Impact of Prescribed Fire on the Terrestrial Orchid, <i>Isotria medeoloides</i> in Virginia's Blue Ridge Mountains

Abstract

Small whorled pogonia (Isotria medeoloides [Pursh] Raf.) is a terrestrial orchid with imperiled G2S2 conservation status. It is native to Appalachia from Georgia to Quebec. This orchid is one of many species whose habitat is being lost to degradation. This mycoheterotrophic orchid is seasonally dormant, but can also remain fully dormant with no above-ground tissue for multiple years. Conservation efforts over the last forty years have provided a management framework that suggests fire may be useful to recovering this flower and restoring its habitat for the benefit of the species that share it. Our study site is one of these habitats where small whorled pogonia grows in a region with a historical regime of frequent fire. The study site is on the western slopes of Virginia's Blue Ridge and is one of many natural area preserves that is managed by the Virginia Department of Conservation and Recreation. This preserve is undergoing ecological restoration with an objective of restoring historical open-canopy oak-hickory woodlands. Open oak woodlands were typical of the historical fire regime in the Shenandoah Valley (Lafon et al., 2017). As a threatened species, small whorled pogonia is protected under the Endangered Species Act of 1973. This requires that publicly funded species management must mitigate harm. Before fire can be used in populations of small whorled pogonia on public lands, its impacts on the orchid and its habitat must be examined. The DCR Natural Heritage Program, in collaboration with Virginia Tech, put together a plan to study the impacts of prescribed fire on the rare orchid population. This observational study gathered multivariate environmental data during three growing seasons leading up to the fire and two seasons following the 2024 fire. The population was geographically divided, and one subpopulation was excluded from prescribed burning. The smaller subpopulation underwent an operational low-intensity prescribed fire in March 2024. Destructive sampling was used to assess fuel loading before and after the fire. Canopy imaging and biodiversity plots were used to assess ecosystem response to fire. Generalized mixed effects models, multilevel pairwise comparisons, and local spatial analysis including Getis Ord Gi and Lee's L were used to explore the individual, population, and community level response to fire. Researchers examined the effects of fire on small whorled pogonia stem counts and life stages and the effects of fire on plot-level alpha diversity and forest floor cover and light transmission. A population differential was calculated by subtracting the burned population stem count from that of the unburned. Results showed that the population differential increased by 17% in spring 2024 (two months after fire) but in 2025, following a steep decline in the unburned population stem count, the differential decreased by 75%. The population persisted after fire with no significant effect on seedling establishment. Dormancy was not measured separately from death. Likelihood of dormancy increased for both populations since 2022, with greater amplitude for the burned population in 2024 and for the unburned population in 2025. Flowering was not affected, but fruiting rate increased in 2024 for the burned population. Low intensity prescribed fire had little effect on forest floor light transmission. Forest floor cover was reduced and favored graminoid and forb cover over woody cover. Woody cover by Pinus strobus saplings was reduced in the shrub layer. Overall, the single observed fire event had short-term effects that reduced small whorled pogonia stem count and did not increase forest-floor richness or light transmission. Further observation of this population will be necessary to disentangle plant death and plant dormancy as fire responses within this population and to assess post-fire population viability. This study describes the isolated effects of a single fire on one subpopulation of small whorled pogonia and its results are not representative of a species-wide fire response. Further study should examine long-term and repeated use of frequent fire across multiple small whorled pogonia populations.