The importance of the diversity of forest to the well being of the adjoining
communities cannot be overemphasized. It supplies various products, which are
consumed by people (Okafor and Lamb, 1994). These include
a wide range of edible products obtained from wild fruit trees such as nuts
and seeds, used as staple foods or main dishes, as well as those used as minor
food supplements, condiments, thickening agents and flavours; leafy vegetables;
fresh fruits; fresh seeds; edible oil; spices; fruit-drinks; alcoholic and non-alcoholic
drinks (Leakey, 1999). Sustainable management options
for most of these forest species and their products are however necessary so
as to curb over-exploitation leading to extinction.
One of the primary focuses of research in forestry is to provide professionals
in charge of tropical forestland management some insight on the structure and
functioning of tropical forest ecosystems and the ecology and regeneration of
tropical trees that may be important for conserving these forest products (Philips
et al., 2002).
The role of regeneration as an ecological strategy in structuring tropical
tree communities has been a focus for tropical ecologists in the recent times
(World Resources Institute, 1985; Cintra
and Horna, 1997).
Enhanced synecological studies on forest management for a satisfactory regeneration
option requires reliable information on the status and conditions of each forest
interpreted from a broad context and change in forest conditions over time (Reed,
1999) coupled with a good knowledge of the history, species composition
and ecology of the forest (Bob, 1992) and an understanding
of the degree of species interdependence within communities, especially the
timber and the non wood forest products (Leakey, 1999;
Ayuk et al., 1999; Fondoun
et al., 1999). Gehlhausen et al. (2000)
and Jerry et al. (2002) observed that forest
edges consist of micro environments that provide habitat for array of species
different from those in the forest interiors. Such attributes could be light,
air temperature, soil moisture and humidity. Biotic components such as seed
dispersal may also give rise to changes in species composition from forest edge
to interior. Distribution of individual species as well as changes in plant
community composition estimated with a similarity index, indicate that competition
may also influence the response of the vegetation from the edge to the interior
Agulli forest reserve over the years has been under pressure with agricultural and other human activities bordering the forest reserve. Therefore, the objective of this study is to undertake the evaluation of the floral composition of Agulli forest reserve through edge and interior analysis. The knowledge of the synecological status of this forest is imperative for appropriate management options for the regeneration of the floral diversity.
MATERIALS AND METHODS
The study area: The study was carried out between 2006 and 2007 at Agulli
forest reserve in the Northwest Province of Cameroon. Agulli forest lies between
longitude 9° 10´ and 10° 20´ East and latitude 6°10´ and 6°
31´ North. The climate is that of a rainforest type with high humidity, high
daily temperatures and high sunshine with average rainfall of 1250 mm annually
which last for more than eight months of the year. Volcanic soil is prominent,
while the rest of the soils are clayey and exhibit ferric properties.
Demarcation of sample plots: Using a sampling intensity of 2.2%, two
sample plots of 1 ha each were demarcated in Agulli forest with total land area
of 89.24 ha (Fig. 1). One plot was located (695 m West and
616 m South) at the left exterior edge, while the other plot was located (617
m South and 694 m West) in the interior of the forest. Each demarcated plots
were thereafter divided into 20 subplots (20x25 m). The layout out of the subplots
is shown in Fig. 2. In each subplot, all trees that were 10
cm dbh and above were identified and their dbh measured. The tree identification
was made by the use of information from herbarium documents and texts (Vivien
and Faure, 1985). Within each of the subplot at the edge and interior of
the forest, parameters such as minimum area; species diversity-dominance index;
importance value index and regeneration status were assessed.
|| Map of Agulii forest
|| Layout of subpolts
Determination of minimum area: The determination of minimum area was
carried out following the procedure of Barbour et al.
(1987). After having identified all the species in the two main plots, separate
list of species composition were established for each of the 20 subplots in
each of the forest edge and interior. Thus each site (edge and interior) had
20 lists comprising the species with dbh of 10 cm and above. Beginning from
the first subplot of each site, the first list was drawn-up with all the species
enumerated appearing. The second list constituted all the species not found
on the first list, that are the additional species. This procedure was repeated
up to the 20th subplot of each of the main plot.
The cumulative number of additional species found in progressive subplots was plotted against the subplot number to produce a line graph for computing and comparing the minimum values in the two main plots (edge and interior of the forest).
Estimation of species diversity/dominance index value: From the 20 subplots
in each main plot, 5 sample plots were selected using the random sampling technique
by Gomez and Gomez (1984). The species diversity was calculated
by using the Simpsons index:
where, C is the summation of the proportion of all individuals tree present
and Pi is proportion of all individuals tree belonging to species
This method was adopted from Keith and Laumonier (2000).
Chi-square test was used to compare the diversity/dominance index between the
edge and interior flora in the forest.
Determination of importance index value: The importance index value
was computed for each species in the 20 subplots using the formula (Pascal
and Pelissier, 1996):
Importance value =Relative density + Relative cover
+ Relative frequency
Assessment of regeneration status: Five randomly selected subplots from
each of the main plot I (subplots 3, 7, 2, 18 and 19) and main plot II (subplots
2, 3, 7, 18 and 19), were used for this assessment. Tables of the different
species with the families, to which they belong, were prepared. The different
species within each of the subplots were grouped into 5 size-classes based on
their dbh. These size classes, based on Igor et al.
(2000) model, were as follows:
Class 1 -------------- 10 to 50 cm
Class 2 -------------- 51 to 100 cm
Class 3 -------------- 101 to 150 cm
Class 4 --------------- 151 to 200 cm
Class 5 --------------- 201 to >250 cm
A t-test to determine the differences among these size-classes between the two main plots (edge and interior) was carried out.
Minimum area determination: Figure 3 and 4
show graphs of the cumulative number of additional species against the increasing
number of subplots sampled at the edge and in the interior. As shown in the
Fig. 3 and 4, the forest edge has no clear-cut minimum area.
As the number of subplots increased, more and more additional species were found.
Thus there were increasing number of new species from subplot 1 to 20 (Fig. 3).
In the interior, a gradual increase in additional new species is observed from subplot 1 to 12. Thereafter, there was a slight increase to subplot 14, which flattens to subplot 20. No additional species were found subplot 19 and 20 (Fig. 4). From this curve, the minimum area was determined subplot 14 which has an area of 700 m2. The forest edge in Aguilli forest therefore, has no minimum area which can be taken to represent the entire community.
Species diversity/dominance index values: The dominant species in the
forest edge of Aguilli were Cola laterita and those classified as unknown.
Both species contributed 21.5% each to an overall dominance index of 0.18.
||Determination of minimum area at the edge of Aguilli forest
reserve (site 1)
||Determination of minimum area at the interior of Aguilli forest
reserve (site 2)
The dominant species in the interior of Aguilli forest was Canthium sp.,
which contributed 54.2% to an overall dominance index of 0.78.
A Chi-square test performed to compare the species diversity of the two sites
shows that, although species dominance index C, is higher in the interior than
at the edge, the values were not significantly different.
||Importance value indices for families identified at the edge
|Importance value = Relative density + Relative cover + Relative
||Importance value indices for families identified in the interior
of Aguilli forest
Importance value: The edge of Aguilli forest had 20 families, with the family Euphorbiaceae having the highest importance value of 64.9 (Table 1). This family had the highest relative density of 15.3 and the highest relative cover of 41.6. The relative frequency of Euphorbiaceae was 8, which was the same with the following families: Mimosoidae, Bignoniaceae, Ceasalpinoidae, Sterculiaceae and Olacaceae.
Fourteen families were identified in the interior of Aguilli forest, with the
family Bignoniaceae having the highest importance value (101.5) (Table
2). This was followed by Moraceae (98.9), Myristaceae (98) and Rubiaceae
(90.7). In the forest interior, Euphorbiaceae had an importance value of 32.2,
a relative density of 5.4 and a relative cover of 18. The families with the
lowest importance values were Chrysobalanaceae (5.4) at the forest edge and
Compositae (7.1) in the interior of the forest.
||Size-class distribution of trees in the edge and interior
of Aguilli forest reserve
|N: No. of trees
The forest interior generally had families with higher importance values than
the forest edge.
Regeneration status: Table 3 is a summary of the number of trees found in the different size-classes at both the forest edge and in the interior of the forest. Within selected subplots at the forest edge, the highest number of trees (40) fall within size-class 51-100, followed by 26 in size-class 10-50. The same trend is observed in the interior of Aguilli forest, with 65 trees in size-class 51-100 and 39 in size-class 10-50 (Table 3).
The t-test conducted to compare the edge (site 1) and interior (site 2) variables indicate that there are no significant differences among the size-classes.
The two sample areas, edge and interior, showed compositional differences in
most of the parameters investigated though there were no significant differences.
The dominance concentration index, which is inversely correlated with species
diversity (Barbour et al., 1987), was in this study,
higher in the interior, than at the edge. This depicts that species diversity
is higher at the edge than in the interior. The Chi-square test shows that there
were no significant differences among species diversity values between the two
sites. However, the dominance concentration index value is significantly different
between the two sites.
The difference in the number of species between the edge and interior sites
of Agulli forest agree with Jon (1999) and
Gehlhausen et al. (2000) both of whom earlier indicated that forest
interior and edge microenvironments differ. Most species are influenced by light
(Jon, 1999) and as such, with more light at the edges,
Agulli forest edge has higher species diversity than the interior.
Several abiotic factors of micro-environments change across the entire gradient,
thus influencing species diversity (Jon, 1999). According
to Gehlhausen et al. (2000), the relative humidity
is higher at edge, while light and soil moisture has the steepest gradients.
Edges bordered by agricultural fields have more extreme changes in microenvironments
than those bordered by trees. Agricultural lands border Agulli forest and thus
such changes may influence dominance concentration attributes between edge values
and interior values, thus agreeing with the findings of Gehlhausen
et al. (2000) and Jon (1999). Biotic factors
such as seed dispersal and competition for decomposed litter also give rise
to changes in flora composition between edge and interior (Nigel
et al., 2000).
Importance value indices are always useful in regeneration programmes because
if these values are not commensurate to usefulness, some species or entire families
could be eliminated from the forest and the forest, maintained through the most
appropriate silvicultural system (Pascal and Pelissier, 1996).
Importance values could also determine the level of dominance of family or species
in terms of relative basal area and relative density (Pascal
and Pelissier, 1996).
At the edge, the family Euphorbiaceae has the highest Importance value index,
while Bignoniaceae has the highest in the interior. Some of the species in these
families could be very competitive and best adapted under conditions of low
light intensity thus out-competing the fewer more useful species requiring higher
light intensities. Bob (1992) reviewed the competitive
nature of grasses at the forest edge with forest species especially at the period
of seedling establishment. Bob (1992) claimed that the
competitive vigour of the non-forest species for factors such as water, nutrients,
space and microbiological factors of soil probably gives rise to forest edges
having a lower dominance concentration value and thus, a higher species density.
This could probably explain the overall higher species diversity at the edge
of Aguilli forest.
The interior flora of Agulli forest has a higher dominance index with lower
species diversity, probably, owing to the superior shade-tolerant adaptive nature
of the fewer species, out-competing and subsequently eliminating the weaker
species or pushing them gradually to the forest edge. Thus the postulate that
there are more interior species at the edge than edge species at the interior
(Whittaker, 1975), holds for Agulli forest which has an
increasing cumulative number of additional species as one moves from one subplot
area to the other.
Establishment of young plants after germination is not much of a problem in
Agulli forest. This was observed from the non-significant values of the lower
size-classes between the interior and edge sites. However, the species at the
edge are subjected to competition from grasses and circulation of air since
they are bordered by agricultural lands (Lugo, 1997;
Gehlhausen et al., 2000). This flow increases
moisture stress in the soil through higher transpiration and evaporation rates
and thus creating xerophytic conditions, which lead to a poor germination of
seeds and a slower establishment of the younger trees as well as their movement
into higher size-classes. The higher air circulation may have its advantage
in reducing moldiness and fungal attack and therefore aid better seed viability.
Trees in the interior of the forest do not experience such stress conditions
thus it was expected that a higher proportion of the lower size-class trees
developed into higher size-classes and thus, agreeing with the report of Harper
and Ellen (2002) on the structure and composition of edges being different
from the forest edge.
The non significant difference between the two sites in size-classes however,
can be attributed to biotic disturbances such as competitive interaction at
the edge between grasses, agricultural crops and other edge trees, coupled with
intense competition for abiotic resources, while within the interior, it can
be attributed to the adaptive nature of the fewer species to less intense abiotic
factors (Luken et al., 1997). This leads to
the more adaptive species in the interior, eliminating the lesser adaptive ones
and hence fewer higher size-class trees.
The TSS has been practiced in many tropical forests and has proven successful
(Lof et al., 1998; Loftis,
1990). This system will be good for Agulli forest since creating canopy
gaps at successive stages will give an assessment of seedlings and seeds present
on the forest floor and the rate at which these can re-establish in new conditions.
The enrichment planting system is recommended as alternative for the development
and regeneration of Agulli forest. Gaps can be created in the forest and the
seeds and seedlings of the desired species introduced in these gaps. This method
has proven successful in the Australian subtropical rainforest (Igor
et al., 2000).