Response of introduced plant Betula nigra and native plant Morus alba to flooding in the draw-down zone in the Three Gorges Reservoir area.
Objective: The growth and photosynthetic characteristics of the native Morus alba and the introduced plant Betula nigra planted in the Three Gorges Reservoir area were compared, and the adaptability of the two woody plants to the periodic flooded habitats of the Three Gorges reservoir zone was investigated to provide a scientific basis for the selection of suitable tree species for the vegetation restoration. Method: In this study, two-year-old M. alba and B. nigra seedlings were planted on the tableland in typical draw-down zone at altitudes of 170 m and 175 m in Wanzhou, Chongqing. Then the seedlings grew naturally, and their survival rate was investigated in early April each year. After the preliminary experiment of testing survival rate, more two-year-old M. alba and B. nigra seedlings were planted on the side of the tableland. After two years, the growth (plant height, ground diameter, crown width) and photosynthetic characteristics (chlorophyll, fluorescence dynamic parameters, gas exchange parameters) were measured in Spring Exposure Period (SEP) of early April, Summer Drought Period (SDP) of mid-July and Pre-Autumn Flooding Period (PFP) at the end of September. Result: The survival rate of M. alba and B. nigra at altitude 170 m was significantly lower than that at altitude 175 m. The survival rate of B. nigra was higher than M. alba at both altitudes, and the difference between the two species was significant at altitude 170 m. B. nigra grew faster than that of M. alba at altitude 170 m, and the height was significantly different. The chlorophyll content of M. alba at altitude 170 m was higher than that at altitude 175 m in SEP. There was no significant difference in Pn, Gs, Tr, Ci and WUE between the two altitudes. In SDP, the chlorophyll content and Pn, Gs, Tr, Ci of M. alba and B. nigra at altitude 170 m were higher than those at altitude 175 m. The differences in Ci and Tr of M. alba were significant, and the difference in Tr of B. nigra was significant between the two altitudes. In PFP, the chlorophyll content of B. nigra at altitude 170 m was significantly higher than that of M. alba, and there were no significant differences in Pn and WUE between the two species at altitude 175 m. In SEP, the qP, ETR and ΦPSII of M. alba at 170 m altitude were higher than those at 175 m altitude. In PFP, the ΦPSII, ETR and qP of B. nigra at both altitudes were basically similar with those in SDP, however qn of B. nigra at altitude 170 m was higher than that at altitude 175 m. Conclusion: The native plant M. alba can rapidly recover the ability of the PSII reaction system of leaves, increase the transfer rate of photoelectron, increase the chlorophyll content and accumulate more organic compound to restore growth after Spring Exposure Period to adapt to the stress of flooding in the Three Gorges Reservoir Area in winter. The introduced plant B. nigra can prolong photosynthesis time, maintain a high efficiency of light energy conversion after M. alba entering dormancy. Thus it can accumulate more organic compound to cope with excessive consumption in winter. Within the range of this experiment, M. alba and B. nigra can adapt to the special habitat of the Three Gorges Reservoir to a certain extent. Compared with the native tree species, the introduced plant had stronger adaptability to the habitat of the Three Gorges Reservoir. It can be used for vegetation restoration in the draw-down zone of the Three Gorges Reservoir area. However, whether or not the introduced plant B. nigra has ecological invasion and whether it can be used as the restoration tree species in the Three Gorges Reservoir area has yet to be studied.