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Recruitment of woody plant species juveniles in Ngel Nyaki Forest Reserve and its potential for forest regeneration.

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The study examined the recruitment of woody plant species juveniles in both the protected portion and unprotected forest fragments of Ngel Nyaki Forest Reserve in Taraba State, Nigeria. A 1m x 1 m (1m 2) wooden quadrat was thrown randomly for thirty times in each of the four study sites – the protected Main Forest (MF) and unprotected adjacent forest fragments A, B and C. This was done far away from parent species in order to avoid counting juveniles that regenerated under the parent plants. Woody plants at three different stages of growth (seedlings, saplings and poles) were identified to species level for each site, and the number of individuals in each juvenile class encountered for each species recorded. The abundance of juveniles in the four sites followed the order: MF < A < B < C; a trend similar to the degree of disturbance in the sites. Although the least number of juveniles was found in MF, it was fairly distributed among the three juvenile classes examined. Juveniles in the fragments were dominated by seedlings of pioneer plant species with the saplings and poles being either scarce or missing completely. There appears to be high mortality in the fragments as juveniles progress from the seedling stage to the pole stage. A very high degree of species compositional variation was observed among juveniles found in the MF on one hand, and the fragments on the other hand; with the highest dissimilarity seen at the pole stage. Clausena anisata was the only plant species with its juveniles found in all the sites. The proportions of seedlings, saplings and poles found among juveniles in the fragments are not likely to sustain effective regeneration of the plant communities. Protection and control of unsustainable practices in the fragments were suggested as a corrective measure.
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I.J.S.N., VOL. 2(4) 2011: 718-722 ISSN 2229 – 6441
718
RECRUITMENT OF WOODY PLANT SPECIES JUVENILE IN NGEL
NYAKI FOREST RESERVE AND ITS POTENTIAL FOR FOREST
REGENERATION
1
Ihuma, J.O.,
2
Chapman, H.M. &
3
Chima, U.D.
1
Department of Biological Sciences, Bingham University, P.M.B. 005, Karu, Nasarawa State, Nigeria.
2
Department of Biological Sciences, University of Canterbury/Nigerian Montane Forest Project, Private bag 4800,
Christchurch, New Zealand.
3
Department of Forestry and Wildlife Management, University of Port Harcourt, P.M.B. 5323 Port Harcourt, Nigeria
ABSTRACT
The study examined the recruitment of woody plant species juveniles in both the protected portion and unprotected forest
fragments of Ngel Nyaki Forest Reserve in Taraba State, Nigeria. A 1m x 1 m(1m
2
) wooden quadrat was thrown randomly
for thirty times in each of the four study sites – the protected Main Forest (MF) and unprotected adjacent forest fragments
A, B and C. This was done far away from parent species in order to avoid counting juveniles that regenerated under the
parent plants. Woody plants at three different stages of growth (seedlings, saplings and poles) were identified to species
level for each site, and the number of individuals in each juvenile class encountered for each species recorded. The
abundance of juveniles in the four sites followed the order: MF < A < B < C; a trend similar to the degree of disturbance in
the sites. Although the least number of juveniles was found in MF, it was fairly distributed among the three juvenile classes
examined. Juveniles in the fragments were dominated by seedlings of pioneer plant species with the saplings and poles
being either scarce or missing completely. There appears to be high mortality in the fragments as juveniles progress from
the seedling stage to the pole stage. A very high degree of species compositional variation was observed among juveniles
found in the MF on one hand, and the fragments on the other hand; with the highest dissimilarity seen at the pole stage.
Clausena anisata was the only plant species with its juveniles found in all the sites. The proportions of seedlings, saplings
and poles found among juveniles in the fragments are not likely to sustain effective regeneration of the plant communities.
Protection and control of unsustainable practices in the fragments were suggested as a corrective measure.
KEY WORDS: Ngel Nyaki, forest disturbance, juvenile recruitment, forest regeneration
INTRODUCTION
Forest regeneration through seedling recruitment is the
evidence that direct gene flow had occurred through the
process of seed dispersal. One major means through which
seeds are distributed spatially into new physical
environment is seed dispersal. Dispersal is a central life-
history trait (Levin et al. 2003), and it is important to
assess seedling recruitment so that possible limitation or
enhancement to forest regeneration in plant communities
can be identified. Seedlings are able to grow into adult tree
species through stages of saplings and poles when there is
reduction of the competition among kin (Hamilton and
May, 1977), avoidance of inbreeding depression
(Bengtsson , 1978) and the temporal heterogeneity of the
environment, such as local population extinction (Comins
et al, 1980).
Seed dispersal by animals plays a crucial role in the
tropics. Fruit-bearing plants serve not only as nutritional
sources for frugivores, but also as seed sources for forest
regeneration and as important foci for the re-establishment
of other plant species by attracting seed-dispersing animals
to their vicinity. Where forests have become fragmented
they are not absolutely isolated, but interact genetically
with each other through the mechanisms of seed dispersal.
Natural forest succession on human-disturbed land is often
slow because the resources necessary for succession are
depleted. In many cases the overriding factor impeding
forest recovery appears to be lack of forest seeds (Nepstad
et al., 1991, 1996; Aide & Cavelier, 1994; Da Silva et al.,
1996). In such landscapes, forest succession may be
dependent on arrival of seeds from off-site, and many of
these seeds are dispersed by fruit eating animals. In many
tropical regions, frugivorous birds and mammals are the
predominant dispersers of pioneer woody plants, playing
an important role in their early establishment (Vierira et
al., 1994; Da Silva et al., 1996).
In a fragmented landscape like that of Ngel Nyaki Forest
Reserve, it is imperative to assess seedling recruitment so
that possible limitation to forest regeneration within the
plant communities can be identified. The study evaluated
woody plant species regeneration both in the protected
portion of the reserve and three unprotected fragments
with varying degrees of disturbance.
MATERIALS AND METHODS
Description of the Study Area
The study was conducted at Ngel Nyaki Forest Reserve,
located towards the western escarpment of the Mambilla
plateau in Taraba State, Nigeria (Figure-1). The plateau is
located between longitude 11
0
00
1
and 11
0
30
1
East and
latitude 6
0
30
1
and 7
0
15
1
North. It is drained by numerous
water courses which unite to form the main rivers to
discharge eventually into the Benue River. It comprises
Woody plant species juvenile recruitment at Ngel Nyaki Forest Reserve, Nigeria
719
approximately 46km
2
of impressive sub-montane to mid-
altitude forest, lying between 1400 - 1500m (Chapman and
Chapman, 2001). Heavy rainfall is recorded from April to
October while the dry season is from approximately
November to March. Ngel Nyaki was formally gazetted a
local authority Forest Reserve under Gashaka - Mambilla
Native Authority Forest order of April 1969, but at present
it is under the management of the Taraba State
Government and the Nigerian Conservation Foundation
(NCF), with the Nigerian Montane Forest Project (NMFP)
as a project partner.
However, Ngel Nyaki Forest Reserve is currently beset
with problems of fragmentation (especially in the riverine
forest strips of the buffer zone). Ihuma et al., (2011) have
observed a remarkable variation in tree species
composition between the main forest (which is protected)
and the adjacent unprotected forest fragments, with the
light demanding pioneer species dominating the latter.
FIGURE 1: Map of Ngel Nyaki Forest Reserve showing the adjacent forest fragments
Experimental Design
The study area comprised of four sites, the main forest
(MF), that is, Ngel Nyaki forest, and forest fragments A, B
and C (Figure-1). The study was designed to incorporate
areas of forest at different locations from large, MF (>
8km
2)
, to small fragments A and B (>200 m
2
) and very
small fragment C (<200m
2
); increasing distance from MF,
310m to 1,590m (fragment A and fragment C,
respectively), and increasing habitat degradation from very
little (MF) to extreme (fragment C).
Method of Data Collection
A 1m x 1 m(1m
2
) wooden quadrat was thrown randomly
for thirty (30) times in each of the 4 study sites – the
protected MF and unprotected forest fragments A, B and
C. This was done far away from parent species in order to
avoid counting seedlings regenerated under the parent
plants. Woody plant species at three different stages of
growth: Seedlings (up 30 cm in height); Saplings (>30 cm
but below 1m in height); and poles (> 1m in height), were
identified to species level in each site and number of
individuals encountered for each of the three height
classes recorded for each species.
Method of Data Analysis
Sorensen’s similarity index was used to ascertain the
extent of similarity or dissimilarity of each pair of the
studied sites with respect to the different stages of the
woody species recruitments (i.e. seedlings, saplings and
poles).
Sorensen’s index is expressed as: RI = 100 * [a / a + b + c ]
Where:
a = number of species present in both sites under
consideration
I.J.S.N., VOL. 2(4) 2011: 718-722 ISSN 2229 – 6441
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b = number of species present in Site 1 but absent in Site 2
c = number of species present in Site 2 but absent in Site 1
RESULTS
Woody Plant Species Regeneration at the Various Sites
The summary of woody plant species regeneration for the
three juvenile classes is presented in Table 1. In MF, a
total of 48 juveniles belonging to 10 species were
encountered. Out of the 48 juveniles, 19 were seedlings,
25 saplings while 4 were poles. In Fragment A, a total of
103 juveniles belonging to 13 species were encountered.
Of the 103 juveniles, 77 were seedlings, 22 saplings while
4 were poles. A total of 209 juveniles from 17 species
were encountered in Fragment B. Of these, 199 were
seedlings, 10 saplings, while no pole was encountered. In
Fragment C, 296 juveniles belonging to 11 species were
encountered. Seedlings accounted for 279 of these,
saplings 15 and poles 2.
TABLE 1: Summary of woody species juvenile recruitment at the various sites
Woody plant Species
MF A B C
Se Sap Pol Se Sap Pol Se Sap Pol Se Sap Pol
Voacanga bracteates 0 1 0 0 0 0 0 0 0 0 0 0
Celtis gomphophylla 5 5 0 0 0 0 0 0 0 0 0 0
Clausena anisata
7 8 3 11 7 0 48 7 0 63 0 0
Pouteria altissima 3 0 0 0 0 0 0 0 0 0 0 0
Deinbollia crossonephelis 1 1 0 1 2 0 0 0 0 0 0 0
Isolona deightonii 1 0 0 0 0 0 0 0 0 0 0 0
Chrysophyllum albidum 0 1 0 0 0 0 0 0 0 0 0 0
Anthonotha noldeae
1 7 0 0 3 0 0 0 0 0 0 0
Synsepalum sp. 1 1 0 0 0 0 0 0 0 0 0 0
Garcinia smeathmannii 0 1 1 2 0 0 0 0 0 0 0 0
Albizia gummifera
0 0 0 0 1 0 0 1 0 0 0 0
Allophylus africanus 0 0 0 1 1 0 107 0 0 126 1 0
Bridelia micrantha 0 0 0 2 0 0 0 0 0 5 0 1
Croton macrostachyus
0 0 0 0 0 3 0 0 0 36 3 1
Ficus sp. 0 0 0 0 1 0 0 0 0 1 1 0
Psorospermum corymbiferum 0 0 0 0 6 0 12 0 0 10 1 0
Psychotria schweinfurthii 0 0 0 1 1 0 0 0 0 0 0 0
Syzygium guineense 0 0 0 59 0 0 23 0 0 30 2 0
Xymalos monospora 0 0 0 0 0 1 0 0 0 0 0 0
Canthium vulgare
0 0 0 0 0 0 2 1 0 0 1 0
Maesa lanceolata 0 0 0 0 0 0 2 0 0 3 2 0
Trema orientalis 0 0 0 0 0 0 0 1 0 1 2 0
Unknown sp
. 1 0 0 0 0 0 0 1 0 0 0 0 0
Unknown sp. 2 0 0 0 0 0 0 4 0 0 0 0 0
Dalbergia heudelotti 0 0 0 0 0 0 0 0 0 4 2 0
Sub-Total 19 25 4 77 22 4 199 10 0 279 15 2
Total 48 103 209 296
Se = seedling; Sap = sapling; Pol = pole
Species compositional variations of juvveniles at the
various sites
Seedlings
The extent of similarity or dissimilarity of plant species
seedlings encountered at the four study sites is shown in
Table 2. The plant species compositional variation was
very high when seedlings of MF were compared with
those from each of the fragments; with the highest level of
dissimilarity (93.75%) observed between MF and
fragment C. However, the extent of variation was less
(though still high) when woody plant species seedlings
were compared for each pair of the fragments; with
fragments B & C having the least percentage variation
(58.33%). Clausena anisata was the only plant species
with its seedlings found in all the sites.
TABLE 2: Sorensen’s Similarity Indices for seedlings
MF A B C
MF * 16.67 7.14 6.25
A * 25.00 30.77
B * 41.67
C *
Woody plant species juvenile recruitment at Ngel Nyaki Forest Reserve, Nigeria
721
Saplings
The similarity of woody plant species saplings between
MF and each of the fragments was generally low (below
30%) and showed a declining trend with increasing
distance from MF; with no sapling being common to MF
and fragment C. The level of similarity of woody plant
species saplings slightly appreciated when each pair of the
fragments was compared, though, figures were lower than
those obtained when seedlings were compared for same
sites.
TABLE 3: Sorensen’s Similarity Indices for saplings
MF A B C
MF * 23.08 9.09 0.00
A * 20.00 21.43
B * 18.18
C *
Poles
The highest level of variation in species composition of
the juveniles was observed in poles (Table 4). No juvenile
of woody plant species at the pole stage was common
between MF and each of the fragments and between each
pair of the fragments except for fragments A & C, where
Croton macrostachyus juvenile at this stage was observed.
TABLE 4: Sorensen’s Similarity Indices for poles
MF A B C
MF * 0.00 0.00 0.00
A * 0.00 33.33
B * 0.00
C *
DISCUSSION
The total number of juveniles encountered increased as
one progressed from MF through Fragment A to fragment
C. This shows an increasing trend with increasing level of
habitat disturbance. Of all the juvenile classes, the
seedlings dominated in all the sites except MF. Generally,
a decrease in the number of juveniles as they grew from
the seedling stage through the sapling stage to the pole
stage was also observed in all the sites except MF where
saplings dominated. This may be probably attributed to
different forms and degrees of exploitation going on in the
unprotected fragments.
The least number of juveniles found in the MF which
represents the climax plant community in the area, is not
out of place. In climax plant communities, seedlings on the
forest floor only grow slowly, and in most species seldom
reach taller than 1m because they eventually die unless
released (Whitmore, 1998). This probably explains why
the majority of the juveniles encountered in the MF were
at the sapling stage. Although, fewer juveniles were found
in the MF than in each of the fragments, their mix with
respect to the three classes (seedlings, saplings and poles)
indicates higher potential for regeneration and
sustainability of the plant communities, than what was
observed in the fragments. The juveniles in the fragments
were dominated with seedlings while the other two classes
(especially poles) were either few or totally missing. With
some good level of protection, this unsustainable trend and
even the much variation in juvenile plant species
composition between the MF and the fragments could be
corrected at the later stage of succession. Clausena anisata
was the only plant species with its seedlings found in all
the sites.
Most of the juvenile plant species encountered in the study
either showed more affinity for the MF on one hand or the
fragments on the other hand. This probably accounted for
the very low similarity indices between the MF and each
of the fragments in all the Juvenile classes.
The evidence of seed dispersal from the MF to fragments
would have been the presence of seedling in the fragments
of adult species only found in the MF. These were looked
for, but none was found. Instead, most of the regeneration
in the fragments was of the pioneer species, many of
which were not found in the MF. In large gaps (similar to
what now obtains in the fragments), pioneers which appear
after gap creation, form the next forest growth cycle.
Ihuma et al (2011) equally observed a remarkable
difference between the tree species composition of the MF
on one and the forest fragments on the other. While the
MF was dominated by shade-tolerant (climax) tree
species, the light demanding (pioneer) tree species have
colonized the forest fragments following varying degrees
of disturbance and exploitation. Thus, the absence or
paucity of juveniles of the shade-tolerant adult tree species
(found in the MF) in the fragments may not likely be due
to failure in seed dispersal by the frugivores. Ihuma, et al
(2011) have observed an appreciably high level of
similarity in frugivorous species between the MF and each
of the fragments, and attributed it to the migratory nature
of the avian fauna which accounted for over 70% of all the
frugivores recorded for the 5 taxonomic classes ( birds,
primates, ungulates, rodents and bats). However, the
absence or paucity of juveniles of adult climax tree species
found in MF, in the fragments may be as a result of failure
in germination and establishment of seeds of such species
due to unfavourable microclimatic conditions orchestrated
by habitat disturbance and exploitation in the fragments.
Such disturbances which encourage the penetration of full
sunlight, favour the germination and growth of seeds of
pioneers in the soil seed bank. This explains why the
population of juveniles increased with increase in habitat
disturbances, and why there were great species
compositional variations between the fragments and the
MF.
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CONCLUSION
Although, less number of juveniles was recorded in the
main forest than each of the fragments, the proportion of
the three classes of juveniles in the main forest is such that
can sustain the regeneration of the plant community unlike
in the fragments. Juveniles in the fragments were
dominated by seedlings, with poles being the scarcest. In
the fragments, there seems to be juvenile mortality as
seedlings move through the sapling stage to the pole stage.
Some appreciable level of protection and control of
unsustainable practices in the fragments may help in
correcting this trend.
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Bengtsson B.O. (1978) Avoiding inbreeding: at what cost?
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Chapman J.D. and Chapman H.M. (2001) Forests of
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... Sorensen's similarity index was used to measure beta diversity using the formula below after Ogunleye et al. (2004), Ojo (2004), Ihenyen et al. (2010), and Ihuma et al. (2011). ...
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