Abstract
Responses of bald cypress (Taxodium distichum) and pond cypress (Taxodium ascendens) saplings in photosynthesis and growth to long-term periodic submergence in situ in the hydro-fluctuation zone of the Three Gorges Dam Reservoir (TGDR) were studied. Water treatments of periodic deep submergence (DS) and moderate submergence (MS) in situ were imposed on 2-year-old bald cypress and pond cypress saplings. The effects of periodic submergence on photosynthesis and growth were investigated after 3 years (i.e. 3 cycles) compared to a control (i.e. shallow submergence, abbreviated as SS). Results showed that pond cypress had no significant change in net photosynthetic rate (Pn) in response to periodic moderate and deep submergence in contrast to a significant decrease in Pn of bald cypress under both submergence treatments, when compared to that of SS. Ratios of Chlorophyll a/b and Chlorophylls/Carotenoid of pond cypress were significantly increased in periodic moderate submergence and deep submergence, while bald cypress showed no significant change. Diameter at breast height (DBH) and tree height of both species were significantly reduced along with submergence depth. Relative diameter and height growth rates of the two species were also reduced under deeper submergence. Moreover, bald cypress displayed higher relative diameter growth rate than pond cypress under deep submergence mainly attributed to higher productivity of the larger crown area of bald cypress. When subjected to deep subergence, both species showed significant reduction in primary branch number, while in moderate submergence, bald cypress but not pond cypress showed significant reduction in primary branch number. These results indicate that both bald cypress and pond cypress are suitbale candidates for reforestation in the TGDR region thanks to their submergence tolerance characteristics, but bald cypress can grow better than pond cypress under deep submergence overall.
Highlights
Dam building has altered the natural hydrological regimes of many rivers [1,2] all over the world
There was no significant difference in photosynthetic rate (Pn) of bald cypress between moderate submergence (MS) and deep submergence (DS), whereas, mean Pn of pond cypress in DS was 46% higher than that of MS (p < 0.001)
Pond cypress showed a higher Pn than bald cypress in MS and DS (MS: p < 0.01, DS: p < 0.001), respectively, but no difference in Pn was found between the two species in SS (p > 0.05)
Summary
Dam building has altered the natural hydrological regimes of many rivers [1,2] all over the world. To operate the TGDR at full capacity, the water level of the TGDR fluctuates between 145 m a.s.l. in summer (i.e. from May to September mainly for flood control and emission sediment) and 175 m a.s.l. in winter (i.e. from October to the following April mainly for energy generation and navigation) [5,6] These extreme fluctuations with an annual 30 m water-level drawdown have led to the formation of a hydro-fluctuation zone around the reservoir. The newly-formed hydro-fluctuation zone in the TGDR encompasses an area of 400 km and 2000 km of shoreline [7] This hydro-fluctuation zone experiences a reversed flooding seasonality and prolonged flooding duration [8], which is opposite to its natural hydrological regime of the Yangtze River [6]. There is a clear need to screen suitable tree species for vegetative restoration of the hydrofluctuation zone of the TGDR [9]
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