Studies on Dormancy, Germination, and Survival of Seeds Buried in Soil of the Rare Plant Species Aeschynomene virginica (Fabaceae)
Jerry M. Baskin
Carol C. Baskin
R. Wayne Tyndall
Dormancy in seeds of Aeschynomene virginica is caused by a water-impermeable (“hard”) seed coat. In the laboratory, dormancy was broken by mechanical scarification, dry heat, wet heat (boiling), dry storage, incubation on a moist substrate for several months, and (to a lesser extent) freezing. Wetting/drying cycles were ineffective in breaking dormancy. Nondormant seeds germinated over a wide range of temperatures in both white light and darkness, in far-red light, at salinities (NaCl, Na2SO4, MgSO4) of 0-1%, and while flooded or buried in soil. Fewer seedlings emerged from nondormant seeds buried > 3 cm in soil than from those buried 1 or 2 cm; emergence was 93% at 1 cm and 19% at 5 cm. Germination of only a few of the dormant seeds sown on the soil surface in a nonheated greenhouse in autumn 1995 was delayed until spring 1997, while 44% of those buried 7 cm deep in soil were still dormant (“hard”) in spring 1999. Thus, it seems likely that A. virginica has the potential to form a persistent seed bank in its natural habitat.
Effects of Experimental Shading and Competitor Removal on Vegetative and Reproductive Performance of Trifolium thompsonii Morton, a Rare Endemic Herb of Eastern Washington, USA
Richy J. Harrod
Charles B. Halpern
We examine how competitive interactions with associated species in the ground layer and shading by overstory trees may contribute to reduced survival, size, and reproductive allocation of Trifolium thompsonii, a rare native forb of north-central Washington State. We hypothesized that its current rarity reflects long-term exclusion of fire and concomitant increases in vegetation cover and overstory shade. We used field and greenhouse experiments to test whether removal of competing species and experimental shading affected survival, vegetative performance, and reproductive allocation. In 3-y field trials, survival of mature plants was greater in community-removal (89%) than in control plots (69%). Among surviving plants, vegetative performance differed little between treatments, but reproductive allocation was significantly greater in removal plots (average of 5.7 vs. 1.2 flower heads per plant in year 3). Experimental shading (30% ambient light) of established plants in the field yielded few significant changes in vegetative or reproductive performance after 2 y of treatment. However, experimental shading of seedlings in the greenhouse had a significant effect on plant performance: seedlings grown under 30% ambient light for 10 wk had less than half the shoot or root mass than did seedlings grown under 70 or 100% light. Our results suggest that competition with associated species and reductions in available light negatively affect particular stages of the life history of T. thompsonii, and may limit its long-term persistence. Future management for conservation of this species should include two strategies: (1) reducing density and cover of conifers in forested habitats through mechanical removal and/or prescribed fire, and (2) limiting development of competing ground-layer vegetation through periodic, low intensity fire.