on Sylvania land system. It occurs on
another 37 land systems as a minor site type.