Recovery of native forest after removal of an invasive tree, Falcataria moluccana, in American Samoa

Abstract

Invasive species are among the greatest threats to global biodiversity. Unfortunately, meaningful control of invasive species is often difficult. Here, we present results concerning the effects of invasion by a non-native, N2-fixing tree, Falcataria moluccana, on native-dominated forests of American Samoa and the response of invaded forests to its removal. We sampled species richness, seedling and stem densities, biomass, and soil inorganic N status in native-dominated forests, and in forests invaded by F. moluccana where it was subsequently removed. While total biomass of intact native forests and those invaded by F. moluccana did not differ significantly, greater than 60% of the biomass of invaded forest plots was accounted for by F. moluccana, and biomass of native species was significantly greater in intact native forests. Biomass of native Samoan tree species following removal of F. moluccana accumulated rapidly from 128 Mg ha−1 immediately after tree girdling treatment to 185 Mg ha−1 following 8 years of post-removal recovery, at which point biomass of F. moluccana-removal plots did not differ significantly from native-dominated forest plots. Native trees exhibiting early successional traits accounted for a large portion of aboveground biomass in these forests where frequent large-scale disturbance events (i.e., tropical cyclones) are a salient feature. We suspect that this is the single most important reason why F. moluccana removal is a successful management strategy; once F. moluccana is removed, native tree species grow rapidly, exploiting the legacy of increased available soil N and available sunlight. Seedling densities of F. moluccana were high in invaded forest stands but effectively absent following only 3 years of forest recovery; a result likely due to the shade cast by reestablishing native trees. Although F. moluccana is a daunting invasive species, it exhibits characteristics that make it vulnerable to successful control: it is easily killed by girdling or herbicides, and its seeds and seedlings do not tolerate shade. These characteristics, combined with the important capacity for rapid growth exhibited by many of Samoa’s native trees, provide conditions and opportunities for successful, long-term control of F. moluccana across lowland forests of American Samoa. Caution should be exercised, however, in anticipating comparable management success in the control of F. moluccana elsewhere, as post-removal responses of invaded forest communities may differ dramatically from what has been documented in American Samoa.