Current understanding of the vulnerability of tropical forests to plant invasion is limited but is widely believed to increase where forests: (1) suffer marked natural or man-made disturbance; and/or (2) are exposed to high propagule pressure of alien species. This study aimed, for the first time, to address the importance of propagule pressure and disturbance by examining the spread of an introduced tree, Cordia alliodora, from a single plantation into a surrounding mosaic of humid forest in the East Usambara Mountains, Tanzania. By assessing vulnerability to invasion along transects radiating from the plantation, the effects of distance (measure of propagule pressure), and disturbance could be discerned. For all life stages, distance from source population was the strongest correlate of density. A marked influence of disturbance was only evident for C. alliodora seedlings. Spatial variation in the densities of later life stages may be a function of past disturbances, less easy to assess from current surveys, especially following the marked self-thinning between seedling and adult densities. Nevertheless, the evidence suggests that propagule pressure is a more important determinant of Cordia density than disturbance. If this is true for other alien tree species in tropical forests, controlling for introduction effort is essential to assess the drivers of plant invasion. Given an annual population growth rate of ca 3.5 percent, equivalent to the population doubling every 20 yr, C. alliodora poses a significant threat to the East Usambaras as well as other humid forests where it is promoted for agroforestry.