Postplanting Microclimate, Germination, and Emergence of Perennial Grasses in Wyoming Big Sagebrush Steppe

Several recent studies have hypothesized that postgermination/preemergence soil freezing can have a major negative impact on spring emergence of seeded perennial grasses in the Intermountain western United States. In this experiment, we measured germination and emergence of 4 fall-seeded perennial grasses at 15 sites in southeastern Oregon, north-central Nevada, and southwestern Idaho in 2 different yr. We used hydrothermal germination and seedbed microclimatic models to estimate planting date effects on germination and preemergence mortality and evaluated these simulations relative to measured germination and seedling emergence in the field. There were clear indications that both germination and emergence were responsive to site-year effects, but premature fall germination before winter freezing events was not a factor in emergence rates. In all cases, planting was sufficiently late in the fall to avoid significant postgermination/preemergence freezing injury for the sites and years tested. Emergence was weakly correlated to seedbed favorability in the late winter and spring after the principal period of soil freezing. Our data show that for the 2 yr of this study, we can only account for a relatively small portion of any abiotic effects on spring emergence. General inferences about planting date effects, however, may have been limited by arid conditions during the 2-yr field study relative to long-term climate normals.