Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Datasheet

Acacia mellifera
(blackthorn)

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Datasheet

Acacia mellifera (blackthorn)

Summary

  • Last modified
  • 20 November 2020
  • Datasheet Type(s)
  • Invasive Species
  • Preferred Scientific Name
  • Acacia mellifera
  • Preferred Common Name
  • blackthorn
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Plantae
  •     Phylum: Spermatophyta
  •       Subphylum: Angiospermae
  •         Class: Dicotyledonae
  • Summary of Invasiveness
  • Acacia mellifera is a dense thorny shrub native to many semi-arid regions in Africa and western Asia. It can be widespread in dry bushland, thornveld and wooded grassland, mainly on clay rich or calcium rich soils. However, it is also an...

  • Principal Source
  • Draft datasheet under review

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Pictures

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PictureTitleCaptionCopyright
A. mellifera ssp. mellifera in the Rift valley, Kenya.
TitleTree habit
CaptionA. mellifera ssp. mellifera in the Rift valley, Kenya.
CopyrightChris Fagg, Depto. Ecologia, Univ. Brasilia
A. mellifera ssp. mellifera in the Rift valley, Kenya.
Tree habitA. mellifera ssp. mellifera in the Rift valley, Kenya.Chris Fagg, Depto. Ecologia, Univ. Brasilia
Multistemmed habit of A. mellifera ssp. mellifera close to the Angolan border in Namibia.
TitleTree habit
CaptionMultistemmed habit of A. mellifera ssp. mellifera close to the Angolan border in Namibia.
CopyrightChris Fagg, Depto. Ecologia, Univ. Brasilia
Multistemmed habit of A. mellifera ssp. mellifera close to the Angolan border in Namibia.
Tree habitMultistemmed habit of A. mellifera ssp. mellifera close to the Angolan border in Namibia.Chris Fagg, Depto. Ecologia, Univ. Brasilia
Thicket of A. mellifera ssp. detinens (foreground) in Northern Namibia.
TitleThicket
CaptionThicket of A. mellifera ssp. detinens (foreground) in Northern Namibia.
CopyrightChris Fagg, Depto. Ecologia, Univ. Brasilia
Thicket of A. mellifera ssp. detinens (foreground) in Northern Namibia.
ThicketThicket of A. mellifera ssp. detinens (foreground) in Northern Namibia.Chris Fagg, Depto. Ecologia, Univ. Brasilia
Foliage of A. mellifera
TitleFoliage
CaptionFoliage of A. mellifera
CopyrightChris Fagg, Depto. Ecologia, Univ. Brasilia
Foliage of A. mellifera
FoliageFoliage of A. melliferaChris Fagg, Depto. Ecologia, Univ. Brasilia
Flowers appear before the leaves on the stems of A. mellifera ssp. mellifera.
TitleFlowers
CaptionFlowers appear before the leaves on the stems of A. mellifera ssp. mellifera.
CopyrightChris Fagg, Depto. Ecologia, Univ. Brasilia
Flowers appear before the leaves on the stems of A. mellifera ssp. mellifera.
FlowersFlowers appear before the leaves on the stems of A. mellifera ssp. mellifera.Chris Fagg, Depto. Ecologia, Univ. Brasilia
Flowers and young green pods on the stems of A. mellifera ssp. mellifera.
TitleYoung pods
CaptionFlowers and young green pods on the stems of A. mellifera ssp. mellifera.
CopyrightChris Fagg, Depto. Ecologia, Univ. Brasilia
Flowers and young green pods on the stems of A. mellifera ssp. mellifera.
Young podsFlowers and young green pods on the stems of A. mellifera ssp. mellifera.Chris Fagg, Depto. Ecologia, Univ. Brasilia
A. mellifera ssp. mellifera.
TitleMature dehiscent pod
CaptionA. mellifera ssp. mellifera.
CopyrightChris Fagg, Depto. Ecologia, Univ. Brasilia
A. mellifera ssp. mellifera.
Mature dehiscent podA. mellifera ssp. mellifera.Chris Fagg, Depto. Ecologia, Univ. Brasilia
Acacia mellifera (black thorn); Dense thicket of A. mellifera having invaded pasture land in the previous ten years. Okandjatu, northern Namibia. April 2016.
TitleHabit
CaptionAcacia mellifera (black thorn); Dense thicket of A. mellifera having invaded pasture land in the previous ten years. Okandjatu, northern Namibia. April 2016.
Copyright©Nick Pasiecznik
Acacia mellifera (black thorn); Dense thicket of A. mellifera having invaded pasture land in the previous ten years. Okandjatu, northern Namibia. April 2016.
HabitAcacia mellifera (black thorn); Dense thicket of A. mellifera having invaded pasture land in the previous ten years. Okandjatu, northern Namibia. April 2016.©Nick Pasiecznik

Identity

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Preferred Scientific Name

  • Acacia mellifera (Vahl) Benth.

Preferred Common Name

  • blackthorn

Other Scientific Names

  • Acacia detinens Burch.
  • Acacia ferox Benth.
  • Acacia mellifera (illegit)
  • Acacia mellifera Benth.
  • Acacia mellifera subsp. detiens (Burch.) Brenan
  • Acacia mellifera subsp. detinens (Burch.) Brenan
  • Acacia mellifera subsp. mellifera (=Senegalia mellifera (Vahl) L. A. Silva & J. Freitas)
  • Acacia mellifera var. almakkiana S. A. Chaudhary
  • Acacia senegal subsp. mellifera (Vahl) Roberty
  • Acacia tenax Marloth
  • Acacia vicioides Ferr. & Galim.
  • Inga mellifera (Vahl) Willd.
  • Mimosa mellifera M.Vahl
  • Mimosa mellifera Vahl
  • Senegalia mellifera (Vahl) L. A. Silva & J. Freitas
  • Senegalia mellifera (Vahl) Seigel & Ebinger subsp. detinens(Burch.) Kyal. & Boatwr.

International Common Names

  • English: black thorn; hook thorn; hookthorn; hook-thorn; wait a bit thorn; wait-a-bit-thorn

Local Common Names

  • Egypt: khashab
  • Eritrea: haq
  • Ethiopia: bilili; radi a kora
  • Germany: Akazie; Hakendorn
  • Kenya: banyirit; bilel; curach; ebenyo; eiti; ekunoit; erenyo; habalkes; iit; iti; kathigira; kathiia; kikwata; kikwatha; kithiia; lanen; magokwe; mgoror; milikin-ki-badda; muthigira; muthiia; ng'orore; ngororo; oiti; oito-orok; panyirit; panyuriit; talamoga; talamogh; talamow
  • Namibia: swarthaak
  • Somalia: bilel
  • South Africa: blouhaak; hakiesdoring (Afrikaans); monga (Tswana); mongangatau (Northern Sotho); monkana (Siswati); munembedzi (Tshivenda); swarthaak; swarthaak
  • Sudan: ajondoke; kitr; kittir; kittur; shok
  • Tanzania: kimodoa; mdugala; militimoko-hadza; miritimoko; mlugala; msasa; oete; oyeti; reketto; wait a bit thorn
  • Zimbabwe: ngaga

EPPO code

  • ACAMD (Acacia mellifera subsp. detinens)
  • ACAML (Acacia mellifera)

Subspecies

  • Acacia mellifera subsp. detinens
  • Acacia mellifera subsp. mellifera

Summary of Invasiveness

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Acacia mellifera is a dense thorny shrub native to many semi-arid regions in Africa and western Asia. It can be widespread in dry bushland, thornveld and wooded grassland, mainly on clay rich or calcium rich soils. However, it is also an aggressive colonizer and forms impenetrable thickets, increasingly so on badly managed pasture lands. It is one the main species involved in what is commonly referred as ‘bush encroachment’, especially in southern Africa, becoming an invasive species within its native range, where is greatly increases its density, linked in part to overgrazing and the reduction or removal of the use of fire in the landscapes. In parts of Namibia, South Africa and Botswana, for example, millions of hectares are now dominated by A. mellifera and the problem is increasing. In Ethiopia and East Africa, it also encroaches on rangeland but is not noted as invasive. Although it does have some value as a browse and for bee forage, the curved thorns make this a particularly undesirable species, with significant impacts reducing the availability of pasture for livestock and corresponding land value and, once established, it has proved to be very difficult to remove.

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Plantae
  •         Phylum: Spermatophyta
  •             Subphylum: Angiospermae
  •                 Class: Dicotyledonae
  •                     Order: Fabales
  •                         Family: Fabaceae
  •                             Subfamily: Mimosoideae
  •                                 Genus: Acacia
  •                                     Species: Acacia mellifera

Notes on Taxonomy and Nomenclature

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Acacia mellifera is divided into two accepted subspecies, subsp. mellifera and subsp. detinens. A variety has also been reported, var. almakkiana, but this is not generally recognized.

The generic name Acacia comes from the Greek word akis, meaning point or barb; the species name mellifera means honey-bearing, referring to the sweet-scented flowers that are attractive to bees; and the subspecies name detinens means ‘to hold’, referring to its hooked thorns (Nonyane, 2013).

Pedley (1986) proposed a classification in which the genus Acacia was subdivided into three genera, Acacia, Senegalia and Racosperma. However, most botanists did not adopt Pedley's 1986 classification, mainly due to insufficient evidence to support the changes (Chappill and Maslin, 1995). Thus, it has been referred to as Senegalia mellifera, but this binomial is generally not accepted.

Description

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The following description is adapted from Nonyane (2013) and Biodiversity India (2019):

Acacia mellifera is a very thorny shrub to small tree, 2-5 m tall. Crown rounded or flat and spreading, with branches that may reach down to the ground. Bark light to dark grey and longitudinally fissured, fissures generally darker. Branchlets flattened, 1.5-5 mm thick, glabrous; stipules deciduous, not spinescent; spines in pairs, 3-5 mm long, black, recurved and in pairs at nodes that are only 5-15(-30) mm apart and thus there are more thorns per unit length of branch than with other species.

Leaves compound, rachis 2-5 cm long, slender, sparsely puberulous to glabrous; with 2-3(-4) pairs of pinnae, 0.5-2.5 cm long, oblong to obovate-oblong, 5-10 x 2-5 mm, unequal (acute + rounded) at the base, rounded to acute-mucronate at apex, chartaceous, green initially, the same shade above and below, leaflet margins are sparsely fringed with white hairs; but become glaucous when older; main vein subcentral with 2 prominent accessory nerves ascending about 2/3rd way up the lamina; lateral nerves prominent. There are (1-)2-3 pairs of leaflets per pinna, relatively large, 3.5-15 x 2-12 mm and 1-4 leaves per node. Petiolar gland more or less midway on petioles or at the base of at least the upper pinna pair, circular, sessile, ca 0.5 mm diameter.

The sweetly scented flowers are cream to white, becoming brownish when fading. Inflorescence are axillary spikes, up to 8 cm long, calyx obconic, ca 2 x 2 mm; teeth triangular, ca 1 mm long; corolla 2.5-3 mm long; lobes oblong, 1.5-2.5 mm long, glabrous; stamens ca 5 mm long; ovary shortly stipitate, glabrous. Pods develop rapidly after flowering, being small, straw-coloured or pale brown, greenish-brown when dry, thin to almost papery, straight, smooth, dehiscent, (2.5-)4.5-6.5(-8) cm long and (1.3-)2-2.5 mm wide, obliquely oblong, flat, thin, unlobed, rounded or obtuse-mucronate at apex, glabrous, prominently and transversely reticulate-veined; seeds 4-5.

The two subspecies differ in pinna pairs, pubescence and flowers, with subsp. mellifera usually having 2 pinna pairs and glabrous leaflets and flowers in distinct spikes, while subsp. detinens tends to have 3 pinna pairs and pubescent leaflets with a fringe of marginal hairs and flowers that appear to be in balls (actually in very shortened spikes).

Plant Type

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Broadleaved
Perennial
Seed / spore propagated
Shrub
Tree
Woody

Distribution

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Acacia mellifera is considered by most sources as native to a broad area in Africa, from South Africa to Egypt, including dry areas in most countries in southern and eastern Africa, the Horn of Africa and in neighbouring Saudi Arabia and Yemen in the Arabian Peninsula. Some extend this range slightly westwards to include Chad (USDA-ARS, 2019) which is likely at least in eastern areas. However, A. mellifera is not native to the Sahel. ILDIS (2017) report an extended range in Asia, north to Syria and east to Iraq, which is accepted here.

Nonyane (2013) provides detail of its distribution in southern Africa. In Botswana, it occurs in the eastern and southern regions and in Namibia, it is found in the Caprivi Strip, Kaokoveld, Otjiwarongo, Outjo, Okahandja and Gobabis Districts and further south. In South Africa, it is reported from the Northern Cape in the south, to the Free State, Gauteng, North West Province, Mpumalanga, the Limpopo Province and northwards to Tanzania. In Northern Cape Province, it occurs mostly in the northwest portion, notably in the Vryburg, Taung, Barkly West and Keimoes areas. In the Free State, it grows in the west: Jacobsdal and Fauresmith areas. In Gauteng, Limpopo, Mpumalanga and North West Provinces, it is found over most of the bushveld areas south to the Springbok Flats north of Pretoria. A. mellifera subsp. detinens is the most widespread subspecies in southern Africa, with subsp. mellifera being found in the far northwest of Namibia and further north (Nonyane, 2013).

There are few reports of A. mellifera as an exotic, but it has been introduced to Pakistan and India (ILDIS, 2017), with a first record of having become naturalized in the Eastern Ghats in Tamil Nadu, India in 2011 (Kottaimuthu et al., 2011). It was also planted in field trials in Mali (Keita et al., 2006), Senegal (Diagne et al., 2006) and Brazil, and is recorded in Mauritius (ILDIS, 2017). However, a record from Australia (ILDIS, 2017) could not be supported and, as such, is treated as unconfirmed.

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Last updated: 20 Nov 2020
Continent/Country/Region Distribution Last Reported Origin First Reported Invasive Planted Reference Notes

Africa

AngolaPresentNative
BotswanaPresentNativeInvasive
ChadPresentNative
DjiboutiPresentNative
EgyptPresentNative
EritreaPresentNative
EswatiniPresentNative
EthiopiaPresentNativeInvasive
KenyaPresentNative
MalawiPresent
MaliPresentIntroduced
MauritiusPresentIntroduced
MozambiquePresentNative
NamibiaPresentNativeInvasive
NigeriaPresentIntroduced1949
SenegalPresentIntroduced
SomaliaPresentNative
South AfricaPresentNativeFree State, Northern Cape, Gauteng, Limpopo, North-West
SudanPresentNative
TanzaniaPresentNative
UgandaPresentNative
ZambiaPresentNative
ZimbabwePresentNative

Asia

IndiaPresentIntroduced
-KeralaPresentIntroduced
-Tamil NaduPresentIntroducedInvasiveNaturalized
-Uttar PradeshPresentPlanted
IranPresentNative
IraqPresentNative
IsraelPresentNative
OmanPresentNative
PakistanPresentIntroduced
Saudi ArabiaPresentNative
SyriaPresentNative
YemenPresentNative

Oceania

AustraliaAbsent, Unconfirmed presence record(s)

South America

BrazilPresentIntroducedHerbarium record from Rio de Janeiro Botanical Garden

History of Introduction and Spread

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Only relatively recently has A. mellifera been introduced into cultivation in India, mainly in experimental trials where its drought resistance and fast growth shows promise for land reclamation in semi-arid areas (Sastry and Kavathekar, 1990). It has also been introduced into countries in the Sahel, but no further information on introduction or potential spread is available.

However, it has spread widely within its native range, increasing in frequency and density and occupying habitats where it was previously less common. As such, it is typical of what were once referred to as ‘densifiers’, now more commonly known as ‘encroachers’ or ‘bush encroachers’, i.e. is behaving as a native invasive species. It is specifically called an invasive species in Namibia, but also shows invasive behaviour elsewhere in countries in southern Africa, East Africa and the Horn of Africa.

Risk of Introduction

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As a drought resilient tree species often included on lists of potential fuelwood and food species that has already been introduced for such purposes, it is possible that it may be further introduced to other countries for the same reasons.

It is unlikely to be introduced accidentally, unless it is with seeds in imported livestock, but cross-border transhumance may be a pathway for introduction.

Habitat

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In southern Africa, A. mellifera occurs in bushveld and semi-desert areas, arid savannah, dry woodland and bush, often on deep sandy or gravelly soils where it often grows in circular groups forming impenetrable thickets in overgrazed areas (Nonyane, 2013).

Habitat List

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CategorySub-CategoryHabitatPresenceStatus
Terrestrial ManagedCultivated / agricultural land Present, no further details Productive/non-natural
Terrestrial ManagedManaged grasslands (grazing systems) Present, no further details Harmful (pest or invasive)
Terrestrial ManagedIndustrial / intensive livestock production systems Present, no further details Harmful (pest or invasive)
Terrestrial ManagedDisturbed areas Present, no further details Harmful (pest or invasive)
Terrestrial ManagedDisturbed areas Present, no further details Natural
Terrestrial ManagedRail / roadsides Present, no further details Harmful (pest or invasive)
Terrestrial ManagedRail / roadsides Present, no further details Natural
Terrestrial Natural / Semi-naturalNatural forests Present, no further details Harmful (pest or invasive)
Terrestrial Natural / Semi-naturalNatural forests Present, no further details Natural
Terrestrial Natural / Semi-naturalNatural forests Present, no further details Productive/non-natural
Terrestrial Natural / Semi-naturalNatural grasslands Principal habitat Harmful (pest or invasive)
Terrestrial Natural / Semi-naturalNatural grasslands Principal habitat Natural
Terrestrial Natural / Semi-naturalNatural grasslands Principal habitat Productive/non-natural
Terrestrial Natural / Semi-naturalRiverbanks Present, no further details Harmful (pest or invasive)
Terrestrial Natural / Semi-naturalRiverbanks Present, no further details Natural
Terrestrial Natural / Semi-naturalScrub / shrublands Present, no further details Harmful (pest or invasive)
Terrestrial Natural / Semi-naturalScrub / shrublands Present, no further details Natural
Terrestrial Natural / Semi-naturalDeserts Present, no further details Harmful (pest or invasive)
Terrestrial Natural / Semi-naturalDeserts Present, no further details Natural
Terrestrial Natural / Semi-naturalArid regions Present, no further details Harmful (pest or invasive)
Terrestrial Natural / Semi-naturalArid regions Present, no further details Natural

Biology and Ecology

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Genetics

The chromosome number for A. mellifera is 2n=26 (Missouri Botanical Garden, 2020)

Microsatellite markers have been developed in A. mellifera (Omondi et al., 2010). Guajardo et al. (2010) used microsatellite loci to investigate spatial genetic structure and to identify potential ecological and geographic barriers to dispersal in A.mellifera, from variation among 791 individuals from 28 sampling locations across Kenya, identifying five well-defined geographic regions within which gene flow was localized.

Physiology and Phenology

In southern Africa, A. mellifera flowers usually appear in August or September before the tree comes into leaf, and a second flowering has been observed in March following heavy rains. The main fruiting period is between January and April (Nonyane, 2013).

In Senegal A.mellifera was found to be colonized by mycorrhiza, with the intensity of colonization greater on A. mellifera (66%) than on A. laeta (42%) and was higher after the rainy season (Diagne et al., 2006). In alkaline soil in Tamil Nadu, India, shoot length, photosynthetic area and dry weight of A. mellifera seedlings were higher when amended with manure and Rhizobium (Manorama et al., 2007).

Seed ripens quickly and, after heavy rainfall, can germinate in great profusion. However, early growth may be slow and seedling establishment and growth of A. mellifera subsp. detinens were found to be influenced by different soils and sub-habitats in semi-arid savannahs in South Africa (Smit and Hagos, 2008).

An assessment of the encroachment of woody species in the grasslands of Nechisar National Park, Ethiopia found that A. mellifera was one of six main invasive species (the others were A. nilotica, A. senegal, A. seyal, A. tortilis and Dichrostachys cinerea). The density and cover of woody species and unpalatable forbs and bare land were significantly higher in highly grazed and fire-suppressed parts of the grassland plain, and woody species cover and density were negatively correlated with total herbaceous and grass cover (Yusuf et al., 2011).

Effects of rain, nitrogen, fire and grazing on tree recruitment and early survival in bush-encroached savannah in South Africa was assessed by Kraaij and Ward (2006), who inferred that above-average rainfall years with frequent rainfall events are required for mass tree recruitment and that overgrazing makes space and resources available for tree seedlings, also through nitrogen removal making nitrogen-fixing trees more competitive. However, in contrast to conventional wisdom that grazing alone causes encroachment, Kraaij and Ward (2006) suggested that there are complex interactions between the above mentioned factors and 'triggering' events such as unusually high rainfall.

Many Acacia species in East Africa, including A. mellifera, evolved in both a high fire and herbivory landscape, and Acacia trees can allocate resources to fire defence by developing thicker bark, or to herbivory defence by developing spines. The trend towards bush encroachment may be due to the reduction or absence of intense fires which is facilitating the expansion of thin-barked species such as A. mellifera (Midgley et al., 2016).

Environmental Requirements

Acacia mellifera is a very drought resistant species (Nonyane, 2013) and well adapted to arid and semi-arid tropical and sub-tropical climates. Young plants are, however, reported to be tender to frost and some die-back has been noted on small unprotected plants following severe cold (Nonyane, 2013).

In its native range in southern Africa, A. mellifera is reported to prefer deep sandy or gravelly soils (Nonyane, 2013).

Acacia mellifera does not occur on clay pans and grows in low numbers only in other clay and sandy soils, with soil rockiness found to increase long-term presence and abundance of A. mellifera due to better soil moisture conditions (Britz and Ward, 2007). However Ward and Esler (2011) concluded that A. mellifera seedlings are more likely to encroach in habitats with low grass density, although they may achieve greater biomass on sandy soils, thus, it may be lower grass density rather than rockiness which increases encroachment in rocky habitats.

From the literature and work within Botswana, it is suggested that bush encroachment of species such as A. mellifera results from the exclusive use of moisture by encroachers, high soil nutrient concentrations, low fire frequencies and high cattle selectivity (Moleele et al., 2002).

Climate

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ClimateStatusDescriptionRemark
As - Tropical savanna climate with dry summer Tolerated < 60mm precipitation driest month (in summer) and < (100 - [total annual precipitation{mm}/25])
Aw - Tropical wet and dry savanna climate Tolerated < 60mm precipitation driest month (in winter) and < (100 - [total annual precipitation{mm}/25])
BS - Steppe climate Preferred > 430mm and < 860mm annual precipitation
BW - Desert climate Preferred < 430mm annual precipitation

Latitude/Altitude Ranges

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Latitude North (°N)Latitude South (°S)Altitude Lower (m)Altitude Upper (m)
22 -30 30 1850

Air Temperature

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Parameter Lower limit Upper limit
Absolute minimum temperature (ºC) 2
Mean annual temperature (ºC) 15 30
Mean maximum temperature of hottest month (ºC) 25 40
Mean minimum temperature of coldest month (ºC) 10 20

Rainfall

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ParameterLower limitUpper limitDescription
Dry season duration210number of consecutive months with <40 mm rainfall
Mean annual rainfall1501200mm; lower/upper limits

Rainfall Regime

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Bimodal
Summer
Winter

Soil Tolerances

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Soil drainage

  • free
  • impeded

Soil reaction

  • acid
  • alkaline
  • neutral

Soil texture

  • heavy
  • medium

Special soil tolerances

  • infertile
  • saline
  • shallow
  • sodic

Means of Movement and Dispersal

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Natural Dispersal

The paper seed pods may be dispersed by strong winds, but are also very likely to be dispersed by water, especially after rainfall events.

Vector Transmission (Biotic)

As seed pods are readily browsed by livestock and by many wild animal species, this is likely to be the main means of seed dispersal.

In Northern Cape Province, South Africa, several rodents (Gerbillurus paeba, Desmodillus auricularis, Tatera brantsii and Tatera leucogaster) were found to assist in seed dispersal of several tree species, including A. mellifera. As most caches were found near grass tussocks, results suggest that the impact of rodents on bush encroachment is highest in habitats where bush and grass species are in close proximity (Kuechly et al., 2011).

Intentional Introduction

Acacia mellifera has been intentionally introduced to several countries for its perceived benefits as a browse species and also as a living fence.

Pathway Causes

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CauseNotesLong DistanceLocalReferences
Animal production Yes
Digestion and excretion Yes
Disturbance Yes
Flooding and other natural disasters Yes
Forestry Yes Yes

Pathway Vectors

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VectorNotesLong DistanceLocalReferences
Livestock Yes
Water Yes

Impact Summary

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CategoryImpact
Economic/livelihood Negative
Environment (generally) Negative

Economic Impact

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In south-eastern Ethiopia, invasive species including A. mellifera were reported having negative impacts on livestock, livestock products and crops, although the specific impacts of each species were not assessed separately (Mussa et al., 2018).

In a study on the impact of cattle ranching on large scale vegetation patterns in a coastal savannah in Tanzania, Tobler et al. (2003) identified high amounts of three bushland types in the region, including one dominated by A. mellifera, and found that the biggest problem faced by the ranch managers was bush encroachment into pasture land.

Environmental Impact

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Impact on Habitats

Acacia mellifera spreads rapidly, forming impenetrable, tangled thickets. By 1970, A. mellifera was already found to be a serious problem on 1.25 million ha in the Northern Cape and at least half of the natural pasture in Molopo, South Africa (Mostert et al., 1971), with encroachment attributed to overstocking with cattle and the absence of wild browse animals and burning, as a dense grass cover is needed to prevent the establishment of A. mellifera seedlings.

In the southern Kalahari, Botswana, bush encroachment is prevalent in semi-arid areas where A. mellifera is widespread in communal areas and private ranches, indicating rangeland degradation has been increased by land tenure changes since the 1970s (Dougill et al., 2016).

The socio-economic and environmental impacts of A. mellifera and other invasive species (A. bussei, A. seyal, A. tortilis, Argemone ochroleuca, Caesalpinia spp., Parthenium hysterophorus and Xanthium strumarium) in south-eastern Ethiopia were assessed by Mussa et al. (2018), but the study did not differentiate between the impacts of individual species. Respondents reported certain economic and ecological benefits provided by the invasive species, but also reported negative impacts on biodiversity, degrading ecosystems, on livestock and livestock products, crops and on animal and human health.

Acacia mellifera was found to modify soil water dynamics and potential groundwater recharge in invaded savannah ecosystems in Namibia (Groengroeft et al., 2018). Impacts were an increase in soil moisture larger than precipitation after some rain events, interpreted as water run-on resulting from surface ponding. There was also an overall reduction in water infiltration in the below-canopy area of A. mellifera compared to the inter-canopy space; and a faster drying of the soil in the below-canopy space because of water uptake by tree roots. These processes resulted in a potential for deep drainage about three times greater in the inter-canopy space than in the area below the canopy and thus invasion is likely to reduce groundwater recharge.

Impact on Biodiversity

Changes in landscape vegetation, forage plant composition and herding structure in pastoralist livelihoods in rangelands of northern Baringo District, Kenya point to major changes in structure and biodiversity composition over the past century, with a landscape of perennial grasses having turned into bushland dominated by A. mellifera, A. nubica and A. reficiens that have increased rapidly since the 1950s, due to high grazing pressure and restriction of pastoral mobility following the increasing numbers of humans, goats and camels (Vehrs, 2016).

Encroachment by A. mellifera appears to have a larger impact on bird diversity in grassland than in open woodland in South Africa, suggesting that shrub encroachment could be one of the main drivers of bird population dynamics in southern African savannahs and if current trends continue, the persistence of several southern African bird species associated with open savannah might be jeopardized (Kaphengst and Ward, 2008; Sirami et al., 2009).

Risk and Impact Factors

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Invasiveness
  • Invasive in its native range
  • Proved invasive outside its native range
  • Has a broad native range
  • Abundant in its native range
  • Highly adaptable to different environments
  • Is a habitat generalist
  • Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
  • Pioneering in disturbed areas
  • Highly mobile locally
  • Long lived
  • Fast growing
  • Has high reproductive potential
  • Has propagules that can remain viable for more than one year
  • Has high genetic variability
Impact outcomes
  • Ecosystem change/ habitat alteration
  • Modification of hydrology
  • Modification of nutrient regime
  • Modification of successional patterns
  • Monoculture formation
  • Negatively impacts animal health
  • Reduced amenity values
  • Reduced native biodiversity
  • Threat to/ loss of native species
Impact mechanisms
  • Competition - monopolizing resources
  • Competition - shading
  • Rapid growth
  • Produces spines, thorns or burrs
Likelihood of entry/control
  • Highly likely to be transported internationally deliberately

Uses

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Social Benefit

The heartwood is termite-resistant and used for fence posts. The dark heartwood becomes almost black when oiled and highly polished and is very attractive and very hard. It is also used as fuel and for making charcoal. However, in Khomani San resettlement farms in the southern Kalahari, South Africa, A. mellifera is abundant but specifically avoided as a fuel wood, with local people preferring other species (Nott and Thondhlana, 2017).

The pods, young twigs, leaves and flowers are all highly nutritious and are eagerly eaten by livestock. A. mellifera is also mechanically harvested and chipped into ‘bush-based fodder’, where woody material is mixed with any remaining foliage and used as a dry season feed, following treatments to reduce the impacts of high levels of tannin and cellulose on animal nutrition and digestion (Pasiecznik, 2016a, b). For nutritive value and tannin content of A. mellifera, see Osuga et al. (2007) and Ondiek et al. (2017) in Kenya, Nassoro et al. (2015) in Tanzania.

In areas where it occurs naturally, it provides good shade for livestock and given its numerous qualities, A. mellifera was also considered to show promise as a species for live fencing (Diagne et al., 2006).

Flowers are sweetly scented and are attractive to bees. Nonyane (2013) also consider that is has ornamental value and that it is worth cultivating for its fine floral display.

Acacia mellifera is widely used in traditional African medicines against various diseases such as pneumonia and malaria, with bark extracts exhibiting anti-bacterial and anti-fungal activity, active against Staphylococcus aureus, Microsporum gypseum and Trichophyton mentagrophytes. These results may partly explain and support the use for the treatment of infectious diseases in traditional medicine in Kenya (Mutai et al., 2009). Plant extracts are also reported to be used in traditional medicine in South Africa (Twilley et al., 2017) and are used to treat chronic joint pain in Kenya (Wambugu et al., 2011). In southern Africa, A. mellifera twigs are also chewed and used as toothbrush sticks (Nonyane, 2013).

Environmental Services

Leaves, small twigs and pods are very nutritious and in southern Africa it is commonly browsed by wild animals such as black rhino (Diceros bicornis), kudu (Tragelaphus strepsiceros), eland (Taurotragus oryx) and giraffe (Giraffa).

Uses List

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Animal feed, fodder, forage

  • Fodder/animal feed
  • Forage

Environmental

  • Agroforestry
  • Boundary, barrier or support
  • Erosion control or dune stabilization
  • Land reclamation

Fuels

  • Charcoal
  • Fuelwood

Human food and beverage

  • Honey/honey flora

Materials

  • Carved material
  • Gum/resin
  • Tanstuffs
  • Wood/timber

Medicinal, pharmaceutical

  • Source of medicine/pharmaceutical
  • Traditional/folklore

Ornamental

  • garden plant

Wood Products

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Charcoal

Roundwood

  • Building poles
  • Posts

Woodware

  • Cutlery
  • Industrial and domestic woodware
  • Tool handles
  • Turnery
  • Wood carvings

Similarities to Other Species/Conditions

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Due to its particularly curved and darkly-coloured thorns, A. mellifera is one of the more distinctive African Acacia species. However, a number of other Acacia spp., such as A. senegal, also have curved thorns, thus initial identification by inexperienced field workers should be confirmed. It is noted by Nonyane (2013) to be similar to A. nigrescens, but A. mellifera has white lenticel markings on the young branches and lacks prickly knobs on the trunk and also has much shorter inflorescences and smaller, more papery pods and that are paler in colour.

Prevention and Control

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Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Physical/Mechanical Control

Mechanical control with bulldozers, stump pullers and cutting with axes or saws proved effective (Mostert et al., 1971) and mechanical eradication of A. mellifera or burning was found to result in improved grass growth. Encroachment can be reduced or avoided by allowing light grazing only, though sowing of grass seed is not effective (Joubert, 1966).

Negasa et al. (2014) tested five treatments for controlling A. mellifera over three years in Borana, Ethiopia, finding that all treatments significantly influenced basal cover, but that cutting trees at 0.5 m above ground and dissecting the stumps and cutting at 0.5 m above ground and debarking the stumps down into the soil surface, gave better control than cutting at 0.5 m above ground and pouring kerosene on stumps, or cutting at 0.5 m above ground alone. Furthermore, this also had also a positive effect in eradicating the tick population. The effectiveness of mechanical control, referred to as de-bushing or bush clearing in southern Africa, was also assessed by Stolter et al. (2018), finding that seedlings reacted to damage of top-shoots, with more branching.

Simulation by Lohmann et al. (2014) showed that prescribed fires on semi-arid rangeland in Namibia would increase carrying capacity of cattle by more than 30%, but when grazing intensity was too high, fire management failed to prevent shrub encroachment. In western Botswana, burning was found to significantly reduce density by 53%, with more multi-stemmed and less tall trees in burned paddocks (Mudongo et al., 2016).

Biological Control

In contrast to sheep, goats were not absolutely deterred by the thorns on A. mellifera, and it is assumed that under controlled grazing regimes, goat browsing could be a useful tool to reduce seedling establishment (Stolter et al., 2018). Goats were also found to be effective in reducing density of many encroaching bush species including A. mellifera in southern Zimbabwe, as was the use of grass competition close to the base of trees in Namibia that choked out and killed them (West, 1964).

On East African rangelands, Augustine and McNaughton (2004) found that browsing by dik-diks (Madoqua kirkii) reduced the rate of increase in shrub density to only 7 new shrubs/ha/y, compared to a rapid rate of increase in the absence of browsers (137 shrubs/ha/y), whereas damage to shrub canopies by elephants (Loxodonta africana) caused significant reductions in cover of A. mellifera, with elephant damage focused on shrubs >2.5 m tall, with intermediate height classes (0.5-2.5 m) showing minimal browser impacts. Elephants also influenced shrubland dynamics by altering shrub height-class distributions, shifting species composition from broadleaved species (e.g. Grewia tenax) to fine-leaved Acacia species (e.g. A. mellifera) and suppressing woody biomass accumulation; but elephants had little influence on changes in shrub density.

Chemical Control

During trials in 1984 in South Africa’s Northern Cape, aerial application of ethidimuron controlled many bush and tree species including Grewia flava, Rhigozum trichotomum, Boscia albitrunca and in particular, A. mellifera and A. erioloba, resulting in more grass tufts and increased grass production and grazing capacity, particularly dramatic in the dry season (Fourie, 1992). Chemical control proved effective with monuron applied to the soil around stems in the rainy season (October-January) and picloram stump treatments also proved effective (Mostert et al., 1971). In Kenya, aerial applications of 2,4-D proved effective (West, 1964).

Integrated control

Short-term ecological implications of chemical and physical removal of 21 sites invaded by A. mellifera in the central highland savannah of Namibia, found that treatments reduced tree density (Haussmann et al., 2016) and Dougill et al. (2016) showed the need for integrated landscape-scale land use planning if the ecological value and biodiversity of the southern Kalahari is to be retained.

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USDA-ARS, 2019. Germplasm Resources Information Network (GRIN). Online Database. In: Germplasm Resources Information Network (GRIN). Online Database. Beltsville, Maryland, USA: National Germplasm Resources Laboratory. https://npgsweb.ars-grin.gov/gringlobal/taxon/taxonomysimple.aspx

Principal Source

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Draft datasheet under review

Contributors

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07/01/20 Original text by:

Nick Pasiecznik, Consultant, France

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