Spatial Heterogeneity in Tree Diversity and Vegetatively Sequestered Aboveground Carbon Stocks in Kakamega and North Nandi Forest ecosystems, Kenya
Authors: Obonyo Amos , Tsingalia Harrison Mugatsia , Agevi Humphrey 
Published: January 2023

Abstract
Monitoring and measuring spatial changes in aboveground carbon stocks distribution in forest ecosystems and how tree diversity influences it is key to understanding the role of vegetation in implementing climate change mitigation strategies in addition to promoting sustainable forest management. This study was carried out to evaluate spatial changes in aboveground carbon stocks and their relationship to tree species diversity in Kakamega and North Nandi Forest ecosystems. Specifically, the study (i) determined the spatial changes in aboveground carbon stocks (ii) evaluated how changes in species diversity affect aboveground carbon stocks. This study was carried out in the North Nandi and Kakamega Forest ecosystems. Three sites based on forest condition sites were selected in each of the forest ecosystems. These were least disturbed, plantations and disturbed sites. Nested sampling was used to study mature trees and shrub and herbs. All trees greater than 5cm in diameter were counted, identified to species level and recorded. Diameter at breast height was measured using the diameter tape, 1.3m above the ground for DBH ≥ 5cm. Tree height was measured using the Suunto clinometer while wood density was obtained from wood density database at http://db.worldagroforestry.org/wd. An estimation of carbon stocks in aboveground woody biomass was done using allometric models based on non-destructive method. Statistical Package for Social Sciences (SPSS) version 25 was used to analyze the data. One-way analysis of variance was used to assess the variation in carbon stocks and tree diversity between forest ecosystems and among forest types. Correlation was used to establish the relationship between tree diversity and above-ground carbon stocks. Kakamega Forest had the highest mean aboveground carbon stocks (157.93 ± 26.91 MgC ha− 1) while North Nandi Forest had (97.83 ± 19.89 MgC ha− 1). Least disturbed forest areas recorded the highest mean aboveground carbon stocks (65.96 ± 8.56 MgC ha− 1). Plantations recorded (26.69 ± 1.12 MgC ha− 1) while disturbed forest type had (3.26 ± 0.11 MgC ha− 1). There was statistically significant spatial variation in aboveground carbon stock (F(5,18) = 6.523; p < 0.05) between the Kakamega and North Nandi Forest ecosystems. In the Kakamega Forest ecosystem, AGC was highest in least disturbed sites (316.31 ± 15.64 MgC ha− 1), followed by plantations (154.96 ± 14.99 MgC ha− 1), and lastly disturbed sites (2.53 ± 0.77 MgC ha− 1). The AGC variation among the forest types in the Kakamega Forest ecosystem was significant (F(2,9) = 5.966, p < 0.05). In the North Nandi Forest ecosystem, AGC was highest in least disturbed sites (211.40 ± 40.82 MgC ha− 1), followed by plantations (58.57 ± 16.06 MgC ha− 1), and lastly disturbed sites (23.54 ± 9.85 MgC ha− 1). Shannon Wiener’s diversity index revealed a relatively higher tree species diversity in Kakamega Forest (H´= 1.82 ± 0.95) relative to North Nandi Forest (H´= 1.24 ± 0.88). There was a significant variation in spatial tree diversity among different forest types (F(5,18) = 7.311; p < 0.05). There was also a significant positive correlation between the aboveground carbon and tree species diversity (r = 0.62, p < 0.05). Both Kakamega and North Nandi Forest ecosystems independently revealed statistically significant positive correlation between AGC and species diversity (r = 0.67; p < 0.05); and (r = 0.60; p < 0.05) respectively. Statistically significant strong positive correlation was recorded between AGC and species abundance (r = 0.68; p < 0.05); AGC and species richness (r = 0.85; p < 0.05) and AGC and DBH (r = 0.92 at p < 0.05). Kakamega and North Nandi forests vary spatially and temporally in their above ground carbon stocks. Tree species diversity positively influences the above-ground carbon stock of the two forest ecosystems.