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Biodiversity and Fire: The effects and effectiveness of fire management

Proceedings of the conference held 8-9 October 1994, Footscray, Melbourne
Biodiversity Series, Paper No. 8

Biodiversity Unit
Department of the Environment, Sport and Territories, 1996

8. Prescribed fire and control of coast wattle (Acacia sophorae (Labill.) R.Br.) invasion in coastal heath south-west Victoria

A.R.G. McMahon, G.W. Carr, S.E. Bedggood, Ecology Australia Pty Ltd
R.J. Hill, Portland Aluminium
A.M. Pritchard, J.P. and D.M. Cleary Pty Ltd

8.1 Abstract

The results suggest that prescribed fire can be an effective primary control strategy for A.sophorae. The vital attributes (sensu Noble & Slatyer 1980) which can potentially be exploited are fire-sensitivity, presumably a relatively small persistent seed-bank, and a primary juvenile period of two to three years. We anticipate that most burns would require a follow-up removal of seedlings, but this may be substantially reduced by burning prior to seed-set, that is, in December rather than autumn. In the study area the present level of seedling recruitment is also likely to be reduced by summer water stress. None-the-less seedlings of A. sophorae and other weed species, namely the exotics Phytolacca octandra and Chrysanthemoides monilifera ssp. monilifera will have to be removed before next spring. Flowering and seed set frequently occurs in C. monilifera in the first season (Weiss 1986) and likewise in P. octandra.

The results also demonstrate the profound effect A. sophorae invasion has on coastal heath vegetation, and the propensity for fire to restore the former floristics and structure. There are strong indications that the potential recovery of the heath is negatively correlated with the age of invasion.

Documenting the recovery of long-invaded areas should improve our understanding of potentially irrevocable changes, and provide a longer-term prognosis for this heath vegetation modified by invasion.

Key words: prescribed fire, coast wattle, invasion control, Acacia sophorae, coastal heath, Victoria

8.2 Introduction

Acacia sophorae (Labill.) R. Br. (syn. A. longifolia (Andrews) Willd. var sophorae (Labill.) F.Muell. Racosperma sophorae (Labill.) Martius) is a common plant of coastal dunes on the Australian mainland from the Maroochy River in southern Queensland to the Eyre Peninsula in South Australia (Morrison & Davies 1991; Whibley 1986; Pedley 1983). The species also occupies similar environments in Tasmania (Costermans 1981; Curtis 1956). In general habit it is a spreading shrub, frequently salt-pruned in exposed situations, but develops as a small tree to 10 m on sheltered sites. In Victoria and South Australia, A. sophorae has become a serious environmental weed of coastal or near-coastal heaths and woodlands, analogous to other out-of-balance indigenous invaders such as Kunzea ambigua, Pittosporum undulatum and Leptospermum laevigatum (Carr 1993; Carr et al. 1992; Cohen 1981; Judd 1990; Gleadow & Ashton 1981; Burrell 1969). A.C. Beauglehole (1992 pers comm) has noted that A. sophorae has become seriously invasive in the last two decades, and that it now infests an estimated 10 000 ha of indigenous vegetation in south-west Victoria. Beauglehole (1993) has consistently sought to draw the attention of the public and management agencies to the problem of Acacia invasion.

The above and several other problematic indigenous taxa are fire sensitive, parent plants being killed by 100 percent canopy scorch. Little use has been made however, of this attribute as a control measure. In Victoria, prescribed fire has been used in Wilsons Promontory National Park to control Kunzea ambigua, a species which requires an additional fire within the primary juvenile period, due to prodigious regeneration from soil-stored seed (Judd 1990). Leptospermum laevigatum has been controlled by wildfire in two small suburban Melbourne reserves where the season of burn proved critical in determining the amount of post-fire recruitment (Molnar et al. 1989); L. laevigatum releases its soft-coated seed annually and maintains minimal soil storage of seed (Burrell 1969; Van Gameren 1977). On the Mornington Peninsula, prescribed fire has been trialled by the Shire of Mornington at Woods Reserve, in an attempt to control Pittosporum undulatum (Bedggood et al. 1989; Narayan 1993).

Despite the threat posed to indigenous plant communities in south-eastern Australia by several highly invasive Acacia species (Carr et al. 1992), scant information on fire-related control methods is available. Anecdotal information suggests that land managers have resisted this method of control either because of logistic constraints or because of the perceived probability of wheat-crop post-fire recruitment from soil-stored seed. It seems that a common opinion is that all or most Acacia are hard-seeded and maintain substantial seed banks. This is not an unreasonable perception from much of the published data or references to fire-related behaviour of Acacia fire (e.g. Floyd 1976; Shea et al. 1979; Clemens et al. 1977; Warcup 1980; Grice & Westoby 1987), but these attributes may not be characteristic of all weedy Acacia.

Ironically, it has been invasion of the fynbos vegetation of South Africa by several Australia Acacia species that has resulted in most information to date. Of particular relevance to A. sophorae has been the work on the 'hard-seeded' and closely related A. longifolia (Boucher & Stirton 1978; Pieterse & Cairns 1986), and on A. cyclops (e.g. Glyphis et al. 1981; Milton & Moll 1982; Gill 1984, 1985). The latter species occupies similar coastal environments in its native Western Australia (Marchant et al. 1987). Encouraging findings from these studies in relation to control include the destruction of a large proportion of the seed bank of A. longifolia by intense fire (Pieterse and Cairns 1986), and the small proportion of the seed crop which is committed to soil storage in A. cyclops populations in Western Australia (Gill 1984).

Clearly, the success of control of Acacia species by fire relates primarily to fire-sensitivity and the dynamics of the seed bank, including hard-seededness, dispersal and seed predation. Almost nothing is known of these attributes in A. sophorae other than unpublished accounts of the species' fire sensitivity. Furthermore, little is known about the floristic and structural changes induced by A. sophorae invasion, nor the potential for recovery of invaded plant communities.

In 1990 a project was initiated to research the control of A. sophorae in heathland south of Portland, in south-west Victoria. Here the species threatens the destruction of most coastal heathland and woodland vegetation as well as populations of nationally rare plant and animal species, notably the endangered endemic Mellbloms spider-orchid (Caladenia hastata) (Carr & Kinhill Planners 1980) and the endangered Heath rat (Pseudomys shortridgei) (B. Wilson, unpub.).

Funds for the project were allocated by Portland Smelter Services Pty Ltd, and the Heathland Committee comprising local interest groups and agencies, formed to facilitate and oversee the project.

This paper is dedicated to Cliff Beauglehole who has worked tirelessly to extend botanical knowledge and raise the profile of environmental weed invasions in Victoria.

8.2.1 The study area

The study area, known locally as Deans Heath near Bald Hill, is located some 6 km south of the Portland township and approximately 1 km west of Point Danger. The area is part of a dune system formed from Quaternary siliceous sands abutting precipitous coastal cliffs with exposures of Tertiary basalt and Quaternary aeolianite (Douglas & Ferguson 1976). The study site is within a broad swale formerly dominated by coastal heath and interspersed landward with Allocasuarina verticillata (drooping she-oak) thickets. Over the past decade or so the majority of heath has been replaced by A. sophorae (A.C. Beauglehole 1992 pers comm; Carr 1994 pers comm). The study site is situated some 40 m ASL and receives about 850 mm of rainfall per annum (Bureau of Meteorology 1988).

8.3 Methods

8.3.1 Pre-burn

Six permanent transects (A-F) from 40-45 metres in length were established across an invasion front of A. sophorae in October 1990 (Plate 2). Pre-burn vegetation data were collected from 51 mostly non-contiguous, 2 x 2 m permanent plots located along the transects. In each plot all vascular plant species were recorded using the Domin-Krajina scale (Mueller-Dombois and Ellenberg 1974):

Rating Criteria % cover
+ Solitary, with insignificant cover  
1 Seldom, with insignificant cover  
2 Very scattered, with small cover <1
3 Scattered, with cover under 1/20 1-5
4 Any number, with 1/20 - 1/10 cover 5-10
5 Any number, with 1/10 - 1/4 cover 10-25
6 Any number, with 1/4 - 1/3 cover 25-33
7 Any number, with 1/3 - 1/2 cover 33-50
8 Any number, with 1/2 - 3/4 cover 50-75
9 Any number, with more than 3/4 but less than complete cover >75
10 Any number, with complete cover 100

Taxonomic nomenclature follows Ross (1993).

Additional data were collected on A. sophorae plants in each plot including stem length and diameter, plant height, and density.

Two soil cores were obtained from each plot in November 1990 using an 8 cm diameter tube. Each core was divided into 0-2.5 cm and 2.5-5 cm profiles, and passed through a 2.5 mm stainless steel sieve to retain A. sophorae seed. These were then counted, and seed of other species remaining were noted. No attempt was made to identify all taxa with soil-stored seed.

8.3.2 Prescribed fire

The 4 ha site was originally specified to be burnt in autumn 1991 and post-burn data collected the following spring. The managing authority, the (then) Department of Conservation and Environment slashed a 30 m perimeter fire break in March 1991 but the site was not burnt until March 1993. The prescribed burn occurred under the following conditions: Core area burnt: 2.1 ha; Time: 1300-1630 hrs; Temperature: 17-20°C; Wind: S-SE at 15-20 km/h; Relative humidity: 50 per cent.

8.3.3 Post-burn data

Vegetation data were collected from 31 permanent plots along Transects B, C, D and E. Transect A was inadvertently included in the slashed firebreak and time constraints prohibited the resampling of Transect F. In addition to cover/abundance values the following data were recorded: mode of regeneration for all species i.e. their primary regeneration strategy (strict seeder or resprouter), and seedling density of A. sophorae and other selected taxa.

8.4 Results

8.4.1 Pre-burn floristics

The shrubby dominants of intact heath vegetation include the woody Banksia marginata, Leptospermum myrsinoides, Spyridium parvifolium, Acacia verticillata var. ovoidea, Correa reflexa and Hibbertia empetrifolia. Lepidosperma canescens, Hypolaena fastigiata, Baumea acuta, Stipa mollis and S. flavescens are important herbaceous species, together with the geophytes Microseris lanceolata and Corybas diemenicus. Other common species include Pteridium esculentum and the leafless parasites Cassytha glabella and C. pubescens.

The extent and age of A. sophorae invasion, as measured by percentage cover, stem diameter and plant height, had a pronounced effect on species-richness and cover of heathland taxa (Figure 1). On uninvaded plots (n = 15) species richness ranged from 30 to 56 species per 4 m² with a mean of 40 species, and mean total cover of 96 per cent.

Invasion proceeds along a characteristic front with the bulk of plants recruiting close to the main stands, but with scattered individuals distributed across the heath. Branches typically rest on the ground at several points, giving rise to adventitious roots. Branches measured up to 9 m in length, with stem diameters often exceeding 12 cm. Roughly assuming a circular outline, older individuals of A. sophorae occupy 200 - 250 m².

At moderate levels of invasion (in general A. sophorae cover 30 - 60 per cent, stem diameter 5 - 10 cm, plant height 50 - 100 cm,) mean species richness of heathland taxa was reduced to 27 species (range 24 - 29) per plot (n = 6), with a mean cover of 65 per cent. This further declined at high levels of invasion (in general A. sophorae cover 60 - 100 per cent, stem diameter > 10 cm, height 100 - 290 cm) to a mean species richness of 16 species (range 4 - 28) per plot (n = 16), and mean total cover of 29 per cent.

Species which survive these latter stages of invasion include Lepidosperma canescens, Spyridium parvifolium, Pteridium esculentum, Baumea acuta, Banksia marginata, Hypolaena fastigiata and Hibbertia empetrifolia.

8.4.2 Soil samples

Acacia sophorae seed was isolated from only eight of the 102 soil cores sampled. The total number of seeds obtained was 15, of which 14 were found in the uppermost 2.5 cm of soil. By comparison 324 seeds of Astroloma conostephioides (flame heath) were isolated from 67 samples, one of the few other species to be separated by the sieve pore size, and which could be readily identified.

8.4.3 Post-burn

The fire killed all parent plants of A. sophorae in the study site. Post-fire recruitment for this species ranged from 0 - 53 seedlings per 4 m² with a mean of 11. This corresponds to approximately 32,000 seedlings per hectare. Seedlings were distributed unevenly with very few present in uninvaded heath, and with most occurring under moderate to dense A.sophorae (Table 1).

Twenty four, or 77 per cent of the post-fire plots, recorded a higher number of species post-fire (Figure 1). This increase was universal for plots under dense A.sophorae, with a mean number of species of 30, or 100 per cent increase. Some heavily invaded plots recorded increases in excess of 300 per cent, an example being from 12 species pre-fire to greater than 40 post-fire (Figure 1c).

The species richness of uninvaded plots (n = 15) exhibited a similar range to their pre-fire counterparts (30 - 56 species) but recorded a higher mean: 45 species, or an increase of 12.5 per cent.

Figures 8.1: Pre- and post-burn species richness of coastal heath at various levels of pre-fire invasion by Acacia sophorae; Transects B, C, D and E, Deans Heath, Portland.

Figure 8.1a: Transect B Figure 8.1a: Transect C Figure 8.1a: Transect D Figure 8.1a: Transect E

Of the 127 species recorded for all plots, 44 species (35 per cent) are regarded as strict seeders: species which lack the capacity to resprout after fire. These consist of 35 indigenous and nine exotic species, which are represented by the following life forms:

Group Indigenous Exotics
Shrubs 12 species (including A. sophorae) 2 species
Perennial herbs 20 species 2 species
Annuals 3 species 5 species

The remaining taxa behaved as resprouters, but no rigorous attempt was made to further subdivide this group into seeders / resprouters or strict resprouters (sensu Purdie 1977; Wark et al. 1987; McMahon 1987).

Table 1: Post-fire A. sophorae recruitment from different levels of pre-fire invasion
Parameter Invasion level
Low Medium High
Estimated no. of seedlings/ha1 160 325 x 102 625 x 102
Low A. sophorae cover < 30% n = 14
Medium A. sophorae cover 30 - 60% n = 6
High A. sophorae cover > 60% n = 10
Table 2: Recovery of heathland taxa, 8 months post-fire from different pre-fire levels of A. sophorae invasion
Parameter measured Invasion level
Low Medium High
Mean species richness per plot 45 43 30
Mean total cover per plot (%) 60 5.9 30

Within eight months of the burn total cover of heathland species had reached about 60 per cent on plots with low or medium pre-fire levels of A. sophorae. For uninvaded plots, this also represented a return to 60 per cent of pre-fire cover. Plots with high pre-fire levels of A. sophorae had attained a total post-fire cover of only 30 per cent (Table 2).

8.5 Discussion

8.5.1 Acacia sophorae invasion

The invasion of coastal heath by A. sophorae results in a dramatic floristic and structural change. The data from all transects show that this change is progressive and closely related to the age of invasion; the older the invasion the more pronounced the change. At worst, as seen at the start of the Great South West walk near the study area, all indigenous species are eliminated.

The structural transformation from a low heath, approximately 30 cm in height, to a closed shrubland of 2 - 3 m is rapid (Plates 2 and 3), particularly as the front extends by lateral branch growth blanketing the heath. Observations over recent years indicate that these branches have a growth rate of at least 1m/year-1 and that at the edge of the front, the structural change is complete within three to five years. A chronosequence of air photos available for the study area reveals that the invasion and virtual elimination of Deans Heath has occurred over the past 20 years, but mostly in the last ten years.

Floristic change appears to be less rapid, but equally marked. All six pre-burn transects broadly represent a temporal sequence, as they traverse invasion fronts from the margins to the centre of A. sophorae thickets. At the margins, knife-edge boundaries exist between species-rich heath and the heath - A. sophorae interface (Plate 2). Mean species richness per plot (4m²) is 40 and 27 respectively, or a decline of 32 per cent. Progressing inwards to where the thicket is probably 10 - 15 years old, mean species richness declines to 16 species, representing an overall fall of 60 per cent. Heath species which have commonly survived to this stage, but are significantly reduced in abundance include Spyridium parvifolium, Banksia marginata, Hibbertia empetrifolia and Correa reflexa. Other species which have generally maintained cover include Cassytha pubescens, Baumea acuta, Lepidosperma canescens and Pteridium esculentum. The latter three species are characteristic of the oldest and most degraded heath remnants under A. sophorae thickets where species-richness was often as low as three to four species per plot.

No data are available on the causes of Acacia invasion, but at least two factors are implicated. First is the increased bird-dispersal of seed. Large flocks of common starling (Sturnus vulgaris) inhabit the study area, and we observed them feeding en masse on A. sophorae. They nest in the nearby coastal cliffs. Comparable observations have been made near Rye on the central Victorian coast (Carr 1993).

Diaspores of A. sophorae are strongly arillate with a high lipid content and are typical of bird-dispersed Acacia seed (O'Dowd & Gill 1986). In Australia, a wide range of native birds (including silvereyes, red wattlebirds and other honeyeaters, brush bronzewing pigeons, Australian magpies and grey currawongs), have been recorded taking arillate Acacia seed (Gill 1985; O'Dowd & Gill 1986), and in South Africa, where birds disperse the weedy A. cyclops (Glyphis et al. 1981). In the study area, starlings roost on nearby powerlines, under which we observed mass germination of A. sophorae. This and the vast number of starlings observed feeding on A. sophorae seed, suggests that they have greatly influenced the mid-range dispersal (sensu Gill 1985) of this species. Explanations for this occurring in Deans Heath mostly in the last decade are speculative. Flocks of starlings may have only targeted Deans Heath once the A. sophorae population reached a certain level, precipitating the exponential increase observed over the past 20 years or so.

The second environmental factor which may have changed markedly over recent decades is the frequency of fire, brought about by fire suppression policies. We have not reconstructed the fire history of Deans Heath, but consecutive fires at a short interval, say three years, may have once been an episodic feature of this landscape. Such a fire regime would have constrained an advancing front of A. sophorae. Other factors such as vehicle dispersal of seed (Wace 1977), the frequent dispersal of seeds as contaminants of road making material in the Portland district (A.C. Beauglehole 1992 per comm), mammal and reptile dispersal (see Gill 1985) and diggings (particularly rabbits), may also be implicated in the spread of this species.

8.5.2 Soil-stored seed

Clearly, an understanding of the seed-bank dynamics is critical to the management of A. sophorae. In this study we investigated the extent of soil-stored seed in late spring 1990, as an indicator of the amount of seed persisting in soil storage. The presence of substantial numbers of seed would suggest a seed bank characteristic of hard-seeded Acacia species, whereas a small seed bank may infer a soft-seeded morphology and limited seed longevity (New 1984). However, the present results are qualified as the soil samples were from only one season and there was no investigation of pre- or post-dispersal predation (see for example Gill 1985; O'Dowd & Gill 1986).

Seed of A. sophorae was isolated from only eight of the 102 cores sampled. In an average year at Deans Heath, A. sophorae sets abundant seed but in some years (e.g. 1992-1993) almost no seeds were produced. In any event it appears that only a small proportion of seed survives in soil-storage. A similar high ratio of seed production to seed storage was found for A. cyclops in Western Australia (Gill 1984), where a significant proportion of seed may be destroyed by insects (Van den Berg 1980a, 1980b). At Deans Heath the roles of insects and birds (e.g. brush bronzewing pigeons) in the predation of A. sophorae seed remains a fascinating and crucial question.

If, in an average year, soil-storage of seed is minimal, control strategies have an additional weakness to exploit. Assuming that the quantity of viable seed declines after ripening, most seed would be available for recruitment following seed shed – January to February in the study area. This implies that burning late in the season, i.e. mid - late December would potentially minimise recruitment from soil-stored seed. The prescribed March burn during the present study should then give some indication of the maximum recruitment from soil-storage.

Some seed may also have been destroyed by the fire, particularly that stored in the uppermost 2 cm of soil. The potential for seeds to be destroyed would depend on fire intensity, and in turn soil temperatures (Floyd 1976), and on the hard-seededness of the stored seeds. For example, Pieterse and Cairns (1986) found that an intense fire in felled A. longifolia infestations in South African fynbos, killed all the seed of this hard-seeded species in the top 1cm of soil.

8.5.3 Post-burn

Our results confirm that A. sophorae is fire sensitive in south-west Victoria, which is consistent with our observations elsewhere in the State. Seedling recruitment was substantial, although this was potentially maximised by the time of burning, and was far short of a wheat-crop response observed for many other weedy Acacia species, e.g. A. saligna and A. longifolia. The distribution of seedlings appeared to correspond to parent plant distribution, with very few seedlings occurring in uninvaded heath. This implies that short-distance dispersal, possibly by ants (Gill 1985; O'Dowd & Gill 1986) is limited, and that isolated seedlings in the heath result from bird or mammal dispersion.

Uninvaded heath rapidly recovered its pre-fire floristics and recorded a mean increase in species richness of 12 per cent. This is comparable to other studies of coastal heath, as is the early dominance of resprouting species (ca. 65 per cent of the heathland taxa at Deans Heath) after fire (Siddiqui et al. 1976; Russell & Parsons 1978; Wark et al. 1987; McMahon 1987). In the study area, a number of herbaceous and woody post-fire colonisers contributed to this increase in richness, including Ixodia achillaeoides ssp. arenicola, Bulbine semibarbata and Olearia ramulosa, as well as less fire-dependent herbs such as Luzula meridionalis sens. lat., Opercularia varia, Isolepis marginata, Xanthosia dissecta sens. lat., and Senecio spp.

On invaded sites, the floristic and structural changes were dramatic. The extent of the floristic changes appear to be correlated with the age of the invasion, but species composition varied between sites. The structural change was more uniform, the fire converting a 2-3 m high shrubland to a low open heath or herbfield.

The floristic changes included increases in species richness from 100-300 per cent. Such increases far exceed those recorded elsewhere for fire-climax vegetation, heath or otherwise (e.g. Russell & Parsons 1978; Specht 1981; Bell et al. 1984; Wark et al. 1987; McMahon 1987), but are comparable to the regeneration of heath from sites invaded by Leptospermum laevigatum (Molnar et al. 1978-87).

The post-fire flora of invaded plots, although variable, displayed several common features. First was the status of the former woody dominants. Those soft-seeded species, e.g. Leptospermum continentale sens. lat. and Banksia marginata, which had virtually died-out under A. sophorae, were absent from the regenerating flora, apart from the occasional resprouting B. marginata. Other former dominants, e.g. Spyridium parvifolium, Acacia verticillata var. ovoidea and Hibbertia empetrifolia which had either died out or been vastly reduced in number, regenerated solely from seed. The former two species are also strict seeders. Second was the return of other woody heath species, from soil-stored seed, which were absent as parent plants pre-fire, e.g. Tetratheca ciliata and Correa reflexa.

In addition, several species typical of the calcareous sands seaward of the study site, regenerated solely from seed. These mostly appeared under formerly dense A. sophorae and were presumably bird or wind dispersed. The most common of these were Rhagodia candolleana, Muehlenbeckia adpressa, Olearia axillaris and Senecio spathulatus. No parent plants of these species were recorded in the pre-fire vegetation.

A third common feature of invaded plots was the co-dominance of mostly short-lived strict-seeder herbs, and vigorously resprouting sedges and/or Pteridium esculentum. The early successional herb dominance was characteristic of most plots and included Opercularia varia, Schoenus apogon, Ixodia achillaeoides, Viola hederacea ssp. seppeltiana, Pelargonium australe, and the resprouting Baumea acuta and Lepidosperma canescens.

Overall, the early seral stage of heavily invaded plots was markedly different from that in uninvaded heath. The vegetation was open (30 per cent cover compared with 60 per cent for heath plots), herb-dominated (the woody component represented by seedlings rather than resprouts) and there were significant changes in species composition. These factors limit the predictability of a successional pathway, which will most likely differ from the self-replacing mechanisms of the uninvaded heath.

A key factor in determining the structure and composition of the mid-seral stage will be the survivorship of seedlings of woody species. Preliminary counts indicated that 300-400 seedlings per plot of major woody species (Spyridium parvifolium, Tetratheca ciliata and Hibbertia empetrifolia) are present in the first season post-fire. Their relative and absolute survival, will influence species composition and the extent of woody species dominance. Near or total loss of these seedlings would result in herb dominated vegetation.

By comparison, 4000 - 8000 seedlings of Spyridium parvifolium alone, were recorded from uninvaded plots. Losses due to competition may however be less on invaded plots as the vegetation is more open, and there are fewer individuals competing for resources. The effect of other factors on seedling loss, for example herbivory, are potentially significant (e.g. Grice & Westoby 1987).

8.6 Acknowledgments

We gratefully acknowledge the advice and assistance of the following people during this project: Cliff Beauglehole, Portland; Adam Muir and Beverley Mussen, Ecology Australia Pty Ltd; Luke Hampshire; J.P. and D.M. Cleary Pty Ltd, Portland; the Heathland Committee; staff of Portland Aluminium, particularly Peter Manley.

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