Life history and modeling of an endangered plant, Penstemon haydenii

Abstract

The demography of the federally listed endangered species blowout penstemon (Penstemon haydenii S. Watson) was quantified from 2005 through 2007 by the survival and growth of marked plants in the Nebraska Sandhills. Factors hypothesized to affect plant fecundity, seed viability, and seed germination rate were tested experimentally for their effect on population growth rate (λ). Average seed production was 518 (SE = 29.01) seeds per stalk across all blowouts for 2005, 2006, and 2007. Experimental treatment with fungicide, insecticide, or both did not reduce damage. Therefore, no differences emerged in seed output, weight, or viability. However, herbivore, granivore, and fungal impacts were significant in some sites. These interactions may be important to population persistence if surrounding vegetation invades blowout penstemon habitat, and increases herbivore habitat. From 14 to 81% of seed buried in permeable enclosures were found to remain viable for 2.5 years, suggesting that a seed bank is possible. However, most field-planted viable seed germinated within the first year and post-dispersal seed losses did occur, which resulted in minimal seed remaining in the soil. Simulated grazing experiments produced no significant differences between clipped or control treatments in mean plant survival, flowering stems, flowering stem height, or number of verticillasters after either the first or second growing season. Spring clipping of flowering blowout penstemon plants was not detrimental to plant survival or reproductive capacity in the subsequent year. Using a Lefkovitch stage based matrix model, the population growth rate of blowout penstemon averaged over all test populations was found to be negative from 2005 to 2006 and positive from 2006 to 2007. Tests varying matrix parameters across a range of values indicated that the juvenile stage is important for long-term survival. The initial population size made little difference in the survival potential of populations in individual habitat fragments. Adjusting carrying capacity to simulate loss of habitat suggested that a closer look at habitat persistence is important to predicting demographic outcome for blowout penstemon.