This study was carried out in Semuliki National Park (SNP) in western Uganda to assess the invasive potential of selected alien plant species. The aim was to determine the density and size class distribution of invasive alien plants and how environmental factors influenced their distribution in disturbed and undisturbed forest. The hypothesis was that alien plant species would be restricted to disturbed forest and not occur in undisturbed forest. The sampling sites were located in Rwakasenyi (SS1), Sempaya (SS2), Bumaga (SS3), Ntotoro (SS4), and Nsaara (SS5). Data collection was done in nested plots of 20mx10m established on alternate sides of transects running from disturbed into undisturbed forest. The results show that Cedrela odorata, Senna siamea, Theobroma cacao, Coffea canephora and Ananas comosus were the most common invasive alien plant species. However, C. canephora and C.odorata appear to be the most invasive. There was a significant difference (P<0.05) in mean density of alien plant species between undisturbed and disturbed forest compartments. There was a significant difference (P<0.05) in size class distribution of alien plant species between undisturbed and disturbed forest compartments. The canonical correspondence analysis (CCA) biplot shows a significant correlation between alien plant species abundance and environmental factors. The soil types (loam, sand, and clay), canopy cover, forest vegetation, cultivation and dense herb cover played a significant role in influencing the distribution of alien plant species in undisturbed and disturbed forest compartments. Consistent with studies from other ecosystems, areas with high anthropogenic disturbance have a high percentage of alien plants probably due to the availability of resources. However, contrary to our hypothesis, natural areas of tropical forests were found susceptible to some alien plant species invasion. Native foraging herbivores like baboons, monkeys and birds probably act as the agents of dispersal for alien plant invasion in Semuliki National Park by dispersing edible seeds of T. cacao and C. canephora, whereas C. odorata the main disperser is wind and water in rivers. Disturbance highly influenced alien species abundance and its interaction with environmental and site conditions thus enabling alien plants to persist in disturbed forest and the resultant invasion of undisturbed forest. Park managers should monitor such interactions to reduce the invasiveness.

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