A Dissertation Submitted in Partial Fulfilment of the Requirements for the Degree of Master’s in Life Sciences at the Nelson Mandela African Institution of Science and TechnologyThis study aimed at determining the extent (distribution, density and structure) of Acacia mearnsii invasion as well as assessing its influence on the performance of Pinus patula by comparing, stem density, basal area and standing volume between invaded and non- invaded areas in Sao Hill Forest Plantation (SHFP). A systematic random samplings technique was used to select sample plantation blocks and plots allocation. The results showed that there was a significant difference between the mean population density per hectare of A. mearnsii in the margins and inside plantation blocks (F-value = 61.4, df=278, p = 0.0000), with greater mean population density being in the margin than inside the plantation blocks. Also, a significant difference between the size class group (seedlings, saplings, poles and adults) was found in the mean population density of A. mernsii (F-value = 26.28, df = 278, p = 0.0000), with the greater mean population density being in seedlings followed by a sapling, sub-adults, and adults. Moreover, there was a significant difference between invaded and non-invaded areas across different age classes of P. patula in all variables measured with greater density, basal area and volume being in non-invaded than invaded areas. The study found that, an invasive tree A. mearnsii has a negative impact (reduction) on the survival and growth of P. patula in all age classes sampled. This study suggests that forest management strategies should incorporate invasive plant control given that the performance of desirable tree species can be influenced by plant invasions. This may include clearing of the A. mearnsii stands before they mature to flowering and thus, limiting seed production.