A review of literature regarding forested wetlands reveals several district focuses relevant to the proposed research. Research on wetlands uses and functions dates back hundreds of years with relevant publications as early as 1928. The literature illustrates not only the importance of wetlands, but also an evolution in societies' view towards these ecosystems. A second focus on the succession of forested wetlands gives insight to the formation of microtopography through the study of plant and microbial oxygen demand. A final focus of the literature clarifies the importance of micro topography in wetland ecosystem functions through survey and study of the supported heterogeneous plant communities and their productivity.
Wetland services
In 1928 Viosca published a statement of wetland values, the most comprehensive of the time (Ewel and Odum, 1986). In a list of direct values Viosca included "the maintenance of constant water levels near the surface in adjoining high lands, recreational and educational values; value as transportation routes; their value in pest control for nearby agricultural fields; and the value as organic matter reservoirs…" Viosca proved to be far ahead of his time as most other early literature focused on the value of wetlands for agricultural purposes, most notably rice (Wright and Wright, 1932) and timber products such as cypress (Mattoon, 1919) and tupelo (Hadley, 1926). These publications set a tradition for assigning value to wetlands. However more recent publication have expanded the traditional evaluation of wetlands and brought new ecosystem services to light (Mitsch date, Gosselink, 2000). It is argued that the primary value of wetland ecosystems is their self-organizing capacity, from which all other ecosystem services are a result (Gren, 1994). The self-organization of wetlands is a response to hydrologic regimes which define the system's biota. The biota in turn influences the hydrology and provides such ecosystem services as trapping sediments, building peat deposits, and storing water (Gosselink and Turner, 1987). The variability in water level and the response of microbial respiration provide value in water purification. Physical, microbial, and chemical processes all conspire to transform and retain nutrients and other flood water constituents. To a lesser degree vegetation also plays a role in nutrient and pollutant retention (Johnston, 1991) (Reddy, 1975). Numerous other publications evaluate the value of wetlands for the treatment of waste water (Verhoeven, 1998) (Vymazal, 2006). The value of wetlands as a result of their abundance and diversity of flora has been extensively studied (Mitsch and Gosselink, 1986) (Ewel and Odum, 1986). The water storage capacity of wetlands is also well known to prevent flooding by buffering the flow through watersheds during and after high intensity or duration storm events (Gren, 1994). While the literature suggests that flooding can be greatly reduced in watersheds where approximately 5% of the area is wetlands (Mitsch and Gosselink, 2000) the organization of multiple wetlands throughout watersheds that make this reduction in flooding possible does not appear to be well studied. Clearly the literature is in agreement on the numerous services provided by wetlands to society and surrounding ecosystem.
Hummock Genesis
Peat accretion is a key factor in the development of wetland microtopography and the effects of inundation on microbial decomposition have been well studied with formal literature dating back to 1935. However it is difficult to make generalizations about the effects of inundation on decomposition due to the complexity of hydrologic controls such as differences in duration, depth and spatial patterns of flooding (Neckles, 1994). Other variable such as litter quality, soil composition, pH and microbial communities further complicate studies on the effects of flooding on decomposition. It has been well documented that litter decomposition is accelerated by inundation over at least a portion of the growing season as compared with litter that is perpetually on dry soil (Ewel and Odum, 1978) (Day, 1982). The literature suggests however that the length on inundation is the greatest controlling factor in decomposition. Studies by Acharya (1935) indicate that long term inundation can greatly reduce rates of decomposition. More recent studies have utilized redox probes to correlate length of flooding duration with a reduction of Oxygen available as an electron acceptor for microbial decomposition. Studies have shown a 50% reduction in the rate of decomposition in wetland redox conditions (Reddy, 1975). A study on perminatley inundated bogs found that anaerobic decay can occur 1000 times slower than aerobic decay (Belyea and Clymo, 2001). It is now commonly accepted that the decomposition of organic matter in wetlands is greatly reduced when oxygen is the soil and water column is depleted (Mitsch, Gosselink, 2000) (Ewel and Odum, 1986). The literature also shows that the alteration of aerobic and anaerobic conditions which frequently occurs on wetland hummocks effect the cycling of nutrients, most notably nitrogen. While studies have shown that alternate aerobic and anaerobic conditions may cause an increase in the loss of nitrogen through alternating nitrification and denirtrification (Reddy, 1975) (Bowden, 1986), little work has been done to explore patterns in nutrient cycling across varied elevations within seasonally inundated wetlands.
The genesis of wetland micro topography is not well studied although much speculation exists and the syntheses of many studies support these speculations. Productivity in wetlands is often increased at greater elevation as oxygen becomes more available to plant roots (Brinson, 1981). In addition anaerobic conditions can cause the formation of toxic byproducts that can also harm productivity as a function of inundation stress (Crawford, 1996). This increased productivity is followed by greater litter fall on the higher elevation hummocks. As mentioned above, decay occurs fasted at the elevation of greatest water level fluctuation, slowest in the waterlogged peat of the "hollows" or deep areas of the wetland. The occurrence of hummock and hollow systems suggests a balance between the increased rate of litter fall and greater respiration on hummocks. Models have been used to show the importance of wet and dry periods in maintaining this balance. Dry periods allow for greater decomposition of hummocks which increases the storage for wet periods which in turn increase the formation of hummocks (Belyea and Clymo, 2001) (Tallis, 1994).
Hummocks and Ecosystem Services
One finale question remains, how dose micro topography tie into the wetland services by which society values these ecosystems. Hummocks increase the productivity of wetlands as well as the biodiversity of the systems. The increase in productivity has been previously illustrated. Biodiversity in wetlands is affected by seedling germination and vegetation preferences. The majority of seedling germination occurs on hummocks, where inundation stress is minimized (Beatty 1984). Microscale variations in topography allow for an array of niches preferring different species (Collins et al. 1982) ( Beatty, 1984) (Kolasa & Pickett, 1991) (Tilman, 1994) (Titus 1990). Hydrophytic vegetation has been shown to exhibit habitat preferences over only 3cm change in elevation (Vivian-Smith, 1997).
Problem Statement
Microtopography is a defining characteristic of fresh water swamps. These microscale variations in topography explain the distribution of plant species throughout wetland ecosystems (Collins et al. 1982, Beatty 1984, Kolasa & Pickett 1991; Tilman 1994 , Titus 1990). Hummocks allow for the germination of a rich diversity of plant species due to the ability of wetland vegetation to exhibit habitat preferences over only 3 cm (Vivian-Smith, 1997). In addition hummocks provide oxygenated soils, reducing inundation stress and increasing productivity. Hummocks are also thought to tightly cycle carbon and nutrients, further promoting productivity in often oligotrophic environments (citation needed).
Despite the inherent importance of hummocks in the development of diverse and productive wetlands, few studies have examined the growth and constraints of this morphology. Understanding the occurrence and formation of hummocks is vital to our understanding of the succession of wetlands, the cycling of nutrients, and the factors that define a healthy wetland ecosystem. We hypothesize that the incidence of hummocks is directly related to two factors, productivity and water table fluctuation. In the case of fresh water swamps, productivity is a function of hydroperiod, nutrient loading, and canopy cover (Mitsch, 2000). As nutrient loading, and therefore productivity, increases the incidence of hummocks is expected to increase. However at higher nutrient loadings productivity does not respond so directly and hydrology is predicted to be the driving force in the formation of hummocks by limiting respiration. Through exploring the responses of wetland morphology to a gradient of nutrient loading and hydrologic fluctuation we hope to better understand the factors that form and maintain wetland ecosystems.
