Individual study: The effects of water level on germination and establishment of a rare annual, Virginia jointvetch Aeschynomene virginica: a greenhouse experiment, USA
Griffith A.B. & Forseth I.N. (2003) Establishment and reproduction of Aeschynomene virginica (L.) (Fabaceae) a rare, annual, wetland species in relation to vegetation removal and water level. Plant Ecology, 167, 117-125
Virginia jointvetch Aeschynomene virginica is a rare annual plant of freshwater tidal wetlands of eastern USA. In conjunction with field trials, a greenhouse study was undertaken to test the effects of water level on seed germination and seedling establishment.
The greenhouse experiment was designed to simulate natural surface soil moisture differences at different tidal elevations where A.virginica occurs. Twelve plastic pots (21 × 21 cm) were filled with a 50:50 mix of sterilized soil and sand and 40 A.virginica seeds were evenly spaced across the soil surface of each pot. The 12 pots were placed randomly in a tank at one of three water level treatments:
i) submerged - soil surface 12 cm below the water level (simulating conditions that a seed might experience in low elevation sites during tidal inundation);
ii) waterlogged - water level 3 cm below the soil surface;
iii) wet - water level 15 cm below the soil surface (producing a wet but not saturated soil surface).
Seeds were checked daily for 17 days for: i) germination (defined as the appearance of a radical); and ii) seedling establishment (i.e. when they produced the first true leaf).
Water level significantly affected seed germination and seedling establishment. The wet soil treatment germination rate (83%) was much higher than the waterlogged (56%) and submerged (44%) treatments. No plants established in any of the submerged treatment pots. The wet treatment group had greater seedling establishment (94%) than the waterlogged treatment (84%).
These greenhouse experiments corroborate results of a field experiment conducted in 1998 and 1999, where jointvetch seed germination and seedling establishment were lowest in submerged and waterlogged treatments. In the greenhouse experiments, germinated seeds were found to be buoyant and in the submerged pots often floated to the surface. Lower germination rates and seedling buoyancy might explain the lower seedling establishment in places with longer and deeper inundation, as would be found in the lower marsh zone. The authors urge some caution when interpreting the greenhouse results, as tides were not simulated.
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