Diversity, abundance and population structure of tree species in Mabira Central Forest Reserve and adjacent fragments

Abstract

Mabira Central Forest Reserve (CFR) and its adjacent fragments face significant anthropogenic and ecological threats, including resource extraction and changes in land use. This has led to degradation and fluctuations in forest size, biodiversity loss, an increase in invasive species, and ecological changes. Understanding the diversity of tree species and the effects of fragmentation in Mabira CFR fragments is crucial for developing informed conservation strategies, restoring habitats, promoting connectivity, and balancing conservation efforts with development to maintain biodiversity. The study aimed to assess the influence of fragmentation on tree species diversity and abundance in Mabira CFR and adjacent forest fragments of Nandagi and Namukupa CFRs. The study specifically sought to compare: a) tree species diversity in Mabira CFR and its adjacent forest fragments, and b) tree density in Mabira CFR and its adjacent fragments. Tree species diversity, community similarity, composition, community structure, and population structure were investigated. Systematic sampling was employed, and along each transect, 30 m x 30 m quadrats were established alternately. Per quadrat, trees’ DBH were recorded under the different size classes, and the total number of trees per species in a quadrat was counted. The tree species were identified at the Makerere University Herbarium. The results revealed a total of 95 tree species, belonging to 66 genera and 37 families. Eighty-six species were found in Mabira, 67 in Nandagi, and 37 in Namukupa. The evaluation of tree density across different size classes revealed that Nandagi and Namukupa exhibited an approximation of an inverse J-curve, indicative of ongoing regeneration and a relatively healthy community structure. Mabira CFR, though the most intact forest, had a less stable and unhealthy structure with inconsistent densities across size classes. Mabira CFR had the highest tree species diversity (H’ = 2.58), followed by Nandagi (H’ = 2.16) and Namukupa (H’ = 1.01), and these differed significantly (p = 1.492e-10, n = 3). The most important tree species based on their Importance Value Index values were Trilepisium madagascariense DC. In Mabira CFR, Cedrela odorata L. in Nandagi CFR, and Broussonetia papyrifera (L.) L'Hér. ex Vent in Namukupa CFR. Namukupa CFR had the highest median stem density among the three study sites (1100 trees/ha), followed by Nandagi CFR (1033.3 trees/ha) and Mabira CFR (944.4 trees/ha). However, these differences in stem density were not statistically significant (p = 0.1185, n = 3). In conclusion, the study revealed that forest fragmentation negatively influences tree species diversity, and that while tree density may increase in small fragments, it does not reflect the ecological health of the forests. Mabira was a more advanced and stable mature forest community than the other two fragments of Namukupa and Nandagi. The dominance of Broussonetia papyrifera - an invasive species in Namukupa shows a shift in community structure, poses a significant threat to native species composition, and underscores the urgent need for management interventions to prevent the degradation of ecosystem integrity. It is recommended that the National Forestry Authority (NFA) explore opportunities to reduce fragmentation and isolation of forest fragments, such as agroforestry on intervening private lands and the establishment of buffer zones around forest fragments. Future studies should assess the spatial distribution and population structures of threatened tree species or those with low IVIs to inform the development of management and restoration strategies. Sites with low stem density should be enriched with indigenous, fast-growing pioneer species to close the canopy gaps and foster forest recovery and advancement to a more primary forest community. Natural regeneration by creating protected zones within the CFRs should be encouraged to allow seedlings to grow into mature stems without interference from human disturbances.