Black wattle (Acacia mearnsii), an invasive species in many countries, has received much attention abroad. It was introduced into China as commercial plant in the late 1950s. Presently, it has become a sustainable part in many secondary forests in China. Few studies, however, have focused on the risk of its invasion to native ecosystems. Sample plots were established in six communities invaded by black wattle in Wenzhou (one sample plot for each community). All individuals whose DBH (diameter at 1.3m height) larger than 1cm were mapped, identified and measured. The competitive strength and the spatial relationship between black wattle and indigenous species, and the replacement relationships among all species in each community were analyzed. The results suggest that (1) black wattle has no significant advantage in competing with indigenous species in a community; (2) black wattle is spatially independent from other species in a community, which is probably related to its weak competition that is not strong enough to lead to the mortality of its competitors; (3) all species, excluding masson pine, in each community are subject to self-replacement. Black wattle, however, is likely to replace other species and becomes the only dominant species in three communities (PY04, CN02 and RN01), though its dominance may not sustain for long periods of time because of its recruitment restriction; (4) overall, the risk of invasion of black wattle to an indigenous vegetation is not very high, and the wide diversity of species in a native community is able to keep black wattle from becoming a dominant species.