The influence of prescribed fire on the rare endemic plant Delphinium pavonaceum (Peacock larkspur)

Abstract

In the wetland prairie of William L. Finley National Wildlife Refuge (FNWR) in western Oregon, we investigated the response of Delphinium pavonaceum Ewan (peacock larkspur, Ranunculaceae), an endangered perennial forb, to four unreplicated dormant season fire regimes of 0, 2, 4, or 10 fires that were applied over a 12-year period. Additionally, an unexpected removal of woody plants by refuge staff within some portions of our control area offered an unplanned opportunity for study. In 2002 we measured the density and vigor of reproductive plants, and performed seed germination trials. In 2003 we repeated previous field measurements, sampled immature plant density, and recorded observations of the insects visiting D. pavonaceum in burned and unburned habitats. We hypothesized that this rare endemic species and its insect visitors would respond positively to prescribed burning or the removal of woody species. Low seedling density was found in the unburned and hand-removal areas, likely due to interference from litter and/or taller, shading plants. We also found low seedling density in sites burned the previous year, implying that fire consumes or damages unprotected seeds in the litter layer or exposed on the soil surface. Seedling density was greater in a site burned three seasons previously, suggesting that fire ultimately leads to the enhancement of seedling density following the replenishment of the seed bank. The largest density of recruits was detected in a subunit recently returned to fire management in 1999, and also burned in 2002 following our first field season. However, the other, more-frequently burned sites did not exhibit an increased density of recruits, possibly due to a reduction of the seed bank following repeated bums, and increased intraspecific competition with mature plants. The elevated density of seedlings and recruits we observed in some burned areas may lead to population growth, as we observed a greater density of reproductive plants in the two most-frequently burned subunits during both years of study. Our results also suggest that fewer plants enter summer dormancy in burned areas, and that increases in flowering plant density may decline after 3 years. Plants in the burned and hand-removal sites were shorter, likely resulting from water stress following the removal of shading plants and litter. Additionally, plants in unburned areas might have experienced greater stem elongation due to competition with tall and dense vegetation. Plants in the burned and hand-removal areas were generally similar to the unburned control site for flower and fruit production, fruit set, seed production and seed mass. However, plants in the burned and hand-removal areas produced more flowers per centimeter of height, indicating that they allocated more energy to reproduction than plants in the unburned area. We suggest that the decreased productivity we observed in some vigor traits is not problematic to D. pavonaceum conservation goals and may be ameliorated after 3 years. Bombus ca1ifornicus, B. appositus, and several large moths were the only insects we observed visiting D. pavonaceum during the two years of this study. We did not detect a difference in bumblebee abundance between a frequently burned and unburned study plot during the peak flowering time of D. pavonaceum. However, our small sample size requires that this result be cautiously interpreted and further studied, as it is possible that our visitation data would change appreciably with a broader range of observations. Our results indicate that the current FNWR fire management plan is not in conflict with D. pavonaceum conservation. The choice of fire-return interval seems to influence D. pavonaceum populations and plant vigor, but because the fire-schedule at FNWR was altered in 1997, our ability to recommend an appropriate fire-regime for this species is limited. Although not directly investigated, we suggest that annual fires, when applied for more than five consecutive years, might lead to population declines for this species because fire appears to consume the seed bank and reduce seedling density. If annual fires are returned to FNWR, the potential for this undesirable result should be investigated for at least 10 years by population monitoring.