Plant survival is one of the most direct reflections of plant adaptation, especially under environmental stress. In our field monitoring experiment, both investigated plant species showed decreased survival percentage during flooding, and this decreased survival percentage was not obviously different between the species. However, in the control experiment, the survival percentage of T. lutarioriparia decreased greatly and that of P. australis did not vary obviously over time, indicating that P. australis exhibited higher survival percentage than T. lutarioriparia. The possible reason for this is that P. australis, which is a widespread species, has higher ability of adaptation to environmental changes than T. lutarioriparia, an endemic species, does15.
Plant growth also can reflect plant adaptation to environmental stress. In our field monitoring experiment, P. australis had higher aboveground biomass before flooding and higher relative elongation rate, whereas T. lutarioriparia had higher aboveground biomass after flooding and higher relative growth rate. However, in the control experiment, plant biomass, relative growth rate, and relative elongation rate were not obviously different between the two species. These different results in plant growth between the field monitoring experiment and the control experiment may be because of flooding. In the field experiment, five-month flooding caused different response patterns in the plants: the well-developed stem porosity in T. lutarioriparia led to increased oxygen transmission from the aboveground parts to the belowground parts and resulted in increased growth, whereas the relatively lower stem porosity in P. australis forced rapid vertical stem elongation so that stems are able to reach the water surface and access oxygen from the air24,25. In the control experiment, the two plants showed similar growth characteristics because of the absence of flooding stress.
Plant competition can be determined by competition intensity and is associated with community characteristics, such as biomass, density, and proportion26. Similar to the growth performance, the competition performance of the two species also differed between the field monitoring experiment and the control experiment. In the control experiment, P. australis showed a higher competition ability than T. lutarioriparia in the early growth stage and lower competition ability than T. lutarioriparia in the middle and later growth stages. However, in the field monitoring experiment, P. australis showed higher competition ability than T. lutarioriparia both before and after flooding. These differences may be related to flooding stress; in the field monitoring experiment, five-month flooding reduced the competition ability of T. lutarioriparia and improved the competition ability of P. australis.
Plant competition ability includes the ability of intraspecific and interspecific competition8,9. In the field monitoring experiment, intraspecific competition was higher in P. australis, and interspecific competition was higher in T. lutarioriparia. However, over time, the competition changed from intraspecific to interspecific in P. australis and from intraspecific to interspecific in T. lutarioriparia. Competition often leads to species occupying different ecological niches27. Relative yield total in both the field monitoring and the control experiment were greater than one, indicating that the two plant species had different niches. We found that P. australis would become the dominant species in the field monitoring experiment, and T. lutarioriparia would become the dominant species in the control experiment.
Altogether, the present study showed that the competition ability of plant species varied depending on environmental conditions. In the non-submergence habitat, the competition ability of P. australis was higher than that of T. lutarioriparia, whereas the opposite was true in the water submergence habitat. In the Dongting Lake wetlands, the five-month flooding period led to P. australis becoming the dominant species, and the seven-month non-flooding period led to T. lutarioriparia becoming the dominant species. Therefore, niche differences between the two species enable their coexistence in the Dongting Lake wetlands under the influence of seasonal flooding.