Residual Effects of Global Warming on Coastal Wetland Vegetation
Janet Frentzel
Writer’s comment: As an environmental biology student with a strong interest in plants and wetland conservation, I could not have found a more appropriate topic for my English 102 literature review than the effects of flooding and salinity on coastal wetland plant species. This paper gave me the opportunity to apply many of the concepts I learned during my stay at UC Davis to investigate a complex and fascinating topic. In addition, it broadened my understanding of global climate change and some of its subtle effects. As awareness of these subtle effects increases, I hope scientists will be able to circumvent further detriment to our wetland ecosystems.
- Janet Frentzel
Instructor’s comment: Janet wrote this literature review for my Advanced Composition course, English 102: Writing in the Biological Sciences. The assignment asks the students to read and understand six to ten articles within a research area of their choice, then synthesize the information so that a reader can quickly understand the present state of research in that area. Janet chose to look at how increased flooding and salinity might affect coastal wetland soils and species, and she organized the material clearly and effectively. What I liked particularly about this review was that it moved logically from the effects of flooding and salinity on wetland soils to the effects on the plants growing there to the overall effects on the wetland, both in terms of species composition and, more drastically, wetland survival. The tie-in with global climate change puts this local chain of cause and effect into a larger perspective.
- Jared Haynes, English Department
Introduction
Global climate change, rising sea levels and anthropogenic factors are creating increased flooding and salinity levels in coastal wetland areas. (Conner, 1994; Flynn et al., 1995; Webb et al., 1995; Conner and Askew, 1993; Allen et al., 1995; McCarron et al., 1998; Baldwin and Mendelssohn, 1998). Increased flooding and salinity levels can affect wetland soil parameters, and in turn, wetland associated plant species (Baldwin and Mendelssohn, 1998). Although some species are more tolerant to these conditions, most species demonstrate physiological responses and decreased survival at increased flooding and salinity levels. Therefore, changes in wetland species composition are expected unless intolerant species can adapt to heightened flooding and salinity levels imposed by changing environmental and global conditions (Allen et al., 1995).
Changes in Soil Parameters due to Increased Flooding and Salinity Levels
Because flooding creates anaerobic soil conditions, it decreases soil redox potential, Eh(Flynn et al., 1995; Webb, et al., 1995; Baldwin and Mendelssohn, 1998). Additionally, Ehhas been shown to decrease even further when soils are flooded with increased salinity levels (Baldwin and Mendelssohn, 1998; Flynn et al., 1995). Under these conditions, the soil medium becomes highly reducing and can affect wetland vegetation in two ways. First, and probably most significant, is the reduction of sulfate (present in seawater) to hydrogen sulfide, a known phytotoxin that has been shown to reduce wetland vegetation growth and/or survival at higher concentrations (Flynn et al., 1995). Secondly, highly reducing conditions can interfere with a plant’s ability to uptake nitrogen by altering its form and availability in the soil (Webb et al., 1995).
Other problems created by high salinity soil conditions include the change in water potential necessary for the plant to overcome the elevated salt concentration in the soil, and the possibility of toxicity due to uptake of Na+ and Cl– ions (Flynn et al., 1995).
Physiological Responses of Wetland Plants due to Increased Flooding and Salinity
Although tolerance to increased flooding and salinity conditions varies somewhat by species, wetland species that survive these heightened conditions demonstrate either some or all of the following characteristics: lowered photosynthetic activity, decreased stomatal conductance, diminished height and diameter growth, and lowered stem and root biomass. For example, Allen et al, (1995) state that bald cypress seedlings had reduced photosynthetic activity, stomatal conductance levels, and decreased height growth when exposed to flooding and salinity conditions. In addition, Conner (1994), found bald cypress to demonstrate slightly reduced root biomass. Conner (1994) also concluded that another wetland species, Chinese tallow, demonstrated a significant root and shoot biomass reduction. This result was consistent with another study in which Conner and Askew (1993) found Chinese tallow to demonstrate reduced shoot and root biomass along with decreased diameter and height growth. Other wetland species such as buttonbush and swamp tupelo have also been found to have altered photosynthetic activity and stomatal conductance levels, as well as decreased stem and root biomass and decreased height (McCarron et al., 1998).
Functional Mechanisms Explaining Plant Responses to Increased Flooding and Salinity
Because wetland species that survive increased flooding and salinity conditions demonstrate similar reactions, researchers have begun to investigate why these physiological changes occur. According to Allen et al, (1995), many plant species respond to increased salinity levels by taking up Na+ and Cl– ions into the leaves and storing them in cell vacuoles. This response appears to have many repercussions. Initially, the taking up of Na+ and Cl– ions lowers the ion concentration in the immediate rhizomial area, making it easier for the plant to uptake water. However, the plant must increase its production of organic solutes in order to osmotically adjust itself once the ions reach the vacuole. This osmotic adjustment takes energy and may use organic molecules that are needed for other functions within the plant.
Allen et al. (1995) also conclude that storing ions within the vacuoles has an effect on stomatal conductance and photosynthetic activity. As ion concentrations within the leaf increase, both stomatal conductance and photosynthetic activity decrease. This may partially be explained by examining stomatal function in the plant. According to McCarron et al. (1998), stomatal closure helps to increase water potential within the plant, thereby reducing water stress that may accompany heavy salt concentrations in the soil. This closure would account for reduced stomatal conductance and possibly some of the reduction in photosynthetic activity, since stomatal openings provide Ca2 necessary to fuel photosynthetic reactions by allowing gaseous exchange to occur between the atmosphere and plant tissues. However, Allen et al. (1995) speculate that the increased leaf ionic concentrations may also contribute to decreased photosynthetic activity by inhibiting carboxylating enzyme function.
Reduced photosynthetic activity is thought to play a role in inhibiting root functions (Allen et al., 1995). Because flood tolerant species adapt to flooding conditions by replacing their roots with more flood tolerant aerenchymatous roots (McCarron et al., 1998), these species require additional energy when faced with flooding conditions. Therefore, the reduction in assimilate production is thought to adversely affect this process and may provide an explanation for reduced root biomass under heavy salinity conditions.
Another explanation for root biomass decline is a breakdown in root membrane integrity (Allen et al., 1995). According to the study, as salinity levels increased, the ratio of Na+ to other cations in the root increased dramatically, suggesting that root functions were being compromised. Similarly, Allen et al. (1995) state that leaf biomass showed a significant decline with increasing salinity levels.
Changes in Species Composition due to Increased Flooding and Salinity
Although successional patterns in coastal wetlands are not well established, there is some evidence suggesting that dominant species response, original wetland composition, seed bank reservoir and the level of flooding and salinity will be significant factors in the composition of future wetland habitat. According to Baldwin and Mendelssohn (1998), species capable of rapid regrowth, seed germination under flooded conditions, or vegetative reproduction (cloning) may prevail under increased flooding and salinity levels. This prevalence would imply a possible loss of species richness in wetland habitat, or in the absence of such species, a loss of wetland habitat altogether. Allen et al. (1995) also conclude that increased flooding and salinity could cause wetland habitat loss if dominant wetland species could not overcome the newly imposed conditions. Alternatively, a shift in species dominance toward more tolerant species could occur. This is supported by Conner and Askew (1993), who found Chinese tallow to have the potential to outcompete less tolerant species. In another study, Flynn et al. (1995) observed changes in species composition due to the differential responses in wetland species. This was mainly a function of the regrowth rate of the surviving plants and the emergence of new seedlings from the wetland seedbank. However, these seedlings must be tolerant to flooding and salinity conditions, thereby naturally selecting for more tolerant species.
Conclusion
Global climate change and rising sea levels are impacting coastal wetland plant species by increasing the flooding and salinity levels within wetland soils. Although all of the impacts incurred from these conditions are not yet understood, researchers are investigating the complex interactions between the soil medium and plant physiology to determine how internal plant responses play a role in aiding survival. Unless many species are able to adapt to changing environmental conditions, species richness and composition within wetland areas will be affected, possibly causing a decline in wetland habitat. Possible interventions to prevent species decline and/or wetland loss may include engineering approaches (i.e. reintroducing freshwater and sediments to wetland areas) and/or genetic hybridization of species more tolerant to flooding and salinity stress (Allen et al., 1995). The latter will require much more investigation of which coastal wetland genotypes exhibit salinity and flooding tolerance, as well as determination of exact mechanisms responsible for the variation between tolerant and intolerant species.
Literature Cited
Allen, J.A., Pezeshki, S.R., and Chambers, J.L. (1995). Interaction of flooding and salinity stress on baldcypress (Taxodium distichum). Tree Physiology16: 307-3 13.
Baldwin A.H. and Mendelssohn, I.A. (1998). Effects of salinity and water level on coastal marshes: an experimental test of disturbance as a catalyst for vegetation change. Aquatic Botany61: 255-268.
Conner, W.H. and Askew, G.R. (1993). Impact of Saltwater Flooding on Red Maple, Redbay, and Chinese Tallow Seedlings. Castanea58(3): 214-219
Conner, W.H. (1994). The Effect of Salinity and Waterlogging on Growth and Survival of Baldcypress and Chinese Tallow Seedlings. Journal of Coastal Research10(4): 1045-1049.
Flynn, K.M., McKee, K.L., and Mendelssohn, I.A. (1995). Recovery of freshwater marsh vegetation after a saltwater intrusion event. Oecologia103: 63-72.
McCarron, J-K., McLeod, K.W., and Conner, W.H. (1998). Flood and Salinity Stress of Wetland Woody Species, Buttonbush (Cephalanthus occidentalis)and Swamp Tupelo (Nyssa sylvatica var. biflora). Wetlands18(2): 165-l 75.
Webb, E.C., Mendelssohn, I.A., and Wilsey, B.J. (1995). Causes for vegetation dieback in a Louisiana salt marsh: A bioassay approach. Aquatic Botany5 1: 28 l-289.