Germination patterns and seedling growth of congeneric native and invasive Mimosa species: Implications for risk assessment

Comparisons of plant traits between native and invasive congeners are useful approaches for identifying characteristics that promote invasiveness. We compared germination patterns and seedling growth of locally sympatric populations of native Mimosa himalayana and two varieties of invasive M. diplotricha (var. diplotricha and var. inermis) growing in southeastern Nepal. Seeds were germinated under a 12-h photoperiod or complete dark, low (25/15 degrees C day/night) and high (30/20 degrees C) temperatures, different water stress levels (0, -0.1, -0.25, -0.5, -0.75 and -1.0 MPa), and soil depths (0, 2, and 4 cm). Plant height, biomass allocations, and relative growth rate (RGR) of seedlings were measured. Invasive M. diplotricha had higher germination percentage, rate, and shorter germination time compared with the native species. Germination of both congeners declined as water stress increased, but the decline was more pronounced in native species. Seedling emergence declined with increasing depth in all taxa. The seedlings of invasive species were taller with higher leaf number and allocated greater proportion of biomass to shoot, whereas the native congener allocated greater biomass to root. The RGR was nearly twice as high in invasive species as it was in the native congener. Seedling height and number of leaves were always higher in invasive than in native species, and the native-invasive differences increased over time. Better germination and higher growth performance of invasive species than the congeneric native one suggests that seed germination and seedling growth can be useful traits for the prediction of species' invasiveness in their introduced range during risk assessment process.