- 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.