Determination of species abundance, diversity and carbon stocks in Kakamega Forest Ecosystem.

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.