CHAPTER VI
PARKLAND PRODUCTION LEVELS

Production of parkland trees

As forest resources decline and demand for some forest products increases, people tend to rely more on obtaining desired products from trees growing on farmland. This trend has led to both the active regeneration of agroforestry parklands (Bergeret and Ribot, 1990; Joet et al., 1998; Cline-Cole et al., 1990) and their degradation (Gijsbers et al., 1994; Lericollais, 1989) in different semi-arid West African locations. Similar patterns of reliance on farmland trees appear elsewhere in Africa and Asia (Holmgren et al., 1994; Falconer, 1990; FAO, 1995). In all cases, parklands, and resources outside forests more generally, have the potential to meet the demand for a variety of forest products, and can sometimes offer higher production of specific commodities than natural forests.

The sustainable management of parklands is therefore of increasing importance, and should begin with a thorough knowledge of parkland growth rates and production levels, including wood, fruit and foliage, as well as factors affecting them. This knowledge is also necessary, especially for forestry planners, in order to define the percentage of supply to be expected from these systems as compared with other forest systems and the degree to which they can meet community needs for these products.

Fig. 6.1 Weighing pod production of a single Parkia biglobosa tree, Thiougou, Burkina Faso.
J.-M. Boffa

In addition, as harvest levels exceed the ability of a species to regenerate, a process of domestication can act to supplement wild resources (Chapter 4). As a result, several aspects related to production need to be elucidated. These include measuring the variation in desirable traits in parkland species, establishing whether this variation is of genetic or environmental nature, and deciding on the most appropriate interventions needed to improve production (phenotypic selection, silvicultural interventions or a combination of both). Knowledge of production patterns will be instrumental in efforts to estimate and match actual and potential supply and demand of parkland products, and in supporting decisions about appropriate improvement levels. Production data are also necessary for cost-benefit evaluations of trees in agroforestry parkland systems (Chapter 7).

A large body of data shows that agroforestry parklands represent a major production site, not only in number but also in quantity of non-timber forest products (NTFPs). In villages bordering the Monts Mandingues Classified Forest in Mali, 63 and 51 percent of NTFPs recorded in the study originated from fallow lands and crop fields respectively (Gakou et al., 1994). As expected from its proximity, the largest proportion (90 percent) of NTFPs also originated from the natural forest. In the Bassila region of Benin, 12 of the 16 main marketed NTFP species were found in fields and fallows as well as, in some cases, their original savanna and forest vegetation types. Fields and fallows greatly outweighed forests and savanna zones in terms of the volume of NTFPs used (Schreckenberg, 1999). In the North Central Peanut Basin of Senegal, fields and fallows provided two-thirds of collected grasses, medicinal plants, and service wood and about 75 percent of fuelwood, fruit, and fodder, while the remainder was collected in unexploited zones of the rural landscape, mostly in bottomlands (Seyler, 1993). Whenever possible, this chapter provides references focused on parkland locations including fields and fallows, but references to forest products which do not differentiate between parkland and other types of forest lands are also included.

Parkland products can be classified into a number of major use categories such as: food, fodder, fuelwood, construction materials, wood for tools and utensils, medicinal items and other products. Many plant parts serve multiple purposes. Rather than attempting to quantify the contribution of agroforestry parklands in each use category, the following sections review information on production of the various parts of parkland trees, including fruit, foliage, gum and wood.

Fruit production

Flower and fruit production in parkland trees vary more than foliage production between species, between individuals of the same species, and from year to year (Breman and Kessler, 1995). Only for a few parkland species is detailed information available on fruit production. Data for V. paradoxa and F. albida are presented in Tables 6.1 and 6.2 respectively. Pod production in P. biglobosa, reviewed in Hall et al. (1997), ranges from 15 to 130 kg/tree. A few general production figures are provided for species producing commonly eaten fruit in Booth and Wickens (1988) and von Maydell (1983). These include:

• Balanites aegyptiaca: 100–150 kg/mature tree

• Boscia senegalensis: 3.9 kg/ha

• Moringa oleifera: 1 000 pods/tree

• Ziziphus mauritiana: 80–130 kg/tree in Africa

• Tamarindus indica: 150–200 kg/tree

• Bombax costatum: up to 1 500 capsules (fibre)/tree.

Suprisingly, few production data exist for several important parkland fruit species, such as Adansonia digitata, Borassus aethiopum, Hyphaene thebaica, Lannea microcarpa, Sclerocarya birrea, Strychnos spinosa and Ximenia americana.

Fruit production in parklands varies between individuals of the same species and fluctuates greatly on an annual basis. For instance, Dunham (1990) reported an inter-annual variation from 5.4 to 290 kg in a given F. albida individual. The average nut production of 50 V. paradoxa trees varied by a factor of five in two consecutive years in southern Burkina Faso (Desmarest, 1958). Also in Burkina Faso, Boffa et al. (1996a) found that 40–50 percent of trees in a natural Vitellaria population contributed only 15 percent of the total stand production, while the top 20–25 percent of the population produced over half of total yields and about 15 percent of trees were consistently high producers.

Table 6.1 Fruit production in Vitellaria paradoxa

Notes:
^a Numbers in parentheses were derived from weight ratios (fresh nut = 0.5 fresh fruit; dry nut = 0.6 fresh nut; dry kernel = 0.7 dry nut) provided by Ruyssen (1957).
^b Measured in 1994 and 1995, two years of relatively high production.

Table 6.2 Pod production in pruned and unpruned Faidherbia albida trees

Annual variations in Vitellaria fruit production are believed to follow cycles of two or more years, yet a relationship with climatic parameters has not been clearly identified. Early initiation of flowering due to higher minimum temperatures during the flowering period may contribute to higher fruit production (Desmarest, 1958). Fluctuations may also result from differential success in pollination. Thus only 25 percent of hermaphrodite flowers produced fruit due to a combination of physiological factors, lethal genes and lack of fertilization which were particularly pronounced in late or inaccessible central flowers of Vitellaria (Guinko et al., 1988, cited in Serpantié, 1997a). A combination of a large number of flowers, synchronized flowering (favouring pollination), high humidity and good site quality resulted in exceptionally high yields in Burkina Faso (Serpantié, 1997a).

Potential fruit production tends to increase with tree size. Vitellaria starts producing at age 15–20; yields rise rapidly and become significant around age 40–50, and start declining only after 200 or 300 years of age (Ruyssen, 1957). A significantly positive relationship between tree diameter and number of nuts produced was found in Thiougou (Boffa et al., 1996a) as well as in the five-year Katibougou data presented by Ruyssen (1957). High production in Vitellaria appears to be associated with specific tree form and high foliage density, which may be genetically controlled. No relationship between tree size and pod yield in F. albida could be established in Watinoma, Burkina Faso, where most trees are pruned and yield variability was very high. In contrast, pod production could be accurately predicted by a regression equation based on crown surface area and tree height in nearby Dossi (Depommier and Guérin, 1996). In Mali, fruit production of Acacia raddiana was correlated to tree diameter on two types of site quality (Cissé, 1983).

Pruning affects production considerably. In F. albida, high-intensity pruning reduces fruit yields in the following year by a factor of two to ten (Table 6.2). The effect of plant parasites (Tapinanthus spp.) on fruit production has also been documented but not quantified (Boussim et al., 1993a, 1993b).

Site conditions also influence yields of fruit. The lack of nutrients available for fruit development may explain spontaneous flower abortion and inter- and intraspecific variation in fruit production (Breman and Kessler, 1995). As soil conditions improve, standing biomass and average plant height increase, as may fruit production. For instance, Acacia raddiana pod yield in trees of a given size was twice as high on deep as on shallow soils (Cissé, 1983). While pruning drastically reduced F. albida fruit production in bottomland locations in Watinoma, on upland sites the initially low yields were not greatly affected by pruning. This finding was attributed to differences in site conditions, along with the possible late effect of pruning in previous years (Depommier and Guérin, 1996). Results of fertilization and weeding experiments for Vitellaria have not been conclusive (Picasso, 1984).

Farmers and researchers often claim that yields of Vitellaria and Parkia trees found in the bush are lower than those of trees in cultivated fields, yet this appears to depend on fallow age, management, and soil type. In Burkina Faso, yields of Vitellaria trees in fields and fallows less than ten years old located on deep fertile soils did not differ significantly (Serpantié, 1997a). However, plant competition in older fallows is expected to reduce fruit production. In the Bassila area of Benin, estimated yields of small and large V. paradoxa were respectively low and high, and not significantly different in bush or field conditions. In contrast, medium-sized trees (28–37 cm in dbh) produced significantly more in fields than in the bush (Schreckenberg, 1996). Regular weeding and a light, early fire (or grazing) lead to higher fruit production, but cannot explain all annual variability.

Fig. 6.2 Juicy fruit of Lannea microcarpa are an energy-rich snack for children in the dry season, Thiougou, Burkina Faso.
J.-M. Boffa

In conclusion, fruit production in parkland trees is highly variable within species and between years as a result of genetic and several environmental factors. This variation is reflected in the various production assessments available. Compounding this variability are the logistical constraints of measuring resources which are highly valued by people and livestock and used on a daily, sometimes open-access basis. Research to establish reliable, standardized participatory assessment techniques for parkland tree production would be beneficial. Consistent with the low intensity of parkland tree management, there is generally limited knowledge of the factors governing fruit production. As domestication of parkland species proceeds, additional research is needed on intraspecific variation, the reproductive biology of parkland species and the influence of climatic and edaphic factors on reproductive success and productivity. Irrigation may be instrumental in assessing the role of soil water status at the beginning of the dry season. Fertilization and weeding experiments could also be pursued.

Foliage production

The measurement of leaf production in agroforestry parklands is important because of the role of leaves as food for people, browse and fodder as well as a litter component for soil nutrient cycles.

Foliage produced decreases as a proportion of standing biomass with increasing tree age and size (Breman and Kessler, 1995). Small young plants have the highest ratio of foliage production to standing biomass. This ratio can reach up to 10 percent in the Sahel zones as compared with about 5 percent in the Sudan zones. In contrast, large mature trees have the highest absolute foliage yield. Foliage production is lower in years of low rainfall, and higher in bottomland sites due to a higher availability of nutrients and water.

Foliage production in trees has mostly been studied in natural ecosystems in the Sahel which may not always qualify as parklands. Studies reviewed in Breman and Kessler (1995) give an order of magnitude for the leaf production per tree for the following species: Acacia senegal - 1 kg; Acacia laeta - 0.8 kg; Acacia seyal - 2.9 kg; Balanites aegyptiaca - 0.8 kg; Combretum ghasalense - 4.5 kg; Commiphora africana - 0.9 kg; Grewia bicolor - 2.1 kg; Guiera senegalensis - 0.2 kg; Pterocarpus lucens - 3.9 to 6.3 kg; Sclerocarya birrea - 14.3 kg.

Fig 6.3 Borassus aethiopum leaves are collected for use in basket-making.
R. Faidutti

More information is available for F. albida than other parkland species. Average foliage production in five F. albida trees with diameters of 40–110 cm was 58 kg/tree or 3 percent of standing biomass in Senegal (Jung, 1969). Miehe's (1986) estimate was 20–50 kg/year for pruned trees in Koronga, Ethiopia.

Insights on foliage production in F. albida were also provided by artificial pruning experiments consisting of 100 percent crown reduction in Watinoma and Dossi, Burkina Faso (Depommier and Guérin, 1996). Foliage production during a one-year interval between two such prunings ranged from 3 to 22 kg/tree for trees with a dbh of 16–48 cm and 15 to 40 kg/tree for trees with a dbh of 48–80 cm. Post-pruning leaf biomass was similar to or higher than pre-pruning in Watinoma and 20–40 percent lower in Dossi. In the same diameter classes, production differences between upland and bottomland sites within village locations were small, whereas they were more pronounced but not statistically significant between villages. There was a higher leaf production and a higher increase in leaf production per unit of crown area from one year to the next (thus also regrowth vigour) in Watinoma than in Dossi. This was primarily related to larger canopies in Watinoma, where pruning had been a more frequent and intense practice, despite similar trunk diameters. Leaf production per unit of crown area on upland sites in both years was slightly lower in the smaller diameter class than in the higher one, whereas both diameter classes had similar foliage yields per unit of crown area on bottomland sites. A second pruning after a year increases the ratio of foliage production to total (leaf and twig) production, especially in younger trees.

Leaf production data in P biglobosa are lacking, despite its good fodder properties. The species has high coppice shoot regrowth potential, with leaf production of 1.2 and 3 kg per tree at 8- and 16-week cutting intervals in 7-year old trees in the humid zone of Nigeria (Sabiiti and Cobbina, 1992). Based on litter collection baskets under four V. paradoxa (mean dbh 42 cm) and four Bombax costatum (mean dbh 39 cm) trees in Thiougou, southern Burkina Faso, leaf production was estimated at 29 and 23 kg per tree respectively (Bambara, 1993) or over 500 kg/ha based on local densities. Information on leaf production is lacking in many other parkland trees which are widely used in human diets, such as Adansonia digitata, Cadaba farinosa, Celtis integrifolia, Crateva adansonii, or as animal food such as Acacia spp., Pterocarpus erinaceus, Pterocarpus lucens and many others. More focus has been placed on the nutrient concentration and digestibility of foliage in tree species of the Sahel. Such information is reviewed in several places, including Le Houérou (1980) and Breman and Kessler (1995).

Fig. 6.4 Gum arabic oozing from an Acacia senegal tree, Barkedji, Senegal.
P. Danthu

Gum production

Several semi-arid African tree species produce gum. The most well-known species is Acacia senegal (commonly known as gum arabic). Estimates of gum production in this species vary, fluctuating between 0.1 and 1 kg, (maximum of about 10 kg in Sudan), and 0.25 kg in a highly stocked stand (von Maydell, 1983). A similar but inferior gum, tahl gum, is produced by Acacia seyal. Sterculia setigera also produces a gum which resembles the high quality gum karaya, derived from Sterculia urens in India, but information on production per tree is not available.

Wood production

Wood production data in semi-arid West Africa are relatively scarce, especially for parkland populations. This may be due to the fact that in past decades wood production schemes focused on plantations rather than parklands. Wood production may not be a primary objective of parklands, but this depends on species and farmers. Nevertheless, because time expenditure is a central concern in wood collection, the proximity of fields and fallows facilitates gathering and these locations can provide a significant share of wood used for fuelwood, tools and construction needs. Substantial quantities of wood are made available when fields are cleared. It is also obtained from fallows, uncultivated woodlands, tree prunings and occasional tree felling in fields.

Fig. 6.5 Wood gathered to build a drying rack for millet, Holom, northern Cameroon.
C. Bernard

Data on annual (stem and branch) wood productivity in unprotected woody plant communities of West Africa with annual rainfall from 500 to 1 600 mm were first reviewed by Clément (1982) and expressed (WP in t/ha/yr) in relation to rainfall (R in mm) by the equation WP = 0.05129 + 1.08171 (R/1000)². In West Africa, values range from 0.32 t/ha/yr for 500 mm rainfall to 1.61 t/ha/yr for 1 200 mm rainfall. Wood production is increased by a factor of 1.25 with protection against fire and grazing. Clément's curve appears to hold and is still used today (Bellefontaine et al., 1997). As noted earlier, standing biomass and average plant height increase with improved soil conditions. Thus wood production is higher in bottomland locations (Breman and Kessler, 1995). As a percentage of total tree biomass, wood production decreases with increasing tree age and varies according to species.

There are isolated estimates of wood production in parkland trees. In parklands west of the town of Kano, northern Nigeria, average standing timber volume (cylinder estimated from tree height and dbh) was 0.56 m³/tree or 8.9 m³/ha, not including Adansonia digitata trees which have large boles of poor burning quality. Further away from Kano, timber volume was 9.5 m³/tree and 108.5 m³/ha in parklands with much larger trees and a lower density (Cline-Cole et al., 1990). Most of these estimates of standing biomass were higher than those of surrounding uncultivated woodlands. Felling a F. albida tree provided 5–6 m³ of fuelwood in Zinder (Lemaître, 1954, cited in Felker, 1978). When managed in 10-year rotations, Acacia raddiana yields 80 to 100 kg/tree in India (von Maydell, 1983). Standing wood biomass of parkland trees in southern Mali was estimated according to dbh using a volume curve established in a project for the inventory of forest resources in the country (Bagnoud et al., 1995b). It ranged from 0.5 m³ for trees in the 20 cm dbh class to 6.5 m³ for trees in the 70 cm dbh class.

Figures of standing tree biomass/ha in parklands are often far higher than figures for woodlands and forests in various degrees of degradation and underline the potential of parklands as a major wood supply source. Survey data in Kenya revealed that the standing volume of woody biomass found within conventional forests, including both state-owned indigenous forests and forest plantations, is lower than the volume outside (Holmgren et al., 1994).

Estimates of the annual wood production available for harvesting appear to vary according to calculation methods, as well as site productivity, density of woody species, and harvesting intensity. Based on projections of parkland density change over the next 50 years rather than measurements of harvested quantities, annual wood production was calculated at 0.15–0.2 m³/ha with V. paradoxa and P. biglobosa densities of 9 to 17 trees/ha in Mali (Bagnoud et al., 1995b). Because it is pruned more frequently and intensely, estimates are higher for F. albida. Jensen and Koné (1982, cited in Miehe, 1986) estimated that harvestable wood in F. albida parklands with 40–60 trees/ha amounts to 1.8–4.7 m³/ha according to rainfall and tree age in Senegal. Given a weight of 200 kg/m³, this is equivalent to 360–940 kg/ha. Wood made available from total crown reduction of F. albida trees through pruning depends on tree diameter. In Burkina Faso, wood production was 30–40 kg for trees with dbh of 16–48 cm, and 80–130 kg for trees 48–80 cm in diameter, giving an overall average of 0.2–0.3 m³ per tree. Taking into account local tree density and pruning intensity, production of pruned wood was estimated at 100–200 kg/ha/yr (Depommier, 1996a). With a 20–35 percent volume reduction, pruning of F. albida in an Ethiopian study site gave 0.4–0.5 m³ per mature tree on a 4–5 year rotation, or about 0.1 m³/tree/yr (Poschen, 1986). Compared with the apparent extent of parklands' use for wood procurement, there are few production data, especially for parkland species often cited for fuelwood or other wood uses, such as Anogeissus leiocarpus, Acacia spp., Azadirachta indica, Balanites aegyptiaca, Combretum spp., Crossopteryx febrifuga, Diospyros mespiliformis, Guiera senegalensis, Pterocarpus spp., Prosopis africana and Tamarindus indica.

Wood quantities to be harvested from parklands appear to play a significant role in farm needs. Wood consumption in rural areas is estimated at around 270–310 kg/person/yr for fuelwood (Ernst, 1978) and around 30–100 kg/person/yr for wood for tools and construction (März, 1986) or roughly 1 800 to 2 400 kg/yr for a family of six. As estimated by Depommier (1996a), a production of 100–200 kg/ha/yr from an average farm of 2 ha would cover at least 10–20 percent of the annual fuelwood needs of a local farm household. However, the thorough evaluation of parklands' contribution to overall household wood needs would require additional quantitative studies.

Species information regarding appropriateness for fuelwood, charcoal or construction use, indigenous knowledge and scientific evaluation of characteristics of fuelwood species are not reported here but can be found in a variety of references including Cline-Cole et al. (1990), von Maydell (1983) and Rocheleau et al. (1988).

Summary

Agroforestry parklands represent a major production site, not only in number but also in quantity, of non-timber forest products. Except for Faidherbia albida and Vitellaria paradoxa, the availability of data on fruit production is limited. Determined by a combination of genetic and environmental factors, yields vary substantially on an annual basis and between individuals of the same species. Potential production increases with size and, in F. albida, is heavily depressed by pruning. Given the high nutrient requirements of fruit, tree productivity depends on site conditions, as well as management practices, phenology and climatic patterns, which need to be defined more clearly.

Foliage production displays a lower variability between species, between individuals of the same species and between years than fruit production. Its proportion relative to standing biomass decreases with increasing tree age and size. Regular pruning of Faidherbia albida appears to have more impact on leaf production than site differences. Leaf production data in parkland species are limited and fragmented. Annual wood production in woody plant communities in West Africa is fairly well established and is favourably influenced by protection from fire and grazing, and soil fertility. Standing woody biomass of individual parkland trees is estimated at 0.5–10 m³/tree, depending on tree size. At a national level, woody biomass in parklands can be significantly higher than in uncultivated woodlands due both to a higher standing biomass per hectare and to a larger surface area. Annual wood harvests in parklands may cover only a small part of household wood requirements but are significant because of their proximity.