Abstract: We conducted a greenhouse experiment to examine the responses of dominant species, Phragmites australis, in the Yellow River Delta to water level and its fluctuation under global climate change. Three different haplotypes (haplotype O, haplotype P and haplotype M) of P. australis were subjected to three stable water levels (0, 15 and 30 cm) and two frequencies of water level fluctuation (every 5 days and every 15 days). We investigated the response strategies of different P. australis haplotypes with respect to the morphological and physiological traits, and explored the differences in phenotypic plasticity among different P. australis haplotypes. The results showed that: (1) Leaf biomass of P. australis at high water level (15 and 30 cm) decreased significantly by 31.99% and 33.54%, respectively (P<0.05), while total biomass, plant height, basal stem and number of tillers decreased, compared with those under the low water level treatment. (2) Compared with stable water level and low-frequency fluctuation, highfrequency fluctuation significantly reduced leaf biomass, number of tillers, total biomass, and other phenotypic traits. Morphological and physiological traits were not significantly affected by the frequency of water level fluctuation. (3) There were significant differences among different haplotypes of reed (P<0.05). Plant height, growth rate, basal stem, photosynthetic rate, leaf nitrogen and phosphorus concentrations, as well as phenotypic plasticity index of haplotype M were significantly greater than those of haplotype O and haplotype P. Therefore, rising water level and frequent flooding in the future will not benefit the survival and spread of P. australis populations, but attentions should be paid on the spread of haplotype M. This study can provide important theoretical references for vegetation protection and the control of biological invasion in wetlands.

Key words: water level, fluctuation frequency, Yellow River Delta, haplotype, phenotypic plasticity.