3 The Physical Environment of Wetland Plants I. An Introduction to the Wetland Environment Water is one of the primary factors that organizes the landscape, doing so through processes such as transport, erosion, leaching, solution, and evapotranspiration (Brown 1985). The hydrologic regime of a wetland is one of the key variables that determine the composition, distribution, and diversity of wetland plants. Hydrologic conditions affect species composition, successional trends, primary productivity, and organic matter accu- mulation (Gosselink and Turner 1978; Brinson et al. 1981; Howard-Williams 1985; van der Valk 1987). Factors related to the hydrologic regime that affect wetland plant communities include water depth (Spence 1982; Grace and Wetzel 1982, 1998), water chemistry (Ewel 1984; Pip 1984; Rey Benayas et al. 1990; Rey Benayas and Scheiner 1993), and flow rates (Westlake 1967; Lugo et al. 1988; Nilsson 1987; Carr et al. 1997). Hydrology also influences the plant community composition and primary productivity by influencing the availabil- ity of nutrients (Neill 1990), soil characteristics (Barko and Smart 1978, 1983), and the depo- sition of sediments (Barko and Smart 1979). The hydrologic regime can be thought of as a master variable with respect to all these factors since it not only determines the hydro- period, but it is also instrumental in carrying nutrients and sediment (and so modifying soil type) into a wetland. In this chapter, we focus on the ways in which hydrology controls plant community structure. We describe the hydrologic budget, with an emphasis on transpiration and its measurement. We also discuss how hydrologic forces affect species distribution, commu- nity composition, and primary productivity. Following the section on hydrology, we discuss the characteristics of saturated soils that render them inhospitable to plants. Low oxygen levels stress plant roots, which require oxygen to maintain cellular respiration. High concentrations of toxic forms of met- als accumulate, and nutrients may become less bioavailable. In saltwater ecosystems, these obstacles to growth and establishment are compounded by osmotic stresses. We discuss special conditions in nutrient-poor peatlands and the influences on substrate pH and nutrient availability. Finally, in the underwater environment, light and carbon dioxide may become limiting factors for submerged plants. II. The Hydrology of Wetlands Wetlands exist in geologic settings that favor the accumulation of water (Winter 1992). A wetland’s hydrology is a major influence on vegetation composition, which in turn L1372 - Chapter 3 04/25/2001 9:33 AM Page 61 © 2001 by CRC Press LLC determines the value of the wetland to other organisms. Differences in wetland type, soil type, and vegetation composition are the result of the geology of an area, its topography and climate (Bedford 1996, 1999). Ultimately, the hydrologic budget and local geology determine the quantity and chemistry of water in a wetland. The distribution, abundance, and type of plants in a wetland are related to the timing and duration of flooding, the tim- ing and duration of soil saturation, and soil characteristics. A. Hydroperiod and the Hydrologic Budget An understanding of hydrology provides a basis for understanding the ecology of wetland plants, particularly their association with flooded or saturated conditions. Plant establish- ment is influenced by a number of hydrologic processes including inflow rates, water depth, internal flow rates and patterns, the timing and duration of flooding, and ground- water exchanges. Changes in water level over time are referred to as the hydroperiod (Mitsch and Gosselink 2000). The hydroperiod is a result of the hydrologic budget, or the balance of a wetland’s water inflows and outflows over time. The annual hydroperiod pre- sents data on water level changes during a year, including flood depth and duration and the amount of soil saturation, but does not tell us explicitly about the topographic and cli- matological factors that cause the changes. A hydrologic, or water, budget is the total of water flows into and out of a site. It is an important tool because it reveals the relative importance of each hydrologic process for a given wetland. Water budgets, along with information about the local soils and surficial geology, can provide an understanding of the hydrologic processes and water chemistry, help explain the diversity and distribution of species in the plant community, and provide insight into the changes that might result from hydrologic disturbance. Water inflows are generally driven by climate and include precipitation, surface runoff, groundwater inflows, and, in coastal systems, tidal ebb and flow. Mass balance equations are often used to describe the flows of water into and out of a wetland (Huff and Young 1980), and are generally calculated to solve for volume such that: ∆V/∆t = water inputs – water outputs (3.1) or more specifically: ∆V/∆t = S i + G i + P n – ET – S o – G o ± T (3.2) where ∆V/∆t = change in volume of water (storage) per unit time, t S i = surface inflow G i = groundwater inflow P n = direct precipitation ET = evapotranspiration S o = surface outflow G o = groundwater outflow T = tidal inflow (+) or outflow (–); not present in inland wetlands L1372 - Chapter 3 04/25/2001 9:33 AM Page 62 © 2001 by CRC Press LLC TABLE 3.1 Examples of Water Budgets for a Variety of Wetland Types Wetland Type and Location Inflows Outflows Tides (∆V/∆t) S i G i P n ET S o G o Great Lakes coastal marsh, Ohio 576 131 a 38 67 653 a +25 Mangrove swamp, Florida 121 108 90 28 in = 1228 –54 out = 1177 Prairie pothole, North Dakota 40 37 64 18 –5 Okefenokee Swamp, Georgia 39 b 131 93 73 4 0 Fen, North Wales 38 b 102 49 100 –9 Green Swamp, central Florida 89–180 86–99 5–79 5–6 –10 to 0 Bog, Massachusetts 145 102 24 c +19 Pocosin swamp, North Carolina 117 67 49 1 0 Note: The units are cm yr -1 . Not all of the terms in the water budget equation apply in each type of wetland, and in some, not every part of the budget was measured (S i = surface inflow, G i = groundwater inflow, P n = Precipitation, ET = evapotranspiration, S o = surface outflow, G o = groundwater outflow). a Groundwater inflow is combined with tidal inflows and outflows. b Surface inflow is combined with groundwater inflow. c Surface outflow is combined with groundwater outflow. Data compiled by Mitsch and Gosselink 2000. L1372 - Chapter 3 04/25/2001 9:33 AM Page 63 © 2001 by CRC Press LLC © 2001 by CRC Press LLC A wetland’s annual water budget may change from one year to the next because of climatic variability, which in turn may result in a change in the magnitude of different components of the budget. A comparison of water budgets for several North American wetlands is shown in Table 3.1. The terms in the water budget vary in importance depending on the type of wetland, and not all terms apply to all types of wetlands. In the examples in which the change in volume is small or zero, the water level at the end of the study period was close to the water level at the beginning of the study period (Mitsch and Gosselink 2000). 1. Transpiration and Evaporation Transpiration (water that passes through vascular plants to the atmosphere) is an impor- tant parameter in wetland plant studies because it represents the interaction between a wetland’s hydrologic regime and its vegetation. Transpiration is the only component of the water budget that is dependent entirely upon plants. Estimates of transpiration are often combined with evaporation (water that vaporizes directly from the water or soil); this measure is known as evapotranspiration (ET). When water supplies are not limiting, mete- orological factors tend to control rates of ET. The rate of evapotranspiration is affected by solar radiation, wind speed and turbulence, available soil moisture, and relative humidity. Rates vary with the difference in vapor pressure at the water surface or leaf surface and the vapor pressure of the atmosphere. As the vapor pressure of the water or leaf surface increases relative to the atmosphere (due to solar energy or increases in temperature, for example), ET rates increase. When differences in vapor pressure decrease, for example when humidity increases or wind speeds decrease, ET rates decrease in response (Mitsch and Gosselink 2000). On an ecosystem level, water outputs due to ET are largely controlled by vegetation (both the species present and their areal extent) and the supply of water (Lafleur 1990a, b; Gilman 1994). In many cases, ET is the largest loss term in the water balance equation (Hollis et al. 1993; Gilman 1994; Owen 1998). For example, Verhoeven and others (1988), working in mesotrophic and eutrophic fens, found ET rates of 482 mm yr -1 . This accounted for 60% of total annual precipitation. Additionally, soil porosity may affect ET by limiting or facilitating the movement of water in the soil to roots or to the soil surface. Mann and Wetzel (1999) demonstrated this in a mesocosm study using Juncus effusus (soft rush). When grown in clay, J. effusus did not cause a decrease in soil water levels. However, in more porous sandy soils, where water movement in the soil is relatively quick, J. effusus caused a decline in the water level. Table 3.2 summarizes the results of some studies comparing the rates of ET from different vege- tation stands. TABLE 3.2 Mean Daily Summer ET Rates for Wetlands in Different Regions Vegetation Type Location ET (mm/d) Ref. Reed swamp Czechoslovakia 6.9 Smid 1975 a Czechoslovakia 3.2 Priban and Ondok 1985 a Freshwater marsh Florida 5.1 Dolan et al. 1984 a Low arctic bog Canada 4.5 Roulet and Woo 1986 a Quaking fen Netherlands 2.5 Koerselman and Beltman 1988 a Coastal marsh (wet) Ontario, Canada 3.1 Lafleur 1990b Coastal marsh (dry) Ontario, Canada 2.6 Lafleur 1990b Reed swamp North Germany ~10 Herbst and Kappen 1999 a Data compiled in Lafleur 1990b. L1372 - Chapter 3 04/25/2001 9:33 AM Page 64 © 2001 by CRC Press LLC 2. Measuring Transpiration and Evaporation Two general approaches to the quantification of ET are direct field measurements and esti- mates calculated using models of atmospheric conditions. Under certain conditions it is possible to isolate and measure transpiration rates directly (i.e., to measure transpiration separately from evaporation). For example, transpiration can be measured directly by tak- ing readings of stomatal conductance of water vapor. Water lost through stomata is a func- tion of atmospheric conditions but it is also influenced by stomata density (number per unit leaf area) and the diameter of the stomatal openings. Measurements are taken with leaf photosynthesis meters (infrared gas analyzers) that provide data on both inorganic carbon uptake and water loss through the stomata (e.g., Mann and Wetzel 1999). Results are generally reported in mol H 2 O m -2 s -1 . Measures of stomatal conductance, while pre- cise, present difficulties when one attempts to apply the results to the population or com- munity level. Using this method, Mann and Wetzel (1999) found transpiration rates in a population of Juncus effusus ranged from 0.16 to 0.43 mol H 2 O m -2 s -1 . The highest seasonal rates were found during the summer and autumn, and the highest daily rates in the early evening. In wetlands without standing water, diurnal fluctuations in the groundwater table (during periods of no precipitation) can be used to estimate ET. Groundwater levels typi- cally fluctuate by several millimeters over a 24-h period. Water levels decline during the day and remain stable or increase slightly overnight. Any increase in water level during the night is due to groundwater influx, which is assumed to occur at a constant rate over a 24-h period. Rates of evapotranspiration can be calculated as follows (Gilman 1994): ET = S/100 (24r ± s) (3.3) where ET = evapotranspiration S = the specific yield of the soil (i.e., how quickly water can move through the soil profile) r = the hourly rate of change in the water table during the night (generally taken from the night before, calculated from 8 P.M. to 6 A.M. or midnight to 4 A.M.) s = the net increase or decrease in the water table over the 24-h period Diurnal fluctuations can be attributed solely to transpiration and not evaporation by comparing changes in groundwater levels in areas that are vegetated with areas that are not (for instance, where plants have been cleared). Transpiration was measured in this way using continuous water level recorders at Wicken Fen in the United Kingdom (Gilman 1994). Groundwater levels showed diurnal fluctuations from mid-June to late September. Early in the growing season, rapid growth rates and high temperatures led to high tran- spiration rates. As the growing season progressed and both the water table and water demand by the plants declined, transpiration rates declined as well (as evidenced by the decreasing amplitude in diurnal groundwater level changes; Figure 3.1). The accumula- tion of plant litter can also affect transpiration. Using this method, lower rates of transpi- ration were found in natural stands of herbaceous plants where standing crop and accu- mulated litter reduced water loss. By comparison, transpiration rates were higher in grazed or mowed areas with little accumulated litter. One technique to measure evapotranspiration is to convert data from pan evaporation. Data from a Class Aevaporation pan, which provide an estimate of open water evaporation L1372 - Chapter 3 04/25/2001 9:33 AM Page 65 © 2001 by CRC Press LLC (E o ), are converted to ET of a vegetated area by multiplying by an empirically derived coef- ficient. ET is assumed to be less than E o and the coefficient 0.7 is often used (Chow 1964). Coefficients vary however, depending on environmental conditions and species. In a study of Typha domingensis, for example, coefficients varied from 0.7 to 1.3, depending on salinity levels (Glenn et al. 1995). Indirect estimates of ET are based on physical variables. These methods tend to ignore the influence that plant species composition may have. For example, Thornthwaite’s equa- tion (Chow 1964) to calculate potential evapotranspiration (i.e., the maximum rate possi- ble when water is not limiting) requires only the input of mean monthly temperature: PET (mm mo -1 ) = 16 (10 T i /I) a (3.4) where PET = potential evapotranspiration T i = mean monthly temperature, °C 12 I = local heat index = ∑ (T i /5) 1.514 i=1 a = location dependent coefficient = (0.675 × I 3 – 77.1 × I 2 + 17,920 × I + 492,390) × 10 -6 Penman (1948) also developed a model to calculate ET. Temperature is also used as a factor in this model, but other meteorological data are also used, including wind speed, solar radiation, elevation, and vapor pressure. This model allows the calculation of daily ET rates. Monteith (1965) modified the Penman equation (commonly referred to as the Penman–Monteith equation) to more clearly take into account the effects of stomatal resis- tance and wind. In essence, the Penman–Monteith model incorporates all parameters that govern energy exchange and the corresponding latent heat flux (i.e., evapotranspiration) from a uniform bed of vegetation. These parameters are either measured directly or calcu- lated from weather data. Souch and others (1998) investigated the effects of disturbance histories (ditching and drainage) on evapotranspiration rates in wetlands in the Indiana FIGURE 3.1 Diurnal groundwater level fluctuations due to transpiration on three dates recorded at Wicken Fen in the United Kingdom. The fluctuations have an amplitude of several millimeters which declines as the growing season progresses. In each case the fluctua- tions were recorded over a 3-day period: (a) July 6–8, 1984, (b) August 17–19, 1984, (c) August 31–September 2, 1984. (From Gilman, 1994. Hydrology and Wetland Conservation. Chichester. John Wiley & Sons. Reprinted with permission.) L1372 - Chapter 3 04/25/2001 9:33 AM Page 66 © 2001 by CRC Press LLC Dunes using this method. Measuring ET as its energy equivalent, the latent heat flux, they found that ET losses were approximately 3.5 mm d -1 whether standing water was present (undisturbed sites) or absent (disturbed sites). B. The Effects of Hydrology on Wetland Plant Communities Wetland plant communities have been shown to respond to different hydrologic regimes in several ways including differences in primary productivity, species diversity, and the distribution of species within the ecosystem. 1. Hydrology and Primary Productivity The duration and frequency of flooding may reduce or enhance primary productivity, depending upon the physiological benefit or stress that is created. Increased water inflows to wetlands carry additional nutrients and facilitate the exchange of dissolved elements (e.g., phosphorus nitrogen, oxygen, and carbon) by decreasing the thickness of the bound- ary layer at the plant surface, thus enhancing primary productivity (Odum 1956; Brown 1981; Madsen and Adams 1988; Carr et al. 1997). However, in some wetland types, pro- longed inundation can cause stress if the soils become anoxic (Mitsch and Ewel 1979; Odum et al. 1979; Brinson et al. 1981; Conner and Day 1982). Several studies concerning the influence of hydrology on primary productivity have been performed in forested wetlands. Studies in Florida (Carter et al. 1973; Mitsch and Ewel 1979), Louisiana (Conner and Day 1976, 1982; Conner et al. 1981), and Kentucky (Mitsch et al. 1991) have shown that stagnant, continuously flooded forested wetlands have lower primary productivity than sites open to flow and with a more pulsing hydrol- ogy. In a review of this relationship, Mitsch (1988) used a parabolic curve to describe pri- mary productivity as a function of water flow (Figure 3.2). His model of forested wetlands designed to investigate this relationship showed that primary productivity is highest when hydrologic inputs are “pulsing.” The high primary productivity of wetlands with pulsing hydrology has been attributed to higher nutrient loads. Brown (1981) found a similar pattern when comparing flow-through, sluggish flow, and stagnant cypress wetlands in Florida. She concluded that phosphorus inflow, which is coupled with hydrologic flow, was the critical variable in determining primary productiv- ity. Brinson, Lugo, and Brown (1981) characterized the link between hydrology and pri- mary productivity in wetlands in order of greatest to least productivity as: flowing water wetlands > sluggish flow wetlands > stillwater (stagnant) wetlands FIGURE 3.2 The relationship between hydrology and net primary productivity in forested wetlands. Productivity is highest when wetlands have “pulsing” hydrology, shown here as a seasonal pattern of flooding. (From Conner, W.H. and Day, J.W., Jr. 1982. Wetlands: Ecology and Management. B. Gopal, R.E. Turner, R.G. Wetzel, and D.F. Whigham, Eds. Jaipur, India. National Institute of Ecology and International Scientific Publications. Reprinted with permission.) L1372 - Chapter 3 04/25/2001 9:33 AM Page 67 © 2001 by CRC Press LLC The data summarized in their review show that stillwater forested wetlands averaged 707 g dry weight m -2 yr -1 ; systems with sluggish flows averaged 1090 g dry weight m -2 yr -1 , and flowing water wetlands (excluding data on shrub wetlands) averaged 1498 g dry weight m -2 yr -1 . In another review of forested wetlands, Lugo, Brown, and Brinson. (1988) stated that ecosystem complexity (i.e., structural and functional characteristics) and primary productivity are correlated with both higher hydrologic energy and higher nutri- ent supply. They called these the “core factors” that govern plant community response. The “fertilizer effect” from hydrologic subsidies may elicit other responses from the plant community. Many studies have shown that inputs of water that contain nutrients not only result in higher biomass production but also higher tissue concentrations of these ele- ments (Barko and Smart 1978, 1979; Jordan et al. 1990; Neill 1990). In Florida, wetland plots receiving high rates of wastewater effluent had increased net biomass production (includ- ing roots, shoots, and rhizomes) and higher phosphorus concentrations in plant tissues when compared to control plots (Dolan et al. 1981). Similarly, at a site in Michigan, Tilton and Kadlec (1979) found higher biomass production in a zone nearest the point of waste- water discharge. Tissue concentrations of phosphorus were significantly higher in this zone when compared to areas farther from the discharge point. Bayley and others (1985) found that primary productivity in a freshwater marsh was more dependent on the simple presence of standing water than nutrient subsidies. In their study, emergent vegetation in peat-accumulating marshes showed no difference in pri- mary productivity when nutrient-enriched wastewater was applied as opposed to unen- riched water. In this case, standing water (in spite of the difference in nutrient status) led to anoxic conditions in the peat and the release of dissolved phosphorus to the overlying water. This internal nutrient input, while a result of hydrology, outweighed any differ- ences from hydrologic inputs. Current velocities have been linked to increased primary productivity in submerged plants. Westlake (1967) found that photosynthesis and respiration rates increased in the submerged species, Ranunculus peltatus and Potamogeton pectinatus, as current velocities increased from 0 to 5 mm s -1 . Over this range, the photosynthetic rate of R. peltatus increased by a factor of 6. This response was attributed to increased exchange rates of gases and solutes as faster flows decreased the boundary layer around plants. Similarly, Madsen and Sondergaard (1983) found that the growth of Callitriche cophocarpa increased as flow rates increased up to 1.5 cm s -1 . In their study, photosynthesis rates increased by 20 to 28% with increasing current velocity after a 30-min incubation period. However, if current velocities exceed an optimal level, primary productivity can be reduced. Madsen and oth- ers (1993) found that for eight species of macrophytes, primary productivity was reduced as flow rates increased from 1 to 8 cm s -1 . Chambers and others (1991) also found that the biomass of submerged plants decreased in the Bow River, Canada as current velocities increased from 10 to 100 cm s -1 . A long-term study in constructed marshes in Illinois was designed to test how differ- ent hydrologic regimes influenced plant community development, including primary pro- ductivity. Phytoplankton and periphyton net primary productivity was greater in two marshes with high hydrologic inflow (48 cm wk -1 ) than in two marshes with low inflow rates (8 cm wk -1; Cronk and Mitsch 1994a, b). However, in the first two growing seasons following construction, the macrophyte community did not respond to the different water regimes (Fennessy et al. 1994a). Differences in mean water depths in the four basins may have confounded the results. The discrepancy in the results of the algal community vs. the macrophyte community may also be a function of the response time of the different com- munities. Given enough time, macrophyte primary productivity may become greater in the high flow wetlands. L1372 - Chapter 3 04/25/2001 9:33 AM Page 68 © 2001 by CRC Press LLC 2. Hydrologic Controls on Wetland Plant Distribution Plant species zonation occurs in response to variations in environmental conditions, par- ticularly water depth. A species’ habitat along a water depth gradient is a result of its indi- vidual adaptations. The shoreline of many wetlands, where hydrological conditions change with elevation and where water levels fluctuate over the long term, supports dif- ferent zones of vegetation (Figure 3.3, top). For example, lacustrine wetlands have sub- merged vegetation where the water is deepest, floating-leaved plants at higher elevations and emergent species along the water’s edge. In coastal wetlands, both tidal and fresh- water inputs influence plant zonation. The most salt-tolerant species are found closest to tidal inputs or where salt water collects. Often, salt-tolerant plants are excluded from less saline areas of the wetland because they are unable to compete with other plants there (see Chapter 2, Section III.A.1, Coastal Marshes; and Section III.B.1, Coastal Forested Wetlands: Mangrove Swamps). Hydrology not only structures plant communities in space, but also in time. For exam- ple, flood duration exerts control on the type of community present in a given location as well as species distribution within the community. Keddy (2000) summarized the rela- tionship between community type and hydroperiod for inland wetlands. He organized inland wetlands into four community types defined by the length of time they are flooded each year: • Forested wetlands (swamps, bottomland forests, riparian, or floodplain forests). These areas are only periodically flooded. Where elevations rise they grade into upland species and where elevations fall they give way to more flood-tolerant species. Lugo (1990) described forested wetlands as areas wet enough to exclude upland species but not wet enough to kill trees. The survival time for selected wetland trees in flooded conditions is shown in Table 3.3. FIGURE 3.3 A conceptual diagram showing how stabilizing water levels can compress the zonation of wet- lands species from four zones (top) to two zones (bottom). Overall species diversity in the commu- nity declines as a result. (From Keddy, P.A. 2000. Cambridge Studies in Ecology. H.J.B. Birks and J.A. Weins, Eds. Cambridge. Cambridge University Press. Reprinted with permission). L1372 - Chapter 3 04/25/2001 9:33 AM Page 69 © 2001 by CRC Press LLC • Wet meadows. These tend to replace forested wetlands at lower elevations. Occasional flooding in this zone tends to kill woody plants and allow germina- tion of wet meadow species from the seed bank. If flood frequency is reduced, woody species tend to move in. • Marshes. Marshes tend to be flooded for the majority of the growing season. Species here can tolerate long periods of flooding, but many still require drawn- down conditions for germination and seedling establishment. • Deepwater aquatic sites. These occur at the lowest elevations where flooding is essentially continuous. Kushlan (1990) also described the distribution of plant associations in the Florida Everglades in terms of duration of flooding (Table 3.4). In the Everglades, plant communi- ties change both in composition and growth form as hydroperiods shorten (from greater than 9 months to less than 6 months of flooding) and as fire frequency increases. 3. The Effects of Water Level Fluctuation on Wetland Plant Diversity One of the major controls on the diversity of any plant community is the ability of each species to become established and persist under existing environmental conditions. The establishment phase is critical, and the conditions that a given species requires to germi- nate and become established might differ markedly from the conditions to which they are adapted when mature. This set of requirements for germination and establishment has been dubbed the “regeneration niche” by Grubb (1977). Subsequent reproduction by the individual is often vegetative. Many wetland plant seeds and seedlings require drawn- TABLE 3.3 Estimated Survival Time When Inundated for Selected Species of Flood-Tolerant Trees Species Survival Time (years) Quercus lyrata 3 Q. nuttalii 3 Q. nigra 2 Q. palustris 2 Q. macrocarpa 2 Acer saccharinum 2 A. rubrum 2 Fraxinus pennsylvanica 2 Gleditsia triacanthos 2 Populus deltoides 2 Carya aquatica 2 Salix interior 2 Nyssa aquatica 2 Taxodium distichum 2 Celtis laevigata 2 A. negundo 0.5 Platanus occidentalis 0.5 Pinus contorta 0.3 After Keddy 2000, data from Crawford 1982. L1372 - Chapter 3 04/25/2001 9:33 AM Page 70 © 2001 by CRC Press LLC [...]... exclusion (see Chapter 7, Section V.B.1, Floods) C Hydrological and Mineral Interactions and Their Effect on Species Distribution Wetland water chemistry is a function of hydrologic links between the landscape and the wetland ecosystem itself For example, the mineral composition of the bedrock and the soils helps control hydrology and water chemistry and so controls the formation of specific wetland types... chemistry and so controls the formation of specific wetland types The position of the wetland in the landscape also dictates the amount and quality of surface runoff to the site The balance of landscape position and © 2001 by CRC Press LLC L 137 2 - Chapter 3 04/25/2001 9 :33 AM Page 73 surficial geology creates hydrologic and geochemical gradients that are strongly correlated to plant species distributions... carbonic acid: CO2 + H2O ↔H2CO3 (3. 5) Because the concentration of carbonic acid is low compared to that of CO2, the two are often considered together to be the concentration of CO2 Some of the carbonic acid dissociates to form bicarbonate and hydrogen ions: H2CO3 ↔ HCO 3- + H+ (3. 6) At high pH (>8 .3) , bicarbonate dissociates to form carbonate and hydrogen: HCO 3- ↔ CO3 2- + H+ (3. 7) During photosynthesis,... stress and cannot afford to lose water through transpiration, taking in carbon dioxide becomes problematic (Pomeroy and Wiegert 1981; Longstreth et al 1984; Bradley and Morris 1991b) © 2001 by CRC Press LLC L 137 2 - Chapter 3 04/25/2001 9 :33 AM Page 80 When salt concentrations rise to extremes, the primary productivity of saline wetlands decreases (Lugo et al 1988; Srivastava and Jefferies 1996; Teal and. .. involved in photosynthesis and reduces the capacity of the roots to respire both aerobically and anaerobically CO2 CH4 –250 to 35 0 Methane is transported through wetland plants’ internal gas spaces and released to the atmosphere presence of NO 3- , which is itself produced by the oxidation of ammonia and nitrite When NO 3- is limited, denitrification is also limited (D’Angelo and Reddy 19 93) In nitrate ammonification,... the surrounding wetland The authors mapped eight distinct vegetation communities including a Quercus velutina (black oak) woodland on upland dunes, an Acer rubrum (red maple) swamp adjacent to the FIGURE 3. 4 The relative contributions of water sources including groundwater, surface water, and precipitation determine the type of wetland that will form (From Brinson, M.M 1993b Wetlands 13: 65–74 Reprinted... 1990) © 2001 by CRC Press LLC L 137 2 - Chapter 3 04/25/2001 9 :33 AM Page 79 3 The Presence of Toxins under Reduced Conditions A wide range of soluble organic compounds found in wetland soils are toxic to plants Some toxins are from the anaerobic decomposition of cellulose and lignin Microbial metabolism brings about a potentially toxic accumulation of acetic and butyric acids, and anaerobic metabolism results... Press LLC L 137 2 - Chapter 3 04/25/2001 9 :33 AM Page 74 upland dunes, intergrading Typha marshes and scrub-shrub communities, a CarexCalamagrostis marsh, and a Phragmites-Typha marsh on a drier site not on the mound itself Thuja occidentalis (northern white cedar) occurs on the mound where soils are drier Larix laricina occurs adjacent to the mound, where water chemistry is rich in calcium and magnesium... submerged plants declined dramatically in the 1960s and 1970s (Orth and Moore 19 83, 1984; Twilley et al 1985) With the restoration of cleaner water, submerged plant communities have reappeared, although with altered species composition (Davis 1985; Carter and Rybicki 1986; Stevenson et al 19 93) © 2001 by CRC Press LLC L 137 2 - Chapter 3 04/25/2001 9 :33 AM Page 83 2 Carbon Dioxide Availability Carbon dioxide... redox reactions to nitrite (NO 2-) , then to N2O, and ultimately to dinitrogen gas (N2), which is released to the atmosphere Denitrification is dependent on the © 2001 by CRC Press LLC L 137 2 - Chapter 3 04/25/2001 9 :33 AM Page 76 TABLE 3. 5 The Oxidized and Reduced Forms of Nitrogen, Manganese, Iron, Sulfur, and Carbon, the Redox Value at Which They Are Reduced in Flooded Soils, and the Effect the Reduced . present in inland wetlands L 137 2 - Chapter 3 04/25/2001 9 :33 AM Page 62 © 2001 by CRC Press LLC TABLE 3. 1 Examples of Water Budgets for a Variety of Wetland Types Wetland Type and Location Inflows. water, and pre- cipitation determine the type of wetland that will form. (From Brinson, M.M. 1993b. Wetlands 13: 65–74. Reprinted with permission.) L 137 2 - Chapter 3 04/25/2001 9 :33 AM Page 73 ©. dry weight m -2 yr -1 , and flowing water wetlands (excluding data on shrub wetlands) averaged 1498 g dry weight m -2 yr -1 . In another review of forested wetlands, Lugo, Brown, and Brinson. (1988)