Invasive trees in Singapore: are they a threat to native forests?

Asian tropical forests are under extreme stress from clearance, fragmentation, and logging. Because Singapore suffered these same impacts in the 19 century, it can act as an early warning system for the region. We identified invasive exotic tree species from a comprehensive survey, assessed their survival and growth in the open and under a forest canopy, and compared their traits with native pioneers and widely planted exotic species that have failed to naturalize. Nine species were invasive: Acacia auriculiformis (ear-leaf acacia), Cecropia pachystachya (trumpet tree), Falcataria moluccana (Moluccan albizia), Leucaena leucocephala (leucaena), Manihot carthaginensis subp. glaziovii (Ceara rubber tree), Muntingia calabura (Jamaican cherry tree), Piper aduncum (spiked pepper), Pipturus argenteus (white mulberry), and Spathodea campanulata (African tulip tree). Under the forest canopy, all grew little and suffered high (75-100%) mortality, with Spathodea surviving best. At the open site, mortality was low (0-25%) and height growth rapid (1.6-8.6 cm wk), with Leucaena growing fastest. The only significant trait difference between native pioneer trees and invasive exotics was the dispersal syndrome; none of the common native pioneers are wind dispersed, while two invasive species are. Among exotics, there were highly significant differences between invasive and non-invasive species in wood density and seed size, with invasive species having only 54% of the mean wood density and 19% of the dry seed mass of non-invasive species. Invasive exotics were also significantly farther from their native ranges than non-invasive species. None of these invasive tree species is a direct threat to the integrity of native closed-canopy forest, but they dominate on newly abandoned land and may pre-empt the recovery of native forest on such sites.