Mesquite (Prosopis spp.) in Sudan: history, distribution and control
Prosopis spp. (mesquite) are ever green leguminous trees or shrubs. The genus comprises 44 species of which 40 are natives to the Americas. Of the remaining species P. Africana is indigenous to Africa, whereas P. kodziana, P. farcta and P. cineraria are natives to the Middle East and Pakistan (Bukart, 1976). Prosopis species grow in arrays of environments and are not restricted by soil type, pH, salinity or fertility. In Sudan the tree flowers year-round. The fruiting period, which peaks in December to June, coincides with the dry season (El Tayeb, Mahir and El Hassan, 2001). Mesquite leaves are unpalatable, while pods, renowned for high sugar (16%) and protein (12%) contents, are
attractive to animals (Mohamed, 2002). Self incompatibility promotes hybridization and results in genetic variability which confers plasticity and allows colonization of a wide range of habitats (Hunziker et al., 1986). Taxonomy of mesquite is complex and often confusing (Abdel Bari, 1986). The identity of the prevalent species in Sudan is controversial. The species, when, introduced, was claimed to be P. juliflora (Broun and Massey, 1929). However, it was, later, identified as P. chilensis (Wunder, 1966). This identity, confirmed by Abdel Bari (1986), was refuted by Elfadl (1997) and Mohamed (2001) who ascertained the species as P. juliflora. The tree is a copious seed producer. In South Africa over six hundred thousand seedswere reported to be produced per tree per annum (Zimmermann, 1991). The seeds, characterized by coat imposed dormancy, germinate in flushes and establish a huge persistent seed bank in soil. Goats, sheep, cows and feral animals, attracted by the green foliage, eat ripened pods and liberate the seeds. The seeds encapsulated in animal droppings, are spread into new sites over long distances (Fisher et, al., 1959). The pods are also transported by floodwaters and run-off. Following germination mesquite seedlings grow vigorously (Mohamed, 2001). The rapidly growing root system and un-palatability of the foliage increase seedling survival
rate and competitiveness particularly in heavy grazed areas and/or on uncultivated fallows (Mohamed, 2001). The high coppicing ability of mesquite ensures recovery of the plant when cut and often results in a multi stem tree. Mesquite species have attracted attention, because of plasticity, ability to survive inhospitable environments and capacity to provide fuel, timber, fodder and edible pods.
The most common species P. juliflora, native to central and South America, was introduced into several countries in the tropics and sub-tropics (Bukart, 1976). The tree, often multi-stemmed with a spreading crown of pendulous branches hanging down to the ground, has a high capacity to fix sand dunes.
Mesquite in Sudan: History
Mesquite (P. Juliflora) was introduced into Sudan in 1917 from South Africa and Egypt and planted in Khartoum (Broun and Massey, 1929). The success attained in establishment of the tree and its abilities to tolerate drought and fix sand dunes provided the impetus for introduction of the tree into various agro ecologies with emphasis on dry areas (Broun and Massey, 1929). In 1938 the plant was introduced into Sinar, Fwar, EL foung (central Sudan), Elghaba, Lietti basin (northern Sudan), Sinkat, ELgalabat, Portsudan (eastern Sudan), Kordofan and Darfur (western Sudan) (Fig. 1). Late in 1947 and subsequently in 1965 mesquite was re-introduced into eastern Sudan, where it was planted in a green belt around Kassala (Abdel Barie, 1986). In New Halfa mesquite was introduced to protect the research farm at inception in 1966 (El Tayeb, et al., 2001). The prevailing drought in the 1970 s rejuvenated the interest in mesquite and further introductions, into eastern Sudan, were made to protect residential and cultivated areas. In 1974 mesquite seeds were broadcast by airplanes in around Kassala and further planted in protected forests (Elsidig, Abdelsalam and Abdelmagid, 1998). In the period 1978-1981 the tree was planted as shelterbelts at Portsudan and Tokar. Moreover,introductions were made into the White Nile province, western and central Sudan. The
tree was planted in shelterbelts around farms, irrigated schemes and along the Nile (Luukkanen et al., 1983) Several species of mesquite (P. chilensis, P. valutina, P. glandulosa var. terreyona, P.alba, P. pallida and P. articulata) were introduced, in the period 1978-1986, with the objective of selecting suitable species for the different ecological zones. Some of the species selected, had their seeds multiplied and distributed in western Sudan around El Obeid, and various other locations (El fadl, 1997). Mesquite seedlings failed to establish on sand dunes, but were well established within oases leading to lowering of water tables and suppression of native vegetation.
At present mesquite has become a noxious weed in Sudan (El houri, 1986). It has invaded both natural and managed habitats, including watercourses, floodplains, highways, degraded abandoned land and irrigated areas. The weed is more of a problem within central, northern and eastern Sudan. In the sandy soils of western Sudan, a part from localized foci, no problems of weedy invasion were reported (Elfadl and Luukkanen, 2003). Mesquite tends to establish, successfully, on clay or alluvial soils which have good water retention (Luukkanen et al., 1983). Currently mesquite infestations cover over 230 thousand hectare. The bulk of mesquite infestation (>90%) is in eastern Sudan, where livestock keeping and subsistence cultivation constitute the main source of income (Fig. 1). The plant is found in the Gash delta from Kassala northwards passing Wager and southwards up to the borders with Eriteria, in Atbara river, a long Khor Baraka extending from the delta up to 130 kilometers upstream and in water collection pits a long Kassala-Gadarif and Portsudan highway (Fig. 1). Rate of spread of the weed in eastern Sudan, as revealed by aerial photographs, successively taken, in 1962, 1978 and 1992 and a survey undertaken in 1996 was initially low. However, a substantial increase in rate of spread, 371 hectares per annum, was observed during 1978-1992. In 1992-1996 the average rate of spread increased to 460 hectares per annum (Elsidig, et, al., 1998). In most of the infested sites mesquite forms impenetrable thickets that smothered and excluded native vegetation and substantially changed community structure. Indigenous tree species were replaced by mesquite (Elsidig et al., 1998).
The most prominent benefit of mesquite to the communities, in addition to sand dune fixation, is provision of wood fuel for the households, charcoal making and other traditional industries. Selling mesquite firewood and charcoal is an important economic activity. Records of commercial production of charcoal and firewood in 1996/97 from Gash and Atbara rivers were 600,000 sacks and 135,000 m3, respectively (Elsidig et al., 1998). Animal rearing constitutes the main livelihood of land and resource less farmers in many of the mesquite endemic areas. Prosopis leaves and pods may offer a cheap source of feed. However mesquite species including P. juliflora and P. chilensis have unpalatable leaves (Fagg and Stewart, 1994). Mesquite leaves fed to Nubian goats were reported to
suppress rumen microbes and increase levels of rumen ammonia and blood urea (Mohamed, 2002). Results from trials on feeding mesquite pods to sheep were also disappointing (Elsidig et, al., 1998). Over 90% of livestock owners in eastern Sudan regard mesquite as a liability. Mesquite pods are not used as animal feed, mesquite thorns are injurious to animals and dense mesquite thickets reduce productivity of grazing enterprises and interfere with mustering of stock (Elsidig et, al., 1998).
Control and eradication
Several efforts were made to control mesquite, but they were not sustainable as longterm funding has never been guaranteed. A programme of uprooting of mesquite was initiated by the federal ministry of agriculture in 1995. A programme for containment of mesquite was planned in New Halfa in 1996, with little success (El Tayeb, et al., 2001). A programme, Food for Work, was run by Oxfam to control mesquite in Tokar delta. Under, the programme families of low income were mobilized to the delta, offered food and two hectares, each, of mesquite infested land. Mesquite pods were swapped for sorghum to encourage collection
In 1996 the government approved a bill on mesquite management. The tree is to be eradicated where it constitutes a threat to agriculture or biodiversity and preserved in areas threatened by desertification. At present active eradication programmes, using both mechanical and manual methods for uprooting mesquite, are implemented in New Halfa agricultural scheme and Tokar delta at costs of 7.4 and 8.7 million US dollars, respectively. A similar eradication programme was implemented in Zeidab irrigated scheme. However, because of high cost eradication was incomplete and due to poor follow-up, a significant proportion of the cleared area is re-infested. Work in progress at the Agricultural Research Corporation indicated that the herbicides triclopyr and clopyralid applied as aqueous sprays to foliage or in diesel formulations to the stem base displayed excellent activity against the weed. Biological control of mesquite has not been thought or researched, adequately. However, recent surveys undertaken by Bashir and co-workers (Bashir, M. personal communication) showed a number of insect species, including the bruchids Algarobius prosopis, associated with mesquite in various locations. A. prosopis, which destroys mesquite seeds, was introduced from the U.S. into South Africa with encouraging results (Zimmermann, 1991). Seed damage associated with A. prosopis, under natural conditions, in various localities in Sudan, was 7.3-24 %. However, seeds stored in intact pods, for 7 months, suffered over 90% damage (Bashir, M. personal communication).
Repeated introductions of mesquite, its deliberate distribution within the country (Fig. 1), prevailing drought, livestock and feral animal’s movement coupled with decreased landuse and over exploitation of natural vegetation have led to spread of mesquite into various locations. The plant is a difficult to control invasive weed, free of natural enemies, competitive (El Sidig et al., 1998) and allelopathic (Mohamed, 2001). Mesquite has become a national pest (Elhouri, 1986) and constitute a threat to biodiversity and may lead to deterioration of natural vegetation and pastures and thus jeopardize the livelihood of a large proportion of the population, particularly, where livestock keeping and subsistent farming are the main avenues for income generation. Mesquite is underutilized. Increased utilization may, in addition to contribution to control and containment of the weed, leads to realization of significant ecological and socioeconomic benefits, especially, to poor communities and in drought stricken areas, where famines and malnutrition are the norm. Mesquite pods were main part of the diet in the Americas for several thousand years (Ibrahim, 1989). In Nigeria a local fermented food condiment “Kpaye” is produced, from P. Africana seeds (Omafuvbe, Abiose and Adaraloye, 1999). Mesquite pods yield flour of good quality (Felker et, al., 2003).
Mesquite species exude a water soluble gum that has been used as a substitute for gum Arabic during periods of restricted trading or international market shortages (Vilela and Ravtta, 2005). Mesquite pods and leaves may be used as feed supplements (Ravikala, et, al., 1995). Moreover, mesquite is a suitable tree for agroforestry in low rainfall areas (Luukkanen et al., 1983) and could be, if properly managed, a good source of high-value mechanical wood products (Felker, 2000). Based on global experience, complete eradication of mesquite is unlikely, principally because of the scattered and remote nature of infestations and the longevity of seeds. Spread of mesquite, as is the case with many invasive alien weeds, occurs through dispersal and repeated establishment of satellite foci from a founder population (Moody and Mack, 1988). To date heavy and extensive infestations are limited to certain areas in northern, eastern and central Sudan, but several scattered small isolated foci are known to exist all over the country (Fig. 1). The area under mesquite in New Halfa and Tokar delta
was estimated to be 18.5 and 27.3 thousand hectares, respectively in 1996 (ElBashir, 2004). However, in 2004 the infested area was estimated to be 88 and100 thousand hectares. The problem is set to increase dramatically if left unchecked. A two-step strategy of first destroying outlying populations and confining core infestations, as was successfully adopted to combat other invasive weeds including barberry (Berberis vulgaris), Striga asiatica and Euphorbia esula in the U.S (Moody and Mack, 1988), is needed. A significant reduction in distribution and rate of spread of mesquite may be achievable by eradicating new and small infestations and strategically important foci such as those high in river catchments, on irrigation canals, drains and abandoned land around agricultural schemes. The resources required to prevent mesquite spread, eradicate small infestations and confine established core infestations, on the short term, and eradicate them on the long term, are large. However, the potential costs and impact of the weed justify commitment of these resources. Successful sustainable management requires landowners and community commitment to follow-up. Widespread education is required to increase awareness of the weed. Information on distribution of mesquite is essential for planning. Advanced methods of obtaining data, including remote sensing and aerial techniques, need to be developed and applied. Research on biological control targeting seeds should be promoted. The seeds are the main vehicle of transport and spread of the weed. Beetle damaged seeds do not germinate. Devitalizing seeds reduces the seed bank, shortens the period for follow-up treatments, reduces rate of spread and helps contains the weed within the boundaries of the core infestation without influencing desirable attributes of the plant. On going work in the Agricultural Research Corporation indicates that effective control techniques are available for scattered mesquite plants. Dense infestations are much more difficult to control and may require an integrated approach involving the preliminary use of fire or mechanical removal prior to follow-up treatments. Management programmes, irrespective of means and methods adopted, should consider replacement of mesquite by indigenous species.
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