Carbon Storage in Agroecosystems: A Case Study of the Cocoa Based Agroforestry in Ogbese Forest Reserve, Ekiti State, Nigeria

doi:10.4236/jep.2011.28123

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Journal of Environmental Protection, 2011, 2, 1069-1075

doi:10.4236/jep.2011.28123 Published Online October 2011 (http://www.scirp.org/journal/jep)

Carbon Storage in Agroecosystems: A Case Study

of the Cocoa Based Agroforestry in Ogbese Forest

Reserve, Ekiti State, Nigeria

David Oke, Ayodeji Olatiilu

Department of Forestry and Wood Technology, Federal University of Technology, Akure, Nigeria.

Email: [email protected]

Received June 8th, 2011; revised August 19th, 2011; accepted September 18th, 2011.

ABSTRACT

Large areas of the indigenous tro pical forests in the sou thwestern part of Nigeria a re being converted into agricultural

lands and this has been repo rted to have serious implication s for biodiversity and the environmen t. Cocoa based agro-

forestry is one of the common agricu ltural practices in this region and comparative information on the carbon storage

capacity of the cocoa agroforests is generally lacking. In this study the above-ground carbon storage and partitioning

in a protected primary forest were evaluated and compared with those of the two categories of cocoa agroforests

(sparse and dense) identified in the area. Above-ground biomass accumulation and carbon stock varied significantly

with land use type, with the primary rainforest having the highest values and sparse cocoa agroforests having the low-

est. A reduction in above-ground carbon stock of 89.82% and 71.20% was observed 10 years after conversion of tropi-

cal rainforest to sparse and dense cocoa agroforests respectively.

Keywords: Carbon Sequestration, Cocoa, Agroforestry, Forest Conversion, Global Climate Change

1. Introduction

Carbon dioxide (CO2) is one of the greenhouse gases and

a primary agent of global warming. It constitutes 72% of

the total anthropogenic greenhouse gases, causing be-

tween 9% - 26% of the greenhouse effect [1]. Reference

[2] reported that the amount of carbon dioxide in the

atmosphere has increased from 280 ppm in the pre-

industrial era (1750) to 379 ppm in 2005, and is increas-

ing by 1.5 ppm per year. Dramatic rise of CO2 concen-

tration is attributed largely to human activities. Over the

last 20 years, majority of the emission is attributed to

burning of fossil fuel, while 10% - 30% is attributed to

land use change and deforest at i on [3] .

Article 4 of the United Nations Framework Conven-

tion on Climate Change (UNFCCC) requires preventing

and minimizing climate change by “limiting anthropo-

genic emissions of greenhouse and protecting and en-

hancing greenhouse gas sinks and reservoirs” [4]. Forest

ecosystem plays very important role in the global carbon

cycle. It stores about 80% of all above-ground and 40%

of all below-ground terrestrial organic carbon [3]. How-

ever, the state of tropical forests has continued to dete-

riorate.

Reference [5] reported that the protection of existing

forests, regeneration of degraded forests and raising of

forest plantations have been contributing to enhanced

carbon stock in India. However, the data available on

carbon sequestration i.e. net woody biomass accumula-

tion in trees for long term storage in tropical forests are

extremely limited and incomplete. Thus, the improved

quantification of carbon pools and fluxes in tropical for-

est ecosystems is important for understanding the con-

tribution of these forests to net carbon emissions and

their potential for carbon sequestration [6].

Agroecosystems play a central role in the global car-

bon cycle and contain approximately 12% of the world

terrestrial carbon [7]. From the perspective of climate

change and the global carbon cycle, agroforestry is at-

tractive because the tree component has the capacity to

fix and store carbon from the atmosphere for many years.

The amount of carbon sequestered largely depends on

the agroforestry system put in place, the structure and

function of which are, to a great extent, determined by

environmental and socio-economic factors. Other factors

influencing carbon storage in agroforestry systems in-

clude tree species and system management.

Carbon Storage in Agroecosystems: A Case Study of the Cocoa Based Agroforestry in Ogbese Forest Reserve,

1070 Ekiti State, Nigeria

Cocoa agroforests are a common farming system in

the humid zone of West and Central Africa, in which

forest trees provide shade and other environmental ser-

vices as well as marketable products [8]. Traditionally,

small holder cocoa farmers establish their farms by re-

moving the forest under-storey and thinning the forest

canopy so that Coco a seedlings can grow into produ ctive

trees, as for example in Cameroon [9]. Many authors

[10-14] have described the physiological, environmental

and economic values of shade trees in cocoa growing

systems. They cited benefits such as shade to cocoa, soil

fertility maintenance, biodiversity conservation, protec-

tion against drought, bush fires and insect attacks as well

as additional income through sales of timber species, fuel

wood, and non-wood forest products.

The trees in cocoa agroforests can store carbon in their

shoots and roots, while performing the aforementioned

roles thereby reducing the greenhouse effect. This study

was carried out to assess and compare carbon storage in

cocoa based agroforestry systems and a relatively un-

touched rainforest in Ogbese Forest Reserve, Ekiti State,

Nigeria.

2. Materials and Method

2.1. The Study Area

The study was carried out in Ogbese Forest Reserve in

Ekiti State, (Lat. 7˚31' and 7˚49'N and Lat. 5˚7' and

5˚27'E). The area lies entirely within the pre-Cambrian

Basement Complex rock group which underlies much of

Nigeria. The elevation reaches 600 m above the sea level

and is situated entirely within the upper Ogbese basin.

The area experiences a tropical climate with distinct

wet and dry seasons. Th e rainy season lasts for 9 months

annually between March and November while the dry

season lasts for 3 months between December and Febru-

ary. The annual mean total rainfall is 1367 mm; the av-

erage number of the rainy days is 112 per annum [15].

Temperature is almost uniform throughout the year

with very little deviation from the mean annual of 27˚C.

The mean annual relative humidity varies between 50%

and 95% and is high est in the rainy season months.

2.2. Sampling

Ten-year-old cocoa farms established in, and around

Ogbese Forest Reserve, Ekiti State were visited to select

appropriate sites for this study. Based on the number of

shade trees (non cocoa trees) per unit area, the farms

were classified into dense and sparse mixtures. Four

farms under each category were selected for detailed

biomass measurement. Sample plots were also demar-

cated within the natural forest whose canopy had not

been disturbed by th e activity of a coco a farmer or wher e

there is little or no evidence of timber extraction. One

plot (25 m × 25 m in size) was located in each of the

selected cocoa farms and four at random locations within

the natural forest.

2.3. Biomass Estimation

All cocoa trees in each plot were measured for stem di-

ameter distribution. Two mean cocoa trees with dbh

nearest to the mean dbh were located in each of the plots

within cocoa farms and selected for destructive sampling

after their heights, diameters at the base, at the middle

and at the top have been measured and recorded. The

biomass measurements were based on the biomass sub-

sampling method outlined by [16]. The two mean cocoa

trees were felled at the ground level. Each felled tree was

sorted into the three main components; bole, branches

and foliage and each component was cut into small

pieces for easy weight measurement. Samples were taken

to the laboratory for dry weight determination. As rec-

ommended by [16], stem material removed in saw cuts

were also considered as 0.5% of the stem biomass. Vo-

lumes of the two (2) mean cocoa trees in each sample

plot were calculated using the Newton’s formula by [17].

Diameter and height measurements of all the non co-

coa trees in each plot were also taken and their volumes

calculated using the Newton’s formula by [17]. However,

due to the variety and size of non cocoa tree species en-

countered and the difficulty in getting the cocoa farmers

to allow felling of such shade trees because of the fear of

massive destruction of non target cocoa trees that could

accompany such an exercise, the above ground biomass

of non cocoa trees in the plots was estimated indirectly

from the volume data using the formula:

Aboveground biomass = VOB × WD × BEF

where:

WD = volume-wei g ht ed average wood dens it y

BEF = biomass expansion factor (ratio of aboveground

oven-dry biomass of trees to the oven-dry biomass of

inventoried volume). WD was estimated as described by

[18].

The wood density values (Table 1) were obtained

from [18]. The biomasses were added for each plot and

expanded to biomass in tonnes per hectare.

2.4. Carbon Estimation

The carbon concentration of different tree parts is rarely

measured directly, but generally assumed to be 50% of

dry weight [19]. Hence in this study, the aboveground

carbon stock was calculated by assuming that the carbon

content is 50% of the total aboveground biomass [5,

Carbon Storage in Agroecosystems: A Case Study of the Cocoa Based Agroforestry in Ogbese Forest Reserve,

Ekiti State, Nigeria

1071

20-24].

2.5. Data Analysis

Correlation analysis was carried ou t to examine relation-

ships between some paired growth parameters. Carbon

storage values estimated for sparse mixtures, dense mix-

tures and the natural forest were compared using one

way Analysis of Variance (ANOVA). The three (3)

stands formed the treatments while the four (4) plots

sampled in each of the stands formed the replicates. The

test was conducted for significant difference in the car-

bon stock for the three different stands. Mean separation

was carried out with Fisher’s Least Significant Differ-

ence (LSD) where significant differences occur (P <

0.05).

3. Results

3.1 Distribution of Cocoa and Shade Tree

Species

Seventy six shade trees were encountered in the 1 ha of

dense cocoa agroforests and these were made up of 9

different species in 5 families while 40 shade trees en-

countered in sparse cocoa agroforests and these were

distributed in 5 species and 4 families. In the one hectare

of natural forest surveyed, 166 trees were encountered

and these were distributed in 15 species and 7 families

(Table 2). Ficus mucuso was found to be the most fre-

quently occurring species with 41, 22 and 18 trees in the

natural forest, dense cocoa agroforest and sparse cocoa

agroforest respectively. This was followed by Antiaris

africana occurring 25, 14 and 10 times respectively. The

highest number of trees (336) were encountered in the

sparse cocoa agroforests and these were made up of 296

cocoa trees and 40 shade trees (Table 3). In the dense

mixture 308 trees were recorded made up of 232 cocoa

and 76 shade trees. There were 166 trees in the natural

forest which contained no cocoa tree.

The summary of the cocoa tree growth data presented

in Table 4 for Sparse and Dense Cocoa Agroforests

shows that Mean Dbh and Mean Height respectively

Table 1. Scientific names, family and wood density (as obtained from [18]) of the non cocoa tree species encountered in the

study area.

Species Family Wood Density (t/m3)

Afzelia Africana Kuntze Leguminoseae 0.63

Triplochiton scleroxylon Schumann Sterculiaceae 0.32

Ficus mucuso Welw ex. Ficalho Moraceae 0.39

Pterygota macrocarpa K. Schum Sterculiaceae 0.52

Terminalia superb Engl. & Diels Combretaceae 0.45

Antiaris Africana Engl. Moraceae 0.37

Sterculia oblonga Mast. Sterculiaceae 0.61

Alstonia boonei De Wild Apocynaceae 0.33

Entandrophragma utile Sprague Meliaceae 0.53

Celtis zenkeri Engl Ulmaceae 0.59

Daniella oliveri Rolfe Caesalpiniaceae 0.40

Terminalia ivorensis A. Chev Combretaceae 0.45

Holoptelia grandis Hutch (Mildbr) Ulmaceae 0.59

Nesogordonia papaverifera A.Chev. Sterculiaceae 0.65

Khaya ivorensis A.Chev. Meliaceae 0.44

Pycnanthus angolensis Welw Myristicaceae 0.40

Daniella ogea Harms Leguminoseae 0.40

Gossweilerodendron balsamiferum Harms Leguminoseae 0.40

Carbon Storage in Agroecosystems: A Case Study of the Cocoa Based Agroforestry in Ogbese Forest Reserve,

1072 Ekiti State, Nigeria

Table 2. Diversity of non cocoa/shade tree species in 1 ha of sampled ecosystems in Ogbese Forest Reserve.

Natural Forest Dense Cocoa Agroforest Sparse Cocoa Agroforest

Species Freq. Species Freq. Freq.

Afzelia africana 10 Triplochiton scleroxylon 9 Ficus mucuso 18

Triplochiton scleroxylon 12 Ficus mucuso 22 Antiaris africana 10

Ficus mucuso 41 Antiaris Africana 14 Sterculia oblonga 4

Pterygota macrocarpa 11 Pterygota macrocarpa 3 Terminalia ivorensis 6

Terminalia superba 13 Entandrophragma utile 9 Alstonia boonei 2

Antiaris africana 25 Afzelia Africana 4

Sterculia oblonga 10 Daniella oliveri 5

Alstonia boonei 7 Sterculia oblonga 7

Entandrophragma utile 12 Terminalia ivorensis 3

Celtis zenkeri 3

Daniella oliveri 3

Terminalia ivorensis 6

Holoptelia grandis 4

Nesogordonia papaverifera 2

Khaya ivorensis 7

Total 166 Total 76 Total 40

Table 3. Distribution of cocoa and non-cocoa trees in 1 ha of sampled ecosystems in Ogbese Forest Reserve.

Stand Type No of Cocoa Trees/ha No of Shade Trees/ha Total no of trees/ha

Sparse Cocoa Agroforest 296 40 336

Dense Cocoa Agroforest 232 76 308

Natural Forest - 166 166

Table 4. Distribution of aboveground biomass in the c ocoa trees of ten-year-old co c o a agr oforests in Ogbese Forest Reserve.

Stand Type Tree Components Biomass (t/ha) Proportion (%) Carbon (t/ha) Proportion (%)

Sparse Cocoa Agroforest Stem 5.40 83.98 2.70 83.85

Branches 0.81 12.60 0.41 12.73

Foliage 0.22 3.42 0.11 3.42

Total Aboveground Biomass6.43 100.00 3.22 100.00

Dense Cocoa Agroforest Stem 4.80 87.11 2.40 86.96

Branches 0.58 10.53 0.29 10.51

Foliage 0.13 2.40 0.07 2.54

Total Aboveground Biomass5.51 100.00 2.76 100.00

Carbon Storage in Agroecosystems: A Case Study of the Cocoa Based Agroforestry in Ogbese Forest Reserve,

Ekiti State, Nigeria

1073

were both higher in Dense Cocoa Agroforest (11.70 cm

and 6.79 m) than in the sparse mixture (11.30 cm and

6.66 m), other parameters i.e. Basal area/ha and Volume/

ha were also higher in the Dense stand th an in the Sparse

mixtures. The Basal area/ha was 2.20 m2 and 2.49 m2

while volume/ha was 10.46 m3 and 12.47 m3 in the

Sparse and Dense Cocoa Agroforests resp ectively. Simi-

larly, the shade trees in Dense Cocoa Agroforests were

higher in all the growth parameters than those in the

Sparse stand as many large trees had been removed from

the Sparse stand while these trees were still providing

much shade in the Dense Cocoa Agroforest. Mean Dbh

and Mean Height of shade trees were 43.60 cm and

21.82 m in the Dense stand compared to 38.30 cm and

21.15 respectively in the Sparse stand. Basal area and

volume of shade trees per ha were 14.92 m2 and 192.65

m3 respectively in the dense stand while 4.46 m2 and

50.80 m3 were recorded for the sparse.

3.2. Aboveground Biomass Accumulation and

Partitioning

The partitioning of biomass in the cocoa trees is pre-

sented in Table 5. A total abo ve ground biomass of 6.43

t/ha was obtained for cocoa trees in the sparse cocoa

agroforest. Stem biomass accounted for an average of

83.98% (5.40 t/ha) while branch and foliage biomasses

accounted for an average of 12.60% (0.81 t/ha) and

3.42% (0.22 t/ha) respectively. In the Dense Cocoa

Agroforests, the total above ground biomass of cocoa

trees was 5.51 t/ha. Stem biomass was found to be

87.11% (4.80 t/ha) while branch and foliage biomasses

were 10.53% (0.58 t/ha) and 2.40% (0.13 t/ha) respec-

tively. The highest total above ground biomass (333.34

t/ha) was observed in the natural forest while the least

was in the sparse cocoa agroforest (Table 6). Only

5.73% of the 96.01 t/ha of biomass observed in the dense

cocoa agroforests was accounted for by the cocoa trees.

The cocoa trees also accounted for 18.97% of the above-

ground biomass in the sparse cocoa agroforests.

3.3 Carbon Storage in the Cocoa Agroforests

and the Natural Forest

Figure 1 shows the aboveground carbon storage in the

three types of ecosystems considered. Statistical analysis

showed that Carbon storage/ha varied significantly

among Sparse Cocoa Agroforest, Dense Cocoa Agrofor-

est and the Natural Forest. The highest value of 184.99

t/ha was obtained for the natural forest while the least

was in the sparse cocoa agrofore st.

4. Discussion

Numerically, there were more trees in the sparse cocoa

agroforests than in the other two ecosystems but a high

proportion of these were cocoa trees which were smaller

in size compared to the non cocoa trees. The summary of

the Cocoa tree growth data shows that the Dense Cocoa

Agroforest had higher Mean Dbh and Mean Height than

the Sparse Cocoa Agroforest and there were more natural

or shade trees in the Dense mixtures than in the sparse

mixtures. The lower growth parameters (Mean Dbh,

Mean Height, Mean Basal Area and Mean Volume) in

the sparse mixture may be attributed to the closer spacing

of the cocoa trees as there were more cocoa trees per

hectare than in the Dense mixtures. The minimum and

maximum Dbh recorded for cocoa trees in the Dense

Cocoa agroforest were also higher than those of the Sparse.

Table 5. Growth of cocoa and non cocoa trees in the cocoa agroforests and natural forest of Ogbese Forest Reserve.

Sparse Cocoa Agroforest De nse Cocoa Agroforest Natural Forest

Cocoa Non-Cocoa Cocoa Non-Cocoa Non-Cocoa

MDbh (cm) 11.30 ± 0.00 38.30 ± 0.12 11.70 ± 0.00 43.60 ± 0.01 49.43 ± 0.08

MHt (m) 6.66 ± 0.51 21.15 ± 1.21 6.79 ± 0.12 21.82 ± 0.86 24.76 ± 1.87

No./ha 296.00 ± 1.29 40.00 ± 0.58 232.00 ± 0.58 96.00 ± 0.82 188.00 ± 1.71

Vol/ha (m3) 10.46 ± 0.02 50.80 ± 0.49 12.47 ± 0.00 192.65 ± 0.24 673.88 ± 1.04

Ba/ha (m2) 2.20 ± 0.00 4.46 ± 0.08 2.49 ± 0.00 14.92 ± 0.01 39.96 ± 0.06

Table 6. Aboveground biomass (t/ha) the 10-year-old c ocoa agrofore sts and natur al fore st of Ogbese F ore st Reser ve.

Sparse Cocoa Agroforest Dense Cocoa Agroforest Non-Cocoa

Cocoa 6.44 5.51 0

Shade tree 27.50 90.50 333.34

Total 33.94 96.01 333.34

Carbon Storage in Agroecosystems: A Case Study of the Cocoa Based Agroforestry in Ogbese Forest Reserve,

1074 Ekiti State, Nigeria

Figure 1. Biomass and carbon storage/ha in the study area.

Growth parameters such as MDbh, MHt, Volume and

Basal area of non cocoa trees were all higher in the Dense

Cocoa Agroforest than in the Sparse mixture. This may

be attributed to the removal of many large shade trees in

the Sparse mixture to open the canopy for the establish-

ment of Cocoa plantation.

The mean Aboveground Biomass (AGB) for Cocoa

trees in the Dense Cocoa Agroforest was higher than that

of the Sparse mixture. This is in agreement with the re-

ports of [25] on the effect of shade tree in an 8-year-old

cocoa Agroforestry system in which they concluded that

Cocoa Biomass was higher under shade. The proportion

of AGB concentrated in the stem was also found to be

higher in Dense Cocoa Agroforest than in the Sparse.

This is due to the fact that more AGB were in branches

and foliag e of Sp arse Coco a Agr ofo re st th an in the Den se

Cocoa Agroforest. Cocoa trees in the Dense Cocoa

Agroforests were found to have more stem Bio- masses

and less branch and foliage Biomasses than those in the

Sparse Cocoa Agroforest.

The aboveground Biomass of shade trees in the Dense

mixture was higher than in the Sparse due to the number

and size of the trees found in the stands. Total AGB/ha

was highest in the natural forest followed by the Dense

and the Sparse mixtures in that order for the same reason.

TAGB was more than three times higher in the natural

forest than in the Dense Cocoa agroforest. Reference [26]

reported that the values for natural forest could be about

3 - 4 times higher. The total aboveground Biomass ob-

tained in the natural forest is comparable to 444.70 t/ha

obtained by [ 2 7] in Indonesia.

The carbon storage va lue of 57.5 5 t/h a observ ed for ten

year old Dense Cocoa Agroforest in this study was less

than that obtained in Mango Agroforestry system (121.1

t/ha) in Indonesia [28] and higher than that of

five-year-old Cocoa-gliricidia (38.86 t/ha) [29].

In the natural forest, the large trees contributed more

than 45% to the total AGB. The greater contribution of

large trees to AGB in natural forest is in agreement with

the findings of previous workers [30-32] who reported up

to 50% contribution to AGB by large trees (of Dbh > 0.7

m). A higher proportion of AGB in the large trees in the

natural forest indicates that such trees play important role

in Carbon storage; no twithstand ing, small trees (of Dbh <

0.6 m) enhance future Carbon storage as they are high in

Carbon storage potential.

5. Conclusions

The study shows that AGB of Cocoa increases with in-

crease in density of shade trees. It was also discovered

that growth characteristics of Cocoa are higher under

shade.

It has been shown clearly by the result of this study

that Cocoa Agroforests store substantial Carbon as seen

in the Dense Cocoa Agroforest. From the perspective of

climate change, Cocoa Agroforests are attractive; not

only that the Cocoa component stores Carbon in addition

to the natural trees but also that the system potentially

slows down deforestation by reducing the need to com-

pletely clear forestland for agriculture. This will be more

acceptable to rural farmers than complete afforestation.

Therefore Cocoa Agroforest is recommended as one of

the options in combating climate change.

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