A.L. McCOMB AND J.K. JACKSON
A.L. McCOMB is FAO manager and J.K. JACKSON silviculturist of a United Nations Development Programme project in operation at the Savanna Forestry Research Station, Samara, Zaria, Nigeria. This paper was prepared for the FAO Committee on Forest Development in the Tropics.
Technical and economic aspects, with special reference to Nigeria
THE AFRICAN SAVANNA has been roughly estimated to cover some 5 million square miles,¹ or approximately half the area of this large continent. These savannas occur on both sides of the equatorial rain forest. Brown (1965) describes the major savanna zones - the Sudanese semiarid transition zone, the Guinea savannas, the Miombo woodlands and the southern bushveld. A more technical treatment of the west African savannas of Nigeria is that of Keay (1953) who recognized five major zones to which the names Sahel, Sudan, north Guinea, south Guinea and derived savanna were applied. The vegetation map of Africa, issued by the Association for the Taxonomic Study of Tropical African Fauna (AETFAT) in 1959, Lists seven different savanna types under the general headings of woodland, savannas (and steppes); the Sahel zone which is not included is shown as wooded steppe.
(¹ For conversion factors into the metric system, see the end of this article.)
The term savanna is broadly, and at times rather loosely, used in Africa and seems to include a great many different vegetation types. Usually a mixture of grasses and small trees is implied. No attempt is made here to describe the various kinds of savanna; the reader is referred to extant publications. Most of the general statements made in this paper refer to the woodlands and savannas as given on the AETFAT map or to the different west African zones described by Keay, the Sahel zone excluded.
Various writers stress the roll of fire in forming and maintaining many existing savannas. [Regardless of what ecological factors give rise to savannas it should be stated here that the present size and appearance of savanna trees can give an entirely erroneous picture of the potential productivity of many sites and, in fact, of their present productivity when planted with some exotics.
Of the 5 million square miles of African savanna, about 300 000 square miles or 190 million acres, representative of all the different west African types, occur in Nigeria. This is about 85 percent of the total area of the country. Of further importance for forestry and agriculture is the present low productivity of savanna land, the poor methods of soil and vegetation management now in use, the relatively great potential for increased production both of forest and of food and fib re crops, and the rapid rate of human population growth and the prospects that population will continue to grow rapidly for some time.
TABLE 1. - ESTIMATED POPULATION NUMBERS, DISTRIBUTION AND DENSITIES IN THE NIGERIAN SAVANNA
|
Vegetation zone |
Population |
Density per sq mile |
Area |
|
Million |
|
1000 sq miles |
|
|
Sudan zone |
18.2 |
130 |
140 |
|
Guinea zones (north and south) |
10.2 |
74 |
137 |
|
Derived savanna . |
7.0 |
250 |
28 |
SOURCE: Thulin, 1966.
The rates of population increase are not known with a high degree of certainty 'but some studies and estimates suggest an annual increase of about 3 percent. It is perhaps significant for forestry that both total population and density are greater in the Sudan than in the Guinea zones while moisture and forest productivity are generally greater in the Guinea zones.
The pattern of present land-use varies with climate, soil and location of markets. Within the former northern region of Nigeria, whose boundaries correspond approximately to those of the savanna, about 16 percent of the land is in crops or fallow, 37 percent is described as forest and 46 percent as uncultivated bush pasture and wasteland; 9.2 percent of the land is in forest reserves. Efforts are being made to increase farm production and decrease costs; an extension service which will eventually include foresters has been set up.
There has been a notable increase in the past 15 years in the number of industries processing the local natural resources of the savanna region. Industries are concentrated in the principal cities on the main transportation arteries, particularly Kano, Kaduna, Jos, Saria, Sokoto and Maiduguri. Few of these, however, use much wood.
The transport system in the savanna is constantly being extended and improved, especially the construction of tarmac roads. Some secondary roads are being improved. The existing railway system has not been extended.
ECONOMICS OF PLANTATION FORESTRY
There are relatively few forest industries in the Nigerian savanna, but there are three sawmills, a match factory, a paper mill using imported pulp and a number of small furniture factories. There is a considerable production of small poles for local use. In 1964-65 2 221 tons of gum arabic (Acacia senegal) was taken from Bornu province and 1000 tons of Acacia nilotica fruits were used for tannin in Kano.
Thulin (1966) estimated the annual consumption of major forest products in the Nigerian savanna as follows:
|
|
Million ft³ |
|
Firewood |
785 |
|
Poles for houses |
22 |
|
Other roundwood in houses |
21 |
|
Sawlogs |
11 |
|
Pulpwood |
1.3 |
|
Transmission poles |
0.04 |
|
Total |
840.34 |
He also estimated the consumption of forest products in the years 1980 and 2000 based on two different sets of assumptions (Table 2). According to A, the population was assumed to increase at the rate of 2 percent per year and the rate of per caput income growth was also assumed to be 2 percent; for B both rates were assumed to be 2.5 percent.
TABLE 2. - ESTIMATED CONSUMPTION OF FOREST PRODUCTS, 1980 AND 2000
|
|
1980 |
2000 |
||
|
A |
B |
A |
B |
|
|
Million ft³ |
||||
|
Firewood |
820 |
950 |
865 |
1 230 |
|
Poles for houses . |
26 |
30 |
31 |
44 |
|
Other roundwood in houses |
21 |
27 |
21 |
35 |
|
Sawlogs |
20 |
22 |
44 |
61 |
|
Paper |
8 |
11 |
45 |
97 |
|
Transmission poles |
0. 2 |
0.3 |
1.2 |
1.8 |
|
Total |
895 |
1 040 |
1 007 |
1 469 |
TABLE 3. - ESTIMATED SIZE OF PLANTATION REQUIRED TO MEET FUTURE WOOD NEEDS, 1980 AND 2000
|
|
1980 |
2000 |
||
|
A |
B |
A |
B |
|
|
Thousands of acres |
||||
|
Firewood |
5 466 |
6 333 |
5 766 |
8 020 |
|
Sawlogs |
200 |
220 |
440 |
610 |
|
Pulpwood |
40 |
55 |
225 |
500 |
|
Total |
5 706 |
6 608 |
6 431 |
9 130 |
|
Total sawlogs and pulpwood |
240 |
275 |
665 |
1 110 |
Few savanna species are suitable for the production of sawlogs, and the only source of this produce in the savanna regions is in small areas of fringing forest. Thus, future requirements of Sawlogs and industrial wood will need to be met from plantations. Natural savanna woodland and farm trees will probably continue to supply most of the fuelwood and rough poles for house construction, though the yields are low. A fairly well-stocked area of Isoberlinia woodland produced an average yield of 270 cubic feet per acre at an estimated age of 50 years, or 5.4 cubic feet per acre per year.
The estimates of size of plantation required to meet future wood needs are shown in Table 3, though as
TABLE 4. - COST PER ACRE OF ESTABLISHING TEAK PLANTATIONS AT NIMBIA, NIGERIA
|
|
Polypot seedlings |
Stumped seedlings |
|
Land preparation |
||
|
Clearing |
12.10.0. |
12.10.0. |
|
Windrowing |
4 0.0. |
4.0.0. |
|
Burning |
6.0. |
6.0. |
|
Additional clearing |
17.0. |
17.0. |
|
Pioneer ploughing |
3. 0.0. |
3.0.0. |
|
Total, preparation |
£ 20.13.0. |
£ 20.13.0. |
|
Establishment |
||
|
Nursery stock |
4. 4.0 |
1.18.0. |
|
Planting |
1. 8.0. |
1.0.0. |
|
Seedling transport |
1. 2.0. |
0.10.0. |
|
Cultivation, 3 years . |
7.15.6. |
7.15.6. |
|
Fire tracing, 3 years . |
1. 6.0. |
1. 6.0. |
|
Total, establishment . |
£ 15.15.6. |
£ 12. 9.6. |
|
Total preparation and establishment |
£ 36. 8.6. |
£ 33. 2.6. |
|
Local overhead. |
£ 11. 0.0 |
£ 11. 0.0. |
|
Grand total |
£ 47. 8.6. |
£ 44. 2.6. |
NOTE. £1 sterling = 2.40 U.S. dollars. Costs are based on 1968 operations covering 300 acres with an average of 691 trees per acre.
The cost of establishing plantations will vary greatly with the size and density of savanna trees which must be removed in preparing the land for planting, the cost of producing and planting seedling stock, and the cost of cultivation and weed control until the canopy closes. Fortunately some good cost figures are now available for a pilot plant scheme now in its fourth year in a section of the derived savanna in the north central state of Nigeria (Barrott, 1969). These are summarized in Table 4. Teak (Tectona grandis) and gmelina (Gmelina arborea) are the species planted. The savanna trees in this locality are larger and the stands denser than those found on most sites available for planting. On one 5-acre area studied by Jackson there were 99 trees per acre, 66 percent in the 3- to 6-inch diameter class, another 19 percent in the 6- to 9-inch class and 5 percent in the 9- to 12-inch class.
In the second rotation, when clearing is not necessary, land preparation costs should be very much smaller. Thus, for the purpose of calculating financial yield, Barrott obtained an average cost per acre by subtracting £6.10.0 from clearing and £1.10.0 for sale of firewood from clearing, giving an adjustment mean of £36.2.6. for stump plants.
Using the cost of establishment of £36.2.6. per acre and Faustmann's formula, Barrott calculated a financial yield of between 10 and 11 percent. He assumed a land cost of £6.10.0. with a 50-year rotation for teak with thinnings for poles at years 6, 10, 15, 20, 25, 30 and 40.
The costs given are for one example only. They can be expected to vary considerably from site to site. For areas with smaller trees in less dense stands they could be appreciably smaller than in the above examples. Costs could also be reduced if the scale of planting was sufficient to allow the use of heavy machinery. In Zambia, in woodland heavier than at Nimbia, Allan (1966) gives a cost for clearing of £3.17.5. to £3.19.6., using D-7 tractors and chains.
Endsjo (1967) has assembled costs of establishing plantations in the Sudan zone. On former farmland, excluding overhead, first year costs were £11 per acre and second year costs £7. On savanna land, first year costs were £13 to £17 per acre; second year costs were not available. If one assumes that the second year costs on savanna are the same as on farmland and that third year costs will be £3, then total costs, excluding overhead, are about £20 for farmland and £23 to £27 for savanna land.
There is relatively little information available on volume and financial yields from savanna plantations in Nigeria. In the Sudan zone mean annual growth of neem (Azadirachta indica) for firewood and small poles on 8-year rotations varies between about 50 and 300 cubic feet per acre annually (Gravsholt et al., 1967). On some sites gmelina yields about 300 cubic feet annually and teak over 100. Eucalyptus on good sites will probably reach 400 cubic feet or more. No estimates are available for pines.
SILVICULTURE AND GENETICS
Selection of species
The species of the natural savanna woodland are of poor quality and slow growth, and it is not believed that present yields of 1 to 10 cubic feet per acre can be greatly increased. Furthermore, plantations of several of the more valuable timber species such as Khaya senegalensis, Anogeissus leiocarpus and others have not been encouraging. Thus, there has been the need to test the suitability of exotics for a diversity of uses on a variety of sites. Such trials are not new in Africa; examples of successful introductions in Nigeria include Azadirachta indica in the Sudan savanna, Gmelina arborea in southern Guinea, Tectona grandis in southern Guinea and derived savanna, and several species of eucalyptus and pine. Systematic species trials are not more than 10 to 15 years old in most African countries.
Trials of exotics have taken place in several stages. Generally, species for trial have been selected from similar climatic environments but this precept has not been rigidly followed because of the many examples of species (such as Pinus radiata) which have succeeded in environments different from their native habitat. Separate trials have been scheduled for the following six distinctly different climatic zones: Sudan, north Guinea, south Guinea, derived savanna, Jos plateau (4 000 to 4 500 feet in elevation), and the Mambilla plateau (5000 to 6 500 feet in elevation).
In Nigeria the different trial stages are called elimination, growth and plantation. The species elimination trial is designed to eliminate from further trial those species clearly unsuited to that environment. Plots are small, and replication and observation are usually completed in two or three years, though there is evidence that for some species, especially conifers, a longer period of trial would be desirable. The species growth trial provides information, for those species emerging successfully from elimination trials, on performance, growth rates, stem and crown form, agencies causing injury and death, etc. One-tenth acre plots are commonly used and replicated within one soil type. Growth trial plots are established, if possible, on three soil types representing a cross-section of soils available in that climatic zone, and when replicated in different climatic zones valuable information on site requirements and best locations for future plantations is obtained. Plantation trials of the better species emerging from growth trials give information on increment, area yield and wood quality. Plots may vary in size from 1 to 5 acres.
Following growth trials and concurrent with plantation trials provenance trials are made with the most promising species. Differences within species are often very great and sometimes as important as differences between species. These trials are replicated at each location and usually established in more than one of the six major climatic zones. They yield information on species variation and on the best sources of seed for each planting zone.
Several naturally occurring Eucalyptus hybrids have been found in Nigeria. A most promising one appears to be E. grandis x E. camaldulensis or tereticornis. In the north Guinea zone individual trees have attained diameters of 8 inches and heights over 65 feet in 5 years. On the Jos plateau 14-year-old trees are 85 to 95 feet tall and trees 40 ± years old are 120 feet. The great individual tree variation in stands of the hybrid suggests they are F2 generation plants. Hence, plantations consisting of a matrix of either E. camaldulensis or tereticornis with individual trees of E. grandis at about 60-foot intervals are being established for production of F1 generation seed. This technique is already in use in Zambia.
In Nigeria well over 100 species have gone through the species elimination trial stage, over 60 are in growth trials, nine are in plantation trials, and provenance trials are now being conducted with Eucalyptus camaldulensis, E. pilularis, E. saligna, E. microtheca, E. grandis, E. citriodora, E. maculata. E. tereticornis and E. decaisneana; also with Tectona grandis, Pinus caribaea, P. oocarpa, P. khasya and P. merkusii. Provenance trials with E. camaldulensis have revealed two very promising provenances neither of which has been tried before in Nigeria. After two years trees of these provenances have attained heights of 30 feet without irrigation, and heights of 45 feet in 18 months with irrigation in the first year. Seed orchards of promising provenances are being established. A report on the results of elimination and growth trials through April 1968 has been prepared. This report covers 89 species and one hybrid and includes short notes on the growth and silviculture of 45 species and the hybrid.
Tentative recommendations of some species for savanna plantations in Nigeria can now be made. These are based on plantings made before 1958 by foresters in the then northern regional government, the work of the Federal Department of Forest Research from 1958 to 1965 and the investigations of the Savanna Forestry Research Station since 1965. It is thought that a minimum of 10 years should elapse before reasonably accurate predictions of suitability can be made; thus another 6 years should pass before the initial Savanna Forestry Research Station plantations will be ready for a first long-term assessment. Nevertheless, based on the information now available, the following suggestions are made.
SUDAN ZONE. Azadirachta indica (neem) is well proven and generally superior to any others. Some provenances of Eucalyptus camaldulensis may be useful on selected sites.
NORTH GUINEA ZONE. A Eucalyptus hybrid (probably E. grandis x camaldulensis or tereticornis), E. citriodora, certain provenances of E. camaldulensis, and possibly E. cloeziana and E. tereticornis. Pinus caribaea and P. oocarpa, although only 4 years old, show promise.
SOUTH GUINEA ZONE. Gmelina arborea (gmelina) is a well-tested species; teak (Tectona grandis) may be used on the better sites; Eucalyptus grandis and its hybrids with camaldulensis or tereticornis.
DERIVED SAVANNA. Teak and gmelina are well-tested species; several species from the rain forest, e.g., Terminalia, ivorensis may be useful also.
JOS PLATEAU. Eucalyptus grandis hybrids, E. camaldulensis, E. cloeziana and E. robusta; Pinus caribaea, P. oocarpa and P. khasya.
MAMBILLA PLATEAU. Eucalyptus grandis and its hybrids. Several species of pines look very promising but need longer trials.
Production of nursery stock
To reduce the cost of establishing plantations requires production of low-cost nursery stock that will survive the planting operations, make quick growth and compete successfully with other vegetation on the site. Experience to date shows that for most species polypot stock is superior to stump plants. Stump plants, however, are cheaper to raise and transport to the planting site, and the method can be used successfully with teak, neem, gmelina and perhaps other broadleaved species. More work is required to determine the species and conditions under which stump plants can be successfully and economically used.
The size of polypots affects handling and transportation and thus plantation establishment costs. Experiments in the Sudan and the northern Guinea savannas of Nigeria have been made with Eucalyptus in normal size polypots and in two smaller sizes. When taken from the nursery the plants in the largest pots were taller and grew slightly better in the field. Differences in growth were not large. Additional field tests are needed.
Production of high quality nursery stock requires close attention to potting mixtures, watering regimes, fertilizers and temperatures. At the Savanna Forestry Research Station potting mixture experiments have been made with both eucalypts and pines. For Eucalyptus camaldulensis a good mixture was two or three parts of well-rotted cowdung with three or four parts of sand plus 5 lb single superphosphate per cubic yard of mixture. For Pinus caribaea a good mixture was sand and deciduous forest top soil in the proportions 1:4 or 2:3 with 28 ounces of superphosphate per cubic yard of mixture and a small amount of soil containing mycorrhizal fungi. A nitrogen supplement may be required. Further experiments are needed on soil mixtures, the best balance of nutrient elements, and levels of water for best plant development and reduction in nutrient losses through leaching.
Black polypots have proved superior in lasting qualities to natural polythene. However, temperatures in exposed black or natural pots can get very high. During late March and early April at Samaru, Nigeria, temperatures up to 128°F (53°C) have been measured just inside the black plastic while in the pot centres, at a depth of 4 inches, temperatures were as high as 122°F (50°(C). Seedlings in outside rows of beds of pots die at these temperatures. Furthermore, the mycelium of at least one species of mycorrhizal fungus will not survive temperatures over 100°F (38°C).
Plantation establishment
Land preparation. The costs of land preparation have been dealt with earlier. Work is needed to see if these costs can be reduced appreciably. To carry out such work requires the availability of a variety of very expensive equipment, so comparative trials have not been made in Nigeria.
There are numerous sites on which the soil possesses a layer of :nearly impermeable ironstone close to the surface and below which there is often good soil which will permit deep tree rooting. Ripping experiments are needed to see if these sites can be made plantable and also whether ripping of sites with hard, dense top soils will promote better growth.
Planting. Methods currently in use for planting polypot seedlings have been very successful in terms of survival and early growth. Additional research may be needed to improve stump plant survival. The application of dieldrin to combat termites which may kill young seedlings is standard practice in Nigeria. In view of the toxicity of dieldrin research on other substances and methods will be needed eventually.
Cultivation and weeding. Experience gained over a period of 10 to 15 years has emphasized the absolute necessity for control of competing plants, especially grasses, if high survival, good early growth and rapid crown closure are to be obtained in new plantations. Erroneous conclusions regarding species suitable for planting have been made because of differences in weeding. With complete weed control on Guinea zone sites the canopies of eucalypts, gmelina and teak will close in two years and Pinus caribaea and P. oocarpa on the Jos plateau and the north Guinea zone in three years.
Weeds are removed by both mechanical and hand methods. The best results are obtained with a rotavator mounted on the rear of a wheel tractor. Hand weeding in the rows is required. A tractor-drawn disk does somewhat less satisfactory work and sometimes cuts lateral tree roots. Three weedings in the first year and two in the second are usually required.
Plans are developing for experiments on the use of phytocides in controlling weeds. On steep, rocky or stump-filled lands, chemical or hand weeding are the only methods suitable. Research is needed to determine if a satisfactory control of weeds can be obtained with chemicals, how the costs of different methods or combinations of methods compare and which methods promote best tree growth.
Spacing. Spacing affects the development and rate of growth of the tree, yields of stands, the time required for crown closure, the cost of weeding, the number of plants required per acre, the time and profitability of thinning, and hence the costs of plantation establishment and their financial returns. The Savanna Forestry Research Station has laid out spacing experiments with neem in the Sudan zone, teak in the derived savanna, eucalypts on the Jos plateau and pines in the north Guinea zone. The experiments are not yet sufficiently developed to give meaningful results.
SOILS
Surveying
Plantation forestry requires knowledge of the best sites for each species and the ability to pick these sites in advance of planting. The purposes of soil surveying at the Savanna Forestry Research Station are:
(a) to describe the soils on all trial plots as one of the bases for determining species site requirements;(b) to describe, classify and map the soils on the forest reserves as a basis for siting further trials and determining the areas to be planted in later large-scale afforestation projects.
Semidetailed soil surveys in the forest reserves of the Nigerian savanna have covered 244102 acres in eight reserves in the Sudan, north and south Guinea, and derived savanna zones and the Jos plateau. The classification is essentially that of d'Hoore (1964). The most suitable soils for tree plantations are generally those deep loams and sandy loams of good drainage without a marked ironstone or plinthite layer. Soils of the Basement Complex are very variable in depth and suitability; those formed from sandstones and shales are less variable. The most fertile are derived from more recent basalts, or they are medium- to fine-textured soils from sedimentary rocks in areas of less than 30 inches of rainfall. In the north Guinea zone, with rainfall of 40 to 55 inches and a dry season lasting five or six months, a soil which permits rooting to depths of at least 10 feet is thought quite suitable for planting. Roots of Eucalyptus propinqua have been excavated to a depth of 10 feet and indirect evidence suggests root penetration of at least 14 or 15 feet with some species.
TABLE 5. AREAS OF NIGERIAN SAVANNA FOREST RESERVES IN EACH SUITABILITY CLASS
|
|
Total area |
Suitable |
Moderately suitable |
Fairly suitable |
Not suitable |
|
Acres |
|||||
|
Afaka |
26 982 |
5 666 |
3 917 |
12 007 |
5 392 |
|
Nimbia |
5 440 |
2 554 |
462 |
2 406 |
18 |
|
Sanga river¹ |
70 054 |
23 676 |
27 336 |
8 901 |
4 680 |
|
Mokwa |
15 123 |
8 881 |
5 754 |
399 |
89 |
|
Osara |
119 040 |
- |
946 |
46 722 |
71 372 |
|
Yambawa |
385 |
- |
- |
380 |
5 |
|
Gindiri-Langai |
3 271 |
684 |
306 |
805 |
1 476 |
|
Rafin Bauna N. |
2 477 |
5 |
5 |
10 |
2 457 |
|
Mongu |
1 332 |
200 |
307 |
201 |
624 |
(¹Remainder of the area not classified covers enclaves within the reserve.)
Table 5 summarizes data on the suitability for planting of areas in the surveyed forest reserves, as judged by the soil surveyor. Without sufficient site and growth data it is difficult to set up valid criteria for a more exact classification. Nevertheless, the tabular data will be useful until better criteria are developed.
One of the principal problems in surveying savanna soils for future plantation establishment is the lack of adequate indicators, visible on the ground or from aerial photographs, for delineating the areas of different soils, particularly as to depth. The site index of savanna trees is difficult, to determine because age determinations may be inexact and tree height differences confused by differences in stand density and the variable effects of shifting cultivation and fire. Differences in the composition and density of shrubby and herbaceous vegetation, of possible indicator significance, are often due to fire and/or grazing, rather than to soil and climate.
Surveying and describing soils to depths of 10 feet or more is difficult, time consuming and costly, even with hydraulic soil-coring equipment. Furthermore, soil physical properties change from wet to dry seasons. Some dense soil layers are so hard in the dry season they cannot be cored and in some sandy soils cores cannot be extracted without first wetting them
Another problem concerns how deep to go in profiling the soil. The rule should be to go as deep as tree roots will penetrate. Present information on root penetration is inadequate.
Soil physics
To grow useful trees in savanna climates with sharp and long dry seasons requires soils that infiltrate and store large quantities of available water during the wet season, and which permit root penetration and :functioning in the deepest layers. To judge how well soils meet these requirements demands knowledge of the physical soil properties and of the soil water cycle. Excess water and poor aeration create problems on some sites during the rainy season. The fertility of soils is also dependent on physical as well as chemical and biological properties.
TABLE 6. - EVAPOTRANSPIRATION FROM SOIL UNDER DIFFERENT TREE COVERS AT AFAKA, NORTH GUINEA SAVANNA
Measurements of soil moisture are being made on eight sites at Afaka in the north Guinea savanna of Nigeria using gravimetric, neutron-scattering and electrical resistance methods. These measurements, which were started in early 1968, are beginning to give a clear picture of how water moves into the soil, how much is stored in the soil profile and is available at different times of the year, and how much is lost as evapotranspiration. These studies show changes in stored moisture to depths of 20 feet and suggest that roots may be making direct withdrawals from depths of 15 feet or more. During an eight month period, from October 1968 through May 1969, evapotranspiration losses were nearly 26 inches for north Guinea savanna woodland and 23.5 inches for 11-year-old Eucalyptus citriodora (Table 6). Since total rainfall for 1968 was about 44 inches and the above losses were for eight months only and did not include the rainy months of June to September, it is evident that annual evapotranspiration losses for 1968 were equivalent to a high percentage of total rainfall. Measurements on a nearby stream showed that only 6 percent of 1968 rainfall showed up as streamflow. In the same area evaporation from an open pan evaporimeter equalled 63 inches.
The 12-year-old Eucalyptus citriodora and the 4-year old E. saligna trees continued to grow in diameter and the 2-year-old pine trees in height throughout the year although the rates in February and March were low. Soil moisture depletion curves or drying profiles showed a steady lowering of the zone of maximum soil moisture loss as the dry season lengthened. During the entire year the water tables were below 20 feet and at the peak of the dry season were probably below 30 feet.
All these data emphasize the importance of deep soils with large available water storage capacities and textures and structures favourable to early and deep root penetration. Soil depth and available moisture storage become more critical with increasing length of dry season. Some Basement Complex soils have bulk densities as high as 1.85 grammes per cubic centimetre and often have thick layers of plinthite or ironstone detrimental to survival and growth. Where soil structures have been modified to permit deep and easy rooting, as on old termite mounds, tree growth is especially good.
Research is needed on practical short- and long-term measures that can be taken to alter soil structure and density. Where annual bush fires are prevented, there is evidence of changes in surface soil structure resulting in lower bulk densities. Deep ploughing and ripping may be a means of deepening the changes in structure and incorporating organic matter in the affected layers. Protection of the surface soils from high temperatures and drying rates may affect formation of hard layers.
TABLE 7. - CHEMICAL CHARACTERISTICS OF SELECTED SOILS FROM SAVANNA FOREST RESERVES, NIGERIA
Soil chemistry and fertility
Infertile soils may be as important a cause of poor tree growth in savannas as lack of soil moisture. During the wet season the rates of growth are determined to a large degree by the soil fertility.
Evidence of the importance of fertility and the chemical properties of soils comes from many sources. Trees on or near termite mounds are often nearly twice as tall as adjacent trees and have dark green foliage. When savanna trees are cut and burned on the planting site, tree growth on the places where burning occurred is usually much superior to that on unburned areas; there are places, however, where the concentration of ash prevents the establishment of plants for several years. Trees planted on abandoned farm land often exhibit great differences in growth rates associated with past cultural and soil management treatments. Finally, there is the direct evidence from fertilizer trials.
Many of the savanna soils have very low cation exchange capacities and low levels of available nutrients, and are moderately to strongly acid (Table 7). Most are intensively weathered and leached. Repeated fires have destroyed organic matter and brought about other changes in physical soil properties affecting fertility; although large quantities of nonvolatile elements are returned to the soil by burning, some are lost by erosion and leaching. Intensive cultivation with little return to the soil of organic matter and nutrients is also apparently a cause of low fertility that explains the need for the agricultural practice of shifting cultivation.
Field fertilizer experiments made with various eucalypts, pines and teak in the Sudan, north Guinea and the derived savannas show that phosphorus, nitrogen and boron are the elements usually deficient; potassium occasionally. Shoot dieback and leaf deformities in several species of Eucalyptus on a number of sites have been corrected by adding boron. Phosphorus is probably the element most often deficient. Nitrogen, especially when applied with phosphorus, is associated with increased growth on many sites but, when applied alone, often depresses growth. On some sites total phosphorus seems adequate yet plants respond to phosphate fertilizers. Large quantities of phosphorus may be fixed due to the high iron oxide content and low pH.
There is evidence that during the latter part of the rainy season nitrogen is deficient for growth on some sites. It is suggested that nitrogen from litter of the previous dry season is quickly mobilized in the early part of the rainy season and then rapidly lost through leaching.
Observational evidence on tree growth in plantations suggests that the genetic potential in many tree species is much greater than was formerly thought to be the case, and that very large increases in growth can be obtained if the correct chemical and physical soil conditions are provided. Important problems to be solved in part by soil chemistry research include determination of the existing levels of :nutrients in soils in relation to the optimum levels and balance; factors determining the availability of phosphorus and methods of increasing availability; the elucidation of the nitrogen cycle in soils and its relation to burning; and the investigation of soil boron including critical and optimum levels and factors controlling availability.
THEE PHYSIOLOGY
It is the function of physiology to explain some of the differences in tree growth and behaviour of significance to afforestation. Areas of special interest include seed production and germination, increases in plant size, efficiency in utilizing growth-controlling factors, differentiation of tissues and organs, maturation changes and death. Physiology should explain the internal processes and the internal and external factors controlling them. Seed storage, germination and production, and factors controlling growth such as water temperature, photoperiod and nutrients are some of the problems which physiology can help to solve.
Seed storage
Where exotics are widely used and seed supplies are uncertain it becomes necessary to store seeds over a period of years. It is important to study the best storage conditions and the effects of temperature, moisture, light and different concentrations of oxygen and carbon dioxide on retention of viability and readiness to germinate.
Seed germination
Teak is a good example of a species which germinates poorly. There is need for systematic and comprehensive research on embryo and seedcoat dormancy and the factors affecting them in teak and other species.
Flowering and seed production
The ability to regulate and control flowering is of practical importance in (a) obtaining early and abundant seed production of promising species and provenances and for purposes of producing hybrid seeds (such as Eucalyptus grandis x camaldulensis), and (b) preventing or delaying development of crooked stems (such as teak) where terminal flowers interrupt the elongation of the terminal shoot.
FACTORS CONTROLLING GROWTH
Water
Seasonal deficiency of water is a major cause of slow tree growth, low yields and death from drought. At the Savanna Forestry Research Station studies are being made of soil water availability, evapotranspiration losses, atmospheric vapour pressure deficits, and temperatures in relation to internal tree water deficits and growth. These and other work on stomate functioning should help to explain differences among species as regards drought resistance and the ability to achieve a satisfactory water balance and growth in dry climates.
At Zaria in the north Guinea savanna, studies are being made of drought injury and death in three- to six-year-old trees of Eucalyptus robusta. Dye injections show differences in water conduction which may be related to stem injuries and gum formation in the cells differentiating from the cambium.
Temperature
Subtropical pines on some sites in Nigeria show growth and developmental abnormalities. These include failure of buds to break dormancy, failure of elongating shoots to develop needles, and differences in the development of lateral buds. In the last case some shoots develop an abnormal number of lateral buds while buds of others fail to develop and a " foxtail " tree results. Observation over a number of sites differing in elevation suggests temperatures as a contributing cause. Controlled environment growth chamber studies are planned to determine temperature requirements.
Photoperiod
Some of the abnormalities described above for pines originating at higher latitudes may be due to differences in the number of hours of daylight. Growth chamber studies are needed to determine photoperiod requirements.
Nutrients
Deficiency symptoms. Pot culture experiments are required to develop nutrient deficiency symptoms in various species so that these deficiencies can be recognized and corrected without resort to pot or field experiments.
Optimum levels. Foliar analyses of seedlings in pot experiments and field trials where different levels of nutrients have been applied will give information to guide plantation fertilization for maximum growth.
FOREST PATHOLOGY AND ENTOMOLOGY
If afforestation in the savanna depends on the use of exotics, outbreaks of diseases and insects are to be expected, according to experience in other parts of the world where exotics have been used or native trees planted offsite. Where pure plantations are made with native species, insects and diseases may be limiting (for example, the shoot borer of Khaya senegalensis, an important savanna tree). In the early and late stages of species trials, pathologists and entomologists are needed to identify harmful organisms, estimate their potential for limiting the use of certain tree species and, in some cases, develop control methods.
In the derived savanna and south Guinea zones of Nigeria, teak has been attacked by a butt and root rot which has caused patchwise mortality in stands, and by a disease (Stemphylium sp. is a possible causal organism) which causes dieback of stems in young stands.
Gmelina has also been observed occasionally as dying in patches. species of pines in the Guinea and derived savannas are suffering lightly from one or more needle diseases; a species of Pestalotia has been isolated. Two species of Eucalyptus in the Sudan savanna have been attacked by an unidentified disease organism.
The satisfactory inoculation of pines with beneficial mycorrhizae-forming fungi is currently being studied by Nigerian pathologists. Since seedlings are grown in polypots the fungi cannot easily spread from tree to tree. A satisfactory source of inoculum must be available and an adequate environment for growth provided. High temperatures apparently limit the growth of Rhizopogon luteolus, one fungus symbiont. When seedlings are field-planted without vigorous mycorrhizae, or with some rot of the root tips, they grow very slowly and establishment mortality is increased. Frequently the equivalent of two or more years' growth is lost before the young pines develop normal vigour.
ASSOCIATION :E on THE TAXONOMIC STUDY OF TROPICAL AFRICAN FAUNA. 1959 Vegetation map of Africa. Oxford University Press. 24 pp.
ALLAN, T.A. 1966 Plantation establishment methods in Zambia.
BARROTT, H.N. 1969 The Nimbia timber plantation project. Min. Nat. Resources and Cooperatives, N. Central State, Kaduna. 8 pp. duplicated.
BROWN, Leslie. 1965 Africa: A natural history. Hamish Hamilton Ltd. London. 299 pp.
D'HOORE, J.L. 1964 Soil map of Africa. Scale 1:5,000,000, explanatory monograph. Commission for Technical Co-operation in Africa, Publication 93. Lagos.
ENDSJO, Fer-chr. 1967 Report on the costs and economic effects of establishment of forest plantations in the savanna region of Nigeria. Savanna Forestry Research Station, Samaru. 25 pp. duplicated.
GRAVSHOLT, Sven, J.K. JACKSON and G.O.A. OJO. 1967 Provisional tables for the growth and yield of neem (Azadirachta indica) in northern Nigeria. Savanna Forestry Research Station. Research Paper 1. Samaru.
KEAY, R.W.J. 1953 An outline of Nigerian vegetation, 2nd Edition. 55 pp.
THULIN, S. 1966 Report on wood requirements in relation to plantation establishment in the savanna region of Nigeria. Savanna Forestry Research Station, Samaru. 53 pp.
Conversion factors
|
1 inch |
-= |
2.54 centimetres |
|
1 foot |
= |
30.5 centimetres |
|
1 yard |
= |
0.9 metre |
|
1 mile |
= |
1.6 kilometres |
|
1 acre |
= |
0.4 hectare |
|
1 square mile |
= |
2.59 square kilometres |
|
1 Cubic foot |
= |
0.03 cubic metre |
|
1 cubic foot per acre |
= |
0.07 cubic metre per hectare |
|
1 ounce |
= |
0.28 hectogramme |
|
1 pound |
= |
0.45 kilogramme |
|
£1 sterling |
= |
U.S. $2.40 |
