Seed germination and seedling growth in native and exotic grasses in the semiarid grasslands of Northern Mexico

Exotic African grasses have been introduced into deteriorated semiarid grasslands in northern Mexico to stop soil erosion and improve land productivity. These species are characterized by larger size and greater seed productivity than natives, traits which may contribute to eventual displacement of native grasses. Invasion of natural ecosystems alters species composition and ecosystem functioning, making identification of the traits and mechanisms used by successful invaders vital to controlling this problem. A comparison was done of seed germination potential and seedling biomass allocation in two native grasses (Bouteloua gracilis and B. curtipendula) and five exotic African grasses (Eragroslis curvula, E. lehmanniana, E. superba, Melinum repens and Panicum coloratum). Treatments consisted of washed and unwashed seeds and sterilized and unsterilized soils. Only E. superba exhibited higher (P<0.05) germination rates than the native species. Due to their overall higher seed production, potential propagule production was highest in two of the exotic grasses (E. superba and E. curvula). Seed washing favored higher germination in the three Eragrostis grasses, whereas soil sterilization inhibited germination in B. curtipendula and E. curvula (P<0.05). Seedling growth variables indicated similar effects in biomass allocation. Propagule production is not necessarily the factor that most facilitates invasive success by an exotic grass species, while the lack of pathogenic microorganisms and/or presence of growth promoting microorganisms may play a vital role.