American Classification Systems
The designers of any wetland classification to be used in inventorying wetlands have to deal with a number of major issues. What will be the “unit” classified? How will data on wetlands be collected? How accurate do the data have to be? Who will use the data generated? In what form will it be stored? How will inventory data be made available/distributed/retrieved? In the United States, the first two national classification systems developed in the last half of the 20lh century were intended to be used as a basis for a national wetland inventory (NWI). Consequently, the practical realities (people, time, money) of collecting data at a large geographic scale were major factors in their design. The practical realities of collecting data and distributing them, however, changed dramatically from the 1950s to the late 1970s. The advent of easily available, high- resolution aerial photography and satellite imagery; inexpensive computers; geographic information system software for collecting, storing, and analyzing spatial data; high speed graphics printers; the Internet; etc. has had an impact on the design of classification systems and on the distribution of inventory data collected using them. This can be seen through the comparison of the Classification of Wetlands of the United States® commonly referred to as the Martin et al. (1953) system and its replacement Classification of Wetlands and Deepwater Habitats of the United States141 commonly referred to as the Cowardin et al. (1979) system.
Martin et al. (1953) developed a national wetland classification system to inventory waterfowl habitat. This classification system was horizontal in that its 20 wetland types, which covered all inland and coastal wetlands in the conterminous United States, had no subdivisions or subtypes. One of the main limitations faced by its developers was the small number of staff available to collect data in the field and their limited expertise in wetland ecology. Wetland types (e.g., inland fresh water meadows, inland shallow fresh marshes, inland deep marshes, and inland open fresh water) were vegetation types largely defined by certain characteristic species and water depth: features easy to observe in the field. This classification system was not consistent in the kinds of categories used, and it mixed vegetation types like inland shallow fresh marshes (distinguishable units) with landscape unit like bogs and sounds and bays. It also put wetlands from different parts of the country with little in common (Cypress-Tupelo Swamps from the southeastern and Black Spruce forests from northeastern United States) into the same category, namely, wooded swamps. The use of water depth to distinguish some freshwater wetland types also caused problems because this was highly dependent on the time of year a wetland was visited. The uneven nature of the categories reflected the main purpose of this classification system to find out how much waterfowl habitat of various types still existed in the United States. It emphasized types of wetlands used by waterfowl (six types of inland freshwater wetlands) and plays down those that were not considered important to waterfowl (e.g., bogs). This classification was used to do a detailed inventory of American wetlands, and the results of this inventory were published in 1956 in Wetlands of the United States. Their Extent and Their Value to Waterfowl and Other Wildlife,|s| The results of this inventory were only available in this Fish and Wildlife report, commonly referred to as Circular 39. A number of regional wetland classifications systems were developed in the United States in the 1960s and 1970s that dealt with many of the shortcomings of the Martin et al. (1953) system.121
By the time that the next American NWI was developed 20 years later, advances in technology, especially the availability of high quality aerial photography and computers to store and retrieve spatial information, greatly influenced the development of the Classification of Wetlands and Deepwater Habitats of the United StatesJ41 The more widespread interest in wetlands in the 1970s is reflected in the makeup of the committee that developed this new classification system. All the authors of the Martin et al. system worked for the U.S. Fish and Wildlife Service. Although the U.S. Fish and Wildlife Service was still heavily involved, the authors of the new classification also came from other federal agencies (U.S. Geological Survey, National Oceanographic and Atmospheric Administration) as well as academia. One of the main differences between the new classification and its predecessor is that it is a hierarchical system. Much like the hierarchical system used to classify plant and animal species, it is possible to select and examine several levels of aggregation. Overall, the developers of the new classification system had many major objectives for it: (1) to build the system using consistent and uniform terminology that could be applied to any wetland in the United States; (2) to be able to group ecologically similar wetlands together at all levels of the classification hierarchy; (3) to make it readily applicable to mapping all the wetlands of the United States from aerial photographs as part of a NWI. The system has three levels in its hierarchy: system, subsystem, and class. Systems and subsystems are based on hydrology (Figure 18.1). There are five systems: marine, estuarine, riverine, lacustrine, and palustrine. Except for the palustrine, each system has two or more subsystems. In turn, each system or subsystem has multiple classes. Classes are mostly types of plant communities (aquatic bed, moss-lichen wetland, forested wetland, etc.) or various kinds of non-vegetated substrates (bedrock, rubble, sand, etc.) that can be identified on aerial photographs. Classes can further be subdivided into subclasses and dominance types. While classes and subclasses are based primarily on dominant life form or easily recognized substrate features for unvegetated areas, dominance types are described by their most abundant plant species or sessile macro invertebrate species (e.g., oysters). In addition, water regime, soil, water chemistry, and special modifiers can also be used to characterize wetlands. Special modifiers are used to describe human-made wetlands (excavated, impounded, diked, farmed, etc.). In short, the Cowardin et al. system is a classification in which the basic units are distinguishable areas on aerial photographs. This classification system has been used successfully by the NWI in the United States for over 30years, and the NWI, which is part of the Fish and Wildlife Service, periodically issues status and trend reports for the country that document changes in different kinds of wetlands. NWI maps are now distributed on the World Wide Web (http://www.fws.gov/wetlands/).
Although it has served its primary purpose well, the Cowardin et al. (1979) wetland classification system has proven to be inadequate for other purposes, especially the evaluation of wetland functions and values. In 1993, another wetland classification system was published by Mark Brinson161 for this purpose that he called A Hydrogeomorphic Classification for Wetlands. It is most commonly known as the F1GM system. Unlike the Cowardin et al. system, the basic unit in the F1GM system is a landscape unit. The HGM system identifies wetland types believed to have similar functions. The position of the wetland in the landscape and the source(s) of water and direction of flow into and out of the wetland are the major classificatory variables. In the HGM system, seven basic HGM classes of wetlands are recognized: depressional, riverine, slope, lacustrine, mineral soil flats, organic soil flats, and estuarine (tidal). The HGM system deals only with wetlands as defined by the Committee on Characterization of Wetlands. It excludes the deepwater systems that are also covered in the Cowardin et al. classification system. Various characteristics of a wetland and its watershed can then be used to assess its functions (flood peak attenuation, sediment retention, water storage, nutrient retention, habitat value, carbon sequestration, etc.). A series of regional manuals have been developed to do functional assessments using the HGM system. These can be downloaded from the U.S. Army Corps of Engineers website (http://el.erdc. usace.army.mil/wetlands/wlpubs.html).