Penstemon gibbensii and Penstemon haydenii are two rare, perennial forbs inhabiting remote areas of the western United States. P. gibbensii is listed as a sensitive species by the Bureau of Land Management (BLM) in Colorado, Utah, and Wyoming (Heidel, 2009). P. haydenii was designated as Endangered by the U.S. Fish and Wildlife Service in 1987 (Heidel, 2012). This thesis research was geared toward helping land managers in their efforts to protect and rehabilitate these species by providing understanding on different aspects of their seed ecology. My first study was a laboratory experiment performed on P. gibbensii and P. haydenii seed germination response to moist chilling and dry after-ripening. Wild harvested seeds were subjected to moist chilling at 2-4 °C for 0, 4, 8, 12, and 16 weeks and held in dry storage for approximately 2 years to determine effective methods for breaking primary dormancy. P. gibbensii seed germination increased consistently with increased length of chilling up to 16 weeks and exhibited habitat-correlated variation in this response. P. haydenii seed germination increased from 1 to 100% germination with 4 weeks of chilling. P. haydenii germination was greatest (96%) when incubated under a cool, diurnally-fluctuating temperature regime (10-20 °C) and responded positively to dry storage, increasing germination from 0 to 15%. My second study was an in situ field study designed to characterize the active seed bank of P. haydenii. We set up a transect line across a P. haydenii population and measured the number of seeds entering the seed bank, lost to predation post-dispersal, and persisting in the seed bank. P. haydenii does not appear to form an ecologically significant seed bank. Approximately 140 seeds/ 10m2 could potentially enter the seed bank but only 1 seed in the upper 10cm of sand persisted. Heavy post-dispersal insect predation resulted in a decrease in viability of nearly 30% in exposed P. haydenii seeds after just 12 hours. My third study explored the effects of burial by sand on P. haydenii. Wild-harvested seeds were planted in pots at 1, 2, 4, 6, 8, and 10cm deep in sand and incubated at 10-20 °C. Seed germination and mortality and seedling emergence were measured. The response of dormant seeds to post-burial incubation was determined. Burial depth decreased seedling emergence and seed germination. Shallow burial appears to induce secondary dormancy for seeds that don't germinate quickly, whereas deep burial appears to impose enforced dormancy in burial.

Variable mortality of Pyrenophora semeniperda–infected Bromus tectorum seeds has been referred to as a “race for survival.” Dormant seeds are highly susceptible to P. semeniperda infection. While much is known about primary dormancy little is known about secondary dormancy in B. tectorum seeds. Dormancy status is not the only variable determining outcomes within the Bromus - Pyrenophora pathosystem. Varying temperature and intermittent hydration may strongly influence germination outcomes of B. tectorum in the presence of P. semeniperda. While it has long been assumed that B. tectorum seeds are infected by P. semeniperda in the fall it was recently suggested that seeds may be infected in the summer; however, there is little evidence to support this. To further characterize the Pyrenophora semeniperda - Bromus tectorum pathosystem two studies were conducted to address the following: (1) characterization of secondary dormancy in B. tectorum seeds and (2) summer interactions between host and pathogen after summer inoculation. Studies were conducted using dormant and/or non-dormant B. tectorum (along with B. rubens in one study) seeds and two strains of P. semeniperda. Study one used laboratory and field experiments to characterize secondary dormancy in B. tectorum seeds in terms of temperature (0.5-20°C), and water potential (-2.0-0 MPa). Data was used in repeated probit regression analysis to determine hydrothermal parameters (ψb(50), σψb, θHT) for secondary dormancy induction and loss. In the second study seeds were inoculated with one of two strains of P. semeniperda then exposed to intermittent hydration or dry storage at warm temperatures (30-60°C). After treatment seeds were rehydrated and outcomes observed. Optimum conditions for secondary dormancy induction were incubation at -1.0 MPa at 5°C. Seeds were likely to enter secondary dormancy through the cold winter months indicated by an increase or more positive ψb(50), while a decrease or more negative ψb(50) is associated with dormancy loss which is generally observed in the hot, dry summer months. When seeds were inoculated in the summer they only escaped death when summer conditions were ideal for after-ripening which allowed them to germinate rapidly under favorable autumn conditions. However, the pathogen caused high seed mortality no matter the treatment when disease progression advanced enough to inhibit seed germination. Thus this research shows that in areas with frequent summer rain storms, it would be highly advantageous to apply P. semeniperda as a biocontrol on seeds at maturity.

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