Germination Syndromes and Native Species Conservation in Highly Disturbed Rangeland Ecosystems of the Intermountain Western US.

Wildfire and introduced annual grasses have caused widespread degradation of Great Basin ecosystems in the Intermountain West. Cheatgrass (Bromus tectorum) and medusahead wildrye (Taeniatherum caput-medusae) aggressively displace native species by pre-empting resources and creating continuous fuel loads that increase the size and frequency of wildfire. Introduced annual grasses have a number of adaptive traits that allow them to successfully compete with native perennial vegetation in this highly-variable weather environment, including high seed production, rapid establishment and growth, and the ability to avoid summer drought in the seed phase. Native perennial species have a much more restrictive set of microclimatic requirements for establishment, growth and survival, even in the absence of competitive annual weeds. Seed germination is a critical life-stage transition for successful range-plant establishment but the predominant cause of seeding failure in these systems appears to be post-germination/pre-emergence mortality from relatively short-term soil freezing and/or drought events. We used long-term weather data, seedbed-microclimatic simulation and hydrothermal-germination models to assess the impact of weather variability and planting date on cumulative germination response and subsequent mortality of both native perennial bunchgrasses and cheatgrass. Long-term field simulations reveal alternative germination syndromes for mortality avoidance. Planting date differences of only a few weeks in the fall may significantly affect the probability of successful seedling establishment.