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The practical application of aerial photography for ecological surveys in the Savannah regions of Africa
Photogrammetria - Elsevier P u b l i s h i n g C o m p a n y , A m s t e r d a m - Printed in T h e N e t h e r l a n d s
T H E P R A C T I C A L A P P L I C A T I O N OF A E R I A L P H O T O G R A P H Y FOR E C O L O G I C A L SURVEYS IN T H E SAVANNAH REGIONS OF A F R I C A ~ G. E. W[CKENS tfunting Technical Services Ltd., Boreham Wood, Herts. (Great Britain)
(Received J a n u a r y 10. 1966)
SUMMARY
After dealing briefly with the resources investigation, this in the recognition of plant Examples are given of how interpretation and deduction.
use and place of an ecological paper considers the practical communities and species on additional information can be
survey in a natural problems involved aerial photographs. obtained by photo
INTRODUCTION
A great deal of work is now being carried out, especially in the undeveloped countries of the world, in assessing the natural resources available for future development. For many agricultural projects the planning has been based solely on soil survey and water availability. Such information, although essential, is often insufficient for a correct appraisal of the areas under consideration. Ecological surveys, if correctly applied, are complementary to a soil survey in that they can provide additional information on conditions at the time of the survey, also on past history and on possible future trends, as well as on soil moisture and plant relationships, forestry and grazing potential. An ecological survey should preferably be executed as part of an integrated survey of the natural environment; it should either precede or coincide with the soil survey. Sufficient time must be allowed during and after completing the soil and ecological surveys in order that such an exchange of ideas and data can take place. Ideally ecological surveys should last at least a year so as to include all the seasonal aspects of the vegetation, for often what appears to be barren wasteland during the dry season may be transformed during the rains. An ecological study may be divided into four stages. First a brief reconnaissance of the area in order to become familiar with the major vegetation V e r n a c u l a r n a m e s of plants (Sudan Arabic) see A p p e n d i x I. Photogrammetria, 21 (1966) 3 3 - 4 1
34
G. E. WICKENS
species. This is then followed by a period of intensive stereo interpretation of the aerial photographs. Both these studies should take place well before the start of the rains. Detailed field work should be undertaken during the rainy period. It should be remembered however that trees, shrubs and geophytes flower before the start of the rains. The final stages of the survey include mapping and checking of boundaries.
MAPPING PLANT COMMUNITIES
Mapping plant communities on the ground is difficult, laborious and expensive. The impression gained from a road traverse is rarely representative of an area, whereas the use of aerial photography brings the whole into a true perspective. Boundaries between plant communities may be either sharp or diffuse. Where sharp there is no difficulty as to where to draw the boundary on the photographs (Fig.l) although it may not be so obvious at first on the ground. When boundaries are diffuse there are two alternatives, either to map as a catena or to find some relationship within the zone of change with a natural feature, such as a change in dune pattern, and make an arbitrary boundary.
Fig. l. Clearly defined community boundaries. An example from 1:40,000 aerial photography of Darfur Province, Republic of the Sudan. 1 ~ Acacia seyal savannah-woodland on grey, cracking clay soils; 2 = A nogeissus leiocarpus-Balanites aegyptiaca mixed woodland on basement complex soils; 3 = Acacia albida woodland on alluvial soils. Photogrammetria, 21 (1966} 33.-41
ECOLOGICAL SURVEYS BY AERIAL PHOTOGRAPHY IN AFRICA
35
Fig.2. An intensely cultivated area near Kano, northern Nigeria. The drainage system is obvious, but no plant communities can be identified under such conditions.
For primary communities there is usually a strong correlation between the vegetation, topography and the soil. Secondary communities tend to be less restricted by natural features although for mapping purposes it may be preferable to limit them, thereby emphasising the general relationship and the exceptions. In extreme cases, as in northern Nigeria (Fig.2) the vegetation may be so debased by man's activities that a uniform fire-grazing cultivation subclimax community prevails irrespective of natural boundaries. In such cases natural features have to be used for boundaries. The plotting of boundaries of plant communities is based on the identification of the constituent species, which, presupposing good photography, is related to the scale. The scale of photography to be used may be limited by economic or time factors, which vary in direct proportion to the scale. The scale of photography must also bear some relation to the degree of detail required and the scale of mapping. Scales of 1 : 40,000 or more are perfectly satisfactory for savannahwoodland communities with a light but not continuous tree canopy, and for higher woodland and forest communities. As the tree canopy lightens, so more emphasis must be placed on the identification of individual trees, necessitating larger scale photography, perhaps 1: 20,000. Likewise vegetation in mountainous country is more difficult to identify than with similar species on flat or undulating ground and an increase in scale is beneficial. Photogrammetria, 21 (1966) 33-41
36
G. E. WICKENS
The timber producing countries of the world are mainly in the temperate and tropical forest zones. The use of aerial photography in these areas for forestry purposes is both well known and amply documented. Less is known about the problems in the arid and semi-arid areas. The two easiest mapping units to recognise on aerial photographs are riparian vegetation and vegetation patterns. The first is self-explanatory. The plotting of riparian vegetation is often of great assistance in areas where accurate topographical maps are lacking and if rock outcrops and hills are also included the vegetation map can often suffice as a topographical map.
Fig.3. Crescent shaped clumps of Terminalia brownii on Nubian sandstone soils near En Nahud, Kordofan Province, Republic of the Sudan. Observe the orientation of the clumps in relation to the drainage. Vegetation patterns are fairly widely distributed throughout the semi-arid areas of the world. They may not be readily indentifiable from the ground and often they are first recognised from the air. In some cases although recognised from the air it is still with great difficulty that they can be recognised as such on the ground. These patterns usually represent a very delicate balance between soil moisture and the vegetation, a factor that may not necessarily be obvious during the course of a soil survey. It is important that they should be recognised since the exploitation of such patterned ground can lead to disastrous erosion Photo~rammetria, 21 li966) 33 4[
t'COLOGICAL SURVEYS BY AERIAL PHOTOGRAPHY IN AFRICA
37
problems. The typical crescent-shaped grouping of Terminalia brownii near En Nahud in the Sudan Republic (Fig.3) is very easy to identify. Where the area has been exploited by overgrazing the resulting increased run-off has led to the trees dying from drought.
I DENTIFICATION OF SPECIES
Not only the grouping of species but also tonal differences between similarly shaped species may lead to their identification: Acacia senegal, for example, has a lighter crown than Acacia tortilis subsp, raddiana, although both are of similar size. Although similar in tone Acacia tortilis subsp..spirocarpa can be distinguished from subsp, raddiana by the larger crown size. Crown size relationships may of course vary with the distribution of the species so that what may be applicable in one district may not apply elsewhere. In Kordofan Province of the Sudan Republic tonal differences were expected to provide an easy means of distinguishing between Acacia seyal var. seyal and var. fistula. In practice, using only moderately good photography of 1 : 4 0 , 0 0 0 scale, no differentiation was possible between the two species, although the former had reddish black and the latter milky white bark. Certain species, such as the baobab (Adansonia digitata) may be identified by their large and characteristic shape. The palms too are readily identified although distinguishing between Borassus aethiopum and Hyphaene thebaica at 1:40,000 scale is extremely difficult and sometimes impossible. In some cases stereo interpretation reveals very little unless supported by ground observations. On the clay soils of the Sudan there is a broad transitional zone between Acacia mellifera and Acacia seyal. Both species are recognisable in the areas where they are dominant, one being a large conical shrub and the other an umbrella shaped tree. In the transitional zone the shrub develops as a small tree and appears similar in shape and tone to Acacia seyal. The time of photography is important. For the actual survey the latest photography available should be used, provided it has been taken at the correct time of the year. If the area is too large to be flown all at once then ideally it should be photographed at yearly intervals until completed so that all the photography covers the same seasonal aspects of the vegetation. This can be very important for pasture surveys. Evergreen and semi-deciduous trees are very distinctive during the dry season. The small number of such species in the semi-arid areas makes identification easy, habitat alone may suffice to determine the species. Some species are semi-deciduous on moist sites, such as alluvial soils and completely deciduous on dry soils. Ziziphus spina-christi is one such species. Other species, such as Khaya ~enegalensis, that remain evergreen throughout the year, can only survive in areas of relatively high soil moisture, such as along the banks of seasonal streams. Photogrammetria,
21 (1966) 33 4[
38
G. E. WICKENS
By contrast A c a c i a a lbida is a tree that has the unusual behaviour of shedding its leaves at the beginning of the rainy season and bearing new leaves during the dry season. Apart from its rather characteristic shape (Fig. l) this behaviour makes identification quite easy at almost any time of the year. Unless specific information is required on non-deciduous species, photography during the dry season should be avoided since the skeletal branches of the majority of tree species do not lend themselves to easy identification. Many trees and shrubs produce new foliage two or three months before the start of the rainy season. These are produced in a rapidly changing series of "autumnal" tints. The identification of many species at a distance on the ground during this period is often very difficult, from the air almost impossible. There may be occasional constituent species with a characteristic colour so that the use of infra-red or colour film may be practicable. It must be emphasised that any such distinctive colouring lasts for a short duration only and that within a limited area it is most unlikely for individual species to exhibit the same colour flush simultaneously. False Colour Infra-red Ektachrome film provides very distinct differences in colour between living and dead vegetation; living vegetation appearing as a bright magenta while dead material appears emerald green, a factor that may be used for checking on fungal diseases. Spectacular results have been obtained from swampy areas, the bright blue of the water contrasting vividly with the magenta of the trees. Such contrasts are very difficult to produce from ordinary black and white films. Although such techniques have great advantages, their cost is likely to restrict their large scale use at present. During the early rains the first grass growth is limited to the low lying areas and drainage lines, a useful period for revealing very gentle topography. Unfortunately climatic difficulties limit the practicability of aerial photography during the rain season although much useful information could be obtained from photographs taken at this time. Photography at the end of the rainy season can provide much information; trees are identifiable and the grasses are flowering, their often distinctive tonal differences due to the inflorescences making identification relatively simple. It must be remembered, however, that the grasses present at the end of the rainy season may not be those occuring earlier. Later in the year leaf fall and more important still, grass fires, make identification very difficult. Even large topographical features may be obscured by the general featureless black coloration following a grass fire.
OBTAINING ADDITIONAL INFORMATION
The information obtained so far only shows the distribution of species according to communities, the bare essentials for a forestry stock map, vegetation or pasture maps. Much more information can be both interpreted and decuded from the photographs, examples of which are taken from studies on the Kordofan and Photogrammetria,
21 ~1966) 33 41
ECOLOGICAL SURVEYS BY AERIAL PHOTOGRAPHY IN AFRICA
39
Jebel Marra United Nations Special Fund Projects in the Sudan Republic. The past and present settlements must also be considered. Evidence of past cultivation is often more readily seen on aerial photographs than on the ground. A good example is near Khuwei, where well-grown A lbizia amara subsp. sericocephala occur on the periphery of the cultivated plots and smaller trees on the now long abandoned fallow land. The reason why land has been abandoned is extremely important for future planning. In this particular example the local population was decimated during the Mahdist era, a factor that does not debar the area for consideration for future development. Similarly, on the piedmont ash soils lying to the southwest of the Jebel Marra massif there is again much evidence of former occupation. The area has now been abandoned to the nomadic grazier yet the soils and water resources are good. Local history reveals that the Fur inhabitants were driven from the area during the Mahdist period and have not yet reinhabited the area. The age and condition of the vegetation is in agreement with the historical evidence and the regenerating species conform to the expected climax type so that on historical and ecological grounds the area is suitable for future development. A further example is the northeastern flanks of the Jebel Marra massif where there are remains of very extensive stone terracing. To-day water is very scarce in this particular area yet the ruins suggest quite a large population in the past. It is possible that the area might have been used for rainy season cropping only and that the inhabitants moved elsewhere for the dry season, a system that is practiced by the local Fur people to-day. Is is more probable that the extensive soil erosion in the area has resulted in a drying up of the water sources and that the area has been abandoned as a consequence. The vegetation is certainly a more xerophytic type than would be expected from the rainfall, and for such an example the ecological evidence would debar the area from consideration for future economic development. On the high plateau of the Jebel Marra massif there are extensive areas of rolling grassland with very few trees or shrubs. The first impression is that because of altitude the area must be unsuitable for afforestation. An examination of the aerial photographs reveals that a very large proportion of the massif has been occupied by man in the past. From a comparative study of the existing vegetation on this and other mountain areas it is concluded that man, aided by fire, has destroyed the trees. It is therefore possible that large scale afforestation may be practicable. The intensity of present day cultivation does not necessarily indicate good soils. In Kordofan very intensive cultivation is being practiced in an area of sands of low fertility receiving as little as 250 mm rainfall, good supplies of drinking water being the factor that encourages the high population density. The intensity of cultivation can be assessed from the proportion of fallow to cultivated plots. If the density of cultivated plots appears excessive in relation to the existing fallow areas, then soil erosion and poor grazing may be expected. Photogrammetria,
21 (1966) 33 41
40
(i. I:. WICKENS
For grassland management studies useful information may be obtained from photographs as to the location of watering points. In the case of streams and standing pools the date of the photography will possibly give a guide to seasonal availability. The plotting of major and minor stock tracks can assist in determining the movement of nomadic graziers. Major tracks will generally converge on some strategic watering point or market while a profusion of minor tracks may suggest a good grazing area. The absence of tracks is most likely to indicate a lack of water or poor grazing, possibly areas of unpalatable grasses such as Cymbopogon spp. It could also be due to disease or possibly some legislative reason; in Kordofan one such ungrazed area marks the no-mans land between two major Baggara tribes (G. Poulsen, personal communication, 1965). Future trends, generally in terms of possible soil erosion, are of interest to forester, grazier and agriculturalist, The past and present history may, as in northern Kordofan, show that extensive occupation has resulted in an increasing area of derived grassland devoid of trees except for remote areas. The lack of seed bearing trees and the present extent of very intensive cultivation would appear to indicate an irreversible change to grassland which may be likely to occur in the not too distant future. Already there are indications of recent dune creeping in certain areas, i.e., desert conditions are beginning to occur in an area that is capable of supporting thorn savannah. The climate has not changed but the microclimate has altered. A further example is the change in tree dominance from Anogeissus leiocarpus or Terrninalia brownii to Albiza amara subsp, sericocephala following a decrease in soil moisture due to increased run-off rather than to a major climatic change. The skilled interpreter can see and deduce much of this information from the photography and with the minimum of field work.
CON C LUSION
The author has endeavoured to show how an ecological survey can be of use in an appraisal of natural resources. The use of aerial photography for this work saves many months of laborious field work and provides a more accurate and detailed map than would be otherwise possible. The writer is grateful for the advice of his colleagues in writing this paper, although much of the information is based on personal experience.
Photogrammetria.
21 {1966) 33 41
41
ECOI,OGICAL SURVEYS BY AERIAL PHOTOGRAPHY IN AFRICA APPENDIX VERNACULAR
I NAMES OF PLANTS
Botanical name
Vernac,lar name (Suda, Arabic)
Acacia albida Acacia melliJera Acacia se,e~al Acacia seyal Acacia seyal vat. fistula Acacia tortilis subsp, raddiana Acacia tortil/s subsp, spirocarpa Adanso,ia di~,itata A Ihizia amara subsp, sericocephala Anogeisstts leiocarp,s Borasstts aethiopttm C y m b o p o g o , spp. Hyphaene thehaica Khaya seneL, alensis Terminalia hrownii Zi~iphtts spitta-christi
haraz kitr
hashab tahl hamra tahl abiad
sayyal saml"
tebeldi (baobab arad sahab deleib mahareib dora
universal name)
mklrFaya
subagh nabbak, or sidr
Photogrammetria. 21 (1966) 33 41
T H E P R A C T I C A L A P P L I C A T I O N OF A E R I A L P H O T O G R A P H Y FOR E C O L O G I C A L SURVEYS IN T H E SAVANNAH REGIONS OF A F R I C A ~ G. E. W[CKENS tfunting Technical Services Ltd., Boreham Wood, Herts. (Great Britain)
(Received J a n u a r y 10. 1966)
SUMMARY
After dealing briefly with the resources investigation, this in the recognition of plant Examples are given of how interpretation and deduction.
use and place of an ecological paper considers the practical communities and species on additional information can be
survey in a natural problems involved aerial photographs. obtained by photo
INTRODUCTION
A great deal of work is now being carried out, especially in the undeveloped countries of the world, in assessing the natural resources available for future development. For many agricultural projects the planning has been based solely on soil survey and water availability. Such information, although essential, is often insufficient for a correct appraisal of the areas under consideration. Ecological surveys, if correctly applied, are complementary to a soil survey in that they can provide additional information on conditions at the time of the survey, also on past history and on possible future trends, as well as on soil moisture and plant relationships, forestry and grazing potential. An ecological survey should preferably be executed as part of an integrated survey of the natural environment; it should either precede or coincide with the soil survey. Sufficient time must be allowed during and after completing the soil and ecological surveys in order that such an exchange of ideas and data can take place. Ideally ecological surveys should last at least a year so as to include all the seasonal aspects of the vegetation, for often what appears to be barren wasteland during the dry season may be transformed during the rains. An ecological study may be divided into four stages. First a brief reconnaissance of the area in order to become familiar with the major vegetation V e r n a c u l a r n a m e s of plants (Sudan Arabic) see A p p e n d i x I. Photogrammetria, 21 (1966) 3 3 - 4 1
34
G. E. WICKENS
species. This is then followed by a period of intensive stereo interpretation of the aerial photographs. Both these studies should take place well before the start of the rains. Detailed field work should be undertaken during the rainy period. It should be remembered however that trees, shrubs and geophytes flower before the start of the rains. The final stages of the survey include mapping and checking of boundaries.
MAPPING PLANT COMMUNITIES
Mapping plant communities on the ground is difficult, laborious and expensive. The impression gained from a road traverse is rarely representative of an area, whereas the use of aerial photography brings the whole into a true perspective. Boundaries between plant communities may be either sharp or diffuse. Where sharp there is no difficulty as to where to draw the boundary on the photographs (Fig.l) although it may not be so obvious at first on the ground. When boundaries are diffuse there are two alternatives, either to map as a catena or to find some relationship within the zone of change with a natural feature, such as a change in dune pattern, and make an arbitrary boundary.
Fig. l. Clearly defined community boundaries. An example from 1:40,000 aerial photography of Darfur Province, Republic of the Sudan. 1 ~ Acacia seyal savannah-woodland on grey, cracking clay soils; 2 = A nogeissus leiocarpus-Balanites aegyptiaca mixed woodland on basement complex soils; 3 = Acacia albida woodland on alluvial soils. Photogrammetria, 21 (1966} 33.-41
ECOLOGICAL SURVEYS BY AERIAL PHOTOGRAPHY IN AFRICA
35
Fig.2. An intensely cultivated area near Kano, northern Nigeria. The drainage system is obvious, but no plant communities can be identified under such conditions.
For primary communities there is usually a strong correlation between the vegetation, topography and the soil. Secondary communities tend to be less restricted by natural features although for mapping purposes it may be preferable to limit them, thereby emphasising the general relationship and the exceptions. In extreme cases, as in northern Nigeria (Fig.2) the vegetation may be so debased by man's activities that a uniform fire-grazing cultivation subclimax community prevails irrespective of natural boundaries. In such cases natural features have to be used for boundaries. The plotting of boundaries of plant communities is based on the identification of the constituent species, which, presupposing good photography, is related to the scale. The scale of photography to be used may be limited by economic or time factors, which vary in direct proportion to the scale. The scale of photography must also bear some relation to the degree of detail required and the scale of mapping. Scales of 1 : 40,000 or more are perfectly satisfactory for savannahwoodland communities with a light but not continuous tree canopy, and for higher woodland and forest communities. As the tree canopy lightens, so more emphasis must be placed on the identification of individual trees, necessitating larger scale photography, perhaps 1: 20,000. Likewise vegetation in mountainous country is more difficult to identify than with similar species on flat or undulating ground and an increase in scale is beneficial. Photogrammetria, 21 (1966) 33-41
36
G. E. WICKENS
The timber producing countries of the world are mainly in the temperate and tropical forest zones. The use of aerial photography in these areas for forestry purposes is both well known and amply documented. Less is known about the problems in the arid and semi-arid areas. The two easiest mapping units to recognise on aerial photographs are riparian vegetation and vegetation patterns. The first is self-explanatory. The plotting of riparian vegetation is often of great assistance in areas where accurate topographical maps are lacking and if rock outcrops and hills are also included the vegetation map can often suffice as a topographical map.
Fig.3. Crescent shaped clumps of Terminalia brownii on Nubian sandstone soils near En Nahud, Kordofan Province, Republic of the Sudan. Observe the orientation of the clumps in relation to the drainage. Vegetation patterns are fairly widely distributed throughout the semi-arid areas of the world. They may not be readily indentifiable from the ground and often they are first recognised from the air. In some cases although recognised from the air it is still with great difficulty that they can be recognised as such on the ground. These patterns usually represent a very delicate balance between soil moisture and the vegetation, a factor that may not necessarily be obvious during the course of a soil survey. It is important that they should be recognised since the exploitation of such patterned ground can lead to disastrous erosion Photo~rammetria, 21 li966) 33 4[
t'COLOGICAL SURVEYS BY AERIAL PHOTOGRAPHY IN AFRICA
37
problems. The typical crescent-shaped grouping of Terminalia brownii near En Nahud in the Sudan Republic (Fig.3) is very easy to identify. Where the area has been exploited by overgrazing the resulting increased run-off has led to the trees dying from drought.
I DENTIFICATION OF SPECIES
Not only the grouping of species but also tonal differences between similarly shaped species may lead to their identification: Acacia senegal, for example, has a lighter crown than Acacia tortilis subsp, raddiana, although both are of similar size. Although similar in tone Acacia tortilis subsp..spirocarpa can be distinguished from subsp, raddiana by the larger crown size. Crown size relationships may of course vary with the distribution of the species so that what may be applicable in one district may not apply elsewhere. In Kordofan Province of the Sudan Republic tonal differences were expected to provide an easy means of distinguishing between Acacia seyal var. seyal and var. fistula. In practice, using only moderately good photography of 1 : 4 0 , 0 0 0 scale, no differentiation was possible between the two species, although the former had reddish black and the latter milky white bark. Certain species, such as the baobab (Adansonia digitata) may be identified by their large and characteristic shape. The palms too are readily identified although distinguishing between Borassus aethiopum and Hyphaene thebaica at 1:40,000 scale is extremely difficult and sometimes impossible. In some cases stereo interpretation reveals very little unless supported by ground observations. On the clay soils of the Sudan there is a broad transitional zone between Acacia mellifera and Acacia seyal. Both species are recognisable in the areas where they are dominant, one being a large conical shrub and the other an umbrella shaped tree. In the transitional zone the shrub develops as a small tree and appears similar in shape and tone to Acacia seyal. The time of photography is important. For the actual survey the latest photography available should be used, provided it has been taken at the correct time of the year. If the area is too large to be flown all at once then ideally it should be photographed at yearly intervals until completed so that all the photography covers the same seasonal aspects of the vegetation. This can be very important for pasture surveys. Evergreen and semi-deciduous trees are very distinctive during the dry season. The small number of such species in the semi-arid areas makes identification easy, habitat alone may suffice to determine the species. Some species are semi-deciduous on moist sites, such as alluvial soils and completely deciduous on dry soils. Ziziphus spina-christi is one such species. Other species, such as Khaya ~enegalensis, that remain evergreen throughout the year, can only survive in areas of relatively high soil moisture, such as along the banks of seasonal streams. Photogrammetria,
21 (1966) 33 4[
38
G. E. WICKENS
By contrast A c a c i a a lbida is a tree that has the unusual behaviour of shedding its leaves at the beginning of the rainy season and bearing new leaves during the dry season. Apart from its rather characteristic shape (Fig. l) this behaviour makes identification quite easy at almost any time of the year. Unless specific information is required on non-deciduous species, photography during the dry season should be avoided since the skeletal branches of the majority of tree species do not lend themselves to easy identification. Many trees and shrubs produce new foliage two or three months before the start of the rainy season. These are produced in a rapidly changing series of "autumnal" tints. The identification of many species at a distance on the ground during this period is often very difficult, from the air almost impossible. There may be occasional constituent species with a characteristic colour so that the use of infra-red or colour film may be practicable. It must be emphasised that any such distinctive colouring lasts for a short duration only and that within a limited area it is most unlikely for individual species to exhibit the same colour flush simultaneously. False Colour Infra-red Ektachrome film provides very distinct differences in colour between living and dead vegetation; living vegetation appearing as a bright magenta while dead material appears emerald green, a factor that may be used for checking on fungal diseases. Spectacular results have been obtained from swampy areas, the bright blue of the water contrasting vividly with the magenta of the trees. Such contrasts are very difficult to produce from ordinary black and white films. Although such techniques have great advantages, their cost is likely to restrict their large scale use at present. During the early rains the first grass growth is limited to the low lying areas and drainage lines, a useful period for revealing very gentle topography. Unfortunately climatic difficulties limit the practicability of aerial photography during the rain season although much useful information could be obtained from photographs taken at this time. Photography at the end of the rainy season can provide much information; trees are identifiable and the grasses are flowering, their often distinctive tonal differences due to the inflorescences making identification relatively simple. It must be remembered, however, that the grasses present at the end of the rainy season may not be those occuring earlier. Later in the year leaf fall and more important still, grass fires, make identification very difficult. Even large topographical features may be obscured by the general featureless black coloration following a grass fire.
OBTAINING ADDITIONAL INFORMATION
The information obtained so far only shows the distribution of species according to communities, the bare essentials for a forestry stock map, vegetation or pasture maps. Much more information can be both interpreted and decuded from the photographs, examples of which are taken from studies on the Kordofan and Photogrammetria,
21 ~1966) 33 41
ECOLOGICAL SURVEYS BY AERIAL PHOTOGRAPHY IN AFRICA
39
Jebel Marra United Nations Special Fund Projects in the Sudan Republic. The past and present settlements must also be considered. Evidence of past cultivation is often more readily seen on aerial photographs than on the ground. A good example is near Khuwei, where well-grown A lbizia amara subsp. sericocephala occur on the periphery of the cultivated plots and smaller trees on the now long abandoned fallow land. The reason why land has been abandoned is extremely important for future planning. In this particular example the local population was decimated during the Mahdist era, a factor that does not debar the area for consideration for future development. Similarly, on the piedmont ash soils lying to the southwest of the Jebel Marra massif there is again much evidence of former occupation. The area has now been abandoned to the nomadic grazier yet the soils and water resources are good. Local history reveals that the Fur inhabitants were driven from the area during the Mahdist period and have not yet reinhabited the area. The age and condition of the vegetation is in agreement with the historical evidence and the regenerating species conform to the expected climax type so that on historical and ecological grounds the area is suitable for future development. A further example is the northeastern flanks of the Jebel Marra massif where there are remains of very extensive stone terracing. To-day water is very scarce in this particular area yet the ruins suggest quite a large population in the past. It is possible that the area might have been used for rainy season cropping only and that the inhabitants moved elsewhere for the dry season, a system that is practiced by the local Fur people to-day. Is is more probable that the extensive soil erosion in the area has resulted in a drying up of the water sources and that the area has been abandoned as a consequence. The vegetation is certainly a more xerophytic type than would be expected from the rainfall, and for such an example the ecological evidence would debar the area from consideration for future economic development. On the high plateau of the Jebel Marra massif there are extensive areas of rolling grassland with very few trees or shrubs. The first impression is that because of altitude the area must be unsuitable for afforestation. An examination of the aerial photographs reveals that a very large proportion of the massif has been occupied by man in the past. From a comparative study of the existing vegetation on this and other mountain areas it is concluded that man, aided by fire, has destroyed the trees. It is therefore possible that large scale afforestation may be practicable. The intensity of present day cultivation does not necessarily indicate good soils. In Kordofan very intensive cultivation is being practiced in an area of sands of low fertility receiving as little as 250 mm rainfall, good supplies of drinking water being the factor that encourages the high population density. The intensity of cultivation can be assessed from the proportion of fallow to cultivated plots. If the density of cultivated plots appears excessive in relation to the existing fallow areas, then soil erosion and poor grazing may be expected. Photogrammetria,
21 (1966) 33 41
40
(i. I:. WICKENS
For grassland management studies useful information may be obtained from photographs as to the location of watering points. In the case of streams and standing pools the date of the photography will possibly give a guide to seasonal availability. The plotting of major and minor stock tracks can assist in determining the movement of nomadic graziers. Major tracks will generally converge on some strategic watering point or market while a profusion of minor tracks may suggest a good grazing area. The absence of tracks is most likely to indicate a lack of water or poor grazing, possibly areas of unpalatable grasses such as Cymbopogon spp. It could also be due to disease or possibly some legislative reason; in Kordofan one such ungrazed area marks the no-mans land between two major Baggara tribes (G. Poulsen, personal communication, 1965). Future trends, generally in terms of possible soil erosion, are of interest to forester, grazier and agriculturalist, The past and present history may, as in northern Kordofan, show that extensive occupation has resulted in an increasing area of derived grassland devoid of trees except for remote areas. The lack of seed bearing trees and the present extent of very intensive cultivation would appear to indicate an irreversible change to grassland which may be likely to occur in the not too distant future. Already there are indications of recent dune creeping in certain areas, i.e., desert conditions are beginning to occur in an area that is capable of supporting thorn savannah. The climate has not changed but the microclimate has altered. A further example is the change in tree dominance from Anogeissus leiocarpus or Terrninalia brownii to Albiza amara subsp, sericocephala following a decrease in soil moisture due to increased run-off rather than to a major climatic change. The skilled interpreter can see and deduce much of this information from the photography and with the minimum of field work.
CON C LUSION
The author has endeavoured to show how an ecological survey can be of use in an appraisal of natural resources. The use of aerial photography for this work saves many months of laborious field work and provides a more accurate and detailed map than would be otherwise possible. The writer is grateful for the advice of his colleagues in writing this paper, although much of the information is based on personal experience.
Photogrammetria.
21 {1966) 33 41
41
ECOI,OGICAL SURVEYS BY AERIAL PHOTOGRAPHY IN AFRICA APPENDIX VERNACULAR
I NAMES OF PLANTS
Botanical name
Vernac,lar name (Suda, Arabic)
Acacia albida Acacia melliJera Acacia se,e~al Acacia seyal Acacia seyal vat. fistula Acacia tortilis subsp, raddiana Acacia tortil/s subsp, spirocarpa Adanso,ia di~,itata A Ihizia amara subsp, sericocephala Anogeisstts leiocarp,s Borasstts aethiopttm C y m b o p o g o , spp. Hyphaene thehaica Khaya seneL, alensis Terminalia hrownii Zi~iphtts spitta-christi
haraz kitr
hashab tahl hamra tahl abiad
sayyal saml"
tebeldi (baobab arad sahab deleib mahareib dora
universal name)
mklrFaya
subagh nabbak, or sidr
Photogrammetria. 21 (1966) 33 41