Acacia Pulchella Research Articles

Some like it hot: Seed thermal threshold variation in obligate seeding Acacia pulchella along a climate gradient

Dormancy in seeds is a key persistence mechanism for many flowering plants. Physically dormant (PY) seeds have water impermeable seed coats, and in fire-prone systems a common mechanism for dormancy release is fire-induced soil heating. However, the thermal thresholds innate to seeds with PY may be influenced by vegetation, climate, and fire regimes, varying substantially between populations of the same species. To investigate intraspecific variation of thermal thresholds in PY seeds, we sampled obligate seeding Acacia pulchella (Fabaceae) which produces PY seeds. Sampling was undertaken from 13 populations across a climate gradient of rainfall and temperature, and between two vegetation communities in fire-prone Mediterranean-type ecosystems of south-west Western Australia. To test a range of weather and fire-induced soil heating dormancy-break scenarios, we conducted dry heat shock experiments between 40 and 140 °C for 10 min and scored germination for 16 weeks. We created population-specific thermal performance curves and extracted the dormancy release temperature at which 50 % of the seeds had germinated (DRT50), the optimum dormancy-breaking temperature to stimulate maximum germination (T0), and the lethal temperature at which 50 % of the seeds were killed (LT50). Generalised linear models were used to examine relationships between thermal thresholds and possible vegetation, climate, and fire regime drivers of intraspecific variation in seed traits. We found that thermal thresholds differed between vegetation communities, with thresholds consistently higher in forest-type ecosystems compared to open woodland, and the influence of climate varied significantly between the two communities. Seeds from Jarrah Forest populations had a DRT50 16.0 °C higher, a T0 9.7 °C higher, and LT50 7.8 °C higher than seeds from Banksia woodlands. A high rate of non-dormancy was identified in one population that had lost fire in its system and displayed significant germination after both summer and fire-related temperatures. The PY thermal thresholds modelled here provide insight into the strong influence of variable soil heating as a function of vegetation and fuel dynamics in fire-prone environments. Our findings highlight the significant intraspecific variation for this species and suggest that fire-induced soil heating generated by vegetation characteristics may be an overlooked element of fire regimes shaping seed traits.

Activated carbon seed technologies provide some protection to seedlings from the effects of post‐emergent herbicides

Invasive plant species create barriers to native species' reestablishment after land degradation. Postemergent herbicides are preferred (over preemergent) for use in restoration as they are not expected to affect native seeds. However, recent studies found that postemergent herbicides can persist in soils and reduce native species' seed germination and seedling emergence. Activated carbon seed enhancement technologies (SETs) can protect seeds from preemergent herbicides but are untested with postemergent herbicides. We explored the effects of two postemergent herbicides, Roundup and Fusilade, on seedling emergence and health of five species native to Banksia Woodlands of Western Australia. We investigated if activated carbon SETs protect seeds and seedlings from any negative effects caused by herbicide application. A randomized block design was used, and herbicide applied to the soil after seeds (nonenhanced and enhanced with activated carbon SETs) were sown under field conditions. Roundup considerably reduced seedling emergence of Acacia pulchella, Banksia menziesii, and Eucalyptus todtiana, whereas Fusilade did not negatively affect emergence. Both herbicides considerably reduced seedling health for A. pulchella, B. menziesii, and E. todtiana. Activated carbon increased A. manglesii emergence in all treatments. Activated carbon also notably increased the percentage of healthy A. pulchella, B. menziesii, and E. todtiana seedlings in herbicide treatments, indicating that activated carbon can provide protection from postemergent herbicides applied to the soil before seedling emergence. Roundup and Fusilade should not be used in restoration when soil seed banks are the source of native seeds, and a time lag is required when direct seeding after herbicide application.

Soil conditioning and plant-soil feedbacks in a modified forest ecosystem are soil-context dependent

There is potential for altered plant-soil feedback (PSF) to develop in human-modified ecosystems but empirical data to test this idea are limited. Here, we compared the PSF operating in jarrah forest soil restored after bauxite mining in Western Australia with that operating in unmined soil. Native seedlings of jarrah (Eucalyptus marginata), acacia (Acacia pulchella), and bossiaea (Bossiaea ornata) were grown in unmined and restored soils to measure conditioning of chemical and biological properties as compared with unplanted control soils. Subsequently, acacia and bossiaea were grown in soils conditioned by their own or by jarrah seedlings to determine the net PSF. In unmined soil, the three plant species conditioned the chemical properties but had little effect on the biological properties. In comparison, jarrah and bossiaea conditioned different properties of restored soil while acacia did not condition this soil. In unmined soil, neutral PSF was observed, whereas in restored soil, negative PSF was associated with acacia and bossiaea. Soil conditioning was influenced by soil context and plant species. The net PSF was influenced by soil context, not by plant species and it was different in restored and unmined soils. The results have practical implications for ecosystem restoration after human activities.

Minerals in the ash of Australian native plants

This study characterized the ash of several Australian native plant species (silver wattle (Acacia retinodes), prickly moses (Acacia pulchella), wandoo/white gum (Eucalyptus wandoo), red gum or marri (Corymbia calophylla), grass tree (Xanthorrhoea preissii), jarrah (Eucalyptus marginata) and harsh hakea (Hakea prostrata)) and investigated the reactions of ash with soil. The ashes were alkaline with pH ranging from 12.3 to 13.8. and contained substantial amounts of plant nutrients. Amounts of calcium, magnesium, sodium or silicon varied depending on plant species and plant part. Many minor elements were also present including elements with no biological function. Available phosphorous (Colwell–P) in the ashes ranged from 30 to 199mg/kg. All elements were mostly present in crystalline compounds which were identified using XRD and SEM. Minerals present in ash included calcite (CaCO3), fairchildite (K2Ca(CO3)2),, nesquehonite (MgCO3.H2O), sylvite (KCl), lime (CaO), scolecite (CaAl2Si3O10.3(H2O)), quartz (SiO2) (derived from dust), portlandite (Ca(OH)2), periclase (MgO), and apatite, probably resembling hydroxyl-apatite (Ca5(PO4)3(OH)) and wilkeite (Ca5((P, S, Si)O4)3(OH,CO3)). Ash has important liming and fertilizer values and its effectiveness in these roles is a consequence of the properties of these minerals and their reaction with soil as was demonstrated in an ash plus soil incubation experiment.

In planta selfing and oospore production of Phytophthora cinnamomi in the presence of Acacia pulchella

This paper provides the first evidence of A2 type 1 and type 2 isolates of Phytophthora cinnamomi producing selfed oospores in planta in an Australian soil and in a potting mix. Oospores were observed in infected lupin ( Lupinus angustifolius) roots incubated for 7 d either in the substrate under potted Acacia pulchella plants, or in soils collected from under and near varieties of A. pulchella in jarrah ( Eucalyptus marginata) forest. The A2 type isolates varied in their ability to produce selfed oospores and none were produced by A1 isolates. The gametangial association was amphigynous and spores were predominantly spherical with diameters from 13–40 μm. The oospores were viable but dormant. Two A2 type isolates produced small numbers of selfed oospores with amphigynous antheridia axenically in Ribeiro's liquid medium within 30 d, and one A2 type 2 isolate produced oospores after mating with an A1 strain. Evidence is presented that the presence of roots of Acacia pulchella, and particularly A. pulchella var. glaberrima and var. goadbyi, enhances the production of oospores.

The introduction and release of Chiasmia inconspicua and C. assimilis (Lepidoptera: Geometridae) for the biological control of Acacia nilotica in Australia

Two geometrid moths Chiasmia inconspicua and Chiasmia assimilis, identified as potential biological control agents for prickly acacia Acacia nilotica subsp. indica, were collected in Kenya and imported into quarantine facilities in Australia where laboratory cultures were established. Aspects of the biologies of both insects were studied and CLIMEX® models indicating the climatically favourable areas of Australia were developed. Host range tests were conducted using an approved test list of 74 plant species and no-choice tests of neonate larvae placed on both cut foliage and potted plants. C. inconspicua developed through to adult on prickly acacia and, in small numbers, Acacia pulchella. C. assimilis developed through to adult on prickly acacia and also in very small numbers on A. pulchella, A. deanei, A. decurrens, and A. mearnsii. In all experiments, the response on prickly acacia could be clearly differentiated from the responses on the non-target species. Both insects were approved for release in Australia. Over a three-year period releases were made at multiple sites in north Queensland, almost all in inland areas. There was no evidence of either insect’s establishment and both colonies were terminated. A new colony of C. assimilis was subsequently established from insects collected in South Africa and releases of C. assimilis from this new colony were made into coastal and inland infestations of prickly acacia. Establishment was rapid at one coastal site and the insect quickly spread to other infestations. Establishment at one inland area was also confirmed in early 2006. The establishment in coastal areas supported a CLIMEX model that indicated that the climate of coastal areas was more suitable than inland areas.

Assessing the potential for biological control ofPhytophthora cinnamomiby fifteen native Western Australian jarrah-forest legume species

Fifteen native Western Australian legumes were assessed for their potential to biologically control Phytophthora cinnamomi. Biological control was assessed in a controlled situation, conducive to P. cinnamomi, and was based on susceptibility to the pathogen, ability to reduce soil inoculum, amount of asymptomatic root infection and ability for P. cinnamomi to effectively sporulate from asymptomatic ally infected roots. Acacia extensa, Acacia stenoptera and Acacia alata along with Acacia pulchella, were identified as species with the highest potential for biological control of P. cinnamomi. Acacia urophylla and Viminaria juncea exhibited the least potential for biological control; these are more likely to harbour the pathogen and provide a source of inoculum when conditions become conducive for P. cinnamomi growth and development. These findings have important implications for managing the rehabilitation of bauxite-mined P. cinnamomi-infested areas and severely disease-affected forest. By manipulating rehabilitation seed mix ratios, the density of legume species that suppress P. cinnamomi inoculum in the soil can be increased and the density of those that harbour the pathogen can be reduced. This could potentially contain the activity of P. cinnamomi soil inoculum in infested areas to protect susceptible species and enhance species diversity. Further research is required to ascertain the action of suppression before implementing control measures.

Fire management implications of fuel loads and vegetation structure in jarrah forest restoration on bauxite mines in Western Australia

Bauxite mines being restored to native jarrah ( Eucalyptus marginata)-dominated forest in Western Australia have accumulated substantial fuel loads. Assessment of fire management aspects is necessary for the effective integration of restored areas into forest-wide management. Fuel characteristics, vegetation structure and fire behaviour of young (5- and 8-year-old) restored bauxite mines in Western Australia were examined. Pre-burn fuel loads were moderate in 5-year-old restoration (15.0 t ha −1) and high in 8-year-old restoration (29.8 t ha −1). Large ranges in available fuel load estimates of sample sites (2.2–60.8 t ha −1) indicated the heterogeneous nature of fuel distribution leading to variable fire behaviour. The vegetation structure of restored areas differed from that of the unmined jarrah forest due to the presence of a prominent mid-storey layer composed of number of acacias ( Acacia pulchella, A. celastrifolia, A. extensa, A. drummondii and A. lateriticola). This mid-storey layer contributed 49% of the total fuel load in 5-year-old restoration, although the high proportion of live material (73.6%) inhibited fire development. In 8-year-old restoration the mid-storey layer contributed 46% of the total fuel load. The lower proportion of live material in these sites (27.9%), due to the senescence and death of the relatively short-lived acacias, led to increased fire intensities, flame heights and higher levels of crown scorch and defoliation. Prescribed burns were conducted in early summer 1997. Burns in 5-year-old restoration were of low intensity (<250 kW m −1) while burns in 8-year-old restoration were of very high intensity (>7000 kW m −1). Fuel re-accumulation was rapid in the first 2 weeks post-burn, with litter-fall rates 2–3.5 times that of unburnt control sites. Thereafter, litter-fall and fuel accumulation in burnt restored and unmined sites was comparable to that of unburnt control sites. Analysis of 1 and 2 years post-burn 5-year-old restoration indicated that the prescribed burns had failed to remove the mid-storey acacia layer and actually increased the proportion of dead standing material, whereas in the 8-year-old restoration, the prescribed burns removed the mid-storey layer of acacia shrubs and stimulated an increase in the proportion of live plant material, particularly near ground level. Maximum soil temperatures recorded by heat sensitive crayons exceeded 300 °C in 8-year-old restoration burns but were less than 100 °C in 5-year-old restoration burns. Hard-seeded species were stimulated to germinate in the top 2–3 cm of the soil following burns in 8-year-old restoration and only in the top 0.5 cm of the soil following burns in 5-year-old restoration. Some seeds in the top 1 cm of the soil may also have been killed by the high temperatures generated in burns in the 8-year-old sites. Fire management of jarrah forest restoration will have to be different to that employed in unmined jarrah forest due to differences in fuel characteristics, vegetation structure and fire behaviour.

Heat pre-treatment and the germination of soil- and canopy-stored seeds of south-western Australian species

Abstract The role of heat (shock) in stimulating the germination of soil-stored hard seeds from fire-following species is well known. However, the effects of high temperatures on germination of canopy-stored (serotinous) seeds are less well understood. In this study, we examined the effect of heat shock at four temperatures (60, 80, 100 and 120 °C) applied for 10 min on the germination of ten co-occurring Western Australian fire-following species (five hardseeded, five serotinous). Unlike previous studies, we distinguished between the effects of heat shock on germination rate, as well as total seedling emergence. In comparison with unheated controls, a heat pulse at one or more temperatures increased total germination and germination rate for three of the hardseeded species ( Acacia pulchella , Daviesia cordata and Trymalium ledifolium ). The precise pattern of germination response for D. cordata was influenced by whether we examined total germination or germination rate. Germination of four serotinous species ( Calothamnus quadrifidus and three Hakea species) was unaffected by pre-treatments at one or more above-ambient temperatures. Only Allocasuarina humilis displayed both increased rate and total germination at higher temperatures. Our results show that germination in some serotinous species may respond favourably to, or at least be unaffected by, the passage of fire.

Ecological aspects of soil seed-banks in relation to bauxite mining. I. Unmined jarrah forest

Germinable seed stores were measured in jarrah forest soils at six sites during one year. The overall mean seed content to a depth of 5 cm was 292 seeds m -2 . There was a significant seasonal difference, with a maximum of 435 seeds m -2 in summer, after the majority of species in this Mediterranean ecosystem had flowered and set seed, and a minimum of 207 seeds m -2 in winter. There were also large site differences in both the densities of seed present and the species represented in the soil seed store. More than 85 species were represented in the germinable seed store from a total sampling area of 17.28 m 2 . The germination of Acacia drummondii, Acacia pulchella, Bossiaea aquifolium, Kennedia coccinea, Lasiopetalum floribundum and Trymalium ledifolium were significantly increased by heating the soil. Smoke produced a significant positive germination response in one species (Trymalium ledifolium). To maximize the contribution of the soil seed store to mine rehabilitation, the ideal revegetation sequence is to collect the topsoil immediately after clearing the vegetation in summer, immediately return the soil to an area to be revegetated, and carry out all earthmoving, landscaping and seedbed preparations prior to the onset of the autumn rains.

Implications of Seedling Emergence to Site Restoration following Bauxite Mining in Western Australia

AbstractSeedling emergence of 12 selected northern jarrah (Eucalyptus marginata Donn ex Smith) forest species were investigated to assist Alcoa of Australia Ltd. in maximizing the establishment of topsoil species in rehabilitated bauxite mining sites. The species, which encompassed a range of seed weights (0.024 mg to 87 mg), plant families, seed‐storage types, life forms, and germination requirements, were placed on the soil surface and at depths of 1, 2, 5, 10, and 15 cm under controlled conditions in a glasshouse. Ability to emerge from deep burial was found to depend on seed size for species that annually release their seed to the topsoil but not for species that store their seed on the plant. All selected species were capable of emerging from 2 cm depth of burial, but eight of the 12 species were either unable to emerge from 5 cm or showed a significant reduction in emergence from 5 cm depth of burial compared to optimally buried seed. This group included two small‐seeded species, Stylidium calcaratum and Chamaescilla corymbosa; the major forest dominant, Eucalyptus marginata; the serotinous canopy‐borne seed of Hakea amplexicaulis; and the wind‐dispersed seed of Xanthorrhoea gracilis. A few seeds of the legume species Kennedia coccinea, Acacia pulchella, and Bossiaea aquifolium established seedlings from depths of 15 cm. Currently, Alcoa removes the upper 15 cm of topsoil separately from the underlying soil prior to the commencement of mining. This topsoil is respread at a similar depth following mining as part of the rehabilitation procedure. It is recommended that Alcoa continue to strip topsoil to a depth of 15 cm but investigate the option of re‐spreading topsoil onto rehabilitated pits at a shallower depth to maximize establishment via the soil seed bank.

Impact and Disease Centre Characteristics of Phytophthora cinnamomi Infestations of Banksia Woodlands on the Swan Coastal Plain, Western Australia

Phytophthora cinnamomi Rands was isolated from either dead plants or soil at 46 disease centres in Banksia woodland at national parks and reserves on the Swan Coastal Plain. Phytophthora cryptogea Pethybridge &amp; Lafferty was also isolated from roots of dead Acacia pulchella R.Br. in one disease centre infected with P. cinnamomi. Dead plants were infected with Armillaria luteobubalina Watling &amp; Kile in four disease centres on the Spearwood Dune System, and these centres were excluded from further analysis. Phytophthora cinnamomi diseased areas ranged from 0.01 to 30 ha in size (mean 1.6 ± s.e. 0.7 ha). The total area infested for the 46 disease centres was 71.5 ha. Impact of P. cinnamomi was high in 17% of disease centres and low in 11% of disease centres. Age of plant death was a mixture of old and recent in 85% of disease centres. Mainly old deaths occurred in only 4% of disease centres. The proportion of species dying in infested areas varied between 10-64% (mean 28 ± s.e. 2%) and was positively correlated with impact type. It was found that infestation decreased species number; on average, there were seven fewer species in infested compared to non-infested areas. Four plant species associated with moist sandy sites tended to occur more frequently in centres of high impact than by chance alone. Occurrence of P. cinnamomi was related to soil association with soils of 60% of the disease centres belonging to the Bassendean or Southern River associations of the Bassendean Dune System. Sixteen percent of disease centres occurred in the Cannington, Guildford and Serpentine River associations of the Pinjarra Plain. No disease centres of P. cinnamomi were found on soils of the Speanvood and Quindalup Dune Systems. A water table was found within 3 m of the soil surface in 48% of the centres. Disturbance was associated with all disease centres. Firebreaks were associated with 72% of disease centres. Banksia woodland remnants on the Bassendean Dune System and the Pinjarra Plain are highly vulnerable to infection by P. cinnamomi and their conservation requires control of existing infestatinns and protection from introduction af the pathogen.

Stimulation of Germination of Acacia pulchella: Laboratory Basis for Forest Management Options

(a) The leguminous shrub Acacia pulchella is a valued understorey species of jarrah (Eucalyptus marginata)forests in south-western Australia. It regenerates from buried seeds mainly after fire, which breaks seed dormancy.

The Loss of Va Mycorrhizal Infectivity During Bauxite Mining May Limit the Growth of Acacia pulchella R.Br.

We used plant bioassays to monitor the decline in infectivity of VA mycorrhizal fungi in topsoil during bauxite mining. In a second experiment, we investigated the seasonal changes in the infectivity of VA mycorrhizal fungi in jarrah (Eucalyptus marginata Donn ex Smith) forest soil. Finally, we tested the hypothesis that Acacia pulchella growing in soil disturbed during bauxite mining would respond to phosphorus and to inoculation with VA mycorrhizal fungi. During bauxite mining in the Western Australian jarrah forest, the infectivity of propagules of VA mycorrhizal fungi in topsoil was destroyed, even when the soil was stripped and respread promptly without stockpiling. Most infectivity was lost within 3 weeks of clearing the vegetation, before the soil was disturbed. The rapid loss of infectivity may be associated with the absence of spores in the soil. In a revegetated, respread soil, the infectivity of VA rnycorrhizal fungi was substantially greater than that of freshly disturbed soil, but less than levels recorded in similar undisturbed forest soil. There were no clear seasonal changes in VA mycorrhizal infectivity, indicating that the loss of infectivity during mining was not a seasonal response. Dry matter production by Acacia pulchella, in a glasshouse experiment, was at least doubled if soil from experiment 1 was inoculated with effective VA mycorrhizal fungi.

Notes onDiphucephala affinis (Coleoptera:Scarabaeidae) associated with flowers ofHibbertia andAcacia in Western Australia

Notes are provided on the association of the beetleDiphucephala affinis (Scarabaeidae) with yellow flowers ofHibbertia hypericoides, H. huegelii (Dilleniaceae),Acacia pulchella, andA. stenoptera (Mimosaceae). Observations were undertaken during September 9–19, 1979 at S. Perth, Western Australia. They indicated thatD. affinis is not a pollinator ofHibbertia as suggested in the literature, but may play a small role in the pollination of someAcacia species.

Potential inputs of nitrogen by seeder legumes to the jarrah forest ecosystem

Summary With an increase in density of seed-regenerated legumes on the jarrah forest floor, induced by an appropriate burning treatment, combined with moderate thinning of the forest to increase available soil moisture, it was estimated that the symbiotic activity of understorey legume species could account for up to 10 kg of fixed nitrogen per hectare per year over a 6 year growth period. Acacia pulchella was the most active legume in the ability to capture atmospheric nitrogen. The contributions of nitrogen from Acacia alata, A. extensa and Bossiaea aquifolium under possible ameliorated field conditions, were estimated to be less than half that of A. pulchella but still a significant input to the ecosystem. The additional nitrogen fixed by understorey stands of seeder legumes, promoted by appropriate burning and thinning treatments, could be sufficient to increase the growth of jarrah by 10 to 25 per cent.

NITROGEN FIXATION (ACETYLENE REDUCTION) BY FOREST LEGUMES: SENSITIVITY TO PRE‐HARVEST AND ASSAY CONDITIONS

SummaryThe effects of environmental and methodological factors on acetylene reduction by nodules of Acacia pulchella R.Br., Kennedia coccinea Vent, and K. prostrata R.Br. grown in pots were studied.The time course of acetylene reduction, measured for each species during assays lasting up to 3 h, showed that there was an initial lag phase before maximum rates were reached and that rates then declined rapidly. This decline in activity was less marked for K. prostrata than for the other two species. Rates of acetylene reduction did not differ significantly between detached nodules, roots with attached nodules and whole plants.Delaying the introduction of acetylene into assay vessels indicated that the decline in nitrogenase activity during assays at higher temperatures was only partly caused by the acetylene substrate. As weights of nodules were small in relation to the volume of assay vessels, the decline in activity is unlikely to result from changes in pO2. The initial lag phase was shorter and the decline in activity with time more marked with increasing temperature. This finding has important implications for conducting and interpreting field assays, and in determining optimum temperatures for nitrogenase activity.Calibration factors relating acetylene reduction to nitrogen fixation were estimated from measurements of nitrogen accumulation and acetylene reduction during growth of young plants in a nitrogen‐free sand. Ratios for A. pulchella (2·5:1) and K. coccinea (2·4:1) were lower than theoretical ratios (3 to 4:1), probably because of the decline in rates of acetylene reduction during assays. Lowering of irradiance to about 4 % of full sunlight over 2 d depressed acetylene reduction rates by 58% for A. pulchella and by 36% for K. coccinea.

Rhizosphere Microorganisms From the Jarrah Forest of Western Australia and Their Effects on Vegetative Growth and Sporulation in Phytophthora cinnamomi Rands

Soil dilution plate techniques were used to compare the numbers of bacteria, actinomycetes and fungi in the rhizospheres of Acacia pulchella, Banksia grandis and Eucalyptus marginata (jarrah). The most frequently isolated microorganisms and those detected in significantly different numbers in the rhizospheres of the three species were tested for their effects on sporangium production, zoospore discharge, zoospore germination and mycelial growth of the jarrah dieback pathogen Phytophthora cinnamomi. The total population of fungi in rhizosphere soil from B. grandis was much greater than that found in the rhizospheres of the other two species while the convesse was true for bacteria and actinomycetes, of which the largest populations were associated with A. pulchella. Penicillium spinulosum outnum- bered the combined population of other fungi in the Banksia rhizosphere but formed a much smaller proportion of the jarrah and Acacia rhizosphere microfloras, particularly the latter. P. spinulosum had no effect on mycelial growth or zoospore discharge in P. cinnamomi; it had some ability to stimulete sporangium production and, although it partly suppressed spore germination, the inhibitory effect was less pronounced than that noted for most other microorganisms. In contrast, microorganisms which strongly inhibited mycelial growth, zoospore discharge and germination represented a greater proportion of the Acacia rhizosphere microflora compared with the other microfloras, especially that of B. grandis. While some actinornycetes and fungi produced antibiotics that inhibited vegetative growth of P. cinnamomi in dual cultures, mycelial inhibition was often attributable to nutrient depletion of agar media by the test microorganisms. Similarly, nutrient deprivation resulting from microbial competition for substrates was also considered to be the stimulus for sporangium production in liquid media. The results are discussed in relation to previously reported suppression of P. cinnamomi in forest soils beneath stands of A. pulchella and the associated implications of this for biological control of jarrah dieback.

Native legume establishment on acidic coal mining overburden at Collie, Western Australia

Nitrogen is often provided to impoverished overburden dumps through the establishment of legumes. Low indigenous soil nutrient levels, summer drought conditions and an acidic mining overburden represent major obstacles to successful rehabilitation of open-cut coal mining at Collie in southwest Western Australia. In this study,Acacia pulchella, a native Western Australian species often used in rehabilitation of mined lands, was shown to nodulate and grow in coal mining overburden with pH values less than 4.0 under glasshouse conditions. Plant growth (both top and root dry weight), nodule fresh weight, and nodulation success was best at a pH near 5.0, a value only slightly lower than the typical soil pH of the native jarrah (Eucalyptus marginata) forest. Acetylene reduction rates were reduced by acidity and ranged from 8.2 μm C2H4g(-1)hr(-1) at pH 6.77 to 3.0 μm C2H4g(-1)hr(-1) at a pH of 3.98. Four additional plant species were found to occur and to nodulate on acid overburden material at Collie.

Indirect Effect of Floristic Composition on Production of Sporangia by Phytophthora cinnamomi in Jarrah Forest Soils

Production of sporangia and zoospore discharge by Phytophthora cinnamomi were investigated in jarrah forest soils collected beneath Acacia pulchella, Banksia grandis and Eucalyptus marginata and incubated under conditions which excluded a direct effect of these species on the soil physical environment. Sporangium production was always significantly greater in soils from B. grandis than in soils from A. pulchella; soils from E. marginata gave intermediate results. There was also evidence that discharge of zoospores was suppressed in soils from A. pulchella. Although three isolates of P. cinnamomi differed in their abilities to sporulate per se, they responded similarly to different treatments.