Vitellaria paradoxa (C.F. Gaertn.), commonly known as the shea butter
tree is an iconic fruit tree in the dry savanna woodlands of Africa (Boffa,
1999). It is a major component of the woody flora of Sudanian regional centre
of endemism (White, 1983) contributing immensely to local
livelihoods, amelioration of micro-climate and nutrient recycling through the
decaying of its leaves and fine roots (Dianda et al.,
2009). It covers an almost unbroken belt approximately 6000x500 km from
Senegal to the northern parts of Uganda (Sanou et al.,
2006). It is a tree species of priority for African genetic resources (FAO,
1988) because of its significant economic potential for livelihood improvement
(Teklehaimanot, 2004; Sanou et
al., 2004). The fruit pulp can be eaten by both humans and animals while
the butter extracted from the seed kernel may be used for local consumption,
manufacturing body care products as well as pharmaceutical and confectionery
industries (Kelly et al., 2004a; Lamien
et al., 2007). Shea butter products are increasingly becoming popular
globally and it is envisaged that as the demands grows there will be need for
sustainable management of the shea butter tree (Teklehaimanot,
2004). When aggregated Africa has a potential of exporting about 263,000
metric ton of shea products annually however, only about 150,000 metric ton
of dry shea kernels are currently exported (Lovett, 2000).
The free on board value of shea export is estimated to be ranging in $37.5-45
million annually (ibid).
Despite its economic and ecological importance, the population of this tree
species has reduced for instance (Djossa et al.,
2008a) reported 5 trees ha-1 of this species in West Africa compared
to 15 trees ha-1 reported by Ruyssen. Chevalier reported a density
of 230 trees ha-1 in the Sudanian savannas which has reduced to 11
trees ha-1 (Nikiema et al., 2001).
According to the World Conservation Union, V. paradoxa is considered
a threatened tree species, i.e., it is vulnerable to extinction in the near
future. This has mainly been due to over exploitation for timber, firewood,
charcoal production and agricultural encroachment due to increasing population
The current decline in the population of V. paradoxa has triggered
research on this tree species in the recent years (Bayala
et al., 2008; Kelly et al., 2007;
Lamien et al., 2007). However, it is noticeable
that understanding variation of V. paradoxa population structure
with land management regimes at a wide geographical scale has not been extensively
More so, there is little information on its regeneration status (Djossa
et al., 2008b). Most of the studies that have attempted to investigate
V. paradoxa populations have been done in West Africa (Odebiyi
et al., 2004; Djossa et al., 2008a;
Raebild et al., 2007). Understanding the population
and regeneration of V. paradoxa in Eastern Africa where nilotica
sp. is endemic has not attracted the attention of researchers. It is only Okullo
(2004) who attempted to investigate its population in Northern Uganda.
In this study, we assessed the population structure and regeneration status
of V. paradoxa under different land management regimes in the
Vitellaria savannas. The specific objectives were: to determine the density
of seedlings, saplings and mature individuals of V. paradoxa under
different land management regimes; to examine the size class distribution and
regeneration status of V. paradoxa under different land management
regimes. We hypothesised that density of seedlings, saplings and mature V.
paradoxa does not vary with land management regime. We envisage that
the findings will help inform management decisions in the dry savanna woodlands
MATERIALS AND METHODS
Study sites: The study covered the districts (highest level of local
government in Uganda) of Moyo (3°37′ 41N, 31° 45′13E), Nakasongola
(1°18′ 32N, 32°27′ 23E), Lira (2°14′ 6N, 32°54′35E)
and Katakwi (1°53′ 28N, 33°57′ 58E) which are part of the
area commonly referred to as the shea belt by traders of shea butter products
(Fig. 1). The shea belt in Uganda covers an area of 29,726
km2 mainly covering North-West, North and North-East of the country
(Ferris et al., 2001). The mean annual rainfall
of these districts ranges between 500-1600 mm and they experience bimodal rainfall
regime (NEMA, 1995). The terrain is generally flat with
some plateaus in certain districts. The soils are mainly sandy, loamy and clay
with Ph ranging between 5.5-6.8 (Nabalegwa et al.,
2006; NEMA, 2000). The vegetation is dominated by
savanna woodlands with Acacia, Combretum, Vitellaria, Annona,
Erythrina, Albizia and Terminalia tree species as the most
common (Katende et al., 2000).
Studied species: V. paradoxa is a characteristic tree
species in the savanna woodlands of Africa (Hall et al.,
1996). It is endemic to the African savanna North of the equator (Maranz
et al., 2004).
It belongs to the family sapotaceae within the genus Vitellaria (with only
a single species worldwide). There two subspecies, i.e., V. paradoxa
ssp. paradoxa and V. paradoxa ssp. nilotica (Henry
et al., 1983; Hall and Hindle, 1995).
|| Map of Uganda showing the location of study sites within
the shea belt
There is no definite distinction between the two subspecies based on leaves,
flowers, fruits or morphology of mature individuals. The difference is in terms
of origin of which V. paradoxa ssp. nilotica occurs in eastern
Africa through south Sudan, northern Uganda, western fringe of Ethiopia and
north eastern parts of Democratic Republic of Congo while V. paradoxa
ssp. paradoxa ranges from eastern parts of Central African Republic westwards
to Senegal (Hall et al., 1996). V. paradoxa
ssp. paradoxa ismainly found between 100-600 m above sea level
(masl) while V. paradoxa ssp. nilotica between 650-1600
masl (Bouvet et al., 2004).
V. paradoxa is a long lived (maturing in 10-20 years) deciduous,
compact crowned tree with a height of 7-25 m (Sanou et
al., 2006). The bole is short on average ranging between 3-4 m but in
some instances, 8 m and diameter is usually <1 m (Hall
and Hindle, 1995). Large sized trees are mostly found in land under cultivation
while small sized trees are found in forests and fallows (Djossa
et al., 2008a).
It reproduces mainly sexually and most often it is insect-pollinated (Cardi
et al., 2005). It flowers in the dry season (Von
Maydell, 1986). Peak flowering season in Uganda is January to February (Okullo
et al., 2004). Fruiting of V. paradoxa may start at
10 years of age and attains full fruit production between 20 years to >50
years (Okullo et al., 2004). Ripening of fruits
takes place during the rainy season and there is still little evidence on occurrence
of fruiting cycles (Lovett, 2000). V. paradoxa
is intolerant to shade and therefore open sites especially plains are conducive
for this species. It preferably grows on colluvial soils that are deep with
free drainage and predominantly sandy-clay top soils (Hall
et al., 1996).
Land-use practices within the shea districts of Uganda: Subsistence agriculture, charcoal burning, livestock rearing and V. paradoxa fruit collection/processing are the major activities supporting the livelihoods of the local communities.
They practice subsistence agriculture characterised by two land management
regimes; continuous cultivation with annual crops such as sorghum (Sorghum
bicolour (L.) Moench), Millet (Eleusine coracana (L.) Gertn.), Beans
(Phaseolus vulgaris L.), Maize (Zea Mays L.), Ground nuts (Arachis
hypogaea L.) and fallow system where land that was previously under cultivation
is left over a period without any farming activity to allow it regain soil fertility
(Okullo, 2004). In each land management regime V.
paradoxa and other trees of economic importance are retained by farmers.
The duration of the fallow period is based on size of land and household needs.
Fallow periods of 1-5 and 5-10 years are common (District Environment Officers
pers.com). Short fallows are common in areas with high population densities
while long fallows in areas that are sparsely populated.
Areas that have been under long fallows are characterised by more dense woody vegetation than under short fallows and continuous cultivation. Land under continuous cultivation has mainly very little woody vegetation apart from economically important trees such as V. paradoxa.
Sampling design: Sampling was done in four districts (lowest local government
units in Uganda) that have sizeable areas of V. paradoxa stands.
One site was selected in each district based on presence of fallow land and
current crop fields covered by V. Paradoxa. These make up the
two most dominant land management regimes in the Vitellaria savanna farming
system (Boffa, 1999; Lovett and Haq,
2000; Okullo et al., 2004). These land management
regimes formed the treatments for this study. Fallow land was further categorised
as old or young on the basis of length of fallow period. Old fallows were sites
not cultivated for over 10 years while young fallows were not cultivated for
3-6 years. The current fields were areas covered by annual crops (at the time
Within each land management regime, four sample plots each of 50x50 m were
established using systematic random sampling (Chazdon et
al., 2005). The first plot was randomly located (Johnson
and Bhattacharyya, 2001) while subsequent plots were established systematically
at least 100 m apart. This allowed the plots to be considered as individual
sampling units (Sanou et al., 2006). Generally,
old fallows, young fallows and current fields were encountered but in Nakasongola,
only old and young fallows were encountered.
Data collection: Individuals of V. paradoxa were measured
(cm) for either stem diameter (saplings and mature trees) or root collar diameter
(seedlings/resprouts) following Rondeux (1999). Sapling
diameter was determined at the point of first branching while mature trees were
measured at breast height (at 1.3 m high) using vernier callipers. Individuals
with stem Diameters at Breast Height (DBH) ≥ 10 cm were considered mature
trees, those with DBH 6 ≤ 10 cm as saplings while those with collar diameter
of 0.1 ≤ 5 cm as seedlings (Rondeux, 1999). Plot
assessments were conducted in 2008 between October and December when the rainy
season ends in most areas of the shea belt of Uganda.
V. paradoxa population density under each land management regime:
Density was calculated as the total number of V. paradoxa ha-1
in each size class (seedlings/resprouts, saplings and mature trees) under each
land management regime following (Djossa et al.,
2008a). The average number of individuals of V. paradoxa and
standard deviation under each land management regime were derived to compare
densities of each size class. A generalized linear model analysis in GenStat
(p<0.05) was applied to determine whether land management regime influenced
the density of each size class under the different land management regime.
V. paradoxa population structure and regeneration status: The
size class distribution under each land management regime was analysed following
(Condit et al., 1998). Diameter sizes (cm) of
measured individuals were grouped into the following size classes (Lykke,
1998): 1, 2, 3-4, 5-6, 7-8, 9-10, 11-13, 14-16, 17-19, 20-23, 24-27, 28-31,
32-35, 36-39, 40-43, 44-47, 48-51, 52-55, 56-59, ≥ 60. All individuals ≥
60 cm DBH were grouped in a 10 cm wide class. This categorisation balances samples
across size classes since number of individuals declines with increasing diameter
(Condit et al., 1998). The size class distributions
were generated using the number of individuals in each size class (Ni) and the
The number of individuals in each size class (Ni) was natural log (ln) transformed
and plotted against the class midpoints to display graphically the size class
distributions under the different land management regimes (Condit
et al., 1998; Mwavu and Witkowski, 2009;
Venter and Witkowski, 2010). The Log transformation
was used to standardise densities of the different size classes. The regression
of the size class distributions was calculated with class midpoints as the explanatory
variable and number of individuals ln (Ni+1) per class as the dependent variable
The addition of 1 to Ni was necessary to cater for situations where a class
had 0 individuals (Mclaren et al., 2005). The
slopes of the regressions were used as indicators of the population structure
(Obiri et al., 2002) under each land management
regime. The slope values were derived using LINEST function in excel and interpreted
based on the types of size class distributions (Everard
et al., 1994; Mwavu Witkowski, 2009). Negative
slopes indicate regeneration since there are more individuals in the small classes
compared to the large classes (Obiri et al., 2002).
Flat distributions with a slope of 0 indicate equal numbers of individuals in
the small or regenerating classes and large size classes or mature trees and
positive slopes indicate poor regeneration with more trees found in larger classes
than in smaller size classes. Steepness of the slope was used to further describe
the regeneration status as steep negative slope indicates better regeneration
than shallow negative slopes (Lykke, 1998; Venter
and Witkowski, 2010).
RESULTS AND DISCUSSION
Population densities under different land management regimes: Seedling density was high in the young fallows compared to old fallows and current fields, respectively. Density of saplings was generally low under old fallows, young fallows and current fields. Mature tree density was relatively high under all the three land management regimes. Land management regime significantly influenced seedling density (p<0.00) but not sapling and mature tree densities (Table 1).
Population structure and regeneration of V. paradoxa under different land management regimes: The old fallows showed inverse J-shaped size class distribution across all sites except in Moyo (Fig. 2). Similarly, all young fallows had inverse J-shaped size class distribution across all sites.
There were many individuals in the smallest diameter classes and the number gradually declined in the middle and larger diameter classes in fallows. Current fields had flat size class distributions in all the sites. There were more individuals in the larger diameter classes than in the small diameter classes. The old and young fallows generally had negative slopes of the size class distributions. Within current fields the slopes of the size class distribution were positive with more trees in larger size classes than in smaller ones. Young fallows had steeper slopes (more negative) across all the sites than old fallow and current fields, respectively (Table 2).
Population densities under different land management regimes: The young
fallows had the highest seedling density followed by old fallow and current
fields respectively. This corroborates Djossa et al.
(2008a) findings in Benin where V. paradoxa seedling density in current
fields was low compared to other land uses. This could be probably because of
the farmers ignorance of the importance of protecting V. paradoxa
seedlings when cultivating. It may also be attributed to excessive cultivation
of land that has the effect of inhibiting regeneration of seedlings (Kelly
et al., 2004b). Seedling density was low in old fallows compared
to young fallows across all the sites studied except in Nakasongola.
|| Seedling, sapling and mature tree density ha-1
within the old fallow, young fallow and current fields across studied sites
|| Size class distributions of V. paradoxa in
old fallow, young fallow and current fields across sites studied
This is probably because old fallows generally had dense tree and grass cover
which may have impeded establishment of V. paradoxa seedlings
through competition for growth resources and was potentially more susceptible
to fire damage.
||Slopes of the size class distributions of V. paradoxa
under old fallow, young fallow and current fields across sites studied
According to Picasso, V. paradoxa is light demanding and therefore
its seedlings could suffer from competition for basic growth requirements like
light, water and nutrients in sites that have dense plant cover like most old
Generally V. paradoxa sapling density was very low in each of the land
management regimes across all sites; suggesting that the V. paradoxa
population risks degradation (Gijsbers et al. 1994).
This finding is contrary to Lovett and Haq (2000) who
found out that local people preferentially preserved significant saplings density
of important tree species like V. paradoxa on their land as they
cultivated. The low V. paradoxa sapling density in the fallows could
be due to the uncontrolled incessant bush burning in the Vitellaria savannas
of Uganda. According to Zida et al. (2007) and
Peterson and Reich (2001), frequent fires can prevent
sapling establishment in savanna woodlands. There was evidence of regular fires
in the all fallows indicated by burnt barks of mature V. paradoxa trees.
The extremely low V. paradoxa sapling density in the current fields
confirms studies done else where by Lovett (2000) where
farmers were found to be only interested in mature productive V. paradoxa
trees and therefore cut down saplings that interfered with their crop production.
The low V. paradoxa sapling density across all studied sites under the
different land management regime suggests that land owners may not be aware
of the value of young individuals in sustaining V. paradoxa population.
This may not immediately have a negative impact on the current adult population
but will certainly in the long run be noticeable when the adult shea trees reach
senesce and there are no young ones to replace them.
The density of mature trees was relatively high and similar under all the three
land management regimes. This is contrary to studies done elsewhere (Djossa
et al., 2008a; Odebiyi et al., 2004)
where the density of mature V. paradoxa trees was highest in the current
fields than under other land management regimes. Lack of significant variation
in mature shea tree density across the different land management regimes may
have been due to the fact that within the shea belt (especially where there
are traditional conservation regulations), all productive mature V. paradoxa
trees irrespective of the land management regimes are preserved (Lovett
and Haq, 2000).
This therefore implies that mature shea tree densities are bound to be more or less similar in areas where this tree is considered important for conservation albeit the difference in land management regimes.
Population structure and regeneration under different land management regimes:
Habitats that have abundant young individuals compared to adults have an inverse
J-shaped size class distribution which is an indicator of a healthy and stable
regenerating population (Condit et al., 1998;
Wilson and Witkowski, 2003). This was predominantly
the case in the current study where all the young and old fallows had inverse
J-shaped size class distributions with negative slopes across all sites except
the old fallow of Moyo. This implies that V. paradoxa population
and regeneration in all fallows is in a stable-healthy condition. However, the
population within the current fields had flat size class distributions with
positive slopes across all sites suggesting that there was no regeneration and
the population was unstable and degraded (Everard et
al., 1994). The regeneration is therefore discontinuous (Poorter
et al., 1996) hence maintaining a relatively stable population of
V. paradoxa in the current fields may be difficult (Lykke,
This is so because a stable population requires having more individuals in
the smaller classes than larger ones (Condit et al.,
1998). The findings from this study corroborate findings of Kelly
et al. (2004b) in Mali in which all fallows had more V. paradoxa
individuals in the lower size classes compared to current fields. According
to Kelly et al. (2004b), this could be due to
the fact that farmers rarely protect seedlings that they encounter when cultivating
crops or when they maintain the land under cultivation for a long time hence
impeding shea tree regeneration.
The slopes of the regressions of the size class distributions were steeper
(more negative) within the young fallows across all the sites except Nakasongola
than the old fallows and current fields. This is contrary to studies done in
West Africa (Kelly et al., 2004b; Djossa
et al., 2008a) where old fallows had better regeneration of shea
trees than the young fallows and current fields, respectively. This is probably
because the climatic conditions in Uganda favour rapid establishment of vegetation,
hence leaving land under fallow for a very long time may result into growth
of more competitive vegetation that may impede regeneration of V. paradoxa.
Because of this, the old fallows are bound to have poor regeneration of V.
paradoxa than young ones where it experiences less competition for growth
This study confirms that land management regimes can influence the population structure and regeneration status of V. paradoxa. Young fallows support high seedling densities compared to old fallows and current fields. Although, land management regime may not have had any influence on the sapling and mature tree density the occurrence of extremely low density of V. paradoxa saplings across all sites suggest that land owners do not appreciate the importance of saplings in maintaining a stable population. This may have long-term consequences on the stability of V. paradoxa population and in the process may lead to its collapse and local extinction. Much as land owners preserve relatively high densities of mature V. paradoxa individuals irrespective of the land management regime; young fallows may maintain more stable-healthy populations and better regeneration compared to old fallows and current fields. There is thus need for the land owners in the shea belt of Uganda to be sensitised on the importance of preserving and protecting seedlings and saplings of V. paradoxa so as to maintain a stable population especially in the current fields where their density was dismal.
Sites targeted for conservation V. paradoxa should not be left under fallow for a long time to minimise establishment and growth of more competitive vegetation that may inhibit V. paradoxa regeneration.
The researchers thank the communities who allowed us conduct fieldwork on their land. The European Union is acknowledged for funding the fieldwork under innovative tools and techniques for sustainable use of the shea tree in Sudano-Sahelian Zone Project (contract no. 032037). Researchers are equally indebted to Carnegie Cooperation of New York and the German Academic Exchange Service that provided logistical support to Patrick Byakagaba (PhD candidate) during data collection and preparation of the manuscript. The researchers appreciate the assistance rendered to us by the field assistants; Joel Buyinza, Paul Okiror, Patrick Abwango, Johny Opio, Jasper Okello, Henry Asindua, Patrick Anyama and Paul Omongin.