Available water influences field germination and recruitment of seeded grasses

Abstract

Periodic summer rainstorms in some semi-arid regions result in variable soil moisture and differential establishment of seeded species. A 2-year study investigated soil water effects on germination and survival of 6 native and 2 non-native southwestern U.S. grass species. Bags of seeds were buried and retrieved before and during the summer rainy season. High field germination in seed bags (20-100%) and limited germination in the laboratory of seeds that were ungerminated in seedbags (0-45%) were exhibited by 6 native grasses following initial rainfall events in which the surface soil was saturated for 2 days or water potential (1-3 cm depth) was above -1.5 MPa for more than 9 days. Fewer Lehmann lovegrass (Eragrostis lehmanniana Nees) seeds germinated in response to initial and subsequent rainfall events (0-49%), but this species retained more residual germinable seeds (49-99%) than all other species studied. For 2 sowing dates, the soil drying front exceeded estimated seminal root depth 13 days after germination. Lack of recruitment for some species sown on these dates was probably due to seedling desiccation before adventitious roots had sufficient time to develop. The ability of Lehmann lovegrass to retain a viable seedbank when rainstorms are separated by long dry periods allows it to establish better than some native grasses that germinate quickly and are then subject to seedling desiccation. During a summer with more consistent rainfall, native species recruitment was greatest when seeds were planted during, rather than before the summer rainy season.