Influence of Invasive Acacia melanoxylon on the Forest Stand Volumes and Annual Increment of Commercial Tree Species in Two Humid Forests (Nabkoi and Timboroa Forests, Kenya)

Background and Objective: The ecological relationship between Acacia melanoxylon invasion and forest ecology is not well understood in tropical humid forest ecosystems. Therefore, the main objective of this study was to determine the influence of Acacia melanoxylon on the volume and annual increment of two commercial forest tree species (Cupressus lusitanica and Pinus patula) in a humid tropical forest (North Tinderet Forest, in Kenya). Materials and Methods: Data were collected using a 10×10 m plot overlaid on a 500 m transect in univaded and invaded sites. Tree density, Diameter at Breast Height (DBH) >1.3 m and tree heights were measured. Volume and the mean annual biomass increment (MAI) were calculated. Results: There were significantly (p<0.05) higher tree densities, DBH and height in Timboroa than Nabkoi regardless of invasion. Further, the increased density of Acacia melanoxylon suppressed the volume of commercial tree species. These findings suggest that Acacia melanoxylon invasion reduced volume through reduced annual growth increment of commercial tree species. The proactive management of invasive Acacia melanoxylon should be considered to prevent the loss of tree volumes. Conclusion: However, the complexity of controlling both invasion and restoration side effects highlights the importance of taking a preventive approach.