The newly-created Three Gorges Dam of the Yangtze River, for purposes of flood control, hydropower generation and navigation, has made a very large reservoir that began to hold water in 2003.To maximize the benefits of the Dam both for flood control and hydropower generation, an annual 30 m waterlevel fluctuation change was adopted, drawing down to 145 m ASL at the end of each May due to the impending rainy season and demand for holding flood waters, and rising to 175 m ASL at the end of each November for maximal hydropower generation.The Three Gorges Reservoir region encompasses about 400 km² and 2 000 km of shoreline.

Because of high mountainous topography, rich karst geology and frequent acidic precipitation in the region, a series of eco-environment problems occur, such as soil erosion, vegetation fragmentation and biodiversity loss.Flow regulation imposed by the Three Gorges Dam further degrades the region's already fragile ecosystem.Although multifaceted socialeconomic factors might be attributed to concurrent ecosystem degradation, one of the root causes can be due to lower forest cover.Forests play a critical role in maintaining the ecological balance in a region.Thus, reforestation is of significance and necessary for increasing forest cover in order to facilitate restoration and rehabilitation of the ecosystems of the region. However, as far as reforestation in this region is concerned, suitable tree species selection has been an ongoing assignment in recent years.Adequate tree species selected for reforestation contributes to guaranteeing successful plantations and thus revegetation in the Three Gorges Reservoir region.

Tree species suitable for reforestation in the riparian zones of the Three Gorges Reservoir region are expected to tolerate and/or adapt to multiple stresses, such as higher soil moisture produced by rising waterlevels, and drought stress engendered by water-level drawdown, as well as other adverse environment conditions.Baldcypress (Taxodium distichum) and pondcypress (T.ascendens), native to southeastern North America, have become widespread throughout the Yangtze River valleys of China since their introduction 80 years ago.Previous studies showed that baldcypress and pondcypress perform well in wetlands (Wang et al., 2002), stream banks (Shu et al., 2003; Wang et al., 2002) and agroforestry (Huang et al., 1994).Both species are widely recognized tree species adaptable and/or tolerant to wet and drought soils, air pollution (Arnold, 2002; Cox et al., 1988; Wasowski et al., 1997), varying nutrient availability conditions, a wide range of soil aeration levels and somewhat extreme pH levels (Watson, 1983).So, baldcypress and pondcypress may be promising candidate tree species for reforestation in the Three Gorges Reservoir region.The purpose of this study was to review the research literatures related to baldcypress and pondcypress, so as to provide a reference to tree species selection for reforestation in the Three Gorges Reservoir region.

1 Relationship between baldcypress and pondcypress

Based on allozyme analysis, Lickey et al. (2002a) found the possibility that these two taxa represent distinct species is unlikely due to the apparent existence of genetic flow and the possibility of clinal variation within populations.Thus, these two taxa are suggested to be best regarded as varieties of the same species.In addition, little genetic difference between baldcypress and pondcypress was found in DNA studies by Tsumura et al.(1995; 1999).In addition, Liu et al.(1990) reported that no genetic difference was detected in baldcypress between individuals remaining alive in the thermal effluent of the Savannah River Power Plant and the individuals not affected by the power plant's cooling waters.Some other authors also supported such a viewpoint by considering pondcypress as a botanical variety of T. distichum (Denny et al., 2007; Lickey et al., 2002b; Murphy et al., 1975; Schopmeyer, 1974; St Hilaire, 2001; Watson, 1986; 1993).However, some authors treat pondcypress as a distinct species (Arnold, 2002; Godfrey, 1988; Griffiths, 1994; Krüssmann, 1985; Turner et al., 1999) due to morphological and genetic variation compared with baldcypress (Allen et al., 1994; Faulkner, 1985; Neufeld, 1983, 1986; Sharma et al., 1978).Such an understanding of the intimate relationship between baldcypress and pondcypress might well explain why there are fewer specific studies on pondcypress than on baldcypress.

Besides the contrasting viewpoint on whether or not pondcypress should be considered a variety of T.distichum, such a debate concerning the existence and nature of intermediate phenotypes between baldcypress and pondcypress has remained unresolved for a long time.Hardin (1971) stated that these intermediates are in existence, while Godfrey (1988) insisted that trees and populations with intermediate morphologies are rarely seen in nature.

In addition, montezuma cypress under the same genus Taxodium as baldcypress and pondcypress, is treated as a distinct species in a name of T. mucronatum (Arnold, 2002; Chen et al., 1996; St. Hilaire, 2001; Turner et al., 1999).However, Watson (1983; 1993) and Denny (2007) treat montezuma cypress as a botanical variety, T.distichum var.mexicana.Furthermore, Denny (2007) insisted that the most appropriate treatment for baldcypress, pondcypress and montezuma cypress is three botanical varieties under one species.Thus, many more studies are needed to identify the true relationship among baldcypress, pondcypress and montezuma cypress.

2 Biological and ecological differences

Although leaves of both baldcypress and pondcypress exhibit the same kind of spiral arrangement around deciduous shoots, baldcypress is characterized by more linear leaves in contrast to awllike to scale-like leaves in pondcypress (Brown et al., 1986; Watson, 1986).Leaves of pondcypress are narrower and thicker than those of baldcypress, with an arrangement from radial in high light to distichous in low light (Neufeld, 1983).The foliage of pondcypress trees may confer greater tolerance to or avoidance of water stress effects than baldcypress trees (Brown, 1981).Furthermore, baldcypress typically has thinner bark than pondcypress (Brown et al., 1986).Neufeld (1986) documented that pondcypress have narrower crowns and are taller at a given diameter than baldcypress, and also have approximately twice as many branchlets as baldcypress in the distal 15 cm of branches with a higher leaf area ratio than baldcypress (2.60 versus 1.60 m²·kg^-1 dw).In addition, pondcypress exhibit reduced internodal elongation and slower growth rates most likely due to the adaptation to the environment.

Baldcypress usually grows along nutrient-rich flood plains of large rivers, whereas pondcypress grows in nutrient-poor bays and creeks (Brown, 1981; Christensen, 1988).Watson (1986) speculated that pondcypress had evolved from those populations of Taxodium on the Florida peninsula where they had experienced a much drier climate during the recent glacial-interglacial cycle.Such drier climate selected drought tolerant, fire resistant individuals with characters typical of pondcypress, such as thicker bark, sclerophyllous, appressed leaves with sunken stomates and ascending branchlets (Lickey et al., 2002a).

Neufeld (1983) found that baldcypress seedlings produced greater biomass, leaf area, height and diameter growth, while significant lower net photosynthetic rates, than pondcypress seedlings under light levels ranging from 5%-100% of full sunlight. The author illustrated that thicker cross-sections along with higher specific leaf weights in pondcypress leaves than in baldcypress leaves may have accounted for the differences in net photosynthetic rates due to a resultantly greater internal leaf surface area available for CO₂ uptake.In contrast, Murphy (1974) reported that pondcypress seedlings grew 50% taller than baldcypress seedlings, and did not differ in diameter growth.Such conflictive results between Neufeld (1983) and Murphy (1974) reflected, at least partially, inherent differences in the maximum potential growth rates, as they both adopted almost the same kind of experiment conditions (Neufeld, 1983). Brown (1981) stated that lower growth rates in pondcypress compared to baldcypress may reflect an adaptation to anaerobic and/or nutrient deficient soils. Other reports documented that baldcypress had higher growth rate in the field (Harper, 1902; Mattoon, 1915; Langdon, 1965).

3 Tolerances to multiple stresses 3.1 Flooding tolerance

Baldcypress and pondcypress survive and grow in flooded habitats partly because of root adaptation (Pezeshki et al., 1990), such as lenticel/aerenchyma formation, adventitious rooting and regeneration of new roots (Kozlowski, 1984).Hook et al.(1980) reported that Taxodium distichum (Taxodiaceae) produces water roots under flooding conditions.Grosse et al.(1992) found that pressurized gas transport improves oxygen supply to roots of T.distichum to help them survive the initial period of soil flooding. Intermittent flooding (flooded for 5 d and drained for 5 d) had no significant effect on stomatal conductance and net photosynthetic rate, growth and biomass in baldcypress seedlings, but root porosity increases 1.4 times as compared to the control (Anderson et al., 1999).Megonigal et al.(1992) also demonstrated that total biomass of periodically flooded baldcypress seedlings was comparable to continuously flooded seedlings due to the production of flood-adapted roots under the prolonged 3-year flood durations.Fisher et al.(1990) revealed that pondcypress is flood tolerant as well.In addition, baldcypress may rapidly resume gas exchange function after floodwater was drained (Anderson et al., 1999; Pezeshki, 1993).Yamamoto (1989) found that in baldcypress deep waterlogging inhibited growth of above-water-surface height and diameter, while facilitating growth of below-watersurface diameter.Furthermore, the tracheid of baldcypress under deep flooding became wider, along with thinner cell wall and bark as compared to the corresponding above-water portions.

Under rhizosphere hypoxia imposed by flooding, baldcypress seedlings reduced ribulose-1, 5-bisphosphate carboxylase/oxygenase (RuBPCO) activity at 48 h, which is among the early signals of flood-stress contributing to the loss of leaf net photosynthetic rate (Pezeshki, 1994).The recovery of baldcypress seedlings in stomatal conductance and net carbon assimilation rate can reach 97% and 87% of control by day 28 (Pezeshki, 1994).Although baldcypress and pondcypress are commonly recognized as two water-tolerant tree species due to their adaptation capabilities, previous studies showed conflictive results in responses to flooding as indicated in table 1.Such conflictive results might be caused by disparity among individuals within the population of the seedlings used in different experiments.Some seedlings/saplings may develop adaptation systems antecedently, thus leading to an earlier production of water roots and development of intercellular air spaces. On the other hand, the effect of the flooding treatments on plant growth and biomass was different for 1-yearold seedlings and 3-year-old saplings (Megonigal et al., 1992).Improved growth in the continuously flooded environment began in the second growing season in coincidence with morphological adaptations to flooding (Megonigal et al., 1992).As all of those studies listed in the table 1 used 1-year-old seedlings or 2-year-old saplings, the results may not be in agreement with those obtained using 3-year-old or older saplings.Planting 3-year-old saplings may be better than 1-or 2-year-old saplings in terms of their adaptive performances in the Three Gorges Reservoir region.

The highest water level of 175 m ASL will be maintained for about 35-40 days annually in the riparian zones of the Three Gorges Reservoir region. Plants grown on the riparian zones will be subjected to full submergence during the time period of highest water-level maintenance.Souther et al.(2000) reported that newly-germinated baldcypress seedlings (10.9 ± 0.3 cm height, under two weeks of age) could be sustained for at least 45 days of full submergence, in contrast to 1-year-old seedlings (52.8 ± 1.9 cm height) subjected to complete submergence up to 150 days with 75% survival.Thus, older and bigger saplings such as 2-or 3-year-old baldcypress and pondcypress, with more established root systems, can be expected to better tolerate continuous flooding and waterlogging, but further verification studies are still needed.Nevertheless, both baldcypress and pondcypress are promising candidates for reforestation owing to their water tolerance.

3.2 Drought tolerance

Although baldcypress seedlings are droughtsensitive (Dickson et al., 1972), root biomass under mild drought stress can still be maintained at normal level (Li et al., 2007; see also table 2).However, root biomass of pondcypress showed significant increase under mild drought condition (Li et al., 2008).Eclan et al.(2002) found significant reductions in net photosynthetic rate and stomatal conductance in baldcypress under mild drought stress.In addition, Li et al.(2005) also found that net carbon assimilation rate was significantly reduced in baldcypress and pondcypress seedlings under mild drought conditions. However, under mild water deficit stress, stomatal conductance in pondcypress seedlings was not significantly affected, in contrast to a significant decline in baldcypress seedlings (Li et al., 2005).

Pondcypress seedlings appear more resilience than baldcypress in response to soil water deficit.Thus, pondcypress should be a high priority candidate tree species for reforestation under drought-prone conditions in the riparian zone of the Three Gorges Reservoir region.Unlike the seedlings that are to some extent drought-sensitive, adult trees of baldcypress and pondcypress may perform well when subjected to drought stresses (Wang et al., 1995).Likewise, saplings are also expected to be more drought-tolerant than seedlings partially due to more established root systems (Islam et al., 2003; 2004).When reforestation in drought areas is needed, saplings and/or adult trees of baldcypress and pondcypress might be a better choice for plantation than the seedlings.

3.3 Salinity and alkalinity tolerance

Denny et al.(2008) documented that there was a strong geographic component to the variation in tolerance of alkaline soils in Taxodium, with T. distichum var.mexicana (i.e., montezuma cypress) being more tolerant of an alkaline site followed by T. distichum var.distichum.Baldcypress seedlings may tolerate water at low (2 g·L^-1) salinity, as shown in table 3, but reduced biomass occurs as watering salinity increased to 6 and 8 g·L^-1, where biomass is reduced first in leaves, followed by roots and finally the stem (Allen et al., 1997).Pezeshki (1990) reported that baldcypress seedlings decreased 46% in carbon assimilation rates and 56% in height growth when combined stresses from flooding (5 cm above soil surface) and salinity (concentration of 51 mol·m^-3) were imposed.More relevant studies related to the effects of combined flooding and salinity on growth and gas exchange of baldcypress are listed in table 4, where significant reductions in gas exchanges and growth can be found.However, variation in salt sensitivity among different populations exists (Allen et al., 1997; Pezeshki et al., 1990).Some individuals of baldcypress and pondcypress species exhibit responses characteristic of moderately tolerant species, while others demonstrate responses typical of salt-sensitive species (Allen et al., 1997).

In the riparian zones of the Three Gorges Reservoir region, karst geology accompanied with limestone soils typical in parts of the region causes difficulty in riparian eco-restoration through reforestation.The fact that more than 30% of the Reservoir region is characterized by karst geology, suggests that salt-and alkaline-tolerant species should be selected for reforestation and could play important roles in eco-restoration and rehabilitation.Although baldcypress and pondcypress may survive the acidic soils with pH = 4.7 (Shu et al., 2003), both species could not endure the soils of pH > 7.5 (Chen et al., 1996).However, Taxodium "zhongshansa 302 ", a hybrid between T.distichum and T.mucronatum, and T." zhongshansa 401 ", a hybrid between T.ascendens and T.mucronatum, exhibit such characteristics of enduring the soil with pH value of 8.0-8.5 and salinity around 0.1% (Chen et al., 1996; Zhou et al., 2000) and then could be used for reforestation.

3.4 Tolerance to pollution and disease

Baldcypress has not been observed to accumulate cobalt under natural conditions, and cobalt uptake by 2-year seedlings can be negligible under a cobalt treatment of 100 mg per pot (McLeod et al., 2007). Guthrie et al.(1979) reported that cobalt concentration in baldcypress growing in an ash basin was 0.1 mg·kg^-1 on a wet weight basis.Interestingly, McLeod et al.(2003) found that leaf manganese concentration increased drastically under manganeseenriched, water-saturated soil conditions, while no manganese accumulation occurred under natural conditions.A previous study (McDonald et al., 2008) showed that Taxodium seedlings from high-rainfall, high-humidity provenance areas had less defoliation associated with the presence of a leaf blight fungus (Cercosporidium sequoiae), even though defoliation varied among provenances within all geographical regions.

Baldcypress and pondcypress are expected to grow well even when subjected to pollutants and diseases. But, much more research is needed to better understand the responses of both species to different kinds of pollution and diseases.In the Three Gorges Reservoir region, environment pollution and tree diseases are generally severity (Xie et al., 2004), despite great efforts to mitigate and/or control these problems.The significance of screening adequate tree species for reforestation cannot be overstated in ecological restoration of the riparian zones.

4 Management implications

We conclude that baldcypress and pondcypress seedlings are susceptible to multiple stresses such as drought, salinity, alkalinity, air pollution, and diseases, although the two species are widely recognized adaptable and/or tolerant to flooding. Thus, care should be taken to mitigate and/or get rid of the stresses that negatively influence the growth and development of baldcypress and pondcypress seedlings. In addition, selecting the highly resistant individuals for reforestation is very necessary and also feasible as variations in capability of tolerance to multiple stresses exist among individuals and provenances of baldcypress and pondcypress.Thus, a reliable and efficient screening technique to select genotypes from regions shown to yield tolerant individuals is of significance. Besides traditional means, some modern molecular and genetic technologies could also be used to screen and/or cultivate the outperformed individuals for tolerance to multiple stresses.

In riparian zones of the Three Gorges Reservoir region, 3-year-old baldcypress and pondcypress saplings, even older and bigger ones, may be selected for reforestation.In case of a need in reforestation under high salinity and alkalinity environments such as in karst geological zones, the hybrids of Taxodium "zhongshansa 302"and T."zhongshansa 401"could be used, instead of baldcypress and pondcypress.

In conclusion, baldcypress and pondcypress are promising candidates for reforestation in the Three Gorges Reservoir region.However, much more research on the performances of baldcypress and pondcypress under deep submergence and under a combination of two or more stresses still needs to be done.