Because we must deal with complex patterns of land use, conditions, ownership, and responsibility, humans draw multiple sets of boundaries on the landscape. The boundaries encompass elements that are more like each other than they are to elements not within the boundaries. For example, the management style of a national park is uniform within the boundaries of a park, but may be different from the management style practiced on bordering parcels. In another example, a plot of ground classified as herbaceous grassland is relatively homogeneous within, and very different from an adjacent woodland in a young stage of succession. The two photos below were taken in Colorado just meters apart, but represent habitat types that are classified very differently and perform very different functions.
We arrive at boundaries through the process of classification, which is a process of arranging items of interest into groups based on their relationships. To classify land and water cover, we identify patterns on the landscape and draw lines around them, with the size of the pattern depending on the scale at which classification is done and the intended use of the classified results.
The most useful and flexible classification systems are hierarchical, meaning that their units are based on increasing levels of detail from very broad (e.g., forest versus herbaceous vegetation) to more detailed (needleleaf versus broadleaf forest; closed needleleaf versus open needleleaf), to species level (black spruce closed needleleaf forest), arranged in a hierarchy of levels. The first level is the broadest and most inclusive (for example, based on vegetation growth form or physiognomy) and contains the fewest units, while the lowest level is the narrowest (usually based on species composition) and contains the most units (Racine 2000). A strength of classification is the ability to either aggregate or disaggregate classes in the hierarchy.
Another strength of classifications is the relationships that can be drawn once a unit is defined. Numerous studies have shown a relationship between landscape classes such as land cover and the specific animal populations that may be present, and between classes of water conditions and aquatic populations. Hence, land and water classification is considered one of the first steps in managing fish and wildlife populations, a critical component of ecosystems.
One example of a simple rating system using land cover classes is shown below. Several riparian reaches in a dry northwest environment were classified according to Cowardin et al. (1979), then the extent of those classes in each reach was measured from remote sensing and limited ground data collection. The data can be arrayed multiple ways, with the end result a mechanism to compare reaches and assist in determining restoration opportunities and objectives. In the example array, the PFO/PEM type is scarce and might reflect a restoration need.
Additional recommended reading on classification is the text of Cowardin et al. (1979) and Racine (2000), a "Review and Comparison of Vegetation Classification Systems," Section 3E in "Guidelines for Vegetation Mapping on Army Installations," http://el.erdc.usace.army.mil/elpubs/pdf/tr00-8/use.pdf.
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