Understanding why some introduced alien plant species become invasive whilst others fail is a fundamental question in ecology, not least because of the considerable ecological and economic damage caused by invasive plants globally. Identifying factors that drive alien plant invasions can inform efforts to predict the nsk of invasion by an introduced plant species. This would allow prevention of introduction of high risk aliens, as well as targeted management of species already introduced that pose the latest threats to the ecolocal integrity of host ecosystems. However, generalisations among the findings of comparative plant invasion studies have been limited by a lack of control of confounding variables such as propagule pressure, time since introduction and phylogeny, a lack of knowledge of introduced species that failed to establish, and inconsistent use of terminology defining the invasion process. This thesis used Amani Botanical Garden (ABG) in the East Usambara Mountains of Tanzania, as a comparative case study system to assess the relative ability of multiple factors to explain invasion success of species introduced to a tropical forest ecosystem.