dc.description.abstract |
Agroforestry is an important land use strategy for carbon sequestration. Trees in these
landscapes mitigate climate change by storing carbon in tree biomass and by raising
soil organic carbon levels. While agroforestry stores larger amounts of carbon, this
potential is poorly quantified because of the high spatial and temporal heterogeneity
of trees on farmlands, and methodological challenges. As a result, the role of
agroforestry in carbon sequestration is poorly understood and often underestimated
and the sector is often left out in most national carbon accounting systems. This gap
has led to dearth of data, variable conclusions and a fragmented understanding of the
role of trees in smallholder farms in climate change and development. In addition, the
relationship between tree species diversity and carbon stocks in different agroforestry
practices is poorly understood. The aim of this study was to i) determine tree species
diversity under different agroforestry practices in Kakamega Forest ecosystem; ii)
determine tree biomass and soil organic carbon under different agroforestry practices
in Kakamega Forest ecosystem and iii) extrapolate biomass and soil carbon stocks in
Agroforestry practices in Kakamega Forest ecosystem for the next 50 years. The
study was conducted at two sites (Kakamega North and Kakamega South) adjacent to
Kakamega Forest. A total of 16 farms were randomly selected, 8 farms from each of
the sites. An inventory of all trees in each of the farms was conducted, capturing
diameter at breast height (DBH), the species name, and the management of trees
within two dominant agroforestry practices-homegardens and hedgerows. Tree
circumference at breast height, 1.3 m from the ground was measured by use of tape
measure for trees ≥5cm. Measurement of circumference was converted to DBH by
dividing pi (π = 3.142) with circumference. Soil samples were taken at 0-10cm and
10-30cm in each of the 10x10m plots in each of the farms. Soil Organic Carbon
(SOC) was determined by Walkley and Black method. Biomass and SOC simulations
were determined using CO 2 FIX model. Aboveground biomass (AGB) of trees was
determined by applying allometric equations to DBH measurements. Two models
0.091× (DBH) 2.472 by Kuyah and 2.134× (DBH) 2.53 for tropical dry forests by Brown
were used for estimation of biomass from tree measurements. Below ground biomass
(BGB) was estimated by using a root-to-shoot ratio of 0.26. Homegardens had both a
high number of tree species (n=48) and high tree density in the two sites combined,
and in each of the sites - 562 trees per hectare in Kakamega North and 408 trees per
hectare in Kakamega South. Shannon diversity index revealed a high tree diversity in
Kakamega north (H ́=1.92±0.13) than Kakamega south (H ́=1.71±0.16), and in
homegardens (H ́=1.98±0.14) than in hedgerows (H ́=1.65±0.14). A total of
13.96±0.37 Mgha -1 (6.4MgCha/ha) of aboveground biomass was estimated for the
study area using the equation by Kuyah. Below ground biomass was estimated to be
3.45±0.09 Mg ha -1 (1.6MgC/ha), giving total biomass held in live trees on farms to be
17.22±1.65 Mgha -1 (8.0MgC/ha). Equation by Brown consistently gave higher
estimates per site and agroforestry practice. Kakamega North had significantly
(F=35.03; p=0.01) higher biomass (9.7Mg/ha) compared to Kakamega South
(7.51Mg/ha); corresponding to the higher tree density in the north compared to the
southern part. Similarly, home gardens had significantly higher (F=45.2; p=0.001)
aboveground biomass (9.85Mg/ha) than hedgerows (7.36Mg/ha). SOC determined in
the two study sites was 14.91MgC/ha. Kakamega North had 6.9MgC/ha while
Kakamega South had 8.01MgC/ha. The two sites showed a decline in SOC with
depth. The CO2FIX model simulated the SOC and total carbon stocks in the studied
agroforestry practices, but the prediction of the biomass carbon stocks could be
improved by acquiring more accurate input parameter values for running the model. |
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