JOURNAL OF THE DRYLANDS 6(2): 489 - 498, 2016
DEGRADATION
OF
Boswellia
papyrifera
DOMINATED
DRY
LOW
LAND
DECIDUOUS FORESTS IN TIGRAY, NORTHERN ETHIOPIA
Kindeya Gebrehiwot1, Aklilu Negussie2, Emiru Birhane1,* Mekonnen Yohannes3, Nigussie Hagazi2 and Sarah
Teweldebirhan1
Kindeya Gebrehiwot, Aklilu Negussie, Emiru Birhane, Mekonnen Yohannes, Nigussie Hagazi and Sarah Teweldebirhan
(2016): Degradation of Boswellia papyrifera Dominated Dry Low Land Deciduous Forests in Tigray- Northern Ethiopia.
Journal of the Drylands, 6(2): 489 – 498
The frankincense tree, Boswellia papyrifera (Del.) Hochst is one of the tree species in the drylands of Ethiopia with
significant economic and ecological importance. However, the species is under threat due to natural and human induced
factors. It needs effective conservation strategies, yet, little is known about its ecology and management. The purpose of
this systematic review paper is to look at the population structure and investigate the long horn beetle and parasitic plant
infestation on B. papyrifera trees in two different land use management systems. Two management systems (open grazed
and closed) were chosen in the natural habitat of B. papyrifera. Thirty two randomly selected plots measuring 20 m x 20 m
for trees and shrubs and 5 m x 5 m for saplings were used to compare the population structure of the two land use systems.
Beetle and parasitic plant infestation was investigated by selecting 40 trees from 3 diameter classes in the two land uses in
both sites. The study showed that B. papyrifera dominated forests composed of very few young trees and very few
saplings, indicating that the natural regeneration dynamics of the species have been disrupted. The nocturnal long horned
beetles Idactus spinipennis Gahan, from the family Cerambycidae (sub family Lamiinae) infestation and the parasitic
plants are serious problems causing degradation and death of B. papyrifera trees. The study revealed that, exclosures have
a better vegetation density. The infestation rates were significantly different in the two sites and diameter classes rather
than on the land use systems investigated. It should, however, be noted that the open grazing sites witnessed a lot of death
of these trees, and these have not been accounted for in this study. Overall, urgent intervention is needed if the regeneration
and future sustainability of B. papyrifera is to be ensured.
Key words: B. papyrifera; horned beetle; parasitic plant; area enclosure; open grazed land
1
Land Resources Management and Environmental Protection Department, Collage of Dryland Agriculture and Natural
Resources, Mekelle University, P.O. Box 231, Mekelle, Ethiopia
2
ICRAF, World Agroforestry Centre-Ethiopia, P.O. Box 5689, Addis Ababa, Ethiopia
3
Biomedical Institute, Collage of Health Sciences, P.O. Box 1871, Mekelle, Ethiopia
* Corresponding author: email: [email protected]
Received: January 19, 2016; Accepted: November 29, 2016
INTRODUCTION
Vast areas of land, approximately 620,000 km2, in
Ethiopia are arid to semi-arid with erratic rainfall
characteristics (NCSS 1994, Lemenih et al 2003).
Most parts of the northern Ethiopia are severely
degraded and left barren as a result of years of
deforestation and indiscriminate removal of
vegetation cover. In Tigray, recurring droughts, war
and frequent crop and fodder failures have made
socio-economic development difficult. Therefore, the
need for supplementary options that diversify the
economy of the households are imperative (Lemenih
et al 2003).
In 1996 the forest cover was less than one percent
of the land area of the Tigray region (TFAP 1996),
while currently, there is a trend of increase which is
especially significant for bush-lands (de Mûelenaere
et al 2014). In this regard, B. papyrifera is one of the
most promising tree species in the drylands of
Ethiopia as an alternative income earning source for
the livelihood of the people. Its product, frankincense
- the white oleo-gum-resin exudate, has a long history
in human civilization. The natural oil contents and
pleasant smell of frankincense have made it desirable
to burn as incense in temple rituals (Groom 1981,
Gebrehiwot et al 2003). It is also a desired global
ingredient in the fabrications of perfume, medicines
and similar products (Lemenih et al 2003).
Ecologically, it has a great contribution in the
reduction of speedy runoff in the hilly areas where
shallow and undeveloped soils dominated. B.
papyrifera provides much of the plant cover and
biomass in the marginal areas of the lowlands of
Northern Ethiopia (Gebremedhin 1997). The species
appears to be able to adapt to harsh growing
conditions where other tree species often fail; in hilly
areas with shallow soils of low fertility within an
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altitudinal range of 950 to 1800 m (Gebrehiwot et al
2003, Ogbazghi et al 2006a,b).
The distribution of B. papyrifera in Tigray is
categorized in two based on the extent of human
interference: these are highly dense in the western
zone of Tigray and highly disturbed and severely
exploited for a long period of time in the central zone
(TFAP 1996, Birhane et al 2010, Birhane et al 2011).
Despite its ecological, economic, cultural and
religious importance, the species is fast declining and
threatened as a result of abiotic and biotic factors
such as, improper incense production, over-tapping,
moisture inconsistency, insecure land tenure systems,
illegal cutting of trees, land clearing for farming
practice and settlement, overgrazing, invasion of
parasitic plants, wind and undetermined insects and
fungi (Gebremedhin 1997, Gebrehiwot 2003, Rijkers
et al 2006, Negussie 2008, Birhane et al 2010, Eshete
et al 2012, Groenendijk et al 2012, Tolera et al 2013)
MATERIALS AND METHODS
Study area and data organization
Tigray is located between 120 15’ to 140 50’ North
and 36027’ to 39059’ East in the Northern most part
of Ethiopia. This study was carried out in Tanqu
Abergelle wereda, Central Tigray and Kafta-Humera,
Western Tigray. The site characteristics as described
by Birhane et al (2010) are presented in Table 1.
Previous studies carried out by Negussie (2008),
Hagazi (2008) and Birhane (2010) were used to
support the paper.
Table 1. Location, elevation, soil types and characteristics of the study sites (Birhane et al 2010)
Locality
Specific area
Area of specific site (ha)
Location
Altitude (m)
Geology
Soil types
Slope position
Disturbance
Erosion type
Abergelle
Jijike/Serabite and Siye
51
13°14’ to 13°42’N,
38°38’ to 39°02’E
1500 -1640
Limestone; mixture of schist &
limestone with quartz
Cambic Arenosols, Chromic Cambisols
and Leptosols
Middle
High
Sheet & rill
Population structure survey
The current population structure of B. papyrifera was
surveyed in both sites (Abergelle and Humera). The
survey was made on two land use systems: open
grazed and exclosures. Tree diameter and total
number of individuals per diameter class were
collected in 20 m x 20 m square plots, while B.
papyrifera sapplings were collected in 5 m x 5 m
concentric square plots in 32 randomly selected
locations, 16 in each site.
Long horn beetle and higher parasitic plant
prevalence
The incidence of Idactus spinipennis and related
beetles in family Cerambycidea and parasitic plants
invasion and their damage was estimated on
randomly selected trees within and near the plots, as
some plots did not have the required number of trees
in the given diameter class. Fourty trees were
selected for each diameter class (<16 cm, 16 to 20 cm
and > 20 cm) for each land use and at each site.
Humera
Tekeze/Adigoshu
295
13°42´ to 14°28´N,
36°20´ to 37°31´E
537-913
Metamorphic terrain, phyllite and
quartzite
Vertisols,
Leptosols,
Eutric
cambisols, Vertic Luvisols
Middle
Medium
Gully
Totally, 480 tree were selected, 120 in each land use
type in each site.
Data analysis
Data were analysed using excel and Minitab 16. To
estimate population structure of B. papyrifera in each
land use type, total number of stem for each diameter
class was counted and calculated on hectare basis.
Number of B. papyrifera trees infested with long
horn beetle and higher parasitic plants and their
intensity was expressed in percentage. Two-way
Analysis of Variance and Analysis of Means
(ANOVA and MONOVA) was used to see the
interaction effects of land use, long horn beetle
damage and parasitic plant infestation.
RESULTS AND DISCUSSIONS
Degradation of Boswellia dominated forest in
Central Tigray and its implication
The population structure of B. papyrifera shows that
Boswellia is facing recruitment problems and is
endangered (Figure 1). Due to this recruitment
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problem, the exploitation potential of this valuable
forest is greatly reduced (Abreham et al 2004). Large
amounts of seedlings regenerate in the rainy season,
however, they disappeared and die in the dry season
with very few re-emerging the next season (Negussie
et al 2008). Normally, the shoots of this seedling will
die back, however most are now not re-emerging.
Saplings were totally absent in all plots laid to
undertake this study.
Trees in the diameter class ranges from 15 to 19
cm (diameter measured at breast height) are
abundant. This implies that the succession of B.
papyrifera is endangered (Figure 1). Similar patterns
of recruitment problem and better populations in
closed as opposed to open sites have been observed
in other studies (Gebrehiwot et al 2003, Eshete et al
2005, Rijkers et al., 2006, Ogbazghi et al 2006b,
Abiyu et al 2010, Eshete et al 2012, Groenendijk et al
2012, Tolera et al 2013). Degradation and over
exploitation of natural resources are common in these
forests, and the degradation of natural resources
further limits local livelihood options (Tilahun et al
2007, Gebrehiwot 2003).
Several studies undertaken on this valuable
resource repeatedly point out that the rural poor
benefit greatly from it. However, the population of
this valuable tree is declining and needs immediate
solutions (Gebremedhin 1997, Gebrehiwot 2003,
Tilahun et al 2007, Tilahun et al 2011).
Figure 1. Population structure and number of trees per diameter class of the different land uses
combined for both sites (Abergelle and Tekeze).
Long horned borer beetle attack
B. papyrifera is also under threat as a result of
infestation by Idactus spinipennis Gahan, 1890 from
the family Cerambycidae, long horned borer beetles
(Negussie 2008, Groenendijk et al 2012, Tolera et al
2013). The long horned beetles of the family
Cerambycidea are a cosmopolitan family of beetles,
typically characterized by their extremely long
antennae, which are often as long as or longer than
the beetle’s body (Encyclopaedia Britannica 2013).
They undergo complete
metamorphosis,
meaning that the life stages consist of egg, larva,
pupa and the winged adult. Mature larvae pupate near
the surface of the wood; adults then find their way
out and emerge to mate and repeat the life cycle.
Many of the 30,000 species worldwide are quite
large. The long white larvae bore into the stems of
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herbaceous plants and trees, and can be very
destructive to trees. This insect is darker in colour
and nocturnal. The colourful species are active during
the day, while the dark ones are nocturnal (Milius
1999).
The beetles attack the whole parts of the tree. In
the beginning, the adult are attracted by food. Later,
other relationships evolve so that the crevices and
bark offer shelter and are attractive sites for laying
eggs. Upon hatching, the tiny larvae bore into the
wood and begin feeding, slowly carving tunnels as
they bore ahead and packing frass behind them
(Wang et al 1992). Their prevalence and damage
caused by these beetles were documented in this
study and are presented in Table 2.
The use of these forests for extracting
frankincense and grazing is good; however over
tapping and overgrazing practices weaken these trees
making them more susceptible to these attacks.
Tappers collect frankincense from creating wounds;
goats and cattle eat the bark during the dry season;
cattle keepers defoliated its leaves for fodder when
the rains start (Gebrehiwot et al 2003, Eshete et al
2005, Gebremedhin 1997). The debarked stems and
damaged twigs serve as an entry point for the beetles
to lay and hatch their eggs (Negussie 2008). In
addition to Idactus spinipennis, termites are common
and frequent problems in the area (Gebrehiwot et al
2003, Gebremedhin 1997, Eshete et al 2005). When
the trees start to dry out, the combined effect of
termites and beetles will hasten the end and fall of
these trees. This overuse and damage is one
explanation as to why the population of B. papyrifera
is higher per hectare in the closed sites as compared
to the open grazing areas (Figure 1).
As can be seen from the study (Table 2), the less
disturbed Tekeze site and lower DBH classes had
significantly less infestation by the beetles. It should
be noted that lower diameter classes will not be
tapped, and as tapping creates entry points for the
beetles, the infestation of larger diameter classes may
be facilitated by tapping. This was clearly noticed in
previous studies (Eshete et al 2005, Negussie 2008).
Statistically, the effect of land use was not found to
be significant. However, when one factors in the
number of dead trees and smaller number of the trees
per hectare for the open grazing sites, and the fact
that other studies did find this significant
(Gebrehiwot 2003, Ogbazghi et al 2006b, Abiyu et al
2010, Groenendijk et al 2012), it brings into question
that site disturbance may also have an effect on the
beetle infestation. Generally, this has implications for
management, showing that over utilization of this
valuable resource is leading to its decline and
threatening its sustainability.
Parasitic plants
The green leaved and white pink flowered parasitic
plant scientifically known as Tapinanthus globiferus
subsp. bangwensis on B. papyrifera is very common
in the area where this study was undertaken (Yirgu et
al 2014). It has pear shaped pink or red fruits when
ripe and green seeds inside with a sticky white pulp.
The flower and fruit of this parasitic plant resembles
with flowers and fruits of B. papyrifera. Tapinantus
globiferus, and grows at the side of the branches and
forms protoplasmic connections with B. papyrifera
for food until it establishes well. After establishment,
though, it produces chlorophyll and has the capability
to photosynthesise its own food it remains dependent
on the host for water and nutrients (Dörr 1987). In
this study, it was observed that the infected area
becomes swollen and sometimes black scares form
which later harbour fungi and other insects (Negussie
2008). These parasitic plants can become numerous
enough that they make up almost half of the green
foliage of the tree branches, particularly on older
Boswellia trees, where up to 30 individual
Tapinantus globiferus plants per tree were observed.
As can be seen in Table 3, the study showed that the
number of parasitic plants present was significantly
higher in the more disturbed Jijke site, and in the
larger diameter sized trees. With these results again,
the land use does not seem to have a significant
effect.
In the dry season, these parasitic plants remain
green while the tree sheds its leaves to reduce
moisture lost through transpiration. This phenomenon
affects the health and physiology of the tree, and
weakens the defence mechanism of the species to
other insect and pest attacks (Dörr 1987). The
infected branches of Boswellia often become
deformed, and the flowering and fruiting delays as
the parasite takes the energy and food needed to
prepare flowers and fruits (Negussie 2008).
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Table 2. Long horn beetle damage quantification based on site, land use and tree size (DBH), ** significant at 1% using ANOVA.
Sites
Land Use
Closed
Protected
Total
B.
papyrifera
trees/Ha
513
Tekeze ** (A)
Open
Grazed
Closed
Protected
303
436
Diameter Classes
Noninfested
< 5% of
the crown
infested
5-15% of
the crown
infested
16- 25 %
of
the
crown
infested
> 25 % of the
No
of
crown infested
infested
and
stem
trees/ ha
damaged
DBH<16 ** (A)
101.25
111.91
0.00
0.00
0.00
111.91
213.16
16 >DBH< 23 ** (B)
64.52
170.11
0.00
0.00
0.00
170.11
234.63
DBH>23 ** (C)
48.31
330.32
4.14
4.14
0.00
0.00
1.38
1.38
53.83
55.21
Total
1.38
167.15
335.85
503.00
DBH<16 ** (A)
33.75
21.09
0.00
0.00
1.41
22.50
56.25
16 >DBH< 23 ** (B)
30.08
3.75
67.58
120.32
65.63
207.03
8.59
5.63
14.22
0.00
0.00
0.00
12.89
0.00
14.30
141.80
171.88
DBH>23 ** (C)
Total
71.25
235.55
75.00
303.13
DBH<16 ** (A)
18.75
43.75
0.00
0.00
0.00
43.75
62.50
38.28
245.00
7.66
15.31
0.00
267.97
306.25
5.04
62.07
28.56
317.31
21.84
29.49
6.72
22.03
5.04
5.04
62.15
67.19
Total
373.87
435.94
DBH<16 ** (A)
25.11
50.23
0.00
0.00
1.93
52.16
77.27
16 >DBH< 23 ** (B)
22.87
171.49
5.72
5.72
22.87
205.79
228.65
DBH>23 ** (C)
1.58
49.55
36.27
257.98
17.35
23.06
7.89
13.60
0.00
24.80
61.50
63.08
319.45
369.00
16 >DBH< 23 ** (B)
DBH>23 ** (C)
Jijike** (B)
Open
Grazed
Number of B. papyrifera trees infested with long horn beetle and intensity/ ha (based on
Damaged twigs number per individual tree)
366
Total
Tree per
ha/
DBH
class
(Sources: Negussie 2008, modified)
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Table 3. Parasitic plants (PP) presence on B. papyrifera trees based on site, land use and tree size (DBH), ** significant at 1% using ANOVA, and Tukey’s
grouping in brackets. (Sources: Negussie 2008, modified)
Number of B. papyrifera trees infested with parasitic plants and intensity of
infestation/ha
Total B. Papyrifera
Sites
Land Use
Diameter Classes
Tree Hosts of PP Tree per ha/ each
trees/Ha
Non1-10
11-20
> 20
infested
PP
PP
PP
/Ha
DBH class
Closed
Protected
503
Tekeze ** (A)
Open Grazed
Closed
Protected
303
436
Jijike** (B)
Open Grazed
366
DBH<16 (B)
213.16
0.00
0.00
0.00
0.00
213.16
16 >DBH< 23 (B)
234.63
0.00
0.00
0.00
0.00
234.63
DBH>23 ** (A)
55.21
0.00
0.00
0.00
0.00
55.21
Total
503.00
0.00
0.00
0.00
0.00
503.00
DBH<16 (B)
56.25
0.00
0.00
0.00
0.00
56.25
16 >DBH< 23 (B)
30.08
30.00
0.00
0.00
0.00
1.88
30.08
171.88
DBH>23 ** (A)
141.80
43.13
31.88
75.00
Total
241.18
60.08
0.00
1.88
61.95
303.13
DBH<16 (B)
45.31
15.63
1.56
0.00
17.19
62.50
16 >DBH< 23 (B)
99.53
199.06
7.66
0.00
206.72
306.25
DBH>23 ** (A)
8.40
48.71
6.72
3.36
58.79
67.19
Total
153.24
263.40
15.94
3.36
282.70
435.94
DBH<16 (B)
48.29
28.98
0.00
0.00
28.98
77.27
16 >DBH< 23 (B)
85.74
137.19
5.72
0.00
142.91
228.65
DBH>23 ** (A)
15.77
149.81
37.85
204.01
3.15
8.87
6.31
6.31
47.31
63.08
219.19
369.00
Total
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middle diameter class trees occurred in Tekeze and
that of larger trees occurred in Jijike. Significantly
lower mean value for infestation is observed in the
middle diameter class trees in Jijike and larger
diameter class trees in Tekeze. When we look at the
main effect of site, we see that there is significantly
higher infestation occurring in Jijike as compared to
Tekeze. Looking at the main effect of diameter class,
it is observed that the lower diameter class has
significantly lower infestation, which agrees with
previous studies (Yirgu et al. 2014) indicating that
tapping aggravates the long horn beetle infestation.
Interaction effects of land use, long horned beetle
and parasitic plant infestation
Land use did not show any significant effect on the
presence of the long horn beetle and parasitic plant
(Table 2 and 3). However, the study site and DBH
class had an effect (Figure 2 and Figure 3). The mean
infestation count for trees with DBH <16 cm was not
significantly different from the overall mean (Figure
2). On the other hand, 16 ≥DBH ≤23 and DBH > 23
showed opposite situations for both sites, where
significantly higher mean value for infestation of
Alpha = 0,05
Interaction Effects
1
Effect
0,591
0
0
-0,591
-1
DBH_
Site
1
1
2
3
1
2
Main Effects for Site
3,326
2,983
2,641
2
Mean
Mean
3
Main Effects for DBH_
4
3
2
3,5
3,564
3,0
2,983
2,5
2,403
2,0
1
2
Site
1
2
DBH Class
3
Figure 2. Two-way Analysis of Variance (ANOVA) for beetled branches; where for site (1) is
Tekeze and (2) is Jijke; and for DBH (1) < 16 cm; (2) 16 ≥DBH≤ 23; and (3) > 23 cm.
In Figure 3, the interaction effect of site and DBH
class for parasitic plan infestation showed that all the
means are significantly different from the grand mean
with trees in smaller and middle diameter classes in
Tekeze. The upper diameter class in Jijke is
significantly higher, while the remaining are
significantly lower. This interaction analysis gives an
opposite image of the main effect analysis. In the
main effect analysis, for site, it shows Jijke and for
the diameter class, it shows trees in the higher
diameter class have significantly higher parasitic
plants. What is clearly seen in this analysis and may
not be clear in Table 3, is that the upper diameter
class trees in both sites have significantly different
number of infestation, with Tekeze being low and
Jijke being high. The other two diameter classes also
have significant differences with respect to the sites,
and go in opposite directions. Overall, the higher
diameter class in Jijike has significantly higher
infestation with parasitic plans relative to the other
diameter classes and the trees in Tekeze.
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Two-Way Analysis of Means for Parasitic Plants
Alpha = 0,05
Interaction Effects
Effect
2
0,501
0
-0,501
0
-2
DBH_
Site
1
1
2
3
1
2
2
Main Effects for Site
Main Effects for DBH_
4,5
2
1,890
1,6
1,310
1
0
Mean
3
Mean
3
3,0
2,092
1,6
1,108
1,5
0,0
1
2
Site
1
2
DBH_
3
Figure 3. Two-way Analysis of Means for Parasitic Plants; where for site (1) is Tekeze; and (2)
is Jijke; and for DBH (1) < 16 cm; (2) 16 ≥DBH≤ 23; and (3) > 23 cm
Generally, looking at the results presented here, it is
clear that B. papyrifera is facing recruitment
problems and the future of the trees is under threat.
The threats are multi-fold as several studies have
indicated. The land use system clearly shows that
there is accelerated degradation and death of the trees
in the openly grazed sites. The more intensively used
site of Jijike had more long horn beetle and parasitic
plant infestation that again accelerate the vegetation
decline. These have implications for management as
they affect both production and future regeneration
related issues, as the middle and upper diameter
classes are the trees being tapped and are the trees
producing seeds. When these diameter class trees get
infested and die, both incense and seed production
are in danger. Again, there is a need to stress that
even if the land use did not show any significant
effect, the total death and decline of trees in the
openly grazed areas is a significant effect on its own.
Further study is, therefore, needed to show how many
trees are dying due to infestation by the long horn
beetle and the parasitic plants.
CONCLUSIONS
Although dry land forests play an important role both
in the economic and ecological wellbeing of the
dryland ecosystem; their sustainability is under
threat. B. papyrifera is one of the potential tree
species for economic development and desertification
control in arid and semi-arid areas of Ethiopia. Its
distribution is restricted mainly to steep rocky
lowlands where the development of the soil is young
and within a limited range of rainfall. Human
pressures from tapping, fire and livestock grazing on
the remaining B. papyrifera dominated forest
increase its habitat loss and fragility.
This study revealed that there is a decline in the
number of trees per hectare with a negative natural
regeneration trend and a high long horn beetle and
moderate parasitic plant infestation. The rapid
expansion and high density of long horned beetle
infestation in central Tigray where this study was
undertaken is a serious problem for the future
existence of B. papyrifera. This needs serious
attention and further study on the control of this
beetle infestatoin, in addition to improving the
management and use of this valuable tree.
ACKNOWLEDGMENTS
Mekelle University and K.U. Leuven (Belgium)
through the joint project of Forest rehabilitation
through Natural Regeneration in Tigray, Northern
Ethiopia provides logistic and material support
during the field study and are highly acknowledged.
The authors also want to acknowledge the People in
Tanqua Abergelle district (Central Tigray) and KaftaHumera (Western Tigray) for their hospitality during
our field stay.
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