Influence of selected soil properties on tree species diversity and above-ground biomass in Budongo Forest Reserve, Uganda

A thesis submitted to the Directorate of Research and Graduate Training in partial fulfillment of the requirements for the award of a Master of Science in Integrated Watershed Management of Makerere University.

Tree species diversity and biomass are important indicators of the forest carbon storage capacity. The role of soil characteristics in shaping these two, however, has not been adequately studied in tropical forests of Africa. This study determined the tree species diversity and composition, then investigated the relationship between observed tree species diversity and above-ground tree biomass with selected soil properties in one of the compartments of Budongo Forest Reserve. Data on vegetation and soil variables was collected using thirty-two 40 x 40 m plots, with soil assessments done in nested 30 X 30 m sub-plots. Multiple linear regression was used to establish the relationships between selected soil properties, tree species diversity, and biomass. A total of 15,239 stems representing 117 species in 96 genera and 44 families were recorded in the entire study area. The stem density for trees 1cm ≤ dbh< 10cm diameter was 6,289 stems ha-1and those ≥ 10 cmdbh, 618 stem ha-1. The basal area for the study site was 44.87 m2 ha-1with the small trees having a basal area of 22.41 m2ha-1and the ≥ 10 cm trees, 36.47 m2ha-1. The overall estimated AGB of the study site ranged from 249.9 Mg ha-1to 3,651 Mg ha-1with a mean of 580.2 ± 102.4 Mg ha-1. The area is dominated by shade-tolerant tree species (38.9 % of the identified species) and the most common dispersal mode of many tree species is animal dispersal (64.4% of the sampled species). The relationship between both tree species diversity and biomass, with soil properties, showed uniqueness with soil depth. Soil pH, K, and P in the topsoil were key in explaining the observed variation in the small tree species diversity. Above-ground small tree biomass variation was greatly explained by SOM and P in the top soil; sand, silt, and K in the sub-soil. Finding from this study improves our understanding of forest ecosystem function, which should help inform the management of Budongo forest to ensure the sustainability of the Budongo watershed. It also calls for the adoption of soil nutrient conservation efforts. The low diversity of light demanders highlights the need for enrichment planting and forest protection, especially given that these tree species contribute much to the production management option of the forest.