Dionaea muscipula (Venus flytrap) establishment, release, and response of associated species in mowed patches on the rims of Carolina Bays

Summary

Located on the southeastern Coastal Plain of the USA, Carolina bays are shallow wetlands of high conservation value that support distinctive plant communities which include some rare species, such as Venus flytrap Dionaea muscipula. This carnivorous plant grows in open habitats around the bay rims. In the absence of fire or management activities that mimic fire, vegetation becomes dominated by evergreen shrubs and Venus flytrap populations, along with other species reliant on open habitat, decline. This project examined the effectiveness of mechanical mowing, soil clearing, transplanting, and seeding as an approach to restoring populations of Venus flytraps which had declined in areas where fire management was absent.

Study site: Restoration sites were located in Lewis Ocean Bay Heritage Preserve (LOBHP), Horry County, South Carolina. LOBHP (3,640 ha) includes 22 relatively intact Carolina bays and extensive pine savanna. The bays support dense, impenetrable thickets of evergreen shrubs and trees, dominants including pond pine Pinus serotina, swamp red bay Persea palustris and loblolly bay Gordonia lasianthus. Abandoned loblolly pine Pinus taeda and slash pine P.elliottii plantations, restored longleaf pine P. palustris flatwoods and stunted stands of turkey oak Quercus laevis, surround the bays. LOBHP is adjacent to the Grand Strand, a 30-mile section of coastal South Carolina, experiencing rapid commercial and residential development which will greatly restrict future prescribed burning.

Scrub removal: Adjacent to a powerline corridor crossing LOBHP (which afforded open habitat with populations of Venus flytrap Dionaea muscipula), nine sites on bay rims were selected. In May 2003, using a front-mounted mechanical mower, dense scrub and pond pines were cut and removed producing nine open patches. Mowed patches were 10 × 20 m, the long axes running around a bay edge. Two 0.5-m² plots were established in each patch, one at the bottom and one at the top of the bay rim. The root mat was removed from plots in an attempt to establish a stable seedbed.

Seeding & transplanting of Venus flytraps: Establishing new Venus flytrap populations was attempted by seeding and transplanting. In June 2003, seeds were collected from within LOBHP. Previous research suggested that Venus flytrap seed germinates within a short window of time after seed dispersal. Therefore, within cleared and mown plots seeds (240/plot) were immediately scattered on the soil surface. Adult plants were also transplanted in June 2003 (3 plants/0.5-m² plot) adjacent to the seeded plots. Planting density was 36 plants/m², a density less than the average density of 48 plants/m² observed in natural populations. Pre- and post-mowing patch surveys were undertaken to record growth of suppressed (volunteer) Venus flytraps responding to the clearance. None were observed prior to mowing but the dense vegetation and thick accumulations of detritus may have hidden some small plants.

Monitoring: Assessment Venus flytrap establishment occurred at the end of the first growing season and at peak flowering in the second year. Seedlings in plots were counted in September 2003 and May 2004. Transplant survivorship was monitored, and in May 2004, flowering and size of transplants were measured. Volunteer plants were counted, and a subset measured in May for size and flowering. The size class distribution and flowering percentage of transplants and volunteers were compared to plants from two reference populations at LOBHP in areas last burned during winter 2002 and which were part of a long-term Venus flytrap study.

Plant community response: The response of the plant community to restoration efforts was also measured in the 0.5-m² mowed and cleared plots seeded with Venus flytrap, in plots mown but not cleared, and in plots placed in undisturbed Carolina bay vegetation. Densities of two other carnivorous plants, sundew Drosera capillarisand bladderwort Utricularia subulata, were measured in early spring of 2004. These occurred only in mowed and cleared plots and were assessed in early spring because their populations are ephemeral. Coverage of other plant species in the three categories of plots was estimated visually in July 2004. Relative cover values (total coverage of a species divided by total coverage of all species) were calculated for each.

Venus flytraps responses: Only 5–7% of the Venus flytrap seeds sown in the mowed and cleared plots resulted in seedling establishment but these persisted through the first winter, with evidence of further establishment between the 2003 and 2004 sampling dates in the moister bay rim bottoms. Average transplant survivorship was 85% at the bottom and 72% at the top of the bay rims (average density of 14/patch in 2004). Volunteer plants were found in four patches (average density of 6/patch). The percentage of plants producing flowers was highest among volunteers (93%), followed by transplants (63%) and was lowest in the two reference populations (20 and 21 %). The size class distributions of transplants and plants from reference populations were similar but the volunteers included some extremely large plants not found in reference or transplanted populations.

Associated species: Sundew and bladderwort became established in the mowed and cleared plots during early spring of 2004. However, later in the growing season most of these become dominated by grasses and rushes. Areas only mown supported communities with species composition similar to undisturbed (i.e. shrubby) bay vegetation, these areas being dominated by the following shrubs: large gallberry Ilex coriacea, fetterbush Lyonia lucida, and dangleberry Gaylussacia frondosa. Significant indicators for mowed and cleared plots were erect-leaf witchgrass Dichanthelium erectifolium, long-ligule witchgrass Dichanthelium longiligulatum, needlepod rush Juncus scirpoides and fascicled beaksedge Rhynchospora fascicularis. Plant richness and diversity were significantly higher in mowed plots than in uncleared ones.

Conclusions: Mowing created suitable habitat for growth and flowering, facilitating establishment of Venus flytrap and two other carnivorous species, Drosera capillaris and Utricularia subulata. Mowing initially produced open sites with little ground-layer vegetation. After two years, Venus flytraps transplanted as adults showed high survivorship (79%) and relatively high leaf number/plant. Suppressed Venus flytraps existing on-site quickly grew in response to mowing. These and the transplants had higher flowering percentages than plants in reference populations. Seedling establishment was low, but seedlings persisted into the second growing season. However, mowing and clearing also facilitated grass and rush invasion, and evergreen shrubs resprouted quickly. Maintaining open habitat by mowing will therefore have to be an ongoing management process.


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