Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Datasheet

Prosopis pallida
(mesquite)

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Datasheet

Prosopis pallida (mesquite)

Summary

  • Last modified
  • 16 November 2018
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Preferred Scientific Name
  • Prosopis pallida
  • Preferred Common Name
  • mesquite
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Plantae
  •     Phylum: Spermatophyta
  •       Subphylum: Angiospermae
  •         Class: Dicotyledonae
  • Summary of Invasiveness
  • While principally a weed of arid and semi-arid natural grasslands, P. pallida is generally more erect, less thorny and less invasive than other Prosopis species where introduced, especially as compar...

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Pictures

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PictureTitleCaptionCopyright
P. pallida as an agroforestry tree in upland Cape Verde, showing substantial regrowth following pollarding. Probably established naturally.
TitleAgroforestry tree
CaptionP. pallida as an agroforestry tree in upland Cape Verde, showing substantial regrowth following pollarding. Probably established naturally.
CopyrightN.M. Pasiecznik
P. pallida as an agroforestry tree in upland Cape Verde, showing substantial regrowth following pollarding. Probably established naturally.
Agroforestry treeP. pallida as an agroforestry tree in upland Cape Verde, showing substantial regrowth following pollarding. Probably established naturally.N.M. Pasiecznik
Young P. pallida as a roadside tree in Cape Verde.
TitleImmature tree
CaptionYoung P. pallida as a roadside tree in Cape Verde.
CopyrightN.M. Pasiecznik
Young P. pallida as a roadside tree in Cape Verde.
Immature treeYoung P. pallida as a roadside tree in Cape Verde.N.M. Pasiecznik
P. pallida as an agroforestry tree in lowland Cape Verde, high pruned.
TitleAgroforestry tree
CaptionP. pallida as an agroforestry tree in lowland Cape Verde, high pruned.
CopyrightN.M. Pasiecznik
P. pallida as an agroforestry tree in lowland Cape Verde, high pruned.
Agroforestry treeP. pallida as an agroforestry tree in lowland Cape Verde, high pruned.N.M. Pasiecznik

Identity

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Preferred Scientific Name

  • Prosopis pallida (H. & B. ex Willd.) Kunth

Preferred Common Name

  • mesquite

Other Scientific Names

  • Acacia pallida H. & B. ex Willd.
  • Mimosa pallida (Willd.) Poiret
  • Prosopis affinis (Sprengel) Ferreyra
  • Prosopis juliflora var. horrida (Kunth) Burkart
  • Prosopis juliflora var. inermis (Kunth) Burkart
  • Prosopis limensis Benth.
  • Prosopis pallida (H. & B. ex Willd.) H.B.K.
  • Prosopis pallida forma decumbens Ferreyra

International Common Names

  • English: mesquite; prosopis
  • Spanish: algarrobo

Local Common Names

  • Brazil: algarobeira
  • Cape Verde: espinheiro; spinho
  • Colombia: algarrobo; algarrobo forragero; anchipia guaiva; aroma; cuji; cuji negro; cuji yaque; manca-caballo; mesquite; trupi; trupillo
  • Djibouti: garawa
  • Ecuador: algarrobo
  • French Polynesia: carobier
  • Peru: guarango; huarango; taco; thacco
  • USA/Hawaii: algaroba; kiawe; mesquite

EPPO code

  • PRCPA (Prosopis pallida)

Summary of Invasiveness

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While principally a weed of arid and semi-arid natural grasslands, P. pallida is generally more erect, less thorny and less invasive than other Prosopis species where introduced, especially as compared to the closely related P. juliflora. P. pallida is a declared noxious weed in USA and Australia, P. pallida is a declared noxious weed in USA and Australia and is recorded as invasive in Western Australia, Northern Territories and Queensland, Hawaii, with invasive tendencies noted in Brazil, Cape Verde, Senegal, Mauritania and Djibouti. It was introduced as a fuel and fodder tree and the seed have been spread widely by grazing animals. It is a nitrogen-fixing and very drought and salt tolerant species which can out-compete other vegetation, especially along dry valleys and areas with a high water table. Although the negative economic effects may be significant, P. pallida has also been identified as the prosopis species with most potential in the dry tropics as a food, fodder, fuel and timber tree. It also appears to be less invasive than P. juliflora with which it often confused, though P. pallida is usually treated together with the genus Prosopis as a whole in invasive risk assessments, including P. juliflora, P. glandulosa and P. velutina.

Taxonomic Tree

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

Notes on Taxonomy and Nomenclature

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Burkart (1976) defined Prosopis as 'a kind of prickly fruit', while Allen and Allen (1981) gave the meaning as 'bardane', a type of thorny plant not related to Prosopis. The origin of Prosopis given by Perry (1998) was 'towards abundance', from the Greek word 'pros', meaning 'towards', and 'Opis', wife of Saturn, the Greek goddess of abundance and agriculture. The name pallida, meaning pallid or light in colour, refers to the greyish foliage, particularly when dry as in herbarium samples.

Prosopis pallida has endured less taxonomical confusion than P. juliflora. It was originally placed in the genus Acacia, and was included in the genus Mimosa before being transferred to the genus Prosopis. Bentham (1875) noted P. limensis (syn. P. pallida) from Peru as the only Prosopis species of section Algarobia he was aware of that was not sympatric with others in the section. This may assume that he was either unaware of P. juliflora and hybrids in Ecuador and northern Peru, or he treated them all as the same species, distinct from the P. juliflora of Central America, Colombia and the Caribbean. The species P. limensis was accepted for many years, but Burkart (1976) brought P. limensis into synonymy with P. pallida when no consistent differences could be found.

Two forms of P. pallida, forma pallida and forma armata, were described by Fosberg (1966) from introduced material in Hawaii, USA, based primarily on differences in armature. Burkart (1976) noted these forms but failed to incorporate them into his monograph, whereas he gave similar differences varietal status in P. juliflora. Although the binomial P. pallida remains intact, further taxonomic confusion has involved the differentiation of the species into forms. Ferreyra (1987) confirmed the two forms of Fosberg (1966) but also described two new forms, forma decumbens and forma annularis. Ferreyra (1987) also gave the first description of P. affinis Sprengel in Peru, while noting that this species had previously been described only in Uruguay and Argentina (Burkart, 1976). Although not referring directly to the work of Ferreyra, Díaz Celis (1995) accepts only the two forms of Fosberg and by default brings P. affinis and P. pallida forma decumbens (Ferreyra, 1987) back into synonymy with P. pallida forma armata, whereas P. pallida forma annularis (Ferreyra, 1987) appears to be the same as the description of P. juliflora var. juliflora by Díaz Celis (1995). This confirms the existence of this variety in Peru made by Burkart (1976) but excluded by Ferreyra (1987). Further changes in nomenclature are expected, especially in northern Peru and Ecuador where all the varieties of P. juliflora and all forms of P. pallida have been identified (Burkart, 1976; Diaz Celis, 1995).

The main taxonomic problem associated with P. pallida is the confusion with other Prosopis species from section Algarobia, mainly P. juliflora. For example, in Hawaii, both the binomials P. juliflora and P. chilensis were used before Johnston (1962) identified the correct species, P. pallida. Similar problems have occurred in South Africa and Australia and doubts still remain as to the taxonomy of Prosopis species in several regions where they have been introduced. In Brazil, Senegal and Cape Verde for example, it appears certain that some introductions of P. pallida have taken place as this is now seen as the dominant species, not P. juliflora as commonly recorded (Harris et al., 2003).

The ‘P. pallida – P. juliflora complex’ was proposed by Pasiecznik et al. (2001) as a means to overcome the observed ambiguities and lack of agreement on how to taxonomically deal with tropical American prosopis, and discusses previous proposals and revisions in detail. However, since then, it has been unequivocally shown that the two are distinct taxa, morphologically and genetically (e.g. Harris et al., 2003; Landeras et al., 2006; Catalano et al., 2008; Trenchard et al., 2008; Sherry et al., 2011, Palacios et al., 2012). Comparing with introduced material however, highlighted a number of serious misidentifications, notable being that the common prosopis in the nordeste of Brazil, Cape Verde and parts of northern Senegal is in fact P. pallida, and not P. juliflora as it has always been referred to (Harris et al., 2003). P. pallida has also been identified in southern Mauritania (Pasiecznik et al., 2006) and Djibouti (Pasiecznik et al., 2013).

The three distinct native range ‘races’ as separated by Pasiecznik et al. (2001) appear to have been confirmed by morphological and molecular analysis, with populations raised to species level by Palacios (2006) and Palacios et al. (2012), including a number of further divisions. Of the three, only the ‘Peruvian-Ecuadorian race’ is detailed below as only refers to P. pallida. For the resolution of taxonomical issues of all three races, see the parallel datasheet on P. juliflora.

Though without undertaking a detail taxonomical analysis, Pasiecznik et al. (2001) continued to accept Burkart’s premise that the Peruvian-Ecuadorian ‘race’ included three varieties of P. julilfora as well as P. pallida, with numerous forms described later by Ferreyra (1987) and Diaz Celis (1995). The work by Mom et al. (2002) was the first to propose a revision of this race, which included the separation of P. pallida and P. limensis as two morphologically distinct species, in contrast to the two binomials being only ever used as synonyms in the past. No mention was made of P. juliflora in this paper.

Molecular and morphological analysis was undertaken on prosopis from Peru, Ecuador and Colombia by Palacios et al. (2012). In it, they confirmed the separation of the three species P. juliflora, P. pallida and P. limensis using molecular markers and some seedling leaflet characteristics. Very importantly, however, is that based on this valid evidence, they no longer consider P. juliflora as a species native to Peru or Ecuador, in contrast to the work of Burkart (1976), Ferreyra (1987) and Diaz Celis (1995). Thus, all references to P. juliflora and its three varieties identified by Burkart (1976) in this region, var. juliflora, var. inermis and var. horrida, are now absorbed into synonymy with P. pallida and/or P. limensis. Sherry et al. (2011) separated P. juliflora and P. pallida with no mention of P. limensis, and for the purposes of this datasheet, P. limensis is still taken as a synonym of P. pallida as a single overarching taxa in Peru/Ecuador.

The Central American race

In one of the most detailed analyses on the North American prosopis, Johnson (1962) observed that the previously described P. juliflora on the Pacific coast of Central America was morphologically distinct from P. juliflora in the Caribbean in terms of leaflet size. This was so conspicuous that he suggested the possible reversion of Pacific coastal material to a former name, P. vidaliana (see also Pasiecznik et al., 2001). In his re-analysis of prosopis in Mexico, Palacios (2006) agrees with this, and formally re-instates the binomial P. vidaliana. However, although additional material was sampled during this work, the morphological data used in the key is exactly that published by Johnson 44 years earlier.

It is suggested here that this nomenclatural change is not accepted, as it would change the name of a globally important species for reasons of taxonomical semanics. However, it is acknowledged that what was previously described as P. juliflora from Sinaloa in Mexico with a northern limit close to the Tropic of Cancer in Panama, is different but related to the true P. julilfora of Colombia, Venezuela and the Caribbean. An alternative option is proposed here, however, is that this population is given a sub-specific rank, and is renamed P. juliflora var. vidaliana. It also appears that both may be entirely tetraploid, being another reason to maintain them as no more than varieties of a single ‘good’ species.

In addition, initial analysis of material collected and analysed by Harris et al. (2003), suggests that this is not the basis for widespread introductions in India and possibly elsewhere. Thus, much of what is considered P. juliflora across Africa, Asia and Australia, would remain P. juliflora. However, it would be of interest to elucidate with of the introduced P. juliflora that originally originated from the Pacific coast of Central America.

The Colombian-Caribbean race

Of the three races, only this remains indisputably, ‘true’ P. juliflora, with Colombia considered as the possible origin. The southern limit appears to be near to the border with Ecuador, in restricted and separated dry rain shadow areas at higher elevation. This patterns continues north and across in Venezuela, with populations along valleys going down to larger areas along parts of the Caribbean coast. However, in the Caribbean, it is considered by some to be adventive. The P. juliflora type specimen was collected in Jamaica, but apparently from introduced material (Burkart, 1976). Palacios (2006) also identifies a small population on the northern coast of Yucatan, which noting its small size and presence of P. juliflora in Cuba, could also be naturalized.

Description

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The following description is taken from Burkart (1976) as this description is still accepted in the absence of a new acknowledged taxonomy.

Tree (or shrub on sterile soils) 8-20 m high, trunk to 60 cm in diameter, unarmed or spiny, with short, axillary, uninodal, geminate (paired), divergent spines less than 4 cm long. Leaves bipinnate, medium to small in size, pallid grayish-green when dry, (1-) 2-4 pairs of pinnae, pubescent, ciliolate to subglabrous; petiole short, with the rachis 0.8-4.5 cm long, pubescent; pinnae 1.5-6.0 cm long, with a sessile, cuplike gland at their junction; leaflets green or gray when dry, 6 to 15 pairs per pinna, approximate without touching or a little distant, pubescent or at least ciliolate, oblong-elliptic to ovate, obtuse or mucronate, firm, pinnately nerved below, 2.5-8.3 mm long x 1.4-4.0 mm broad. Racemes 2 to 3 times longer than the leaves; rachis and short peduncle pubescent, together 8-15 cm long; florets dense (200 to 250 per raceme), short-pedicelled, greenish-yellow; calyx ciliolate, 0.5-1.5 mm long; petals 2.5-3.0 mm long, free, villous within; stamens 5-7 mm long; ovary stalked, villous. Legume straight or subfalcate, very similar to that of P. juliflora (Sw.) DC., but thicker, straw-yellow when ripe, with parallel margins, fleshy, sweet, edible, subcompressed, long or short stipitate with rounded base, and acuminate, sometimes nearly subquadrate-rectangular in transection, (6-)10-25 cm long x 1.0-1.5 cm broad x 5-9 mm thick; endocarp segments to 30, broader than long; seeds oblong, brown, 6.5 mm long.

Plant Type

Top of page Broadleaved
Perennial
Seed propagated
Shrub
Tree
Woody

Distribution

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P. pallida has a limited native range, mainly in Peru and Ecuador, also southern Colombia, and possibly extending into Bolivia. However, it must be emphasised that previous taxonomies also recorded P. jullifora as present throughout most of the native range of P. pallida, existing in sympatry. However, several hypotheses suggesting that this could not be the case were finally settled with the study of Mom et al. (2002) that confirmed that material in northern Peru (and thus, by assumption, also in neighbouring areas) formerly identified as P. juliflora, is in fact, P. pallida.

The distribution of P. pallida where introduced is probably may be more widespread than shown. Further populations of P. pallida may yet be identified from areas where only P. juliflora is recorded to date, on the basis of on-going taxonomical studies following the work of Harris et al. (2003) and Pasiecznik et al. (2004). It is only found in entirely frost-free tropical regions. However, it is certainly the ‘common’ Prosopis species in Brazil and Cape Verde (Harris et al., 2003; Landeras et al., 2006, Trenchard et al., 2008), although many scientific publication to this day still mistakenly refer to it as P. juliflora. P. pallida also appears to be the common species in Senegal (author’s own observations, December 2016). Whereas it has been positive identified along the coast (Harris et al., 2003; Landeras et al., 2006, Trenchard et al., 2008), all trees in the interior that were observed (East of Dakar, to beyond Kaffrine) were identified as P. pallida.

Records from Botswana appear to be misidentifications, and records from South Africa are questionable, and though the tree may be present as rare individuals, it is unlike to be one of the main invasive species of Prosopis found in southern Africa.

For the most up to date information on the global distribution of prosopis species at the country level, including P. pallida, see Shackleton et al. (2014).

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.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasivePlantedReferenceNotes

Asia

IndiaPresentPresent based on regional distribution.
-HaryanaPresent, few occurrencesIntroduced Not invasive Planted Tewari et al., 2000
-MaharashtraPresent, few occurrencesIntroduced Not invasive Planted Tewari et al., 2000
-RajasthanPresent, few occurrencesIntroduced Not invasive Planted Tewari et al., 2000
-Tamil NaduPresent, few occurrencesIntroduced Not invasive Planted Harris et al., 2003
-Uttar PradeshPresent, few occurrencesIntroduced Not invasive Planted Tewari et al., 2000
IsraelPresent, few occurrencesIntroduced Not invasive Planted Pasiecznik et al., 2001
JordanPresent, few occurrencesIntroduced Not invasive Planted Harris et al., 2003

Africa

BotswanaPresentIntroducedBotswana Government, 2009Unconfirmed record
Cape VerdeWidespreadIntroduced Planted Harris et al., 2003
DjiboutiPresentIntroducedPasiecznik et al., 2013
KenyaRestricted distributionIntroduced Not invasive Planted Choge et al., 2002
MauritaniaPresentIntroduced Planted Harris et al., 2003
SenegalPresentIntroduced Planted Harris et al., 2003

North America

USAPresentPresent based on regional distribution.
-HawaiiWidespreadIntroduced Invasive Natural Fosberg, 1966

Central America and Caribbean

Puerto RicoPresentIntroducedJohnston, 1962; Burkart, 1976
United States Virgin IslandsPresentIntroduced Planted Johnston, 1962

South America

BrazilPresentIntroduced Invasive Lima and Pasiecznik, 2006
-BahiaPresentIntroduced Planted Harris et al., 2003
-CearaWidespreadIntroduced Planted Harris et al., 2003
-ParaibaWidespreadIntroduced Planted Harris et al., 2003
-PernambucoWidespreadIntroduced Planted Harris et al., 2003
-PiauiPresentIntroduced Planted Harris et al., 2003
-Rio Grande do NorteWidespreadIntroduced Planted Harris et al., 2003
ColombiaRestricted distributionNative Not invasive Burkart, 1976; Díaz Celis, 1995
EcuadorPresentNative Not invasive Burkart, 1976; Díaz Celis, 1995
PeruWidespreadNative Not invasive Burkart, 1976; Díaz Celis, 1995

Oceania

AustraliaPresentPresent based on regional distribution.
-Australian Northern TerritoryPresentIntroduced Invasive Planted Panetta and Carstairs, 1989
-QueenslandPresentIntroduced Invasive Burkart, 1976; Panetta and Carstairs, 1989
-Western AustraliaPresentIntroduced Invasive Planted Panetta and Carstairs, 1989
French PolynesiaPresentIntroduced Planted Burkart, 1976
Papua New GuineaRestricted distributionIntroduced Planted Perry, 1998

History of Introduction and Spread

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The best recorded introduction of P. pallida is to the semi-arid zones of north-east Brazil. The account of Azevedo (1982) is quoted in Silva (1989) and in many other papers presented at the FAO symposium 'The Current State of Knowledge on Prosopis juliflora' in 1986 (Habit and Saavedra, 1990). Azevedo states that the first introduction of Prosopis into Brazil was by J.B. Griffing, who in 1942 introduced seed from New Mexico, USA (presumably P. glandulosa var. torreyana and/or P. velutina), to the Sierra Talhada area of Pernambuco state. This was followed by a second introduction to Rio Grande do Norte by S.C. Harland in 1947 of seed from Peru (now assumed to be P. pallida) and in 1948 of seed from Sudan (Silva, 1990). Only two trees of each of these later two introductions are believed to have survived and it is stated that these provided the basis for the entire population of P. pallida in the north-east of Brazil (Pires and Kageyama, 1990). There are no records of either P. glandulosa or P. velutina existing today as naturalized species, so it can be assumed that the introductions from the USA did not survive. P. juliflora is also present in the north-east of Brazil, but is more common in northern, lowland and coastal areas.

Pacific islands have naturalized populations of both P. juliflora and P. pallida recorded for the Hawaii islands and the Marquesa islands of French Polynesia (Burkart, 1976). It might be assumed that they were introduced from Pacific coastal areas of Peru and Central America where they are native. The first introduction into Hawaii is credited to the Catholic missionary Father Alexis Bachelot in 1828 (Perry, 1998) or 1838 (Esbenshade, 1980). Seed came from a tree in Paris, France, which was thought to have originally come from Brazil (Esbenshade, 1980) or South America (Perry, 1998). P. pallida dominates coastal areas in Hawaii and has been revered as the most useful of all species ever introduced to those islands. It is from here that introductions to other Pacific islands such as the Marquesas were probably made. The distinction between P. pallida and P. juliflora is apparently clear in the literature from Hawaii but much less so elsewhere in the Pacific.

Species of Prosopis were introduced into Australia around 1900. Isoenzyme studies have shown that the P. pallida present there is identical, except for one rare allele, to the P. pallida in Hawaii, strongly suggesting that this species was introduced from the Pacific (Panetta and Carstairs, 1989). The Hawaiian P. pallida may have been introduced to other Pacific islands before reaching Australia. Perry (1998) reported that Hawaiian seed (designated as P. chilensis) had been sent to Australia from Hawaii. No exact records of the first introductions exist, but Prosopis species were first introduced into Australia as a tree for shade, fodder and erosion control, with major planting and possibly further introductions in the 1920s and 1930s (Csurhes, 1996). Of the Prosopis species present in Australia, P. pallida is widespread in the northern part of the continent, possibly due to an earlier introduction (Panetta and Carstairs, 1989). P. pallida is also found in Papua New Guinea (Perry, 1998).

Baron Roger, governor of Senegal, is credited with the introduction of P. juliflora, now accepted as P. pallida (Harris et al., 2003), to the town of Richard Toll in the River Senegal estuary in 1822. The origin of this material is uncertain but was thought by Diagne (1992) to be Mexican, although P. pallida was introduced into Hawaii by a French missionary from South American seed via Paris a few years earlier (Esbenshade, 1980; Perry, 1998) and it is possible that some of this seed also came to Richard Toll. Plantations were established from Ecuadorian seed in Senegal in the 1980s and seed from these plantations has since been distributed as P. juliflora, although it could possibly be P. pallida and this requires confirmation.

There are few other records of the introduction of P. pallida around the world and in most countries other than those mentioned above, the species appears localised, with the exception of Cape Verde where it is by far the dominant species. P. pallida was introduced into Kenya in 1973 in the Baobob farm reclamation project (Choge et al., 2002). Several introductions have been made in India with seed obtained from Israel as part of field trials from the 1970s (Tewari et al., 2000).

Risk of Introduction

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P. pallida ‘beans’ may be locally traded as a fodder, but not internationally, and may only rarely be introduced accidentally e.g. naturally by water, down rivers or along coastlines, or inside live animal exports. P. pallida was introduced around the world intentionally, due to their value as a fuel/fodder species and also as an ornamental in some regions. However, the infamy of Prosopis as invasive species has led to several governments banning further importation of planting stock, and the current risk of introduction is perceived as low. P. pallida is a declared noxious weed in USA and Australia and is recorded as invasive in Western Australia, Northern Territories and Queensland, Hawaii, with invasive tendencies noted in Brazil, Cape Verde, Senegal, Mauritania and Djibouti.

Habitat

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The presence and depth of the water table is a decisive factor in the distribution, size and growth of P. pallida. In its native range, it is common in xerophytic formations, either coastal as in northern Peru, or montane or intermontane valleys as in Ecuador and central/southern Peru, or as part of tropical or subtropical thorn scrub or forest formations. Where introduced, it is found in a variety of similar habitats.

Habitat List

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CategorySub-CategoryHabitatPresenceStatus
Terrestrial
Terrestrial – ManagedDisturbed areas Present, no further details Harmful (pest or invasive)
Rail / roadsides Present, no further details Harmful (pest or invasive)
Terrestrial ‑ Natural / Semi-naturalNatural forests Present, no further details Natural
Natural grasslands Present, no further details Harmful (pest or invasive)
Natural grasslands Present, no further details Natural
Riverbanks Present, no further details Harmful (pest or invasive)
Riverbanks Present, no further details Natural
Deserts Present, no further details Natural
Littoral
Coastal areas Present, no further details Harmful (pest or invasive)
Coastal areas Present, no further details Natural

Hosts/Species Affected

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P. pallida has been noted as a weed of certain habitats, notably natural grasslands in Australia and dry river valleys in north-east Brazil and West Africa. It is not noted as a weed of any specific crops or plants.

Host Plants and Other Plants Affected

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Plant nameFamilyContext
Poaceae (grasses)PoaceaeMain

Biology and Ecology

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Genetics

P. pallida has a chromosome number of 2n=28, common to almost all Prosopis species. The chromosome numbers of most recognised species of Prosopis have been ascertained and all taxa are diploid with a haploid number of n=14 (2n=28), with the exception of P. juliflora which also has tetraploid forms (2n=56) (Hunziker et al., 1975; Solbrig et al., 1977). Harris et al. (2003) suggests that P. juliflora is entirely tetraploid, and ploidy can be used to separate this species from P. pallida, which is entirely diploid (Pasiecznik et al., 2004), confirmed by Trenchard et al. (2008) and Sherry et al. (2011). Karyotype morphology of all species investigated was similar, with somatic chromosomes, which are very small (0.8-1.3 µm), showing slight variations in size within the complement. Chromosomes are only slightly differentiated, with median to sub-terminal centromeres, one pair of which displays a terminal microsatellite in most species (Hunziker et al., 1975).

Reproductive Biology

Styles emerge from most flowers prior to anthesis but they are probably not receptive at this stage and the flowers remain in this state for some days. Anthesis occurs when the flowers fully open, occurring simultaneously in all flowers of a single inflorescence (Díaz Celis, 1995). However, flower maturation often begins at the proximal end while flowers at the distal end are still immature. Species of Prosopis are generally assumed to be self-incompatible (Solbrig and Cantino, 1975; Simpson, 1977) and this was confirmed for this species by de Oliveira and Pires (1990), who reported that no successful pollination or fruit set occurred after bagging and selfing flowers in Brazil.

The flowers attract large numbers of potential pollinators with the production of copious amounts of pollen, and pollen grains are produced and released singly, rather than in polyads. Anther glands may exude a sticky substance to attach the pollen to the body of the insect, and to protect the anthers and ovaries. They may also exude an odorous chemical attractant. Percentage pollination is always low in P. pallida (de Oliveira and Pires, 1990). This is thought to be due to a number of factors such as poor pollen viability, short periods of pollen release or stigma receptivity, lack of synchronization between pollen release and pollen reception, few pollinating insects (or too few at times of maximum receptivity), flower sterility or high rates of ovary abortion. Long periods of asynchronous flower production would assume a long period of pollen release and floral receptivity.

There appears to be sufficient numbers of pollinating insects with little host specificity. However, should climatic conditions stimulate heavy flowering over extensive ranges, it is possible that sufficient numbers of pollinating insects may not be available. Bees are thought to be the main type of insect responsible for pollination with cross-pollination thought to be by larger species of bee. Increased pollination is noted in honey-producing areas and has positive effects on fruit production of P. pallida (Esbenshade, 1980). Very few legumes are produced compared with the large numbers of flowers produced per tree (Solbrig and Cantino, 1975). From 10,000 P. pallida flowers, de Oliveira and Pires (1990) estimated that 129 mature fruits would be produced, an efficiency of 1.29%.
Seedlings are rarely observed under the canopy of a mature tree, possibly because of shading, allelopathic effects, or the presence of seed-eating insects. Beetles of the family Bruchidae are responsible for destroying a substantial percentage of seeds produced by Prosopis spp. in their native range, and dispersal mechanisms may be an evolutionary response to destruction by such insects.

Physiology and Phenology

The seeds of P. pallida possess an inherently high level of dormancy. The hard seed coat must be broken or weakened to allow water absorption by the seed and for germination to occur. The passage of seed through different animals has varying effects on germination, through the removal of the mesocarp or endocarp, or other mechanical or chemical factors (Pasiecznik et al., 2001). The optimum temperature for germination is 30°C, or temperatures alternating between 20 and 30°C (Torres et al., 1994), with germination decreasing rapidly above 45°C and below 15°C (Perez and Moraes, 1990). Dry heat plays a role in increasing germination but only appears to have a small role in breaking seed dormancy. Germination is rapidly followed by the establishment of the root system, young shoots and leaves.

All Prosopis species are able to survive in areas with exceptionally low annual rainfall or very lengthy dry periods, but only if the roots are able to tap ground water or another permanent water source within the first few years. Being adapted to arid and semi-arid climates, germination and establishment of P. pallida generally occurs during the brief rainy season and seedlings must be sufficiently well established to survive the first dry season. The existence of two root systems, a deep tap root to reach ground water and a mat of surface lateral roots to make use of infrequent rainfall events, puts P. pallida firmly in the category of phreatophytes (Diaz Celis, 1995).

Prosopis species exhibit high levels of variability in morphological characters, and rainfall and temperature both affect wood structure (Gomes and de Muñiz, 1990) and pod sugar content (Lee and Felker, 1992). Variation in the onset of flowering can be expected between populations of all species due to climatic variation within existing ranges. Flowering is also variable within and between trees of the same population. Almost continuous year-round flowering of P. pallida is seen in Brazil with a principal fruiting period and a smaller, secondary season later in the year (Pasiecznik and Harris, 1998). With continuous flowering, periods of major fruit production may correspond to periods of increased pollinator activity and not necessarily to genetic controls, particularly with introduced material.

Associations

Mares et al (1977) summarised the ecological associations, describing American Prosopis tree species as "representative large desert trees which provide protection from grazing animals, shade, a moist microhabitat, a substrate for climbing or perching, and a reliable supply of nutrients for parasitic and semiparasitic plants. In providing these habitat components, such desert scrub trees allow an increase in plant density and richness in the community as a whole. These species, which would be rare or absent without the presence of trees and shrubs such as Prosopis, in turn contribute to the support of other trophic levels by providing food sources (leaves, flowers and fruits) for desert scrub animals". In native range Peru, several plant genera are common associates including species of Capparis and Cordia and species of the woody legume genera Acacia, Caesalpinia, Cercidium, Parkinsonia and Pithecellobium. P. pallida has evolved a symbiotic relationship with Rhizobium and other nitrogen-fixing bacteria and also mycorrhizal associations to varying degrees.

In the native range, mammals use P. pallida along with other desert trees for shade, protection and food and it is thought that the removal of these trees, where they are common, would cause a significant decrease in the populations of small wild mammals. Invertebrates are important in the overall ecology of Prosopis trees and stands as they feed on living or dead tissue or use the tree for shelter and as hunting and mating ground. Seed-feeding beetles, many of the family Bruchidae, have evolved alongside Prosopis and are very important in the ecology of American Prosopis species. Of the species of beetles found to feed on the pods of native American species of Prosopis, 93% were obligately restricted to Prosopis, showing a high degree of specialisation. Secretory glands are present on the leaves of P. pallida (Burkart, 1976; Ferreyra, 1987) but their exact purpose is unclear as associations with ants that are common in Acacia species are almost unknown in Prosopis.

Environmental Requirements

P. pallida, being truly tropical, requires relatively high temperatures for growth. The mean annual air temperature in the shade where the complex is found is generally above 20°C, with optimum temperatures for growth being in the range 20 to 30°C. There appears to be no natural upper limit to temperature, and introduced P. pallida is known to tolerate day-time shade temperatures of over 50°C. Such high temperatures are rarely recorded in coastal or montane environments in its native range. A major limitation to the distribution of P. pallida is mean minimum temperature and the frequency and duration of winter frosts. Light frosts cause dieback of branches, harder frosts cause complete stem mortality, and complete death of the plant occurs with more severe or longer-lasting frosts (Felker et al., 1982). P. pallida and P. africana are the most frost-sensitive Prosopis species, tolerating several frosts of -1.5°C, but dying with a frost of -5°C (Felker et al., 1982). P. pallida was damaged by temperatures below 5°C in Peru (FAO, 1997).

P. pallida grows well in areas of low rainfall, thrives in a wide range of rainfall zones, can survive in areas with an annual rainfall of 50-250 mm (FAO, 1997) if there is a water table, and is found at higher densities along seasonal watercourses. It is known to tolerate saline sites in its native range and can often dominate in lowland flats and coastal dunes. P. pallida can tolerate salinity levels of up to 18000 mg NaCl/l with no reduction in growth or survival, and can even grow at 36000 mg/l NaCl, equivalent to sea water (Felker et al., 1981). When found in valleys, the trees tend to congregate along the valley bottom where there is likely to be a permanent supply of sub-surface water. In montane areas, Prosopis species tend to inhabit dry valleys in the rain shadow of large mountains with rainfall of up to 1500 mm m.a.r. Altitude does not appear to limit distribution directly. In the native range, P. pallida is abundant at altitudes below 200 m, is less common as the altitude increases to 500 m, and becomes more abundant again above this altitude, with some trees found up to 1500 m altitude in the Andes. P. pallida is generally well adapted to the different altitudes where it is introduced.

Soil depth is important, with tree growth limited by soils that are thin or have a calcareous or iron pan. Poor drainage or waterlogging can reduce tree growth and survival, and poor oxygen content in the soil is also thought to affect root growth. Soil nutrient status is rarely a limiting factor to distribution. Nitrogen is very rarely limiting, with nitrogen fixation and soil improvement leading to an increase in soil fertility as the trees mature (Geesing et al., 1999).

Latitude/Altitude Ranges

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Latitude North (°N)Latitude South (°S)Altitude Lower (m)Altitude Upper (m)
5 20 0 1500

Air Temperature

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Parameter Lower limit Upper limit
Absolute minimum temperature (ºC) 5 0
Mean annual temperature (ºC) 25 35
Mean maximum temperature of hottest month (ºC) 20 50
Mean minimum temperature of coldest month (ºC) 10 25

Rainfall

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ParameterLower limitUpper limitDescription
Dry season duration612number of consecutive months with <40 mm rainfall
Mean annual rainfall501500mm; lower/upper limits

Rainfall Regime

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Summer
Winter

Soil Tolerances

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Soil drainage

  • free
  • impeded

Soil reaction

  • acid
  • alkaline
  • neutral

Soil texture

  • heavy
  • light
  • medium

Special soil tolerances

  • infertile
  • saline
  • shallow
  • sodic

Natural enemies

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Natural enemyTypeLife stagesSpecificityReferencesBiological control inBiological control on
Algarobius bottimeri Herbivore Seeds
Algarobius prosopis Herbivore Seeds
Oncideres limpida Herbivore Stems Lima, 1994
Prosopidopsylla flava Herbivore Growing point/Stems

Notes on Natural Enemies

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For an extensive list of pests and pathogens known to attack P. pallida and related species, refer to Pasiecznik et al. (2001). Some such as nematodes that are known to attack the roots of P. pallida (Díaz Celis, 1995) may also occasionally infest neighbouring crops. Defoliating insects vary in their severity of attack and twig girdlers (species of Oncideres ) are damaging in some areas (Silva, 1990) with adult beetles girdling small branches before ovipositing. Wood-boring beetles are rarely specific to Prosopis, attacking several taxa of woody perennials.

Means of Movement and Dispersal

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Natural Dispersal

Water is an important dispersal agent in desert ecosystems. Water dispersal ensures the widespread dissemination of seed during flooding or other high rainfall events when seedling establishment is favoured. P. pallida is often found colonising ephemeral watercourses and dispersal is aided by water flow in the rainy season, particularly during very wet years (Solbrig and Cantino, 1975). Oceanic dispersal is important for coastal populations, as pods and endocarps float and are impervious to water infiltration, protecting the seed from the harmful effects of extended periods in sea water.

Vector Transmission

Prosopis pods have a high sugar content, are low in anti-feedants, and are widely sought after by a variety of animals. Disjunct stands of trees near to old centres of population suggest that man has also been a dispersal agent in historic and prehistoric times. Livestock are now the primary dispersal agents, although the fruit are also avidly consumed by a wide variety of wild animals, which play a major role in seed dispersal. Birds, bats, reptiles and ants also feed on Prosopis fruits and are potential, if only minor, agents of dispersal, but it is generally accepted that the fruits and seeds are specialized for animal dispersion. Pods are eaten off the tree or off the ground and seeds are deposited in the faeces. Voided seed are given a positive advantage by being placed in faeces, due to its improved water-holding capacity and high levels of nutrients. Livestock may tend to spend more time on better pasture or by water sources but voiding of seed in preferential locations is not guaranteed. However, different animals have very different effects on seed survival.

Accidental Introduction

It is thought that accidental introduction of Prosopis seed as a contaminant is unlikely, though there remains a possibility for introduction via live livestock imports where the animals have been fed on Prosopis pods either just before, or during, transit. Pods and seed may adhere to agricultural machinery, but this is considered as a minimal cause of spread. The principal reason for agriculture increasing the spread of Prosopis is due to habitat modification such as over-grazing, which creates conditions favourable for spread.

Intentional Introduction

This has been the main reason for spread of P. pallida around the world during the previous 200 years, as a fuel and fodder species able to tolerate the most arid sites and poorest soils where little else would grow.

Pathway Vectors

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VectorNotesLong DistanceLocalReferences
MailMail order seed catalogues Yes
Soil, sand and gravelOceanic dispersal Yes

Impact Summary

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CategoryImpact
Animal/plant collections None
Animal/plant products Negative
Biodiversity (generally) None
Crop production None
Environment (generally) None
Fisheries / aquaculture None
Forestry production None
Human health None
Livestock production Negative
Native fauna Positive
Native flora None
Rare/protected species None
Tourism None
Trade/international relations None
Transport/travel Negative

Economic Impact

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P. pallida is less widespread than other introduced Prosopis species and much less is known about its economic impact. However, in Australia, where it is a common weed, significant losses in livestock production are noted due to a reduction in the available forage and thus stocking densities, as well as effects on animals from the thorns. In Brazil, illnesses from constipation to 'wooden tongue' (a stiffness in the face of cattle) have been reported, and death of cattle is noted where whole pods make up the only source of feed during dry periods. In other countries where it is common, the positive effects from the sale of tree products far outweigh any negative impacts (Pasiecznik et al., 2001).

Environmental Impact

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Prosopis species are widely noted to exploit soil water and lower water tables, being phraetophytic and are known to possess very deep roots which will use subterranean water when no surface water is available. However, there is some debate as to the real effects of Prosopis on water tables. In India, Cape Verde and elsewhere in the Sahel, Prosopis species have been blamed for the lowering of water tables, while some researchers suggest that this is due to the increase in the number of boreholes and the amounts of water being extracted for irrigation. A recent study (Dudley et al., 2014) looked at how groundwater availability mediates the ecosystem effects of an invasion of P. pallida in Hawaii, but not how P. pallida affects groundwater, rather, noting that “Groundwater levels in arid environments are dropping worldwide due to human extraction, and precipitation events are predicted to become rarer and more intense in many arid areas with global climate change.” Also in Hawaii, Burnett et al., (2014) found that in some cases, management instruments can be designed to target simultaneously both groundwater and an interdependent resource such as invasive P. pallida that has been shown to reduce groundwater levels, and that results from a groundwater P. pallida management model suggest that at the optimum, the resource manager should be indifferent between conserving a unit of groundwater via tree removal or via reduced consumption.

There are also certain positive impacts on the environment due to soil stabilisation by the roots and reduced soil erosion from windbreaks and within plantations, and the reduced salinity and alkalinity and improved soil fertility and physical characteristics. However, the genus as a whole is perceived as having a major negative impact due to massive and hugely environmentally impacting invasions of P. juliflora and P. glandulosa. Further detailed studies are clearly needed though, to differentiate the relative invasiveness and environmental impacts of the different species.

Threatened Species

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Threatened SpeciesConservation StatusWhere ThreatenedMechanismReferencesNotes
Hylaeus facilis (easy yellow-faced bee)USA ESA listing as endangered species USA ESA listing as endangered speciesHawaiiEcosystem change / habitat alterationUS Fish and Wildlife Service, 2014a
Hylaeus hilaris (hilaris yellow-faced bee)USA ESA species proposed for listing USA ESA species proposed for listingHawaiiEcosystem change / habitat alterationUS Fish and Wildlife Service, 2014b
Hylaeus kuakea (Hawaiian yellow-faced bee)USA ESA listing as endangered species USA ESA listing as endangered speciesHawaiiEcosystem change / habitat alterationUS Fish and Wildlife Service, 2014c
Panicum niihauense (Niihau panicgrass)CR (IUCN red list: Critically endangered) CR (IUCN red list: Critically endangered); NatureServe NatureServe; USA ESA listing as endangered species USA ESA listing as endangered speciesHawaiiCompetition (unspecified); Ecosystem change / habitat alterationUS Fish and Wildlife Service, 2008
Scaevola coriacea (dwarf naupaka)NatureServe NatureServe; USA ESA listing as endangered species USA ESA listing as endangered speciesHawaiiCompetition (unspecified)US Fish and Wildlife Service, 2010a
Sesbania tomentosaNational list(s) National list(s); USA ESA listing as endangered species USA ESA listing as endangered speciesHawaiiCompetition - monopolizing resources; Ecosystem change / habitat alterationUS Fish and Wildlife Service, 2010b
Tetramolopium capillare (pamakani)USA ESA listing as endangered species USA ESA listing as endangered speciesHawaiiCompetition (unspecified)US Fish and Wildlife Service, 1997
Tetramolopium rockii (dune tetramolopium)USA ESA listing as threatened species USA ESA listing as threatened speciesHawaiiCompetition - shadingUS Fish and Wildlife Service, 1996b

Social Impact

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The principal cause for concern is from the strong and sometimes profuse thorns seen in some provenances of P. pallida, which are able to pierce tyres and all but the toughest of shoes or hooves. However, many P. pallida trees are thornless and erect (Pasiecznik et al., 2001) and these would not cause such negative social impacts.

Risk and Impact Factors

Top of page Invasiveness
  • Proved invasive outside its native range
  • Highly adaptable to different environments
  • Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
  • Highly mobile locally
  • Has high reproductive potential
  • Has propagules that can remain viable for more than one year
Impact outcomes
  • Damaged ecosystem services
  • Ecosystem change/ habitat alteration
  • Negatively impacts agriculture
  • Negatively impacts animal health
  • Negatively impacts tourism
  • Reduced amenity values
Impact mechanisms
  • Competition - monopolizing resources
  • Competition - shading
  • Competition
  • Produces spines, thorns or burrs
Likelihood of entry/control
  • Highly likely to be transported internationally deliberately
  • Difficult/costly to control

Uses

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P. pallida is a valuable multipurpose tree. Principal uses are wood for fuel, posts, poles and sawn timber, and pods for fodder. There are numerous other tree products including chemical extracts from the wood or pods, honey from the flowers, medicines from various plant parts, exudate gums, fibres, tannins and leaf compost. The tree is also widely planted for soil conservation, in hedgerows, as an urban and general amenity tree. For a comprehensive review of the uses of the species, refer to Pasiecznik et al. (2001), though there have many more recent investigations on potential uses, such as pods for ethanol production in India (Purohit et al., 2013).

Uses List

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Animal feed, fodder, forage

  • Forage

Environmental

  • Revegetation

Fuels

  • Charcoal

Human food and beverage

  • Honey/honey flora

Materials

  • Wood/timber

Wood Products

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Charcoal

Furniture

Roundwood

  • Building poles
  • Posts
  • Roundwood structures
  • Stakes

Sawn or hewn building timbers

  • Carpentry/joinery (exterior/interior)
  • Exterior fittings
  • Fences
  • Flooring
  • For light construction

Woodware

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

Similarities to Other Species/Conditions

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P. pallida is often mistaken for P. juliflora, although Johnston (1962) noted that it was the only species within section Algarobia not sympatric with any others. It is generally distinguishable due to its very small leaves and leaflets. A long history of modern misidentification is still being resolved, however, that started with the publication of a new field guide (Pasiecznik et al., 2004), which differentiates P. pallida from seven other Prosopis species also found in tropical regions, and includes a key to separate P. juliflora from P. pallida, the two most frequently confused species (Harris et al., 2003). See the Taxonomy and Nomenclature section for a detail review.

Prevention and Control

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Prevention

Eradication

Methods of eradication that have been attempted for over half a century in the Americas have proved very expensive and largely unsuccessful in the long term. Total tree kill may be possible with some treatments, but adequate techniques for preventing the re-introduction of seeds and re-establishment of trees have yet to be developed. The potential environmental damage from the widespread use of herbicides must also be taken into consideration. It is accepted that eradication is not possible using these techniques and, at best, only some form of control is feasible. Mixed mechanical and chemical methods have proved more effective than either alone in several cases. Several integrated programmes that mix mechanical, chemical and fire have had reasonable success but are costly and require a high level of management input.

Control

Cultural Control

Hand clearance was the first method used to deal with Prosopis as a weed. Work teams are sent into invaded pasture to fell the trees and uproot all stumps. Although very effective, the operation is labour intensive and hand clearing remains practical only for small land holdings of high value, such as for agriculture, or where labour is relatively cheap. Hand clearing can also be used in conjunction with some mechanical or chemical methods, such as chemical stump treatment. In Pakistan, hand grubbing was cheaper than chemical stump treatment (Khan, 1961). Grubbing is more cost effective for lighter infestations.

Fire, probably one of the original management tools used in American grasslands, has undergone limited assessment for controlling species of Prosopis. Young seedlings are sensitive to fire but older trees become increasingly protected by thick bark as they mature and will resprout rapidly after fire. Fire can, however, be used successfully as a management tool for preventing the re-establishment of young Prosopis seedlings while also improving forage production. Fire has been used in conjunction with other methods in the development of integrated eradication programmes. For example, spraying with herbicides produces dead wood that will ignite and support a sustained fire with more likelihood of killing the remaining trees. New integrated systems are being assessed in Australia.

Studies on succession suggest the possibility of 'ecological control', by leaving succession to take its natural course. The invasion of Prosopis species into rangeland has been observed and studied for over a century in the USA (e.g. Archer, 1995) and for long periods in South America (e.g. D'Antoni and Solbrig, 1977) and India (e.g. Chinnimani, 1998). Long-term ecological observations and the use of models indicate that dense thickets associated with the problems of invasion are only a temporary stage in the process of succession. The initial stages of invasion involve the introduction of small numbers of trees, which eventually produce seed and act as centres of dissemination (Archer, 1995). The density of the stand increases if land-use systems allow the establishment of seedlings, leading to the formation of dense thickets where conditions allow. Chinnimani (1998) showed that density eventually declines as other species become established and, if left to take a natural course, a new vegetation complex will occur with Prosopis as only a minor component. Felker et al. (1990) observed that self-thinning occurred in stands of P. glandulosa over time. The dense thickets identified as weedy invasions in many countries may only be indicative of the stage of invasion and, if left alone, ecological control may reduce Prosopis numbers.

Mechanical Control

Mechanical site clearance involves tractor operations developed for removing trees, in which roots are severed below ground level to ensure tree kill. These include root ploughing and chaining, which are often the most effective mechanical means, using a mouldboard plough pulled behind a Caterpillar tractor, or a heavy chain pulled between two machines. For root ploughing, large trees must first be felled by hand, but this treatment has been used to remove stumps up to 50 cm in diameter without difficulty and has a treatment life of 20 years or more (Jacoby and Ansley, 1991). Other advantages are that only a single pass is required, whole site cultivation is affected leading to improved soil water conservation, and there is an opportunity to reseed with improved forage species. However, this method is one of the most expensive control treatments and is recommended only on deep soils that have a high potential for increased forage production (Jacoby and Ansley, 1991).

The soil should be neither too wet nor too dry for effective root ploughing. Chaining involves pulling a heavy chain between two slow-moving Caterpillar tractors, with the effect of pulling over larger trees and uprooting them. A second pass in the opposite direction ensures that roots on all sides are severed to ease tree removal (Jacoby and Ansley, 1991). Soil moisture is again important, with soil that is dry on the surface and moist below providing the optimal conditions. If too dry, the tree stem breaks leading to coppicing and if too wet, the soil and understorey will be damaged (Jacoby and Ansley, 1991). Smaller, unbroken trees have to be removed by other means. Although it is an expensive treatment, it is effective where there are many mature trees. It is most widely used following herbicide application to remove dead standing trees. Clearance with a biomass harvester produces wood chips that can be sold for energy production, off-setting the operational costs (e.g. Felker et al., 1999).

Chemical Control

Chemical treatments involve the use of herbicides to kill trees, with the most effective being stem or aerial applications of systemic herbicides. Effectiveness is dependent upon chemical uptake, which in Prosopis is limited by the thick bark, woody stems and small leaves with a protective waxy outer layer. The formulation and application of chemicals for trees of mixed ages and sizes within a stand is difficult. Many herbicides and herbicide mixtures have been tested. Until the banning of its use in the 1980s, 2,4,5-T was the herbicide of choice in the USA (Jacoby and Ansley, 1991) and Australia (Csurhes, 1996). The most effective chemical for high tree kill in the USA is clopyralid, but dicamba, picloram and triclopyr have also been used successfully, either alone or in combination (Jacoby and Ansley, 1991). In India, ammonium sulfamate was successful in killing P. juliflora trees and as a stump treatment (Panchal and Shetty, 1977).

Biological Control

Several biological control programmes using species of seed-feeding bruchid beetles have been developed and implemented. The advantage with bruchids is their observed host specificity, with many species found to feed only on Prosopis, and some only on single species. Other insect species known to have a deleterious effect on native and exotic Prosopis in the Americas, mainly twig girdlers and psyllids, have also been suggested as possible biological control agents. The twig girdler Oncideres limpida attacks P. pallida in Brazil (Lima, 1994) and Oncideres rhodosticta was seen as a serious pest of P. glandulosa in the USA (Polk and Ueckert, 1973). Psyllids are known to severely affect the growth of Prosopis (Hodkinson, 1991) and have been suggested for use in controlling invasions.

Most work on the biological control of Prosopis to date has been carried out in South Africa, where several programmes are underway. The seed-feeding insects Mimosetes protractus and Neltumius arizonensis were introduced to eastern South Africa in conjunction with the bruchid beetles Algarobius prosopis and A. bottimeri for the control of invasive Prosopis species. N. arizonensis and A. prosopis were successful in establishing themselves in large numbers and having a significant effect on Prosopis, whereas the other species were only found in low numbers (Hoffmann et al., 1993). Maximum damage to seed was recorded where grazing was controlled, as multiplication and progress is hampered by livestock devouring pods before the insects destroy them.

The same two bruchid species were also introduced to Ascension Island in an attempt to control P. juliflora, which is present on 80% of the island, often in dense thickets. Two other species, one a psyllid and the other a mirid, were identified as attacking P. juliflora on Ascension Island and were thought to have been introduced accidentally from the Caribbean. The mirid Rhinocloa sp. causes widespread damage and is thought to lead to substantial mortality of trees (Fowler, 1998). In Australia, Prosopis infestations are at a relatively early stage and extreme care is being employed in the selection of suitable biological control agents, following the long history of problems caused by plant and animal introductions in that country. Insect species continue to be tested for their efficacy and host specificity as possible biological control agents of Prosopis species in Australia (e.g. van Klinken, 1999; van Klinken et al., 2009). Besides the two Algarobius species, the sap-sucking psyllid Prosopidopsylla flava and the leaf-tying moth, Evippe sp. have both been found to provide some control in Australia (Anderson et al., 2006).

Prosopis species continue to spread widely in parts of their native ranges where many insects including bruchids, twig girdlers, psyllids and other injurious pests are common components of the ecology. These insects regularly attack Prosopis but the trees have adapted to infestation by these pests and are still able to become invasive weeds over large tracts of land. Although there has been some success in the control of exotic Prosopis species following the introduction of bruchid beetles and other insects, it appears that biological control alone may be insufficient.

Integrated Control

Control of species of Prosopis could also include the use of animals, other than insects, to eat and kill Prosopis seed. One factor common to most Prosopis invasions is over-grazing with cattle, which spreads Prosopis seed widely. Prosopis seed found in cattle faeces have much improved germination compared with uningested seed (Peinetti et al., 1993; Danthu et al., 1996). In contrast, the percentage of P. juliflora seed excreted after ingestion by sheep and goats was much lower (10-15%) (Harding, 1991; Danthu et al., 1996). Marked differences in the germination of ingested seed following passage through different animals were noted by Mooney et al. (1977); seed germination was 82% with horses, 69% with cattle, but only 25% with sheep. P. flexuosa seeds were killed completely followed ingestion by pigs (Peinetti et al., 1993). Replacing free ranging cattle with other livestock, particularly sheep and pigs, possibly in conjunction with other control methods, could drastically reduce the spread of Prosopis species.

Weedy invasions of Prosopis can be successfully adapted to agroforestry systems by a conversion process developed by Felker et al. (1999) and adapted by Tewari et al. (2000) and Pasiecznik et al. (2001). This conversion requires three main management interventions: thinning, pruning and treatment of the understorey. Weedy thickets with 1000-2500 trees/ha and dense infestations with over 2500 trees/ha need to be thinned to 100-625 trees/ha. This thinning operation is the most problematic and costly aspect of conversion and limits the uptake of such a system. The use of a tractor-mounted flail-mower to cut rows through the stand is the most economical means of initial thinning. The harvested biomass is then sold to offset some of the cost of the operation (Felker et al., 1999). The aim is to leave rows of undisturbed Prosopis at least 1 m wide and at 5-10 m intervals across the site. Clearing by hand is a laborious and unpleasant task owing to thorns and difficult access, and rows of thorny brash must be left on site. However, most of the nutrient-rich foliage is retained on site with this method and adds to soil fertility. All tree stumps, of Prosopis species need to be killed.

Following initial systematic thinning to a stand density of approximately 500-1000 trees/ha, a secondary selective thinning is required to create the desired final density of 100-625 trees/ha or less. Although trees do not necessarily have to be equally spaced, leaving open rows 5-10 m apart will facilitate access and increase the number of understorey management options possible with tractor operations. Trees with desirable characteristics at defined spacings should be marked, and all others removed and the stumps treated. Trees should be selected on the basis of large size, erect form, straight trunk, pod production, lack of thorns and good tree health. Selected trees are then pruned to improve form, by removing any basal shoots and side branches to at least one-half of tree height for timber production, leaving a clear bole preferably over 2 m. For pod production, a shorter bole and broader crown is preferred. Treatments can be applied to reduce resprouting from the tree base and wounds (Pasiecznik et al., 2001). Pods can be browsed or collected as a source of food or fodder. Incorporating bees into the system would produce honey and wax and increase pod production, and the trees could be a minor source of other raw materials. Further thinning of trees in later years can also be carried out, or trees can be removed entirely from the site and the land returned to agriculture (e.g. Bhojvaid et al., 1996).

References

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Allen ON, Allen EK, 1981. The Leguminosae: a source book of characteristics, uses, and nodulation. Madison, USA: University of Wisconsin Press, 812 pp

Anderson LJ, Klinken RD van, Parr RJ, Climas R, Barton D, 2006. Integrated management of hybrid mesquite: a collaborative fight against one of Australia's worst woody weeds. In: 15th Australian Weeds Conference, Papers and Proceedings, Adelaide, South Australia, 24-28 September 2006: Managing weeds in a changing climate [ed. by Preston, C.\Watts, J. H.\Crossman, N. D.]. Victoria, Australia: Weed Management Society of South Australia, 239-242

Archer S, 1995. Tree-grass dynamics in a Prosopis-thornscrub savanna parkland -reconstructing the past and predicting the future. Ecoscience, 2:83-99

Azevedo CF de, 1982. Algarobeira na alimentacao animal e humana. Simposio Brasileiro Sobre Algaroba 1, Natal. Natal, Brazil: EMPARN, 283-299

Bentham G, 1875. Revision of the suborder Mimoseae. The Transactions of the Linnean Society of London, 30:335-664

Bhojvaid PP, Timmer VR, Singh G, 1996. Reclaiming sodic soils for wheat production by Prosopis juliflora (Swartz) DC afforestation in India. Agroforestry Systems, 34(2):139-150

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GISD/IASPMR: Invasive Alien Species Pathway Management Resource and DAISIE European Invasive Alien Species Gatewayhttps://doi.org/10.5061/dryad.m93f6Data source for updated system data added to species habitat list.

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19/02/2017 Updated by:

Nick Pasiecznik, Consultant, France

28/01/2004 Original text by:

Nick Pasiecznik, Consultant, France

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