Spatiotemporal photosynthetic physiology responses of remnant Myricaria laxiflora populations to regulated water level fluctuations.

The construction of the Three Gorges-Gezhouba Dam cascade hydropower station has changed the water level fluctuation pattern of the habitats for remnant rare and endangered Myricaria laxiflora populations downstream of the dam. The present study utilized biochemical markers of photosynthetic physiology to evaluate the spatiotemporal responses of remnant populations to human-regulated water level fluctuations. The results showed that the photosynthetic physiological activities of remnant M. laxiflora populations underwent a period of rapid growth, followed by a gradual decline in the growth recovery phase after flooding. During the entire experimental period, photosynthetic physiological activities of remnant M. laxiflora populations changed with prolongation of emergence time: specifically, net photosynthetic rate and stomatal conductance initially decreased and then subsequently increased, intercellular carbon dioxide concentrations peaked at mid-phase and transpiration rate continuously increased. The maximum net photosynthetic rate, apparent photosynthetic quantum efficiency and dark respiration rate in the light-response curves of the plants continuously increased during growth. The water level gradient also significantly affected the photosynthetic physiological activities in the remnant populations, i.e. the photosynthetic physiological activities of high-altitude plants were significantly higher than the middle- and low-altitude plants. The changes in photosynthetic pigment content of plants in remnant populations during the growth recovery phase and the entire growth period were similar to those occurring in photosynthetic activities in plants. Further, canonical correspondence analysis showed that photosynthetic physiological activities in the plants were significantly correlated with changes in water levels, emergence time, elevation gradient, soil water and soil nitrogen contents. Therefore, the artificial regulation of water level fluctuations by large hydropower stations will inevitably affect the photosynthetic activities and growth of remnant M. laxiflora populations.

[Soil heterotrophic respiration and its sensitivity to soil temperature and moisture in Liquidambar formosana and Pinus massoniana forests in hilly areas of southeast Hubei Province, China].

Field monitoring was conducted to study the annual dynamics of soil heterotrophic respiration and soil temperature and moisture in Liquidambar formosana and Pinus massoniana forests in hilly areas of southeast Hubei Province, China. At the same time, laboratory experiment was performed to study the heterotrophic respiration rate along soil profile, and the sensitivity of surface soil (0-5 cm) heterotrophic respiration to soil temperature and moisture. Then, a model was established to valuate the potential effects of warming change on the soil heterotrophic respiration in study area. In L. formosana and P. massoniana forests, the soil heterotrophic respiration rate in 0-5 cm layer was 2.39 and 2.62 times, and 2.01 and 2.94 times of that in 5-10 cm and 10-20 cm layers, respectively, illustrating that soil heterotrophic respiration mainly occurred in 0-5 cm surface layer. The temperature sensitivity factor (Q10) of soil heterotrophic respiration in 0-5 cm, 5-10 cm, and 10-20 cm layers was 2.10, 1.86, and 1.78 in L. formosana forest, and 1.86, 1.77, and 1.44 in P. massoniana forest, respectively. The relationship between surface soil heterotrophic respiration and temperature (T) well fitted exponential function R = alphaexp (beta3T), and that between surface soil heterotrophic respiration and moisture (W) well fitted quadratic function R = a+bW+cW2. Therefore, the relationship of surface soil heterotrophic respiration with soil temperature and moisture could be described by the model lnR = a+bW+cW2 +dT+eT2, which suggested that the response of soil heterotrophic respiration to soil moisture was depended on soil temperature, i.e., the sensitivity decreased with decreasing soil temperature. The calculation of the annual soil heterotrophic respiration rate in the two forests with the established model showed that the calculated respiration rate was a slightly higher in L. formosana forest but close to the measured one in P. massoniana forest, illustrating the applied importance of the model. Our results suggested that the soil heterotrophic respiration in the L. formosana and P. massoniana forests in hilly areas of southeast Hubei Province would have an obvious increase under the background of global warming.

[Effects of waterlogging on ecophysiological characteristics of Salix variegate seedlings].

Salix variegate, a widely distributed species along the riverbank in Three Gorges Reservoir area, plays an important role in soil conservation and riverbank stabilization. Waterlogging from April to May was simulated in 2006 to test the adapting ability and mechanism of S. variegate seedlings to water environment change resulted from the construction of Three Gorges Project. The results showed that under waterlogging, all of the S. variegate seedlings could survive, but their maximum photosynthesis (Pmax), transpiration rate (Tr), water use efficiency (WUE), and stomatal conductance (Gs) decreased significantly with the prolongation of waterlogging. In the early period of waterlogging (20 days), these indices changed little, with the Pmax and Tr decreased by 18.5% and 2.2%, respectively, compared with the control. After 30 days, Pmax and Tr decreased by 53.4% and 23.7%, respectively, and then, kept stable. The contents of chlorophyll and carotenoid and their ratio decreased significantly with the prolongation of waterlogging, while the ratio of chlorophyll a to b increased significantly. The ratio of chlorophyll to carotenoid ranged in 4.873-6.883, and that of chlorophyll a to b ranged in 2.855-3.912. Adventitious roots were developed after waterlogging, which would benefit the oxygen supply for the plants. It was suggested that S. variegate had good adaptability to waterlogging, and could be used as a pioneer species in the restoration of degraded water-level fluctuation zone.