Wetland Environments

The use of aerial photography has resulted in an essential advance in the understanding of mire structure.

14.1. OVERVIEW

Wetlands include myriad environmental types: bayou, bog, fen, mangrove, marsh, moor, muskeg, pan, playa, sabkha, swamp, tundra, and several more terms in other languages. Many of these names are united under the general term mire nowadays, but that still does not specify the basic characteristics of wetlands. A definition for what is a wetland often depends on who is asking the question and what development or study is proposed for a particular wetland site. The fact that wetlands may dry out from time to time complicates the attempt to describe wetlands in a simple fashion, and some wetlands may be dry more often than they are wet (Tiner, 1997).

Many definitions for wetlands have been proposed and utilized over the years. Among the most widely accepted is that of Cowardin et al. (1979), which was adopted by the U.S. Fish and Wildlife Service.

Land where an excess of water is the dominant factor determining the nature of soil development and the types of animals and plant communities living at the soil surface. It spans a continuum of environments where terrestrial and aquatic systems intergrade.

This definition comprises three aspects—water, soil, and living organisms, which are accepted by wetland scientists as the basis for recognizing and describing wetland environments (Schot, 1999; Charman, 2002).

• Water: Ground water (water table or zone of saturation) is at the surface or within the soil root zone during all or part of the growing season (Fig. 14-1A).

• Soils: Hydric soils are characterized by frequent, prolonged saturation and low oxygen content, which lead to anaerobic chemical environments where reduced iron is present (Fig. 14-1B).

• Vegetation: Specialized plants are adapted for growing in standing water or saturated soils, such as moss, sedges, reeds, cattail and horsetail, rice, mangroves, cypress, and cranberries (Fig. 14-1C).

This triad is the modern approach for wetland definition under many circumstances that include greatly different environments. Notice that water quality is not spec-ified—salinity varies from fresh, to brackish, to marine, to hypersaline; acidity may span the entire scale of naturally occurring pH values. Emergent vegetation ranges from heavily forested swamps to nearly bare playas and mudflats. Wetlands are present in all climatic and topographic settings around the world, covering substantial portions of the land and shore areas of the Earth. Existing wetlands comprise an estimated 7-10 million km2 or 5-8% of the land surface of the globe (Mitsch and Gosselink, 2007). However, as much as half of the world's wetlands have been lost to human development during the past few millennia.

For various environmental and economic reasons, much scientific research is directed toward wetlands, and many different techniques have been utilized to collect, compile, analyze, and synthesize data. In recent years, traditional ground mapping methods have been supplemented with the use of geographic information systems and remote sensing techniques for wetland research (Jensen et al., 1993; Juvonen et al., 1997; Ahvenniemi et al., 1998; Barrette et al., 2000).

Tiner (1997) noted the difficulty of wetland airphoto interpretation because of highly variable water, topographic, and vegetation conditions that may apply. He considered color-infrared imagery as best for recognizing vegetated wetlands, such as marshes, swamps, and bogs, and he emphasized the importance of photographic scale for setting spatial limits on wetland mapping. In this regard, small-format aerial photography (SFAP) has distinct advantages for certain types of wetland investigations (Aber and Aber, 2001).

• High-resolution (2-5 cm), large-scale imagery is suitable for detailed mapping and analysis.

• Equipment is light in weight, small in volume, and easily transported by foot, vehicle, or small boat under difficult field conditions—peat, mud, and water.

• Minimal impact on sensitive habitat, vegetation, and soil. Silent operation of kites and blimps does not disturb wildlife (Fig. 14-2, see also Fig. 1-3).

• Repeated photography during the growing season and year-to-year documents changing environmental conditions.

• Visible and infrared imagery in vertical and oblique vantages in all orientations relative to the sun position, shadows, and ground targets (Aber et al., 2002).

• Special lighting effects, such as sun glint, may aid in recognition of small water bodies (Amsbury et al., 1994).

• Lowest cost by far relative to other manned or unmanned types of remote sensing to achieve comparable high spatial, spectral, and temporal resolutions.

The following examples are drawn from raised bogs of Estonia in north-central Europe and prairie marshes and playas of the central Great Plains in Kansas, United States. SFAP for the former was undertaken exclusively with kite aerial photography (KAP), and the latter combines kites and a helium blimp for lifting camera rigs. Climatic, hydrologic, and geographic settings for

FIGURE 14-1 Primary aspects of wetlands. (A) Water pools of various sizes and shapes occupy much of the surface of Nigula bog in southwestern Estonia. A narrow, wooden footpath can be seen across bottom of view, September 2001. (B) Salt crust forms as soil moisture evaporates from mudflat at Dry Lake in western Kansas, May 2008. (C) Curly dock (Rumex crispus L.) forms a distinct, rust-colored band along the shore of Lake McKinney in southwestern Kansas, October 2006. Kite aerial photographs by SWA and JSA.

these two regions are substantially different, as demonstrated by their wetland conditions.

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