Hydrothermal germination response and the development of probabilistic germination profiles

Abstract

Millions of acres of rangeland in the western United States are now dominated by introduced annual weeds such as cheatgrass ( Bromus tectorum L.). The ability to germinate rapidly at low temperatures has been proposed as one of many mechanisms that confer a competitive advantage to this species. Previous studies of this phenomenon, however, have been limited to relatively simple comparisons of total germinability and germination rate under selected constant-temperature treatments. In this study, we germinated seeds of cheatgrass and two native perennial bunchgrass species, bluebunch wheatgrass [ Pseudoroegneria spicata (Pursh) Löve] and big squirreltail [ Elymus multisetus (J.G. Smith) M.E. Jones], over the temperature range of 0–36 °C and water potential range of 0 to −2.5 MPa. A hydrothermal germination model was constructed to predict germination rate response for the 10, 25 and 50th percentile seed subpopulations. Germination rate estimates were predicted for each species, seedlot and subpopulation for every hour of a 38-year simulation of seedbed temperature and water potential. The seasonal and annual distributions of potential germination rate were integrated to assess relative seedlot response across a wide-range of microclimatic conditions. The integrated response-potential of cheatgrass was consistent with previous studies but the absolute magnitude of enhanced germination rate was relatively small compared to expected differences in seed numbers between cheatgrass and planted species in the field. Historical patterns of seedbed microclimate and predicted germination response may be useful in assessing and optimizing alternative field planting scenarios. Inclusion of weather forecasting and seedbed modeling may provide real-time management options for improving rangeland seeding success.