Regeneration pattern of four threatened tree species in Sudanian savannas of Burkina Faso

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Agroforestry SystemsAn International Journal incorporatingAgroforestry Forum ISSN 0167-4366 Agroforest SystDOI 10.1007/s10457-012-9505-9

Regeneration pattern of four threatenedtree species in Sudanian savannas ofBurkina Faso

Amadé Ouédraogo & AdjimaThiombiano

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Regeneration pattern of four threatened tree speciesin Sudanian savannas of Burkina Faso

Amade Ouedraogo • Adjima Thiombiano

Received: 25 April 2011 / Accepted: 27 February 2012

� Springer Science+Business Media B.V. 2012

Abstract Research was carried out on four threa-

tened tree species across north and south Sudanian

phytogeographical zones in Burkina Faso. A quanti-

tative inventory of regeneration populations was

completed and sapling dynamics were monitored

through surveys of survival and growth in permanent

plots. Laboratory and nursery experiments were

carried out to test germination and the performance

of transplanted seedlings. Results revealed difficulties

that saplings faced during their development. In

natural stands of Sterculia setigera and Afzelia afri-

cana, saplings were rare or irregularly distributed. A

greater number of individuals of Bombax costatum

were observed, which was the only species with

significant density variation between phytogeograph-

ical zones (p = 0.021). Size class showed irregular

population structures. A high mortality between the

first and second year, followed by a stabilization trend,

was observed. The initial height of saplings was highly

correlated to their survival rate. The rate of new

sapling spontaneous recruitment was better in Bombax

costatum and Boswellia dalzielii stands, but heir

growth was unpredictable in all species. Most saplings

grew from lignotubers and root suckers; however, the

potential for sexual reproduction was good. Sterculia

setigera and Bombax costatum seedlings showed

potential for use in silviculture, as evidenced by the

fact that both showed high post-transplantation rates

of survival and growth.

Keywords Germination � Growth �Population dynamics � Regeneration � Tree species

Introduction

The semi-arid Sahelo-Sudanian zone of Africa, which

is one of the world’s dryland ecosystems, is subject to

major impacts from human activities (Rasmussen et al.

2001; Sop et al. 2010). When humans alter the

landscape, as they do through cropping or ranching,

the system become more vulnerable to climatic

variability, particularly droughts (Nicholson et al.

1998). Weak and irregular precipitation causes stress

to woody plants, whose regeneration then becomes

unpredictable. Indeed, these ecosystems experience

dramatic changes in rainfall, which affect vegetation

growth, habitat and human livelihoods (White et al.

2002). For example, annual rainfall has declined since

the 1970s in the Sahel (Agnew and Chappell 1999) and

subsequent droughts have caused high mortality in

many woody species in northern Burkina Faso

A. Ouedraogo (&) � A. Thiombiano

Laboratoire de Biologie et Ecologie Vegetales,

UFR Sciences de la Vie et de la Terre, Universite de

Ouagadougou, 09 BP 848, Ouagadougou 09,

Burkina Faso

e-mail: [email protected];

[email protected]

A. Thiombiano

e-mail: [email protected]

123

Agroforest Syst

DOI 10.1007/s10457-012-9505-9

Author's personal copy

(Ganaba et al. 1998). Because many other species are

associated with these trees, the shift in the structure of

their woodlands has large-scale community conse-

quences (Mueller et al. 2005). The drought has

sometimes affected other areas of the country, which

usually have sufficient rainfall. Some tree species

exhibit intrinsic adaptation strategies of growth and

survival to face dryness (Weber et al. 2008). The

impact of human activities, through extensive agri-

culture and livestock farming, also negatively affect

woody plant dynamics (Ouedraogo et al. 2006b).

Saplings are the most vulnerable stage in the woody

plant population, as these individuals are less sturdy,

particularly in the recruitment stage (Ouedraogo et al.

2006c). Seedlings and young shoots are the first to

wither, and they are also more exposed to livestock

damage, water stress and fire. For many woody species

in savannas, recruitment during the rainy season is

abundant, but very few individuals live through the dry

season (Gijsbers et al. 1994; Setterfield 2002). In the

long term, disturbance factors lead to the selection of

well-adapted species, which in consequence leads to

the reduction of diversity due to the regression of

sensitive species. For example, studies of woody plant

recruitment in the Cerrado savannas of Brazil have

revealed that frequent fires have caused a shift in

species composition, favouring those species that can

propagate vegetatively (Hoffmann 1998). A similar

situation exists in Africa savannas, where vegetative

propagation of woody species is favoured (Zida et al.

2007; Bognounou et al. 2010). Assessing woody

species diversity along a climatic gradient in eastern

Burkina Faso, Ouedraogo (2006) found that the

sapling layer is less rich than the adult one, introducing

a dynamic trend of diversity loss. In addition, those

woody saplings that tolerate semi-arid conditions are

trapped in a continual cycle of growth, then cessation,

and then re-growth (Ouedraogo et al. 2006b). The

advantage of this adaptation is ‘‘self-conservation’’,

rather than the rejuvenation of ageing populations.

This study aims at analysing the structure of the

regeneration population in natural stands and docu-

menting the juvenile individuals’ behaviour as well as

the potential for reproduction by seeds of four tree

species. Such research will contribute to understand-

ing the sapling response to environmental conditions,

which is helpful in supporting future adequate

approaches for conservation of these threatened

woody species. This investigation is especially

critical, as according to Zida et al. (2008), monitoring

of sapling field performance has received very little

attention in Africa, especially in west Africa.

Materials and methods

Study species

Four tree species were chosen, which are Afzelia

africana Sm. (Fabaceae), Bombax costatum Pellegr. &

Vuillet (Malvaceae), Boswellia dalzielii Hutch. (Burs-

eraceae) and Sterculia setigera Delile (Malvaceae).

These species have a high economic value in the west

Africa Sahelo-Sudanian zone, and especially in Burk-

ina Faso (Guinko and Pasgo 1992; Mertz et al. 2001;

Lykke et al. 2004), but they are also threatened (Belem

et al. 2008; Mbayngone and Thiombiano 2011). They

have a wide distribution range across the Sudanian

zone, with ageing population (Sinsin et al. 2004;

Ouedraogo 2006).

Afzelia africana is among the largest trees in the

savannas whose northernmost limit in Burkina Faso is

the south Sahel, at about 13�N (Terrible 1984), where

it is sparsely distributed (Ouedraogo 2006). The

species is mainly used for timber, firewood and forage

(leaves). In the Sudanian zone, its natural stands

exhibit ageing populations (dhp = 35–50 cm) with

many individuals damaged by livestock and pruning.

A similar situation was observed in Benin—to the

southeast of Burkina Faso—where uncontrolled util-

isation leads to the degradation and reduction of its

habitat and populations (Sinsin et al. 2004). Direct

sowing techniques were unsuccessfully tested in the

field as an alternative to relying upon weak natural

regeneration (Bationo et al. 2001a). In natural pro-

tected areas, it is not unusual to observe cohorts of

seedlings underneath parent plants during the rainy

season, but their survival rate is almost nil.

Bombax costatum is a common tree in the Sudanian

zone, where its natural stands are sometimes large

with 6–10 trees per 1,000 m2. Populations are char-

acterised by the predominance of individuals with

5–15 cm-diameter stems in the south Sudanian zone,

while 25–40 cm-diameter trees are most frequent in

the north Sudanian one (Ouedraogo 2006). This

species is mainly used for food and timber. It is

threatened by its apparently poor regeneration (Ou-

edraogo et al. 2006b), and also by the intensive

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harvesting of its flowers for domestic and commercial

use as a vegetable (Belem et al. 2008).

Boswellia dalzielii is a gregarious tree that is widely

distributed and prefers to grow on rocky hills. The

natural groves are quite open, with 6–9 individuals per

1,000 m2, dominated by trees with 20–40 cm-diame-

ter stems (Ouedraogo et al. 2006a). The bark of

Boswellia dalzielii is used for medicinal purposes. It is

declining despite ample fruit production. The inten-

sive de-barking causes high mortality in natural

populations.

Sterculia setigera is a sparsely distributed tree,

rarely forming significant populations. The species

occurs widely, but without an apparent habitat pref-

erence. The few natural populations are rather open

with 4–5 trees per 1,000 m2. Most trees have stems in

the 25–40 cm-diameter class, reflecting old popula-

tions (Ouedraogo 2006). The species is pruned for

forage and also cut for timber. Its natural stands are

characterised by a lack of regeneration, despite ample

fruit production.

Study area

Field work was carried out in eastern Burkina Faso

across two phytogeographical zones, the north Suda-

nian and south Sudanian (Fontes and Guinko 1995).

This area is situated in the Sudanian regional centre of

endemism (White 1983). The rainy season lasts from

May to October, and temperature variation is limited

(Fig. 1). The mean annual rainfall (1974–2004) varies

from 805.30 ± 187 mm to 862.46 ± 158 mm.

According to the FAO classification (WRB 2006),

common soil types in the study area include luvisols,

lixisols, leptosols, cambisols, vertisols and gleysols,

which are common across most of eastern Burkina

Faso.

The vegetation varies from steppes in the north, to

tree and bush savannas in the south. The herbaceous

layer is dominated by grasses, while the woody

vegetation is dominated by shrubs. The region is a

livestock-farming and agricultural area, with exten-

sive land use. Livestock farming is on the increase, and

for over 85 % of farmers natural pasture is the main

source of fodder (Ministere des ressources animales

2004). The extent of cultivated land has also increased,

as cotton cultivation and internal immigration has

expanded.

Field surveys

Field work consisted of inventorying and monitoring

regeneration individuals of each species in natural

stands across the phytogeographical zones. Regener-

ation individuals refer to saplings, with main stems

less than 5-cm diameter at 1.30-m height. Inventories

were carried out in 18–33, 5 9 5 m2 plots depending

on the species (Table 1), randomly chosen within

stands of trees where regeneration individuals were

counted and their height was measured. The monitor-

ing was carried out within five plots, each of 25 m2, at

five different sites per species and for both phytogeo-

graphical zones. The initial stock of saplings in the

plot at the beginning of the survey were marked with

paint and observed for 24 months.

Monitoring was carried out at four periods: begin-

ning of the rainy season (June); early end of rainy

season (end of September); beginning (November)

and end (May) of dry season. The following param-

eters were recorded four times a year: number of

Fig. 1 Ombrothermic

diagrams of the main

localities in each

phytogeographical zone.

Data are from the period

1974 to 2004 (sourceNational Office of

Meteorology, Burkina Faso

2006)

Agroforest Syst

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surviving individuals; number of new recruitments

(these were subsequently marked with a different

colour paint); height and sturdiness of saplings. The

origin of saplings (seedlings, root suckers, lignotubers

or coppice) was distinguished by gently digging

around the base of the trunk. Sapling density was

calculated and the height class distribution was

computed for each species.

Laboratory and nursery trials

Experiments consisted of germination tests which

were carried out for each species using seeds from

different provenances (10 populations 9 3 trees per

population = 30 trees) in each phytogeographical

zone. The experimental design was randomised blocks

with ten repetitions of ten seeds (from the composite

samples of 30 trees) per species. Seeds were sown

without any pre-treatment, in pots filled with a

substrate consisting of two-thirds sand and one-third

humus. Each pot received 250 ml of water daily.

Conditions in the nursery were similar to those of the

surrounding environment (temperature = 25–38 �C;

humidity = 79.5 % ± 11.24). The number of seeds

that germinated or died was recorded daily. Ten

seedlings were marked at germination, and both their

height and number of leaves were recorded every

2 days for a period of 30 days to assess their initial

growth. Afterwards, they were transplanted into larger

pots (70-cm tall 9 25-cm diameter) containing topsoil

to assess their response. Survival rate, diameter and

height growth were measured, and vitality was

visually assessed.

Data analysis

The means of regeneration population parameters

(density, recruitment, survival and growth rates) were

calculated for species in each plot and compared

according to the phytogeographical zones and species.

The growth performance of saplings in natural stands

was compared according to the regeneration mode.

For the germination, both latent period of germi-

nation (LPG, number of days before the first germi-

nation) and speed of germination (SG, number of days

for 50 % germination) were calculated. The increase in

mean height, diameter and number of leaves of

seedlings was calculated to assess the post-germina-

tion and post-transplantation growth performance. For

each parameter, test of equal variance was performed

and multiple comparison test of Tukey was applied to

the series of data through one-way analysis of variance

(ANOVA). The Pearson correlation coefficient was

calculated to measure the degree of linear relationship

between variables for some growth parameters. Sta-

tistical analyses were performed with Minitab 14.

Results

Regeneration populations’ structure

Saplings were rare or absent in Sterculia setigera and

Afzelia africana natural stands. On the other hand,

populations of Bombax costatum and Boswellia dalzi-

elii often contained saplings. Across all inventories,

sapling density showed considerable heterogeneity

according to stands (apart from B. costatum in the north

Table 1 Sapling density per plot and mean number of stems per individual (mean ± SD) in natural stands

Species Number of plots Sapling density (number of individuals/25 m2) Number of stems per individual

NS SS NS SS p-value NS SS p-value

Afzelia africana 16 11 0.93az ± 0.29 1.55ay ± 0.49 0.12 1.15ax ± 0.10 1.09ax ± 0.09 0.66

Bombax costatum 19 14 2.93bx ± 0.31 6.64ax ± 2.10 0.021 1.34ax ± 0.08 1.14bx ± 0.04 0.010

Boswellia dalzielii 12 8 1.92ay ± 0.43 1.88ay ± 0.69 0.96 1.18ax ± 0.08 1.25ax ± 0.16 0.69

Sterculia setigera 12 6 0.75az ± 0.25 0.83az ± 0.31 0.83 1.21ax ± 0.26 1.06ax ± 0.17 0.62

p-value 0.023 0.011 0.112 0.139

Mean values of parameters were compared (ANOVA) for each species according to the phytogeographical zone (lines) and between

species per phytogeographical zones (columns). Values with the same letter are not significantly different at p = 0.05 (Tukey’s test).

First letter is for comparison between the columns and second letter for comparison between the lines. SD standard deviation, NSnorth Sudanian phytogeographical zone, SS south Sudanian

Agroforest Syst

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Sudanian zone). Significant difference in sapling

density between phytogeographical zones was found

only for B. costatum (p = 0.021). This species showed

the highest densities in both north and south Sudanian

zones, whereas S. setigera showed the lowest densities,

although it was not different from the one of A. africana

in the north Sudanian zone (Table 1). The tendency of

saplings to branch at the base was limited. Individuals

rarely had more than one stem, and the mean number of

stems did not vary significantly in species according to

phytogeographical zones, except for B. costatum

(p = 0.010). However, the number was constantly a

little higher in the north Sudanian zone (Table 1).

The height size distribution demonstrated irregular

population structures, characterised by large gaps

between size classes and the notably small number of

individuals in each class (Fig. 2). In general, the

recruitment class (0–0.5 m) had the highest number of

individuals. These population structures reveal prob-

lems of recruitment, survival and growth of saplings.

Only B. costatum showed a relatively good

regeneration population structure with the five height

classes well represented in both north and south

Sudanian zones (Fig. 2). B. dalzielii had a better

structure in the south Sudanian zone.

Regeneration population dynamics

The mean densities of the initial stocks of saplings in

the permanent plots are shown in Table 2. They were

significantly different between phytogeographical

zones for all species, with higher density for B.

costatum in the south Sudanian zone and lower one for

S. setigera in both zones. The saplings of S. setigera in

both zones died or their number decreased consider-

ably from the first to the second year. The maximum

survival rate did not exceed 45.3 %, which was the one

of B. dalzielii’s natural stands in the south Sudanian

zone (Table 2). Across the second and the third years,

survival rates did not vary considerably. There was a

high correlation (r = 0.85, N = 98) between sapling

initial height and their survival rate.

Fig. 2 Height size

distribution of saplings. The

proportion of individuals

from global inventory is

represented per size class for

each species in the north

Sudanian (NS) and the south

Sudanian (SS) zones

Agroforest Syst

123


Recruitment in the permanent plots is illustrated by

the cumulative mean number of new saplings in the

stands over 3 years (Fig. 3). A higher recruitment

number was observed for B. costatum in both north

and south Sudanian zones (11.8 ± 3.4 & 16.4 ± 2.7).

A. africana and S. setigera showed the lowest

recruitment rates (no recruitment for S. setigera in

north Sudanian zone). Sapling growth in the perma-

nent plots was unpredictable, as illustrated by the

curves in Fig. 4 (not estimated for S. setigera because

of the lack or weakness of saplings). Increases in

height were irregular across the 3 years of monitoring.

During the rainy season (June–September), height

increased, but it decreased during the dry season

(November–May) due to drought, grazing and

bushfire.

Natural regeneration modes and saplings’

responses to environmental disturbances

The modes of natural regeneration were seedlings,

lignotubers and root suckers. The proportion of each of

these modes of recruitment varied significantly

according to species, but rarely according to the

phytogeographical zone (Table 3). Saplings of S.

Table 2 Survival rates of initial stock of saplings in five permanent plots per species in natural stands

Species Year 1—initial stock Surviving saplings (%)

Number of individuals per plot Height (cm) Year 2 Year 3

NS SS p-

value

NS SS p-

value

NS SS NS SS

Afzeliaafricana

10.85ax ± 4.10 6.21by ± 2.51 0.04 52.60ax ± 8.50 65.60ax ± 9.00 0.27 36.0 38.2 34.0 38.2

Bombaxcostatum

10.87bx ± 2.40 16.74ax ± 1.19 0.02 32.36ay ± 3.7 48.24ay ± 7.10 0.19 20.4 28.1 18.5 28.1

Boswelliadalzielii

9.66ax ± 1.90 6.41by ± 1.50 0.04 43.18bx ± 5.86 60.54ax ± 12.00 0.04 39.6 45.3 36.7 45.3

Sterculiasetigera

2.12by ± 1.20 4.51az ± 0.71 0.02 9.61bz ± 1.40 16.09az ± 1.80 0.01 0.00 20.6 0.00 20.6

p-value 0.022 0.034 0.021 0.047

Mean number and height of individuals were compared (ANOVA) according to the phytogeographical zones (lines) and species

(columns). Values with the same letter are not significantly different at p = 0.05 (Tukey’s test). First letter is for comparison between

the columns and second letter for comparison between the lines. NS north Sudanian phytogeographical zone, SS south Sudanian

Fig. 3 Total mean number of new recruited saplings over

3 years in the two phytogeographical zones in permanent plots

Fig. 4 Fluctuation of sapling growth over 3 years. The month

of June corresponds to the beginning of rainy season, September

to its early end, November to the beginning of dry season and

April to its end

Agroforest Syst

123


setigera and A. africana grew mainly from lignotubers

(50.8–61.6 and 60.8–64.7 % respectively), while those

of B. costatum and B. dalzielii originated almost

entirely from sprouting of root suckers (94.3–95.1 and

94.7–97.9 % respectively). The highest proportion of

seedlings was recorded in S. setigera and A. africana

stands, with 38.4–49.1 % and 25.2–26.4 %, respec-

tively. Water stress due to drought, annual bushfires

and repeated grazing affected the survival and growth

of saplings. Saplings of all species except B. dalzielii

were systematically grazed by cattle and goats. The

combined effects of these factors caused very high

mortality among seedlings. Individuals growing from

lignotubers and root suckers had a higher survival rate,

but their growth was periodically reduced. Indeed,

saplings often lost all or part of their stems and

survived in the form of stumps. When environmental

conditions became favourable again (in the rainy

season), new stems grew from lignotubers or roots.

Most individuals in the regeneration stage followed

this cycle of growth re-start, having survived

disturbance.

Trees’ potential for sexual reproduction

The germination rates of freshly harvested seeds were

30 % or more for all species. Those with the highest

germination rates were A. africana (68 and 74 %) and

S. setigera (60 and 62 %) for south and north Sudanian

provenances, respectively. On the other side, average

germination in B. costatum (55 and 36 %) and B.

dalzielii (35 and 30 %) was lower for the north and

south Sudanian provenances, respectively. Differences

in germination rates between provenances were only

significant for A. africana (p = 0.038) and B. costatum

(p = 0.006). All provenances taken together, germi-

nation was rapid for B. dalzielii (6.8 ± 1.2 and

8.5 ± 1.3 days, respectively) for the latent period of

germination (LPG) and the speed of germination (SG),

but relatively slow for A. africana (LPG = 14.8 ± 2.7

and SG = 17.1 ± 3.1), B. costatum (LPG = 17.8 ±

3.9 and SG = 23.2 ± 3.9) and S. setigera (LPG =

14.2 ± 3.9 and SG = 17.0 ± 2.7).

The seedlings after 1 month revealed highly vari-

able growth rates between species. The size of the

seedlings was significantly different according to

species after 30 days (Table 4). Seedlings of A.

africana and S. setigera included the tallest individ-

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Agroforest Syst

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height of the 30-day-old seedlings was positively

correlated with their initial height (r = 0.72; N = 40;

p \ 0.01). The rate of height increase showed three

modalities. In the first, growth followed an asymptotic

evolution, with an initial rapid growth followed by a

period of slowing down, e.g. A. africana (Fig. 5a). The

second modality was characterised by a regular

growth rate (B. costatum and B. dalzielii). In the third

modality, the growth model showed two active periods

separated by a period of slow growth (S. setigera).

Leaf production also varied significantly between

species (Table 4). S. setigera showed the highest

number of leaves. Leaf production was more or less

regular for all seedlings during the 30-day period

(Fig. 5b). The total number of leaves on the 30th day

was correlated to their initial number (r = 0.89;

N = 40; p \ 0.01). There was a medium positive

correlation between height and leaves number at the

germination (r = 0.50; N = 40; p = 0.001) and

1 month later (r = 0.55; N = 40; p \ 0.01).

Seedling response to transplantation

The 30-day-old seedlings showed good post-trans-

plantation survival rates. Mortality decreased sharply

1 month after transplantation. S. setigera showed the

best performance, with 98.3 % survival rate 1 year

after transplantation—at this time, saplings were

vigorous enough to survive and continue to grow.

Seedlings of A. africana were the most sensitive to

transplantation, with 18.5 % mortality within 1 year—

they lost all their leaves and often a part of their stem

by withering, but produced offshoots later. B. costa-

tum and B. dalzielii had, respectively, 89. 8 and 86.9 %

survival rates. S. setigera and B. costatum showed best

annual growth rates in diameter as well as in height

(Fig. 6a, b). The growth of B. dalzielii seedlings was

relatively weak compared to the other species, but it

increased more than twice both for diameter and

height 1 year after transplantation (Fig. 6a, b), as

observed in S. setigera and B. costatum. The growth of

A. africana seedlings was very low, particularly in

height due to the partial withering following trans-

plantation. Except for A. africana (r = 0.03; N = 10),

there was a good correlation between annual growth of

diameter and height for all species: B. costatum

(r = 0.62; N = 10), B. dalzielii (r = 0.78; N = 10)

and S. setigera (r = 0.79; N = 10).

Table 4 Comparison of 30-day-old seedlings’ height and number of leaves: mean ± SD

A. africana B. costatum B. dalzielii S. setigera Probability

Height (cm) 27.33a ± 7.01 12.40c ± 2.76 6.21d ± 0.95 17.25b ± 3.76 p \ 0.0001

Number of leaves 6.12c ± 0.35 5.30d ± 0.95 7.20b ± 0.92 11.20a ± 1.23 p = 0.0004

Means followed by the same letter on the same line are not significantly different at p = 0.05 (Tukey’s test). SD standard deviation

Fig. 5 Growth in height (a) and leaf production (b) of seedlings

30 days after germination

Agroforest Syst

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Morphological behaviour of seedlings

Our observations in the nursery for over 1 year

suggested that seedlings reduced their aerial organs

(all leaves and parts of branches) and specialised their

roots systems to survive water stress during the dry

season. Seedlings of B. costatum, B. dalzielii and S.

setigera hypertrophied their root stock (Fig. 7b–d).

This specialisation allowed their seedlings to store

water and other nutrients in these swollen roots to

Fig. 6 Post-transplantation

growth in height (a) and

diameter (b) of seedlings

A. africana B. costatum B. dalzielii S. setigera

dcba

Fig. 7 Root systems of 6-month-old seedlings: expanded root system (a Afzelia africana) and hypertrophied root systems (b Bombaxcostatum, c Boswellia dalzielii and d Sterculia setigera)

Agroforest Syst

123


survive the drought as geophytes. The root system in

A. africana seedlings was not hypertrophied, but much

expanded with long branching roots (Fig. 7a).

Discussion

Regeneration population structure and dynamics

The density of saplings and their size distribution

illustrate the regeneration capacity of woody species

populations. The perceptible reduction of natural adult

populations is certainly the result of a previous long-

term disturbance of regeneration populations. The gap

between the regeneration population and the adult one

will enlarge as the disturbance of reproductive

processes continues. The absent or very small sapling

population in S. setigera and A. africana natural stands

demonstrates this situation. Many authors report the

degraded status of the populations of both species in

the Sahelo-Sudanian zone of West Africa (Lykke

1998; Sinsin et al. 2004; Ouedraogo et al. 2005), as

well as their unpredictable regeneration (Traore 1998;

Bationo et al. 2001a). B. costatum and B. dalzielii had

substantial sapling populations; the variation in den-

sity between both species stands seems to be due to the

intensity of local environmental disturbances. In

similar climatic conditions, Zida et al. (2007) proved

that grazing and fire affect the density of saplings,

largely by reducing the density of single-stemmed

individuals. According to their analysis, the multi-

stemmed state is advantageous in disturbed environ-

ments in terms of chance to conserve aerial organs

alive across the time. They also noticed an increasing

tendency towards multi-stemmed individuals on

grazed sites. Even though the variation is not signif-

icant, the increased number of multi-stemmed saplings

in the north Sudanian sites for all four species could

reflect the environmental pressures in this zone. In

terms of adaptation, B. costatum and B. dalzielii

showed a greater tendency to become multi-stemmed.

Regeneration is affected by the recruitment mech-

anism and the growth conditions of newly recruited

individuals. The small number of saplings and their

irregular height class distribution suggest both recruit-

ment and growth difficulties. The density in the

recruitment class (0–0.5 cm) is too low to ensure

successful regeneration. Indeed, this class is most

vulnerable (Ouedraogo et al. 2006c, 2009) and

requires a very high quantity of individuals to offset

the losses due to disturbances. The low survival rate

between the first and the second year in the initial

stock is mainly attributable to mortality in the

recruitment class. Natural selection due to distur-

bances would favour the tallest saplings which can

hopefully get away from livestock trampling and

bushfire intensity. A positive correlation between

height size and survival rate of saplings is also

reported by Ouedraogo et al. (2006b)and Bellefon-

taine et al. (2000). The better survival rate of A.

africana, compared to B. costatum, could be due to the

origin of its saplings, which were for the most part

‘‘old’’ saplings growing from lignotubers.

Among the four species, B. dalzielii and S. setigera

have a high fruit production in the field, which could

favour an abundant recruitment. However, environ-

mental conditions are limiting factors. For the former

species, there is a relatively high rate of seed

infertility, which accounts for its poor germination

(Ouedraogo et al. 2006a). In addition, the rocky

habitats where the species grows do not favour

recruitment by seeds. The scarcity of S. setigera

seedlings is common across its range, and was also

reported to be the case in Senegal (Traore 1998). In

addition to the difficult terrain, probable predation of

the large seeds, as was reported for A. africana

(Bationo et al. 2001a), may be a problem. For S.

setigera and B. costatum, sexual recruitment potential

is highly reduced by pruning, which reduces fruit

production. Based on the unpredictable recruitment

patterns, the chance for satisfactory regeneration is

weak. Indeed, the upper height classes, resulting from

the survival and growth of the first recruitment, will be

of limited size, as indicated by the gaps in the

population structure. The capacity for rapid growth at

the beginning of the rainy season is an advantage in

semi-arid environments, but the recurrent and some-

times fluctuating drought in the dry season, together

with human and livestock pressures, quash the efforts

of saplings to grow, restricting them to a cycle of

perpetually restarting growth. Considering the pres-

ence of all height classes, and the reverse ‘‘J’’ shape of

their size distribution, the species with the greatest

potential for good regeneration are B. dalzielii and B.

costatum. For these two species, the main threat is

direct and indirect human pressure, as Glele Kakaı

et al. (2009) have observed for Pterocarpus erinaceus,

a common savanna tree that is under threat. A. africana

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123


and S. setigera appear to be more sensitive to climatic

variability.

Natural regeneration modes and saplings’

responses to harsh environmental conditions

A variety of recruitment modes offer alternative ways

for plant regeneration in difficult environmental

conditions. Facing the unpredictable growth of their

saplings, tree species develop recruitment strategies

that allow them to secure the survival of individuals.

This could explain the highly significant predomi-

nance of recruitment by root suckers for B. costatum

and B. dalzielii, where vegetative propagation is more

common than sexual reproduction. These observations

are in line with those of Bognounou et al. (2010) who

reported a dominance of asexual propagation in five

Combretaceae species in Sahelo-Sudanian zone of

Burkina Faso. The advantages of vegetative propaga-

tion are low-cost renewal of regeneration populations

and rapid growth of saplings, as previously docu-

mented by Bellefontaine et al. (2000). For A. africana

and S. setigera, most saplings originate from lignotu-

bers, which represent in most cases the living ‘‘ves-

tige’’ of several previous years of growth. For an

individual, losing all its aerial parts and conserving

itself as a geophyte during the dry season then growing

early in the rainy season is an effective survival

strategy. Bationo et al. (2001b) observed a similar

behaviour for Detarium microcarpum in Sudanian

savannas of Burkina Faso. Although the annual

renewal of saplings from lignotubers is similar to root

suckers sprouting because of the seasonal growth of

stems, it has a seedling origin. Bognounou et al. (2010)

have distinguished ‘‘true seedlings’’ from ‘‘seedlings

sprouts’’.

New seedlings are more vulnerable than root

suckers and lignotuber shoots. For all species, direct

recruitment by seeds is low, revealing the unsuitability

of this mode of regeneration, given the actual climatic

and environmental conditions. The dominance of

vegetative propagation in natural populations is often

the result of an inefficient recruitment by seeds.

Indeed, vegetative propagation is documented as an

adaptative strategy or response to disturbance and

climatic variation in species with normal sexual

reproduction (Hoffmann 1998; Higgins et al. 1999;

Dickinson et al. 2000). This mode of reproduction

cannot ensure the long-term conservation of natural

populations due to limited dissemination capacity of

individuals (Puig et al. 1998; Setterfield 2002; Tremb-

lay et al. 2002). A wide genetic base in agroforestry

trees is essential to prevent inbreeding depression and

allow adaptation to changing environmental condi-

tions and to altering markets for tree products (Daw-

son et al. 2009).

Species potential for sexual reproduction

and adaptation strategies

Excluding eventual dormancy or parasite attack, the

germination rates of freshly collected seeds reveal the

initial fertility rate. The high germination rates of A.

africana and S. setigera seeds are means to mitigate

the environmental instability in natural habitats.

Indeed, the recruitment of seedlings will be reduced

for species with a low germination rate, under threat of

predation and bushfire (Bationo et al. 2001a; Setter-

field 2002; Danthu et al. 2003; Thiombiano et al.

2003). If the moderate germination rate of B. dalzielii

reflects its seed infertility (Ouedraogo et al. 2006a), as

also observed in Anogeissus leiocarpa (Kambou

1997), then the rate for B. costatum is probably due

to dormancy, since Belem (2009) observed a germi-

nation rate of 80 % following pre-treatment that

consisted of soaking seeds in water for 48 h. Dor-

mancy could also explain the difference of the

germination rates between the two provenances of

both A. africana and B. costatum. Indeed, Ouedraogo

(2006) observed variable seed dormancy rates for

these species according to their north and south

Sudanian origins. The rapid germination in B. dalzielii

is an undeniable advantage for seedlings facing the

constraint of a short rainy season. Compared with the

other three species, the germination in B. dalzielii is

faster giving more time to seedlings to gain vigour and

resist poor weather conditions in the dry season.

Endogenous control of natural seed stock is vital for

plants facing irregular environmental factors (Al-

Helal 1996; Schutz and Milberg 1997), latent periods

that allow avoiding harmful effects in the event of

sporadic rainfall. The orthodox seeds of certain

species can conserve their capacity to germinate for

several years (Danthu et al. 2003). This is a useful

adaptation to survive an extended drought following

fruit production.

After germination, the early growth capacity of

seedlings is decisive for their future in disturbed areas.

Agroforest Syst

123


Indeed, observations have shown that shorter saplings

are most vulnerable. Rapid growth permits seedlings

to gain in size during the short rainy season and have a

greater chance of survival. A. africana showed better

height and S. setigera higher leaf number than the

other species 30 days after germination. Thus, the

relatively low germination rate of A. africana and S.

setigera is compensated for by the rapid early growth

of their seedlings. However, for A. africana, seedling

growth slows down early and saplings show limited

development 1 year after transplantation. This obser-

vation agrees with those of Bationo et al. (2001a), who

noticed in the same species a long period of rest after

4–5 weeks of continuous growth. The seedlings of S.

setigera and B. costatum maintained a good growth

rhythm and showed better post-transplantation per-

formances, suggesting good potential for silviculture.

The vigorous growth of B. costatum indicates that the

post-germination height is not bound to have an

influence on subsequent post-transplantation growth.

The development of the root system in seedlings is an

essential factor that determines the difference in their

survival capacity (Dauro et al. 1997). Hypertrophy of

the root system remains an effective survival strategy

because it allows the storage of moisture and nutrients,

which keep the individual alive. Seedlings can thus

lose all their leaves and survive as geophytes until the

next favourable season, as observed in B. costatum and

B. dalzielii. While nutrient storage in hypertrophied

root confers certain survival advantages, root expan-

sion in A. africana is only effective when water is

available in the soil, which could explain its poor

adaptation to drought.

Conclusion

This study provides evidence that the efficiency of

natural regeneration in the four woody species is

uncertain in the actual environmental conditions of

Sudanian savannas. The regeneration is under threat

and its extent varies more according to the species than

to the phytogeographical zones. For Sterculia setigera

and Afzelia africana, the quasi-absence or very low

density of saplings suggests that the conservation of

their natural population is in jeopardy. Although the

sapling densities are relatively high in Bombax cost-

atum and Boswellia dalzielii, their regeneration is also

threatened considering the high mortality during the

dry season. Lack of saplings and high mortality rates

are evidence that the natural populations face a

regression trend in both north Sudanian and south

Sudanian zones. Natural reproductive capacities of

species seem to be surpassed by the worsened harsh

environmental conditions which are characterised by

drought, grazing and bushfire. Thus, the conservation

of these species in parklands would be only possible

through plantations and protection of saplings. The

results have shown some potential for such activities

considering the observed germination capacity, post-

transplantation survival and growth rates. However,

for the purpose of conservation management, the

results from natural regeneration monitoring suggest

that fencing new plantations until saplings reach

sufficient height to be less vulnerable to current

disturbances would be necessary. In addition, further

research should focus on understanding the biological

mechanism of seedlings adaptation to drought in order

to mitigate water stress that is very damageable in

young plantations.

Acknowledgments Our thanks go to the German Federal

Ministry for Education and Research through BIOTA

(Biodiversity Monitoring Transect Analysis in Africa)

program for funding this research. We are very grateful to our

field assistant Nicolas Thiombiano who helped us in data

collection and also to Dr. Ross Bayton for his assistance in

improvement of the written English. We are grateful to the

anonymous reviewers whose pertinent comments considerably

improved the quality of our paper.

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Regeneration pattern of four threatened tree species in Sudanian savannas of Burkina Faso