Emergent wetland plants often alleviate the effects of anaerobic soils on root respiration by passively transporting oxygen belowground through continuous air spaces (aerenchyma) in leaves and roots. Oxygen leaked from the roots into the rhizosphere may oxidize minerals in the soil or become available to other plants. Some spatial patterns in marsh plant communities suggest interspecific facilitative interactions, but there is little experimental evidence for interplant facilitation via soil oxygenation. We investigated the capability of the widespread, highly aerenchymous wetland plant, Typha latifolia, to aerate sediments and affect the growth of two non-aerenchymous neighbors, Salix exigua and Myosotis laxa, both in greenhouse experiments and in a natural pothole pond in western Montana. At soil temperatures of 11°-12°C in the greenhouse, mean dissolved oxygen (DO) ranged from 2.75 ± 0.41 to 4.43 ± 1.11 mg/L (mean ± 1 SE) in pots with Typha, whereas in pots without Typha DO content ranged from 0.40 ± 0.06 to 0.65 ± 0.06 mg/L. At soil temperatures of 18°-20°C, DO in the Typha treatment was lower than in the low-temperature experiment and did not differ significantly from DO in the pots without Typha. At substrate temperatures of 11°-12°C, all rooted Salix cuttings survived when planted with Typha, whereas none survived when planted alone. At the same substrate temperature, Myosotis transplants grew significantly larger when planted with Typha than when alone, also indicating that facilitation occurred. When grown with Typha at soil temperatures of 18°-20°C, however, Myosotis root mass was significantly less than when grown alone, suggesting that competition occurred. At the pond margins where Typha and Myosotis coexist in water-saturated soils, naturally occurring Myosotis plants adjacent to transplanted Typha had significantly larger shoot and fruit mass than did control Myosotis without nearby Typha. In this experiment DO levels in soil water and available soil nitrogen did not differ among treatments. These experiments indicate that aerenchymous wetland plants have the potential to facilitate neighbors via soil oxygenation, and that facilitation and competition may shift in importance with changes in the physical environment.
- Soil temperature