Erythrophleum

Barrett, Russell L. & Barrett, Matthew D., 2023, Taxonomic revision of Australian Erythrophleum (Fabaceae: Caesalpinioideae) including description of two new species, Australian Systematic Botany 36 (5), pp. 401-426 : 402-406

Utilisation and cultural significance of Erythrophleum

The genus Erythrophleum has a significant place in the northern Australian tree flora ( Addicott et al. 2018; Hunter et al. 2022), and specific notes on its importance are provided here. Because Erythrophleum was previously considered monotypic in Australia, no distinction among the three species recognised here has been made in the literature. In some cases, published records can be linked to one or other of the revised species’ concepts, but in many cases the record is insufficient to discriminate between the sympatric species E. chlorostachys and E. pubescens . The disjunct distribution of E. arenarium allows the relevant literature records to be reliably attributed to that species. Specific utilisation of the three Australian species recognised here should be reviewed to determine whether usage may have been species-specific.

Chemistry and toxicity

Erythrophleum as a genus includes the African ordeal trees ( Maroyi 2019; Chauke and Kritzinger 2020; De Meyer 2023), so it is not surprising that the Australian species are highly toxic and are also utilised for medicinal purposes ( Quattrocchi 2016). Australian Erythrophleum is well known as a stock poison ( Hall 1964; Cribb and Cribb 1981; Harborne and Baxter 1996). The first detailed chemical investigations based on leaves and seeds provided by Walter Hill in Darwin were undertaken by James Petrie (1921 a, 1921 b), who confirmed the presence of erythrophleine and demonstrated its highly toxic effects. Griffin et al. (1971) recognised only a single species of Erythrophleum in Australia, but identified at least two chemical varieties differing markedly in their alkaloidal constituents. A voucher from Mareeba ( W. Griffin in V. K. Moriarty HN 557; CANB 335549) is E. pubescens . Definite vouchers for the other samples cited by Griffin et al. (1971) from near Darwin and Cooktown could not be traced at CANB or BRI for lack of detailed information, but the Darwin samples may represent E. chlorostachys sensu stricto, whereas the Cooktown samples will belong to E. pubescens . This suggests regional variation in chemical constituency, something also noted for African species ( Fernández-Marín et al. 2017; Delporte et al. 2021).

Erythrophleine has also been isolated from populations here included in E. arenarium ( Gardner and Bennetts 1956) . Loder et al. (1972) further documented the chemical structure of alkaloids in Australian Erythrophleum . Bisby et al. (1994) listed numerous terpenoids and alkaloids recorded from E. chlorostachys (sensu lato). The complex chemistry associated with toxic compounds in Australian taxa has been further refined by Qu et al. (2006) and Sim (2023), with there being similar studies on African ( Armah et al. 2015; Kablan et al. 2020; Imolede et al. 2022) and Asian ( Yu et al. 2005; Huang et al. 2018) species. The diterpinoid alkaloids, particularly erythrophleine, which are found in wood dust, wood, bark and leaves, are highly toxic, and contact can cause nausea, headaches, asthma, blindness, skin irritations and dermatitis ( McKenzie 2012). The extreme toxicity of erythrophleine, which can cause heart failure if ingested, has been responsible for large numbers of stock deaths following consumption of even small quantities of the foliage, particularly from root suckers, but also from fallen crown leaves ( Bailey 1900; Everist 1974; Milson 2000; McKenzie 2012). Cattle, sheep, goats, horses, donkey and camels are all known to have been poisoned by this species, often fatally, whereas several native possums and parrots such as little corellas appear to be immune to the poison, at least within their native ranges where they co-occur ( Cribb and Cribb 1981; McKenzie 2012; Carmelet-Rescan et al. 2022). Caterpillars of the large saturn moth, Neodiphthera excavus (Lane) , a species that is unusual in that the caterpillars pupate underground, do feed on the leaves ( Lane 1995); however, no butterfly larvae is known to feed on Erythrophleum .

Utilisation

All plant parts of Australian Erythrophleum species have been extensively utilised by First Nations Australians or post-colonial humans, including timber, gum, pigments, and various parts for ceremonial and medicinal purposes, making it one of the most utilised native taxa in tropical Australia ( Si 2020; Thompson 2020). Early colonial records of wooden utensils include spears, spear-point and prongs, spear-throwers, digging-sticks, throwing sticks, mallets, clubs, and as a handle for a hafted elouera (scraper knife) ( Palmer 1883, 1884; Blackman 1904; Roth 1909; Tindale 1925, 1926; Spencer 1928; Thomson 1936, 1939; Setzler and McCarthy 1950). In northern Queensland, bark was used to cover huts ( Roth 1910). In some regions, trees were culturally modified, reflecting their traditional utilisation, especially cutting into the cambium to retrieve honey from native bees ( Morrison et al. 2010; Cole 2022). Because utilisation of the genus is so significant in northern Australia, a review of each usage is provided here.

The commercial timber use of E. chlorostachys (sensu lato, including E. pubescens ), usually referred to by the common name of Cooktown Ironwood, has been documented by Boland et al. (1984), Bootle (2010) and Lake (2015). An English author commented on Australia that ‘You could laugh at the idea of wooden weapons until you saw the kind of wood that grew here’ ( Pratchett 1998). Erythrophleum chlorostachys sensu lato has a density of 1220–1300 kg m −2 (at 12% moisture), being one of the densest native timbers in Australia ( Boland et al. 1984; Cause et al. 1989; Bootle 2010). In fact, it is one of the densest timbers in the world, with 7 of the 10 densest woods globally having density very similar to that of E. chlorostachys sensu lato, with only Acacia cambagei R.T.Baker , Guaiacum officinale L. and Schinopsis Engl. species having significantly higher density ( Meier 2015). The strength of the timber is a key reason that many trees were uprooted, rather than having most branches broken off, from the intense winds of Cyclone Tracey, which devastated Darwin on 25 December 1974 ( Stocker 1976). Missing branches are probably more commonly associated with lightning strikes, and large scars and hollows are also commonly attributable to past cultural modification. Australian Erythrophleum species are long-lived, and large trees may be over 500 years old ( Taylor 2002).

A tightly interlocking grain, good termite resistance, and a smooth finish enable use for fencing, railway sleepers, boat building, firewood, charcoal, decorative turning and joinery, balls for lawn bowls and replacement of worn-out machine bearings, with potential for musical instrument manufacturing ( Swain 1928; Boland et al. 1984; Bootle 2010; Lake 2015; Zich et al. 2020). The exceptional qualities of the timber mean that it was one of the earliest timber products exported from Australia, being utilised by Macassan fishermen since about the 17th century for both masts and anchors ( MacKnight 1976; Clarke 2007, 2008), and some export of wild-harvested timber continues today. A private catalogue from 2019 listed the timber at A $20 kg−1 (∼ A $25 000 m −3). It was also utilised as a favoured timber in the earliest construction of European settlements in the Northern Territory ( Woinarski et al. 2002). Commercial wild-harvesting continues today ( Taylor 2002; Cook et al. 2005). It is noted that dried timber can be susceptible to lyctine beetles ( Cookson et al. 2009). The tightly interlocking grain does make Erythrophleum species generally resistant to white-rot fungi ( Nguyen et al. 2018), although some brown and white rot (particularly Truncospora Pilát ) fungi have been observed on Australian Erythrophleum ( M. D. Barrett, pers. obs.).

Australian Erythrophleum species have been extensively used by First Nations Australians for medicinal purposes, for manufacturing canoes, tools and ceremonial artefacts ( Brock 1988; Aboriginal Communities of the Northern Territory 1993; Lazarides and Hince 1993; Dunlop et al. 1995; Kenneally et al. 1996; Clarke 2012). Some specific examples identifiable to species are listed here, with more extensive records presented in Table 1. Erythrophleum pubescens has been used to make boomerangs and ceremonial clap-sticks in the north Kimberley ( Karadada et al. 2011) and also harpoon points and axe handles on the Cobourg Peninsula ( Blake et al. 1998). It is interesting to note that the wood is of sufficient density that it was historically used to make flat fighting swords by the Alawa people, from south-east of Katherine in the Northern Territory ( Wightman et al. 1991). Stems of young saplings have been utilised for spear shafts ( Wightman et al. 1992). Many uses are recorded for E. pubescens on Groote Eylandt, the dense wood being used for woomera pegs, the heads of hooked spears, bamboo spears and harpoons, for roasting sticks and for grinding food ( Levitt 1981).

In Australia, Erythrophleum bark produces a red dye, the extruded gum contains a tanning agent and can be eaten raw and a resin from the roots has been used to affix spearheads to shafts and pegs to spear throwers ( Blake et al. 1998; Clarke 2007; Beasley 2009). A gum from the roots of young E. pubescens plants has also been used to affix spear and axe heads ( Powell et al. 2013); a gum from saplings being used as a poison; and leaves being used in cleansing ceremonies on Groote Eylandt ( Levitt 1981). The unique chemistry of these gums allows identification of Erythrophleum products associated with historical First Nations artefacts using spectroscopy, confirming their long-term use in First Nations Australians societies ( Georgiou et al. 2022). Smoke from burning bark can cause female sterility, whereas burning leaves repel mosquitos and sandflies ( Beasley 2009). Smoke from burning leaves is also used in cleansing ceremonies ( Blake et al. 1998; Clarke 2007). Crushed bark is effective as a fish poison and in African species; crushed seeds have been used to poison arrows ( Lewis et al. 2005). In northern Queensland, medicinal properties from bark of E. pubescens are reported as effective for the treatment of sores, skin lesions, wounds, cuts, pain, and sprains ( Turpin et al. 2022). Numerous traditional medicinal uses have been documented (e.g. Devanesen and Henshall 1982; Aboriginal Communities of the Northern Territory 1993), and bark material is known to be active against tumour cell cultures ( Collins et al. 1990). African species have been reported to have medicinal and toxic properties similar to those of the Australian species ( Dongmo et al. 2001; Coates Palgrave and Drummond 2002; Okeyo 2012; Son 2019; Teclaire et al. 2019).

Although published records probably represent only a small portion of actual utilisation, identified uses are summarised in Table 1 to reflect the current state of recorded knowledge. Most of these records may apply to either E. chlorostachys or E. pubescens , but it is likely that most apply (at least primarily) to E. pubescens as the more common and widespread species, especially on plains. A few records can be categorically assigned to E. pubescens or E. arenarium on the basis of distribution.

Ecology

All species are trees, usually growing in savanna or seasonally dry forests, but sometimes in tropical wet forests or rainforests. Members of Erythrophleum species are semideciduous, maintaining only a sparse canopy in spring, when new foliage is produced ( Williams et al. 1997). Reproduction can occur from seeds, root suckers, or lignotubers ( Lacey and Whelan 1976; Fensham and Bowman 1992). Germination is reasonably straightforward for Erythrophleum species because the seeds behave like most legumes ( Missanjo et al. 2017). Stem survival following fire is low (often <10%), although survival of lignotubers is high (88%; Williams et al. 1999 b) because greater resource allocation is made to root mass relative to co-occurring eucalypts ( Paramjyothi et al. 2020). Interestingly, both smaller ( DBH <20 cm) and larger ( DBH > 30 cm) trees are most likely to die following fire, with mid-aged trees being most resilient ( Williams et al. 1999 b). This is directly related to bark thickness ( Lawes et al. 2011), so they may be useful indicators of recent fire history ( Paramjyothi et al. 2020). Whitau et al. (2018) provided evidence for the fluctuation in abundance of Erythrophleum related to long-term fire regimes in the northern Kimberley on the basis of campfire deposits spanning a 45 000-year occupation sequence. In Australia, Erythrophleum species increase significantly in abundance when fire is excluded ( Bowman et al. 1988; Fensham 1990; Bowman and Panton 1995).

Erythrophleum species provide important habitat hollows for animals, with a high density of hollows in the landscape compared with other northern savanna trees, although not uniformly across the landscape ( Braithwaite et al. 1985; Taylor and Chisholm 2005). The flaky bark of Erythrophleum arenarium provides habitat for at least one species of pseudoscorpion ( Harvey 1987). Although flowering can be infrequent ( Williams et al. 1999 a), Erythrophleum species also provide important nectar resources for birds and insects ( Woinarski et al. 2000) and to some extent also for fruit bats ( Mickleburgh et al. 1992; Fleming et al. 2009).

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Extrafloral nectaries found on the leaf rachis are likely to provide an important food resource to ants ( Pascal et al. 2000; Marazzi et al. 2019). Erythrophleum chlorostachys sensu lato has been reported to form vesicular-arbuscular mycorrhizae, although the specific fungal species involved was not identified ( Brundrett et al. 1995; Wang and Qiu 2006). A number of different Bradyrhizobium species are associated with Erythrophleum across the range of the genus, often being unique to distinct species (and multiple Bradyrhizobium Jordan species are commonly present; Yao et al. 2015).