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2.3.1 Environmental protection
The Acacia species recorded as being used for environmental protection are shown in Table 2.3.1; there are doubtless others that could be similarly used.
Although often phreatophytes, many species, especially in the drier regions, have extensive, subsurface lateral roots to take advantage of any light rainfall that might occur. Such subsurface root systems help to stabilize the soil. The aerial system too has a role in reducing wind erosion and ameliorating the microclimate although information on changes in humidity and temperature is generally lacking. Such species as A. nilotica subsp. indica, A. senegal, A. tortiliis and F. albida have been widely planted to fix and stabilize sand dunes and combat wind erosion.
The shade effect by reducing air temperatures also reduces soil evaporation and lowers soil surface temperatures; factual information on shade and soil temperatures is poorly documented. Working in Kenya, Amundson et al. (1994) report that midafternoon soil temperatures beneath the canopy of A. tortilis subsp. spirocarpa were reduced by 3-12°C. The depths at which these temperatures were measured is not records but measurements of soil surface and subsurface temperatures with or without a grass cover at El Obeid, Sudan and illustrated in Fig. 2.3.1 (Hunting Technical Services, 1964), suggest soil depth may not be important. However, in the absence of shade the high surface temperatures attained by bare soils must have an adverse and possibly lethal effect on any dormant or germinating seed lying on the soil surface, discouraging revegetation and encouraging desertification.
The lower shade temperatures also encourage a more mesophytic and generally more nutritious ground cover, which is further encouraged by the higher soil moisture in the vicinity of the trunk as a result of stem flow. The rainfall collected as a result of stem flow is not immediately available to the lateral roots, whose root hairs are more able to take advantage of leaf drip around the periphery of the canopy. So far unpublished work in USA suggests that stem flow infiltrates through the soil immediately surrounding the lateral roots to where it can eventually be utilized by the trees.
Measurements of total soil water content under F. albida taken just before the start of the rainy season gave a value of 8% beneath the canopy and 4% outside (Radwanski and Wickens, 1967). The authors hypothesize that the improved physical conditions beneath the canopy could be a contributing factor; lower shade temperatures could a be an additional factor.
2.3.2 Soil Fertility
The direct contribution of Acacia species to soil fertility is two-fold. Firstly, there is the contribution through nitrogen fixation (Table 2.3.2) and, secondly, that due to litter fall. The contribution of the latter is from the recycling of the minerals extracted from the soil by the root system (Radwanski and Wickens, 1967). The contribution in terms of nitrogen is probably minimal since the foliage on the ground probably undergoes two periods of rapid degeneration. The first is the rapid dehydration of the litter and loss of any volatile compounds, so that by the end of the long dry season the second decomposition will be of the remaining fibrous mass and minerals.
The exception is that of Faidherbida albida, where leaf fall occurs at the start of the rainy season and the litter is consequently readily decomposed and the nutrients incorporated into the soil. It has been demonstrated by Giffard, 1964) that the leaf litter from an average stand of 50 trees of A. albida per hectare returns to the soil an annual equivalent of 75 kg N. 12 kg P. 13 kg K, 20 kg S. 25 kg Mg and 120 kg Ca per hectare (Giffard, 1964). The growing of millet (Pennisetum glaucum) beneath the tree canopy can result in a 2.5- to 3-fold increase in yield without additional fertilizers.
There is also an indirect contribution from livestock browsing on the fallen litter, the decomposition of the dung following similar cycles to that of the litter. The quantitave effect of this contribution does not appear to have been evaluated.
2.3.3 Hedges, Shade and Shelter
The species of Acacia reported to be used for live and brushwood hedges, shade and shelter are recorded in Table 2.3.3. Doubtless other species can also fulfill these functions equally well. The shade value of these trees is evident from the large number of livestock crowding beneath a tree canopy during the midday heat.
Gillet (1983) comments adversely on the practice by the pastoralists of cutting large branches of such thorny species as A. nilotica, A. tortilis subsp. raddiana and A. seyal to construct zeribas (overnight brushwood enclosures for livestock); the practice is even more extensively used by agriculturalists to keep livestock off of their cultivated land. These zeribas last about two years before being destroyed by termites or even earlier by bush fires. The brushwood is either obtained by drastic mutilation of the trees or, as is often the case with the agriculturalists, from trees felled during land clearance. In many areas of the Sahel, where large areas have been cleared in an attempt to counteract the low crop yields due to desertification and low rainfall, there are no longer any trees remaining to provide the brushwood hedges. Live fences using A. mellifera and A. tortilis would be a solution, although the eratic rainfall, often compounded by problems of land ownership and protection during early establishment, do not encourage planting.
2.3.4 Wildlife Resource
The Acacia species recorded as attractive to mammals, birds, edible insects are shown in Table 2.3.4; trees browsed by wild herbivores are recorded in Table 2.2.1.1 and a source of bee food in Table 2.2.1.2; those species listed as being browsed by only livestock are doubtlessly browsed by wild herbivores but their use by them has not been recorded in the literature studied.
Management for game refuge requires a high standard of management, not only to ensure against over-grazing but also to ensure that a proper balance is maintained between the different species, especially predators and prey, as well as between the browsing animal and the vegetation strata utilized. Giraffe, for example, usually browse trees higher than 2 m (Pellew, 1983b; Prins and Jeugd, 1993). In Tanzania, Prins and Jeugd (1993) found a close correlation between rinderpest and anthrax epidemics among wild herbivores, A. tortilis recruitment and associated bush encroachment resulting from removal of grazing pressures. The authors also consider elephant-induced tree mortality as over-rated, with impala as the main culprit in preventing natural regeneration.
As already discussed in section 2.2.1.1, the degree to which browse is utilized by herbivores is species dependent, some are mainly grazers, but during the dry season balance their diet by browsing while other species, can thrive satisfactorily on a purely browse diet. This can be explained in terms of the nutritive value and intake of browse by the two groups of herbivores. Since the maintenance requirements for cattle are 0.09 MJ/kg (0.65 FU/kg) of dry matter and the nutritional value of browse varies from 0.04-0.06 MJ/kg (0.25 to 0.40 FU) per kg of dry matter, browse alone cannot maintain cattle but can maintain sheep, who require only 0.05 MJ/kg (0.35 FU/kg) of dry matter, but this browse does not allow for production. However, goats, who only require 0.03 MJ/kg (0.19 FU/kg) can obtain maintenance and production from a purely browse diet. This explains why only goats, camels and some wild herbivores are able to survive in those arid and semi-arid where browse is the main dietary intake and also explains why these animals are less susceptible to catastrophic droughts than cattle and sheep. It also follows, that for the yearlong productivity of sheep and cattle, a balance must be maintained between grazing and browsing, with browse forming 20 to 30% of the dry season diet. In West Africa a density of 25-50 trees per hectare is considered optimal in a Faidherbia albida - millet agricultural system with a canopy cover of 30-60% (Le Houérou, 1983c).
There is a significant difference between the grazing of domestic livestock and that of indigenous game animals, the former being more selective in their grazing habits as well as being less well adapted to coping with the range of vegetation available. For these reasons, Erkkilä and Siiskonem (1992), working in Namibia where bush encroachment by Acacia mellifera subsp. detiens and Dichrostachys cinerea can become a serious problem, recommend encouraging browsing by indigenous game, especially giraffe. However, it should be pointed out that bush encroachment is mainly a problem in southern Africa and is either absent or only a relatively minor problem in much of semi-arid northern Africa.
2.3.5 Amenity and ornamental value
Species reported to be planted as street trees or garden ornamentals are shown in Table 2.3.5. They appear to have rather limited popularity and their wider use is probably hampered by their relative slow growth; the often faster growing exotic species are generally preferred, especially in the Middle East.
2.3.6 Agroforestry
Agroforestry is defined as "a collective word for all land-use systems and practices in which woody perennials are deliberately planted on the same land management unit as crops and/or animals. This can be either in some form of spatial arrangement or in a time sequence. To qualify as agroforestry, a given land-use system or practice must permit significant economic and ecological interactions between the woody and non-woody components." (ICRAF, 1983).
The traditional agricultural system of rotating cultivated land with a period under bush fallow until such time as the soil has recovered its fertility is effective but slow, and is largely dependent for its speed of recovery on the natural regeneration of indigenous woody species. With an ever increasing population demanding more and more land under cultivation to provide more food, a more certain and rapid means of restoring soil fertility is required.
Agroforestry systems would appear to ensure a more rapid recovery and longer maintenance of soil fertility than under traditional fallow; with proper planning it should also offer greater protection against erosion in addition to providing a source of fuel, pole timber, wild food, browse, herbal medicines, etc. But is this necessary true for the drier regions?
The development of agroforestry systems have tended to be concentrated in the more humid tropics, where tree growth is relatively rapid and soil fertility is more speedily restored. There have been relatively few attempts to develop agroforestry in the drier regions. Kessler and Breman (1991) point out that water availability limits primary production in the northern Sahel of West Africa, with nutrient availability the critical factor in the southern Sahel and Sudanian zones. The woody species influence the water balance through rainfall inception, evapotranspiration and water infiltration as well as influencing nutrient redistribution by recycling minerals and increasing fertility. In a southwards direction the benefits of agroforestry increase more than proportionally with the rainfall. The writers conclude that the potential to increase nutrient availability through agroforestry systems in the drier regions is limited and that windbreaks would be more beneficial. In fact, Le Houérou (1989) recommends a reticulated system of windbreaks for the Peanut Basin of Senegal where increasing human pressure on the land is destroying the existing millet-Faidherbia-millet rotation. His recommended solution is for an increased integration of agroforestry and animal production in order to increase draught power and restore fertility without reducing the area cropped. This involves a perimeter box of primary windbreaks of A. senegal, or F. albida subdivided internally by secondary windbreaks of A. holosericea and further subdivided by tertiary windbreaks of fodder species, according to rainfall, Combretum aculeatum, Bauhenia rufescens or Feretia apodanthera interplanted with Ziziphus mauritiana for protection.
The traditional 'gum gardens' of Kordofan Province, Sudan were a form of agroforestry based on a rotation where the land is cropped for 4-6 years, after which it is abandoned to an Acacia senegal, bush fallow from stump coppice regrowth for 9 or more years, the gum garden is then felled end the lend again cropped (Hunting technical Services, 1964, Seif el Din 1981). Unfortunately the combination of the Sahel drought, pressure for more cultivated land, low prices received by the tappers for their gum and the high prices obtained for fuelwood have virtually destroyed the system. Attempts to reinstate the gum gardens by planting seedling trees have been only partially successful due to the number of years with lower than normal rainfall.
Similar attempts to grow Faidherbia albida in agroforestry systems in Chad have also been only partially successful due to low rainfall and internal strife. It is also possible that species provenance might be a factor (see section 4.4). F. albida is an ideal tree for agroforestry in the higher rainfall regions of the semi-arid zone, the lack of foliage during the rainy season ensures a suitable light regime for the growing crops. The incorporation of leaf litter and the faces of browsing animals into the soil at the start of the dry season without excessive desiccation are condusive to improving soil fertility, an improvement that is manifested by the enhanced yields obtained from crops grown beneath the canopy.
In the drier regions of India Acacia nilotica subsp. indica has been widely grown in agroforestry systems there have been a number of recent reports of crop yields growing below the canopy being depressed. Root competition for moisture appears to be a factor but whether high tannin compounds in the soil could also be a factor has not been investigated (Adjers and Hadi, 1993). Comparative trials should also be carried out with subsp. cupressiformis, which has a narrow canopy, to see whether shade too could be a factor since both subspecies would be expected to more or less equally compete with crops for soil moisture.
An important factor that can inhibit the progress of agroforestry systems is that of land and tree tenure; without ownership there can be little incentive for development. Where the land is owned by the state, usage by local customary rights does not necessarily grant ownership of the trees. Changes in governmental doctrine can be a further complication, especially where land has been redistributed under a communist regime and later reverts to some form of democracy, resulting in conflicting rights of ownership. A number of examples of the intricate legal problems involved are discussed in Raintree (1987). Problems vary not only from country to country but also within countries. Before starting administrators of potential agroforestry development projects must ensure that there are no land and tree tenure pitfalls involved.