Acacia Mill., Gard. Dict. Abr., ed. 4, [25]. 1754

Bruneau, Anne, de Queiroz, Luciano Paganucci, Ringelberg, Jens J., Borges, Leonardo M., Bortoluzzi, Roseli Lopes da Costa, Brown, Gillian K., Cardoso, Domingos B. O. S., Clark, Ruth P., Conceicao, Adilva de Souza, Cota, Matheus Martins Teixeira, Demeulenaere, Else, de Stefano, Rodrigo Duno, Ebinger, John E., Ferm, Julia, Fonseca-Cortes, Andres, Gagnon, Edeline, Grether, Rosaura, Guerra, Ethiene, Haston, Elspeth, Herendeen, Patrick S., Hernandez, Hector M., Hopkins, Helen C. F., Huamantupa-Chuquimaco, Isau, Hughes, Colin E., Ickert-Bond, Stefanie M., Iganci, Joao, Koenen, Erik J. M., Lewis, Gwilym P., de Lima, Haroldo Cavalcante, de Lima, Alexandre Gibau, Luckow, Melissa, Marazzi, Brigitte, Maslin, Bruce R., Morales, Matias, Morim, Marli Pires, Murphy, Daniel J., O'Donnell, Shawn A., Oliveira, Filipe Gomes, Oliveira, Ana Carla da Silva, Rando, Juliana Gastaldello, Ribeiro, Petala Gomes, Ribeiro, Carolina Lima, Santos, Felipe da Silva, Seigler, David S., da Silva, Guilherme Sousa, Simon, Marcelo F., Soares, Marcos Vinicius Batista & Terra, Vanessa, 2024, Advances in Legume Systematics 14. Classification of Caesalpinioideae. Part 2: Higher-level classification, PhytoKeys 240, pp. 1-552 : 1

Acacia Mill., Gard. Dict. Abr., ed. 4, [25]. 1754 nom. cons.

Figs 237 , 238 , 239 , 240

Acacia sect. Phyllodineae DC., Prodr. [A.P. de Candolle] 2: 448. 1825. Type not designated.

Racosperma Mart., Hort. Reg. Monac.: 188. 1829, nom. inval. (name not accepted by author)

Phyllodoce Link, Handbuch 2: 132. 1829, non Phyllodoce Salisb., Parad. Lond. ad t. 36. 1806 ( Ericaceae ). Type not designated.

Racosperma Mart., Index Seminum [ München (Monacensis)]: 4. 1835. Lectotype (designated by Pedley 1986): Racosperma penninerve (Sieber ex DC.) Pedley [≡ Acacia penninervis Sieber ex DC.]

Cuparilla Raf., Sylva Tellur.: 120. 1838. Lectotype (designated by Pedley 1986): Cuparilla myrtifolia (Sm.) Raf. [≡ Mimosa myrtifolia Sm. (≡ Acacia myrtifolia (Sm.) Willd.)]

Drepaphyla Raf., Sylva Tellur.: 120. 1838. Lectotype (designated by Pedley 1986): Drepaphyla lanigera (Cunn.) Raf. [≡ Acacia lanigera A. Cunn.]

Hectandra Raf., Sylva Tellur.: 120. 1838. Lectotype (designated by Pedley 1986): Hectandra suaveolens (Sm.) Raf. [≡ Mimosa suaveolens Sm. (≡ Acacia suaveolens (Sm.) Willd.)]

Zigmaloba Raf., Sylva Tellur.: 120. 1838. Type: Zigmaloba sulcata (R. Br.) Raf. [≡ Acacia sulcata R. Br.]

Chithonanthus Lehm., Pl. Preiss. 2: 368. 1848. Type: Chithonanthus restiaceus (Benth.) Lehm. [≡ Acacia restiacea Benth.]

Tetracheilos Lehm., Pl. Preiss. 2: 368. 1848. Type: Tetracheilos tetragonocarpa Meisn., nom. illeg.

Acacia sect. Phyllodoce (Link) Kuntze, in T.E. von Post & C.E.O. Kuntze, Lex. Gen. Phan.: 2. 1903. Type not designated.

Phytomorula Kofoid, Univ. Calif. Publ. Bot. 6: 38. 1914. Type: Phytomorula regularis Kofoid. [described as an alga but shown to be pollen of Acacia sp., vide Copeland 1937].

Type.

Acacia penninervis Sieber ex DC.

Description.

Trees, shrubs or subshrubs, never lianas; prickles absent. Stipules normally present and caducous, rarely spinose. Leaves bipinnate or modified to polymorphic phyllodes, rarely reduced to scales or absent; extrafloral nectaries normally present. Inflorescences globose or oblongoid capitula or spikes, axillary or aggregated in racemes or infrequently panicles, pedunculate or sometimes sessile. Flowers bisexual or staminate and bisexual within a single inflorescence, 5-merous or sometimes 4-merous, uniform, white to golden, rarely mauve-pink or red; sepals free to united, very rarely absent; corolla connate, valvate; stamens numerous, normally free, rarely basally united; pollen in polyads normally 8, 12 or 16 grains, the grains extraporate or sometimes porate, surface with pseudocolpi, exine granular (i.e., lacking columellae that occur in Vachellia Wight & Arn.); ovary solitary or very rarely 2-5 (e.g., A. celastrifolia Benth.). Fruits variable, dehiscent, rarely indehiscent. Seeds normally with a pleurogram and without endosperm; funicle arillate or sometimes exarillate.

Chromosome number.

Mostly 2 n = 26 with ca. 100 species examined ( Hamant et al. 1975; Goldblatt and Johnson 1979 -; Rice et al. 2015), but there are a few notable exceptions and examples of polyploidy. In several members of the ‘Mulga’ group (i.e., Acacia aneura F. Muell. ex Benth. and close relatives ( Miller et al. 2002; Maslin and Reid 2012), which predominate in the arid zone, tetraploids (2 n = 52), triploids (2 n = 39) and pentaploids (2 n = 65) were reported by Andrew et al. (2003). Acacia brachystachya Benth., a close relative of the ‘Mulga’ group, is tetraploid ( Hamant et al. 1975). There are a few other arid zone species not closely related to the ‘Mulga’ group that are polyploids ( Hamant et al. 1975; Moran et al. 1992; Maslin and Thomson 1992). Two extra-Australian tetraploid species are A. koa A. Gray (Hawaii) and A. heterophylla (Lam.) Willd. (Reunion Island), whereas their close relative, the widespread, eastern Australian species, A. melanoxylon R. Br., is diploid ( Brown et al. 2012). Natural triploids and tetraploids have also been recorded in populations of the Australian bipinnate-leaved species, Acacia dealbata Link. ( Blakesley et al. 2002; Nghiem et al. 2018).

Included species and geographic distribution.

1082 species, the majority in Australia. These species are distributed throughout the continent with the major centre of species-richness located in south-west Western Australia, and secondary centres of richness in eastern Australia south of the Tropic of Capricorn associated with the Great Dividing Range, and in northern and north-eastern Australia. Although species of Acacia are a conspicuous component of the central Australian arid zone, this is a relatively species-poor region ( Hnatiuk and Maslin 1988). Only 17 species occur naturally outside Australia (of which seven species plus one subspecies also occur within Australia), where they extend to South East and East Asia (north to Taiwan), the Pacific Ocean (east to Hawaii) and Indian Oceans (i.e., Mascarene islands); see Pedley (1975), Brown et al. (2012), WorldWideWattle (2022) (Fig. 240 ). Species of Acacia are widely distributed globally as weeds, ornamentals or cultivated for economic, social or environmental purposes.

Ecology.

Species of Acacia within Australia grow in a wide range of habitats, from coastal to subalpine, tropical to arid ecological vegetation classes. They are particularly conspicuous and common in many semi-arid and sub-tropical shrublands and woodlands, and while they dominate much of the arid zone the species numbers are, relatively speaking, not especially high. The diverse shrublands of Western Australia and Eucalyptus woodlands of eastern Australia associated with the Great Dividing Range, located between the arid and temperate zones, are especially species-rich, but Acacia is often not a dominant element of the vegetation ( Hnatiuk and Maslin 1988).

Etymology.

It is normally regarded that the name Acacia is derived from the Greek, ake (= a point), in reference to the spiny stipules that characterised the first (African) species described as Acacia ; these species now belong to the genus Vachellia .

Human uses.

Acacias have had extensive utilisation for economic, social and environmental purposes. A discussion of these uses is presented by McDonald et al. (2001) and a listing of them is given in the "Info Gallery/Utilisation" at WorldWideWattle (2022).

Commercially, the most important use of Acacia is for wood products where the species A. auriculiformis A. Cunn. ex Benth., A. crassicarpa A. Cunn. ex Benth., A. mangium Willd. and A. x mangiiformis Maslin & L.A.J. Thomson are especially important in tropical forestry plantation industries, particularly in south-east Asia and India ( Griffin et al. 2011; Maslin et al. 2019b). Acacia mearnsii De Wild., which was initially grown for tannin production, is now highly regarded by the pulp and paper industries in several countries, most notably, Brazil, India and South Africa ( Griffin et al. 2011). Within Australia the wood of A. melanoxylon (Blackwood) provides a niche market for fine furniture ( Beadle and Brown 2007). The potential of Acacia species as a new woody crop plant to assist with salinity control in the agricultural regions of southern Australia (within the 250-650 mm rainfall zone) was assessed by Maslin and McDonald (2004).

As noted by Griffin et al. (2011), a suite of multi-purpose Acacia species have been introduced in dry zone regions of Africa and elsewhere to assist with meeting the demand for food, fodder, fuelwood, poles, and site amelioration ( Doran and Turnbull 1997). Acacia saligna (Labill.) H. Wendl. is the most widely planted of the non-timber species, with around 600,000 ha established worldwide; it is used as an animal fodder, in landscape amelioration projects, and for a range of other purposes ( Midgley and Turnbull 2003; Maslin 2011). Acacia colei Maslin & L.A.J. Thomson has been used as a human food and has been incorporated into local farming systems in Sub-Saharan Africa ( Rinaudo and Cunningham 2008). The potential of this and other species of Acacia as a human food within Australia was assessed by Maslin et al. (1998).

Several species of Acacia have displayed significant invasiveness in places where they have been introduced. Examples include A. cyclops A. Cunn. ex G. Don, A. baileyana F. Muell., A. dealbata Link, A. mangium , A. melanoxylon , A. pycnantha Benth. and A. saligna . Factors promoting the spread of such species as weeds include the production of large quantities of tough, long-lived seeds, prolific seedling recruitment following fire or other environmental disturbances, and the absence of natural invertebrate predators and fungal pathogens. Richardson et al. (2011) provides a good introduction and review of this subject.

Indigenous Australians have long used species of Acacia as a source of food and medicine, tools and weapons, and various other purposes (e.g., Aboriginal Communities of the Northern Territory 1993; Latz 1995; Searle 2004; WorldWideWattle 2022).

As discussed at WorldWideWattle (2022), Acacia has great symbolic significance to Australians where it is the National Flora Emblem, is incorporated into the Australian Coat of Arms, and more.

Notes.

In recent years there have been substantial changes to both the classification and nomenclature of Acacia . The reasons for this are twofold. Firstly, molecular and other evidence have shown that the former broadly circumscribed, pantropical genus Acacia is polyphyletic and should be treated as comprising at least seven genera (see below). Secondly, the name Acacia is now conserved with a new type, namely, the Australian species Acacia penninervis Sieber ex DC. which replaces the Afro-Asian species, Acacia nilotica (L.) Willd. ex Delile. This retypification has meant that about half of the non-Australian species formerly called Acacia are now Vachellia and the name Racosperma Mart. [preferred by Pedley (1986) for most Australian species] is now a synonym of Acacia . There is an extensive literature regarding these two interrelated matters, but useful summaries are presented in Murphy (2008), Miller and Seigler (2012), Maslin (2015) and WorldWideWattle (2022). Summaries of issues concerning the retypification for the name Acacia are provided in McNeill and Turland (2010), Moore et al. (2010) and Thiele et al. (2011). Some of the more important phylogenetic studies that have demonstrated the non-monophyly of Acacia s.l. include Miller and Bayer (2000, 2001), Luckow et al. (2003), Murphy et al. (2003), Kyalangalilwa et al. (2013), Miller et al. (2017) and Koenen et al. (2020a). The current correct names for all species of the former broadly defined genus Acacia are available at WorldWideWattle (2022).

For several years prior to the fragmentation of Acacia s.l. the genus was commonly viewed as comprising three subgenera as defined by Vassal (1972), namely, Acacia subg. Acacia (now Vachellia ), Acacia subg. Aculeiferum Vassal (now Senegalia Raf.) and Acacia subg. Heterophyllum Vassal (= subg. Acacia Phyllodineae (DC.) Ser., now Acacia ). Vassal’s classification was based primarily on characters of seedling ontogeny and seed morphology from 127 species (representing less than 10% of those recognised for the genus at that time), supplemented by data on other characters, including pollen of Guinet (1969). These subgenera broadly corresponded to groupings of six series of Acacia that Bentham (1875) had previously recognised and which had been used to accommodate species of the genus. Discussions of Vassal’s classification in relation to Bentham’s is provided in Maslin (2001) and Murphy (2008) for Australian species and by Ross (1979) for African species. Although Vassal’s subgenera (but not his lower-order categories) were adopted by some authors (e.g., Nielsen 1992), Pedley (1986) elevated them to genera: Acacia (= Acacia subg. Acacia sensu Vassal, now Vachellia ), Racosperma (= Acacia subg. Heterophyllum ≡ subg. Phyllodineae , now Acacia ) and Senegalia (= subg. Senegalia Aculeiferum ). However, Pedley’s (1986) classification was not widely adopted at that time and it was not until later that genetic evidence clearly showed that Acacia s.l. was polyphyletic (e.g., Luckow et al. 2003; Miller and Seigler 2012) and that multiple genera for Acacia s.l. were accepted and the fragmentation of the genus began. This resulted in seven currently recognised genera: Acacia s.s., Acaciella Britton & Rose (15 species, New World), Mariosousa Seigler & Ebinger (14 species, New World), Parasenegalia Seigler & Ebinger (11 species, New World), Pseudosenegalia Seigler & Ebinger (two species, New World), Senegalia Raf. (219 species, pantropical) and Vachellia (164 species, pantropical).

Although all genetic studies have shown Acacia s.s. to be monophyletic, the morphological and other characters that separate this genus from the six genera excised from Acacia s.l. have been poorly studied. Nevertheless, 95% of Acacia s.s. species possess phyllodes which readily distinguishes them from the bipinnate-leaved species of the other six genera (but there are also 73 bipinnate-leaved species in Acacia s.s.; WorldWideWattle 2022). Other attributes that help characterise Acacia s.s. include their extraporate pollen grains which possess pseudocolpi on their exine surface (a character otherwise not found elsewhere in Mimoseae , Guinet 1981a), the sequence of their seedling leaf development ( Vassal 1972) and some biochemical and biological characters summarised by Pedley (1986). However, with the possible exception of the pollen, these attributes have not been comprehensively surveyed across Acacia s.l.

The infrageneric classification adopted for Acacia today is that of Pedley (1978) in which the species are arranged in seven sections, Acacia sect. Acacia (= sect. Acacia Phyllodineae DC.), Acacia sect. Alatae (Benth.) Pedley, Acacia sect. Botrycephalae (Benth.) Taub., Acacia sect. Juliflorae (Benth.) C. Moore & Betche, Acacia sect. Lycopodiifoliae Pedley, Acacia sect. Plurinerves (Benth.) C. Moore & Betche, and Acacia sect. Pulchellae (Benth.) Taub. Although this is a convenient scheme for grouping species of this large genus, it is artificial and does not define monophyletic groups ( Murphy 2008). Molecular phylogenetic studies have provided useful insights into evolutionary questions and informal names have been proposed for some of the major clades, but these studies have been hampered by lack of resolution for some relationships and incomplete sampling of species to date (e.g., Murphy et al. 2010; Mishler et al. 2014).

Taxonomic references.

Andrew et al. (2003); Bentham (1875); Guinet (1969, 1981a); Hnatiuk and Maslin (1988); Maslin (2001, 2011, 2015); Maslin and McDonald (2004); Maslin and Reid (2012); Maslin and Thomson (1992); Maslin et al. (2019b); McDonald et al. (2001); Miller and Bayer (2000, 2001); Miller and Seigler (2012); Miller et al. (2002); Mishler et al. (2014); Moran et al. (1992); Moore et al. (2010); Murphy (2008); Murphy et al. (2003, 2010); Pedley (1975, 1978, 1986); Ross (1979); Thiele et al. (2011); Vassal (1972).

Figures

Figure 237. Diversity in habit and foliage of Acacia A arborescent habit of A. cyperophylla var. omearana Maslin B ' Minni Ritchi' bark of A. grasbyi Maiden C prostrate habit of A. pulviniformis Maiden D shrub habit of A. brachybotrya Benth. E A. argyrophylla Hook. showing 1 - veined phyllodes and globose inflorescences F bipinnate foliage of A. leucoclada subsp. argentifolia Tindale G A. spondylophylla F. Muell. showing phyllodes arranged in regular whorls along branches H A. alata var. biglandulosa Meisn. showing decurrent phyllodes that form bifarious wings along branches I A. tetraptera Maslin showing unusually small phyllodes (B. R. Maslin 5792). Photo credits A, C, E, H, I B Maslin B A George D D Murphy F L Jessup G K Brennan.

Figure 238. Diversity in inflorescences and flowers of Acacia A heads in terminal panicle and racemes of A. pyrifolia DC. var. pyrifolia (B. R. Maslin 8425) B heads in racemes of A. neriifolia A. Cunn. ex Benth. C axillary heads of A. heterochroa Maslin subsp. heterochroa (B. R. Maslin 4766) D axillary spikes with densely arranged flowers of A. fecunda Maslin (B. R. Maslin 8762) E spikes with loosely arranged flowers and arranged in short racemes of A. leptostachya Benth. Photo credits A, C, D B Maslin B unknown E IB Armitage.

Figure 239. Diversity in fruits and seeds of Acacia A pods of A. catenulata subsp. occidentalis Maslin that readily break into 1 - seeded loments (B. R. Maslin 8880) B fruits of the ' Mulga' species A. incurvaneura Maslin & J. E. Reid (B. R. Maslin 9304 B) C A. ancistrocarpa Maiden & Blakely fruits are superficially similar to those of Calliandra D A. ampliceps Maslin showing brittle fruits with seeds having bright red aril (bird dispersed) (B. R. Maslin 8660) E A. coolgardiensis Maiden showing terete fruits F irregularly coiled fruits of A. auriculiformis A. Cunn ex Benth. showing seeds encircled by showy aril. Photo credits A-E B Maslin F K Brennan.

Figure 240. Distribution of Acacia based on quality-controlled digitised herbarium records. Note that Acacia confusa Merr. is widely planted in the Philippines and Taiwan, making it difficult to distinguish native from introduced occurrences. See Suppl. material 1 for the source of occurrence data.