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Africa's shrinking savannas



Tropical savannas are arid or semiaridgrasslands with a sparse
cover of trees and shrubs. They extend across 65% of Africa's
30 million square kilometres, occupying a broad band between
the moist, equatorial rain forests and the deserts. They are
described as marginal environments, where even minimal human
intervention can produce a dramatic environmental impact.
Throughout Africa, the annual loss of savanna woodlands,
resulting in land degradation and desertification, is
estimated to be 2.3 million hectares. Some of the trees and
shrubs are cut to make  vay for cash crops, others are used
for firewood and for browse for domesticated animals by the
rural population, which is expanding at a rate of
approximately 3% per annum. Wood shortages are acute in many
areas and the situation is deteriorating rapidly as a result
of the recent severe droughts, described as the "worst in
living memory" in many parts of Africa. Wood from multipurpose
trees is traditionally regarded as a free resource by millions
of rural people, whose low incomes preclude the purchase of
alternative energy supplies, animal feed or proprietary
medicines. The plight of these people, though emphasised by
international bodies such as the United Nations Food and
Agriculture Organisation for more than a decade, is still
worsening. What can be done?

Historically, governments have turned to Forestry Departments
for solutions. Most of these departments are understaffed and
underfunded and their response to the problem has been the
increased planting of readily available stocks of exotic trees
such as Eucalyptus in managed woodlots or plantations, often
inaccessible to local people. The susceptibility of such non-
native trees to severe drought and to pests has been evident
in recent years. Further, Eucalyptus trees tend to dry the
soil and are therefore unsuitable for agroforestry projects.
Their comparatively fast growth produces high quality poles
but low density, poor quality fuel. In 1992, at an
international conference held at Victoria Falls, the consensus
of foresters was that the escalating problem of shrinking
savanna woodlands can only be reversed by the increased
planting of local trees, well adapted to drought and pests and
used by rural people for millennia. Yet because such trees are
slow growing and of no commercial importance outside Africa,
little is known of their distribution, structure, functioning
and productivity and few stocks exist.
In June 1989, European Union funding enabled a three year,
international research programme (TS2 0211) to be set up to
undertake the first detailed study of the structure,
functioning and productivity of four popular, multipurpose
African savanna species. The main aim of the programme was an
investigation of four drought tolerant tree populations at
different seasons of the year. Each population, which
contained trees of varying ages, was widely separated
geographically and was therefore likely to be genetically
distinct. Some of the mechanisms conferring drought tolerance
were determined and their relative importance evaluated. Seed
collected from the trees was used in field trials, so that
inheritable adaptations could be separated from those arising
merely as short term responses to adverse environmental
conditions. The four species chosen, all multipurpose and
popular in rural areas, belong to two commonly occurring plant
families of the savannas, the legumes and the leadwoods. The
legumes Acacia karroo (the sweet thorn) and Acacia tortilis
(the umbrella thorn) are deep rooting "pioneer" species that
can establish themselves rapidly in badly degraded areas, on
account of the nitrogen fixing bacteria in their root nodules,
which increase soil fertility. Mopane (Colophospermum mopane)
is the legume said to produce "the best firewood in Africa
"and Combretum apicularun (the red bush willow), is a
leadwood. The last two species are shallow rooting "secondary
colonisers", trees that gradually replace the pioneers to form
part of the woody component of mature savannas. All four
species though slow growing, are naturally multi-stemmed. This
means that they can be easily harvested, by coppicing or
lopping, by the women and children, the village wood
collectors. Such traditional harvesting methods involve the
cutting of a proportion of each tree every year. A renewable
source of wood is therefore assured. The EU funded programme
was carried out in Zimbabwe, where arid or semiarid savanna
cover over 60% of the country. The four experimental sites and
their mean annual summer rainfall were at Carats, S.E.
Zimbabwe (564 mm), Matopos, south of Bulawayo (250-1400 mm),
Kadoma in Central Zimbabwe (780k mm) and the Zambezi Valley in
the north of the country (750 mm). The highest annual mean
maximum temperatures were recorded from Carats (29.8 ºC)
Zambezi Valley (34.1ºC). Zimbabwean partners were based at the
Forestry Commission and the University of Zimbabwe.
Collaborating European institutes were the Westfalische
Wilhelms Universitat, Germany, the Rijksherbarium, The
Netherlands, the Royal Botanic Gardens, Kew and Imperial and
University Colleges, London.
     The results showed that several criteria known to be
indicative of drought tolerance in herbaceous plants also
apply to African savanna trees. Structurally, the leaves of
all four species are well adapted to heat and moisture stress.
Acacia leaves are covered by a thick layer of wax platelets
which reflect the sun's rays. When water stress is acute, the
trees will shed leaflets and later, small branches, to offset
moisture loss. Field studies using an infra-red gas analyses
shown that all the trees at each experimental site
photosynthesised rapidly at high light intensities. High
levels were maintained during the hottest parts of the day by
the two acacia species, whose leaves were also able to
transpire continuously at a high rate, using the soil water
available to their deep tap roots. Moisture drawn up by the
leaves for use functional mechanism was not available to the
two shallow rooting secondary colonizers. They behaved more
like trees from Mediterranean regions, cutting their carbon
dioxide intake by photosynthesis around noon, when
temperatures may exceed 40ºC. This minimised transpiration
loss through the stomata or pores on the leaf surfaces. The
effect of stress on their leaves was reduced in two other
ways.
     Mopane leaves consist of two leaflets and towards noon,
these closed up and hung down, thus reducing the surface area
exposed to the fierce rays of the sun. In both mopane and the
leadwood, leaves throughout the trees adapted to drought by
reducing the number of stomata on their more vulnerable, upper
leaf surfaces. Mopane and leadwood leaves from Carats, the
driest site, therefore had fewer stomata (35 and 16% less
respectively) than leaves from Kadoma.
     Two other functional mechanism of the trees were also
directly affected by drought. In several herbaceous species,
it has been shown that water use efficiency is correlated with
delta carbon values, obtained using powdered leaves and mass
spectroscopy. This technique measures the ratio of two carbon
isotopes, C12 and C13. Under conditions of severe water
stress, plants have to use alternative enzyme pathways,
resulting in less C13 being fixed in the products of
photosynthesis. The results shown that all the experimental
Zimbabwean savanna trees exhibited a considerable amount of
water stress when compared with samples taken from a wide
range of environments in other parts of the world. Leaf
samples analyzed from mopane and leadwood trees at Carats and
Matopos showed the highest stress levels. The two acacias were
less stressed at all sites, presumably because of the water
available to their deep tap roots. Chromatographic techniques
were used in Germany to see whether the four experimental
species were able to modify the composition of their cell sap
in response to drought. Herbaceous plants, such as  those
found on salt marshes are known to produce soluble compounds
(proline, sugar alcohols, sugars etc.) when water availability
is reduced. These protect the plant by minimising water lost
into the environment. In the three legumes, pinitol, a sugar
alcohol was found in the leaves, twigs and wood and the
amounts were greatest in the leaves during the stressed dry
season. The highest amounts were found in mopane leaf samples
>from  Carats, the driest site, and these concentrations were
highly correlated with delta carbon values obtained from the
same powdered leaf samples.
     The structural and functional modifications described
above went some way towards explaining the success of the
experimental savanna trees, particularly the secondary
colonisers, in hostile environmental conditions during the dry
winters.
Photosynthesis was maintained, but was the food produced
sufficient to sustain reasonable levels of productivity and
vindicate the selection of such trees in future replanting
schemes? All the experimental trees were cut down in June,
1991. Discs cut from the trunks were air freighted to London
and sanded, so that growth rings, if present, could be
measured. Little or no information concerning wood production
in these species was previously available. Inconclusive
results were obtained for the acacias. Their wood showed some
ring structure, but the increments were incomplete, suggesting
spasmodic growth throughout the year, as and when adequate
water was available. 
Both mopane and the leadwood produced small, measurable growth
rings during each wet season The leadwood produced wider rings
than mopane at all sites (a maximum of 3.5 and 2.2 mm
respectively). The widest rings were recorded from Carats,
where soil fertility was highest, but increments of not less
than 0.85 mm were maintained even in infertile areas covered
by saline or shallow, stony soils. Such areas, common in
degrading savannas, cannot support any Eucalyptus species.
     It has often been assumed that the calorific value of a
wood is a measure of its efficiency. Most slow growing savanna
trees produce wood of similar calorific value. So what is it
that makes mopane the "best fuelwood in Africa"? Microscopic
sections of the discs of all four species showed the bulk of
the cells to be thick walled fibres, which would burn slowly.
Many of the cells seen in the sections were packed with
crystals of calcium oxalate. But how did the crystals get into
the wood? When tree roots take up salt-rich water from the
soil, excess calcium ions, which might upset the overall water
balance and kill the tree, are removed. They are combined with
oxalic acid, a common constituent of cell sap, to form
insoluble, harmless crystal. These crystals, most numerous in
mopane, have an interesting effect upon the burning properties
of the wood. They begin to decompose at temperatures above
370ºC, producing considerable amounts of carbon dioxide, a
flame retardant. Mopane wood used on the open fires of rural
areas, where temperatures range from 300-800ºC, will therefore
produce quantities of slow burning, glowing embers and a long
lasting, hot fire. Quantities of unpalatable calcium oxalate
crystals also deter wood predators such as termites, which
often decimate exotic trees. Termite attack tends to be worse
in drought, when the trees are weakened. Savanna trees have
greater inherent stress resistance and under the driest
conditions, when evaporation greatly exceeds precipitation,
upper soil levels will be particularly salt rich and shallow
rooting species, such as mopane and Combretum apicularun, will
produce most calcium oxalate.
     Field trials, which are being carried out by the Zimbabwe
Forestry Commission, are still ongoing, as they cannot be
completed within a three year period. Industrial sponsorship
has been obtained by the Royal Botanic Gardens, Kew, so that
the trials  can be continued near Bulawayo for another two or
three years. Seed was collected from experimental trees
wherever possible. Germination test were carried out and the
optimal methods of pre-sowing seed treatment was determined.
Germination was generally good and the seedlings were kept in
the nursery for 12-18 months and planted out before the rains
in 1992. Most of the young trees, grown from seed collected
>from  all four sites, are thriving. They are being weeded but
not artificially watered. Detailed structural and
physiological measurements should ultimately provide
information concerning the inheritability of the more
important adaptations. Meanwhile, an interesting series of
laboratory experiments carried out at the University of
Zimbabwe in Harare, showed that germination of seed from
different  species and different sites varied greatly after
seed treatments simulating water and temperature stress.This 
is the first proof that some stress adaptations are
inheritable. Both the secondary colonisers were more tolerant
to both types of stress than the acacias. The leadwood seeds
tolerated drought better than mopane, whilst the latter showed
superior temperature tolerance. Different batches of mopane
seed behaved differently; those collected from the hot
Zambiezi Valley withstood the highest temperatures, whereas
those originating from the dry Carats area had superior
drought tolerance.
     What then, is the future for this type of work and what
are its economic and social benefits? This is the first time
the reasons behind the success of African trees in African
savannas have been studied. A wide range of adaptations exist,
particularly in the case of the leadwood and mopane. Some, at
least, are inheritable. More may prove to be so when the final
results of the ongoing field trial have been evaluated. These
vindicate the current thinking amongst foresters and go some
way towards explaining the superiority of native trees in the
harsh conditions characterising the marginal, tropical
savannas where the majority of rural people live. Since both
mopane and the leadwood have been used by villagers for
millennia, particularly for fuel, browse and medicinal use,
there seems little doubt that increased stocks of these and
other traditionally used species would be popular. Greater
numbers of trees with superior drought resistance would
enhance the dwindling savanna woodlands and reverse the land
degradation without causing any adverse environmental impact.
In the short term, enrichment planting of existing woodlands
could be carried out, using cuttings or seeds originating from
trees with superior drought tolerance. Longer term, breeding
programmes could enhance future stocks by selecting those
inheritable adaptations thought to be particularly desirable
for specific areas. Many villagers, increasingly short of
vital wood supplies, are planting their own trees on a small
scale. Here again, enhanced stocks would be advantageous.
Forestry and Agricultural Extension Services are widespread in
rural Africa, so distribution of stocks would be comparatively
simple.
     In the past, the main argument advanced against the
increased use of native trees has been their slow growth rate.
yet trees such as mopane maintain growth in areas such as
those covered by saline soils, where little else survives.
They are comparatively resistant to the ubiquitous termites
and their wood is dense and burns slowly, so a smaller volume
is required for each fire. In many areas, mopane is
traditionally used for charcoal, a high energy fuel which is
light to transport. The conversion rate of wood to charcoal is
6:1, so large numbers of mopane trees in natural woodlands are
continually destroyed. It may however, be economic to grow
plantations of superior trees specifically for this process.
The growth rate of native trees in the driest areas can never
be fast. The majority are however multistemmed and many
regenerate by root suckers. The frequency and extent of
traditional harvesting methods, such as lopping, coppicing and
pollarding is now being studied by some forestry departments.
Future recommendations, based on widespread trials within
Africa, may considerably enhance renewable wood supplies.
     In 1994, further EU funding enabled an international
workshop to be held in Harare to discuss future plans.
Foresters and extension workers were well represented and
several important points emerged. A great deal of basic
research on African savanna trees is still needed. Many other
popular species need to be studied. Much of the work described
above can best be undertaken outside Africa, with
sophisticated equipment which can be regularly maintained.
Once the ecology, anatomy, physiology, biochemistry and
productivity of the trees and shrubs is known, then future
long term work such as germination and field trials and
enrichment planting can be continued in Africa at low cost. If
this type of work can be expanded along the lines suggested by
Africans, then it will not be too late to reverse the
destruction of savannas and the devastating effect of this
upon millions of rural dwellers.
     For further information contact Dr. Juliet Prior, project
co-ordinator, or Dr. David Cutter, Royal Botanic Gardens, Kew,
Surrey TW9 3AB. UK.



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