Summary Coastal wetlands of the southeastern United States are threatened by increases in flooding and salinity as a result of both natural processes and man-induced hydrologic alterations. Furthermore, global climate change scenarios sug-gest that, as a consequence of rising sea levels, much larger areas of coastal wetlands may be affected by flooding and salinity in the next 50 to 100 years. In this paper, we review studies designed to improve our ability to predict and amelio-rate the impacts of increased flooding and salinity stress on baldcypress (Taxodium distichum (L.) Rich.), which is a domi-nant species of many coastal forested wetlands. Specifically, we review studies on species-level responses to flooding and salinity stress, alone and in combination, we summarize two studies on intraspecific variation in response to flooding and salinity stress, we analyze the physiological mechanisms thought to be responsible for the interaction between flooding and salinity stress, and we discuss the implications for coastal wetland loss and the prospects for developing salt-tolerant lines of baldcypress.
Keywords: multiple stress, salt tolerance, stress interaction, waterlogging.
Introduction
Forested wetlands in which baldcypress (Taxodium distichum (L.) Rich.) is a dominant overstory tree species are common in the southeastern U.S. coastal plain (Larsen 1980, Wilhite and Toliver 1990), often at elevations of 2 m or less above mean sea level (Salinas et al. 1986, DeLaune et al. 1987). Many of these coastal wetlands have recently been exposed to increases in flooding or salinity stress, or both, as a result of a combination of natural processes (e.g., land subsidence) and man-induced alterations of hydrology and sedimentation (Figure 1; Craig et al. 1979, Wicker et al. 1981, Templet and Meyer-Arendt 1988, Conner and Toliver 1990, Allen 1992). If predicted climate changes occur, the consequent rise in sea level will cause flooding and saltwater intrusion in many coastal areas (Titus 1988, Smith and Tirpak 1989, Kerr 1991, Daniels 1992, Wigley and Raper 1993). In the southeastern U.S., such
intru-sion will compound the already significant problems of degra-dation and loss of forested coastal wetlands (Figure 2).
Although much is known about the general responses of baldcypress and associated species to flooding and salinity (Pezeshki et al. 1986, 1987, 1988, 1990, Pezeshki 1990, 1993, Conner and Askew 1992, Conner 1994), less is known about the responses of baldcypress to both stresses combined. How-ever, in many other plant species, a combination of flooding and salinity stress reduces survival and growth more than either stress alone (Van der Moezel et al. 1988, 1991, Marcar et al. 1993, Noble and Rogers 1994). For example, Pezeshki (1992) found that flooding with 3 g l−1 artificial seawater reduced net photosynthesis of loblolly pine (Pinus taeda L.), a species moderately tolerant of soil waterlogging (Hook 1984), by 96% compared to flooding with freshwater.
In this paper, we review species-level effects on baldcypress of flooding alone, salinity alone, and flooding in combination with salinity. We then summarize two studies on intraspecific variation in response to a combination of flooding and salinity stress. We also consider possible mechanisms that may under-lie the interaction between flooding and salinity stress on baldcypress, and briefly discuss possible strategies for the protection, management and restoration of coastal baldcypress forests. We focus on baldcypress because of its economic and ecological importance and its perceived vulnerability to both current environmental changes and predicted global climate change.
Species-level effects of flooding and salinity
Effects of flooding
Baldcypress is considered to be highly tolerant of flooding and soil waterlogging (McKnight et al. 1981, Hook 1984, Brown and Montz 1986, Keeland 1994). Nevertheless, the increasing depth and duration of flooding to which coastal swamps are being subjected is threatening the long-term existence of bald-cypress in some areas, even without the additional factor of salinity (DeLaune et al. 1987, Conner and Brody 1989). One reason for this is that baldcypress seeds generally do not
Interaction of flooding and salinity stress on baldcypress (
Taxodium
distichum
)
JAMES A. ALLEN,
1S. REZA PEZESHKI
2and JIM L. CHAMBERS
31 National Biological Service, Southern Science Center, 700 Cajundome Blvd., Lafayette, LA 70506, USA
2 Biology Department, Division of Ecology and Organismal Biology, University of Memphis, Memphis, TN 38152, USA
3 School of Forestry, Wildlife and Fisheries, Louisiana State University, Louisiana Agricultural Experiment Station, Baton Rouge, LA 70803, USA
Received March 2, 1995
germinate under water (Demaree 1932), and therefore, natural regeneration is unlikely or even impossible under conditions of permanent flooding. Also, growth is substantially reduced in deep (> 1 m), permanently flooded swamps compared to swamps with intermittent flooding (Conner and Day 1976). In some cases, permanent deep flooding has accelerated the de-cline and death of established baldcypress trees (Demaree 1932, Eggler and Moore 1961, Harms et al. 1980, Brown and Montz 1986).
Several studies have indicated that baldcypress seedlings
generally tolerate shallow flooding, but that flooded seedlings go through an initial period of stress and adaptation during which they are outperformed by unflooded well-watered con-trols. For example, Shanklin and Kozlowski (1985) reported that, following 14 weeks of flooding at 2 cm above the soil surface, baldcypress seedlings were 30% shorter, had 56% less leaf area, and had 51% less total dry weight than unflooded controls. Flynn (1986) observed that seedlings in a drained treatment had slightly higher above- and belowground biomass, and significantly greater total biomass than seedlings
Figure 1. Louisiana Gulf Coast forests (including baldcypress–water tupelo swamps) currently experiencing increased flooding or saltwater intrusion, or both. Reproduced from Pezeshki et al. 1987, by permission of the Society of Wetland Scientists.
flooded to a depth of 15 to 20 cm for 126 days.
Seedlings generally recover from the stress imposed by continuous shallow flooding and may grow as rapidly as seed-lings subjected to well-watered conditions or periodic flood-ing. Megonigal and Day (1992), who conducted a 3-year study in large outdoor rhizotrons, found that, after 1 year, continu-ously flooded seedlings had approximately one third the biomass of seedlings subjected to periodic flooding. By the second year, growth of the continuously flooded seedlings had improved substantially, and by the end of the third year, there were no significant differences in total biomass between seed-lings in the two treatments. Improved growth in the second year coincided with morphological changes in the roots, in-cluding the development of water roots and changes in root distribution.
Physiological studies have demonstrated that although bald-cypress seedlings survive and grow when flooded, their physi-ological performance is initially impaired. For example, root elongation is inhibited in baldcypress seedlings subjected to hypoxic (+200 mV soil redox potential) soil conditions for 2 weeks (Pezeshki 1991). Short-term declines in Rubisco ac-tivity have also been reported in response to flooding (Pezeshki 1994b). Pezeshki et al. (1986) found that net photo-synthesis (A) and stomatal conductance (gw) of baldcypress seedlings subjected to shallow flooding for 40 days declined by 21 and 41%, respectively, compared with the values for unflooded controls; however, both A and gw recovered to 80 and 90%, respectively, of pretreatment values within 2 to 3 weeks (Pezeshki et al. 1987, Pezeshki 1993). This recovery may be related to a resumption of root function following development of intercellular air spaces (aerenchyma), which begin to develop in the first 2 weeks of flooding (Pezeshki 1991), and have been shown to play a vital role in maintaining root functioning of numerous wetland species (Blom et al. 1994, Pezeshki 1994a)
Effects of salinity
Baldcypress seedlings are moderately salt-tolerant. Pezeshki (1990) found no significant effects on height growth, net pho-tosynthesis or stomatal conductance when baldcypress seed-lings were watered with a 3 g l−1 saltwater solution for 60 days (Figure 3). Even in seedlings regularly watered with a 10 g l−1 saltwater solution for 3 months, survival was 100% and their mean height was 83% of controls watered with freshwater (Conner 1994).
Effects of a combination of flooding and salinity
Several studies have shown that a combination of flooding and salinity is considerably more detrimental to baldcypress seed-lings than the effect of either stress alone, and the detrimental effects of a combination of flooding and salinity increase with increasing salinity (Javanshir and Ewel 1993, Conner 1994). Flooding seedlings with 3 g l−1 saltwater for 60 days reduced both height growth and A by about 50% compared with well-watered controls; however, the differences were only signifi-cant for height growth (Pezeshki 1990) (Figure 3). In contrast, seedlings watered with 10 g l−1 saltwater survived and grew
reasonably well, whereas seedlings continuously flooded with 10 g l−1 saltwater all died within 2 weeks (Conner 1994). Wicker et al. (1981) concluded that baldcypress wetlands are limited to areas where salinity does not exceed 2 g l−1 for more than 50% of the time that the trees are exposed to inundation or soil saturation (cf. Chabrek 1972). However, Myers et al. (1995) report that seedlings planted from 1 to 4 years pre-viously in a frequently flooded marsh were thriving despite a nearly constant groundwater salinity of 2.8 g l−1.
Intraspecific variation in tolerance to combined flooding and salinity stress
Baldcypress seedlings exhibit intraspecific variation in toler-ance to a combination of flooding and salinity stress (Allen 1994, Pezeshki et al. 1995). Pezeshki et al. (1995) evaluated the flooding and salt tolerance of baldcypress seedlings from two sources in Louisiana----a coastal, brackish area and an inland freshwater area----by subjecting the seedlings to shallow flooding with 0, 4 or 8 g l−1 salt in a controlled-environment chamber for 6 weeks. Although gas exchange rates of seed-lings from the two sources were comparable in the various treatments, seedlings from the freshwater source had signifi-cantly greater growth than seedlings from the brackish source. In the shallow flooding with 4 g l−1 salt treatment, net shoot and root biomass of freshwater plants were 86 and 97% greater, respectively, than those of brackish plants. In the
shallow flooding with 8 g l−1 salt treatment, shoot biomass was 60% greater in freshwater plants than in brackish plants but root growth was not significantly different between sources.
Allen (1994) evaluated intraspecific variation in responses to combined flooding and salinity stress of seedlings from 15 open-pollinated families. Ten families were from coastal loca-tions in Louisiana or Alabama that had surface water salinities between 0.4 and 15.3 g l−1 at the time of seed collection in November (brackish sources). The other five families were from areas slightly further inland that were not subject to saltwater intrusion (freshwater sources). Seedlings were flooded to about 5 cm above the soil surface with 0, 2, 4, 6 or 8 g l−1 artificial seawater for 4 months.
All 15 families were tolerant of 2 g l−1 salt in combination with shallow flooding, although there was a significant overall decline in mean leaf area per seedling between the 0 and 2 g l−1 treatments. In the shallow flooding with 4 g l−1 salt treat-ment, some families exhibited moderate declines in survival and substantial decreases in leaf area and biomass. In the shallow flooding with 6 and 8 g l−1 salt treatments, survival ranged from 42 to 97%, and there were large differences among families in mean leaf area and biomass. In general, families from brackish sources were more tolerant of the com-bined salinity and flooding stresses than families from fresh-water sources, suggesting a moderate degree of heritability. In the shallow flooding with 6 g l−1 salt treatment, the mean leaf area per seedling for the 10 families from brackish sources (93 cm2) was more than four times greater than the mean for the five families from freshwater sources (22 cm2). Overall differences between the two sources were substantially less in the shallow flooding with 8 g l−1 salt treatment, although several families from brackish sources still greatly outper-formed all five families from freshwater sources.
A notable difference among families was the differential pattern of leaf retention and loss. The more tolerant families gradually lost older basal leaves, while retaining or producing healthy younger leaves. At the end of the study, individual seedlings often had only a small cluster of leaves at the top of the stem. These individuals did not exhibit dieback at the top of the plant. Less tolerant seedlings exhibited partial stem dieback and partial refoliation along the lower portion of the stem. These leaf responses are similar to those described in Australian tree species differing in tolerance to salinity or flooding plus salinity stress (Van der Moezel et al. 1988). Thus it appears that there is a substantial degree of variation within the baldcypress species, with some individuals exhibiting re-sponses characteristic of moderately tolerant species and oth-ers having responses more typical of salt-sensitive species.
Significant differences in gas exchange were found among the 15 families (Figure 4). Two of the families (LS1 and SW1) maintained rates of A consistently above the overall mean for the 15 families. Two other families (PB1 and PR1) had consis-tently below-average photosynthetic rates. Within-family vari-ation was also high, and some individual seedlings in the shallow flooding with 6 and 8 g l−1 salt treatments maintained rates of photosynthesis as high as the overall mean for the controls.
Although significant differences among families were found for all physiological response variables except midday leaf water potential, only differences in nutrient uptake were corre-lated with tolerance to flooding and salinity. The three most tolerant families had 34% lower Na+ and 20% lower Cl− con-centrations in their leaf tissue than the overall mean for the 15 families.
Possible mechanisms for flooding and salinity interaction
Although many adaptations to saline environments are known to exist in plants, exclusion of Na+, Cl− and other ions from leaf tissue is believed to be the primary means by which most glycophytes tolerate salinity (Greenway and Munns 1980, Lauchli 1984, Ashraf 1994). The consequences of failure to exclude Na+, Cl− and other ions from leaf tissue may include water stress, ion toxicity or imbalances, and hormonal imbal-ances (Flowers et al. 1977, Greenway and Munns 1980, Poljakoff-Mayber 1988). However, some species of glyco-phytes tolerate salt by a strategy more typical of haloglyco-phytes, whereby Na+ or Cl−, or both, is taken up into leaves and compartmentalized in cell vacuoles (Van Steveninck et al. 1982), usually with the concomitant production of organic solutes in the cytoplasm for osmotic adjustment (Flowers et al. 1977).
In baldcypress, sensitivity to salinity is probably related to its relative inability to exclude ions or effectively compartmen-talize them in cell vacuoles. The finding that the more tolerant families of baldcypress had lower Na+ and Cl− concentrations in leaf tissue than the less tolerant families (Allen 1994) suggests that that regulation of Na+ and Cl− uptake is a critical factor affecting salt tolerance of baldcypress. Pezeshki et al. (1988, 1990) and Allen (1994) found that leaf tissue ionic concentration of baldcypress seedlings increased with increas-ing salinity of the floodwater, and relatively strong and nega-tive relationships between leaf ionic content and A were found
for all ions tested (Figure 5). The results of both studies suggest that the excess accumulation of ions disrupts photosynthesis, perhaps through inhibition of carboxylating enzyme activities. The relationship between A and leaf ion concentrations also indicates that the ion compartmentation process was not effi-cient enough to prevent metabolic dysfunction.
The decline in photosynthesis associated with relatively small increases in leaf Na+ and Cl− concentrations may inhibit root functioning. For example, some adaptations to flooding, such as the development of new aerenchymatous roots, which oxidize the rhizosphere and make temporary use of anaerobic metabolic pathways (Flynn 1986, Pezeshki 1991, Yamamoto 1992, Kludze et al. 1994), require energy, and therefore may be adversely affected by declines in assimilate production. Little is known about the mechanism of ion exclusion in baldcypress roots, but if it is an active process, then it may become increasingly ineffective as the energy status of the plant declines, thereby allowing more ions into the leaves, further disrupting photosynthesis and eventually causing leaf mortality.
Allen (1994) found that as salinity of floodwater increased from 0 to 4 g l−1, leaf biomass declined substantially more than root biomass. The Na+/Ca2+ and Na+/K+ ratios in root tissue remained stable at low salinities. Above 4 g l−1, however, the relative proportion of biomass allocated to roots declined. At the same time, the Na+/Ca2+ and Na+/K+ ratios in the roots began to increase dramatically, suggesting that ion imbalances were occurring in the roots, and that root functions (including active exclusion mechanisms) were being severely impaired. The high Na+/Ca2+ and Na+/K+ ratios may also indicate a breakdown in root membrane integrity, which would affect passive uptake of ions (Alam 1993).
Conclusions
We conclude that baldcypress is highly susceptible to the combined stress of flooding and salinity. Depending on the severity of the stress imposed, one possible outcome of a rise in sea level and other alterations of baldcypress-dominated forests is the loss of wetlands on the scale depicted in Figure 1. Alternatively, a shift in species dominance toward native asso-ciates or exotics such as Chinese tallow (Sapium sebiferum (L.) Roxb.) (Pezeshki et al. 1990, Conner 1994) could occur.
It may be possible to slow or even reverse the effects of flooding and salinity stress on coastal wetlands by a combina-tion of engineering and genetic approaches (sensu Epstein et al. 1980). Engineering approaches designed to reintroduce freshwater and sediments to coastal wetlands have been widely advocated and some projects have been initiated (Templet and Meyer-Arendt 1988, Wetland Conservation and Restoration Task Force 1990).
The genetic approach seeks to develop lines with increased tolerance to combined flooding and salinity stress. The poten-tial of this approach has not been realized; however, it is encouraging to note that intraspecific variability in tolerance of baldcypress seedlings to the two stresses combined has been demonstrated (Allen 1994, Pezeshki et al. 1995). Although the most tolerant families in Allen’s (1994) study came from the more brackish sites, the study of Pezeshki et al. (1995) sug-gests that tolerant genotypes may also come from freshwater sources, indicating the need to screen a wide selection of genotypes.
The development of reliable screening methods will depend on a detailed understanding of the mechanisms by which salinity and flooding affect baldcypress. In particular, we need more information about intraspecific variation in root mor-phology, and translocation and storage of Na+ and Cl− in young and old leaves, as well as a better understanding of ion exclu-sion mechanisms and how they are affected by a combination of flooding and salinity stress. Early formation of Casparian bands may be a major factor affecting differential ion exclu-sion of Eucalyptus camaldulensis Dehnh. genotypes (Thom-son 1988). Casparian band development at the lateral root--endodermis interface (Marschner 1986, Shannon et al. 1988) and at the periderm and exodermis (Moon et al. 1986) may also be important in determining flooding and salinity tolerance.
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