Riparian Ecosystems

Definition of Terms

Wetlands Versus Drylands (Uplands)

The recently recognized field of riparian ecology is employing new terms, defining new conceptual phrases, and developing classification schemes. To demonstrate fully the inclusive scope of what may be identified as "riparian" and the complex and unique plant/animal interactions that contribute to the diversity of life forms within riparian ecosystems, a number of terms and concepts must, at the outset, be reviewed. The relationships among certain of these terms are indicated in table 1.

Terrestrial systems have traditionally been differentiated into two basic and mutually exclusive categories, "upland" and "wetland." These are terms not to be confused with "highland" and "lowland," which simply denote elevational differences; uplands and wetlands may be differentiated on the basis of soil moisture. "Uplands" has been used in wildlife literature in opposition to "wetland," but uplands are also commonly thought of as "high ground," denoting an elevational context. To circumvent this problem, Cowardin (1978) used the term "dryland" in contrast to "wetland." We concur with Cowardin's choice in coining a much-needed technical term. The Oxford Unabridged English Dictionary (1979) defines dry land (two words!) as "land not submerged or under water; land as opposed to sea." Upland may be simply defined as "ground above the floodplain" (ibid .), whereas the term "wetland", referring to "land having wet or spongy soil," (Random House Unabridged Dictionary 1967) is not contained in the Oxford Dictionary.

By beginning with a classification system which categorizes systems as either uplands or wetlands, we would agree with other western authorities in considering virtually all riparian areas as wetlands (Brown etal . 1979, 1980). An outstanding discussion of wetlands in the arid Southwest appears in an extensive regional vegetation classification (Minckley and Brown 1982). This treatise characterizes wetlands as follows:

Wetlands are periodically, seasonally, or continuously submerged landscapes populated by species and/or life forms differing from immediately adjacent biotas. They are maintained by, and depend upon circumstances more mesic than those provided by local precipitation. Such conditions occur in or adjacent to drainageways and their floodplains (riparian zones) on poorly drained lands, along seacoasts,

Table l.—Relationship of terms used to delineate wetland systems.   *"Dryland" used by Cowardin (1978) as an antonym to wetland and thus as a synonym for upland.

and in and near other hydric and aquatic situations, i.e., springs and their outflows, ponds, margins of lakes, etc. The various wetland and riparian communities may be represented as forest, woodland or scrubland, marshland or strand, or be composed largely or entirely of submergent vegetation. [emphasis added]

One of the most perceptive, early characterizations of riparian communities was by Lowe (1964):

A riparian association of any kind is one which occurs in or adjacent to drainageways and/or their floodplains and which is further characterized by species and/or life-forms different from that of the immediately surrounding non-riparian climax. The southwestern riparian woodland formation is characterized by a complex of trees, and their plant and animal associates, restricted to the major drainageways that transgress the landscape of desert upward into forest. It is incorrect to regard this biotic formation as merely a temporary unstable, seral community. It is an evolutionary entity with an enduring stability equivalent to that of the landscape drainageways which form its physical habitat. That is, it is a distinctive climax biotic community . Moreover it is, as are all ecologic formations and their subdivisions, locally subject to, and often dissolved by, the vicissitudes of human occupation. In Arizona, the riparian woodlands have been rapidly dwindling just as the water table has been rapidly lowering. And its trees are now the native phreatophytes of the waterusers. [emphasis added]

In their outstanding book on wildlife habitats, Thomas etal . (1979) did not use the term "wetland" for their work in Oregon and Washington. Instead, they used the term "riparian zone," which they defined as:

. . . an area identified by the presence of vegetation that requires free or unbound water or conditions more moist than normally found in the area.

In addition, we would add the barren, rocky, gravelly, or sandy banks along a body of water.

At the other extreme to the southwestern arid hydrological regime (e.g., the humid northwestern United States), the Oregon Coastal Conservation and Development Commission has a still different definition of wetlands (Metzger 1968 fide Akins and Jefferson 1973):

. . . areas on which standing water, seasonal or permanent, has a depth of six feet or less and where the wet soil retains sufficient moisture to support aquatic or semiaquatic life.

We offer here definitions for various terms, including "riparian wetlands," in full awareness of problems inherent in attempting to categorize and define riparian zones, since biological communities rarely have definitive boundaries. Indeed, the riparian community requires the moisture of the truly aquatic zones from one direction and the substrate of the terrestrial zones from the other. According to Cowardin (1978) ". . . wetland is part of a continuum of land types between deep water and dryland." Thus, as with all classification efforts, a subjective judgment must be made as to where to draw a boundary (where no clear boundary exists) between aquatic, terrestrial wetland, and dryland (upland) zones. The problem of specific definitions becomes even more complicated when certain transitional terms are included. For example, the term "semiaquatic" is used in the extensive treatment of Oregon's coastal wetlands by Akins and Jefferson (1973); however, the term defies definition in their glossary. Semi-aquatic is obviously a term designed to identify vegetation or a zone of vegetation found in the area of transition between aquatic and terrestrial zones.

Semi-Riparian and Pseudoriparian

Another transitional connotation is reflected by the term "semi-riparian zones." The term "semi-riparian communities" was used by Zimmerman (1970) and "semi-riparian habitat" by Hubbard (1971) in their work along the Gila River valleys of southwestern New Mexico, to describe vegetation consisting of "true" riparian species mixed with dryland species. Areas containing such vegetative mixtures are usually found on a gradient extending away from and upslope of the stream or watercourse. The semi-riparian zone, or semi-riparian vegetation, then, is an ecotonal transition between the terrestrial wetland (riparian) zone and the dryland (table 1). As with other ecotones (Johnson 1978b), the semi-riparian zone supports plant and animal species from both the bordering riparian type and the dryland type, as well as species that are almost exclusively specific to these transitional areas. It is important to emphasize here that we are attempting to categorize the various elements of the continuum between wet and dry zones, not merely for the sake of habitat or vegetation classification, but because we can often recognize distinct vegetation and wildlife communities within each of the vegetative types and subtypes.

In general, the semi-riparian community is on a soil substrate intermediate in moisture content between the riparian wetland and the dryland. In the semi-riparian community, where the typically upland species occur on soils containing moisture in excess of their typical habitat, these upland plants frequently attain a popula-

tion density or size-class and vigor greater than that found in the uplands. When a dryland species migrates into the semi-riparian zone and as a result attains a greater density and rate of growth, it may be referred to as a "pseudoriparian" plant (Campbell and Green 1968).

"Pseudoriparian" plants have been described by Campbell and Green as "facultative" in contrast to "riparian" plants, which are "obligate." Unfortunately, the biological definition of facultative as able to live under more than one set of conditions does not indicate a preference for one situation or the other. Therefore, Campbell and Green (1968) have lumped three basic categories of plants under the term pseudoriparian: 1) species which may be best adapted to riparian zones but which will grow in drylands, e.g., velvet mesquite (Prosopisvelutina ); 2) species which are well adapted to either riparian or dryland zones, e.g., desert hackberry (Celtispallida ); and 3) species which are more common in dryland zones but which will grow in riparian zones, often attaining greater size and/or population densities, e.g., Arizona cypress (Cupressusarizonica ). Table 2 demonstrates a classification scheme which further clarifies the concept of "pseudoriparian" in relation to a species' ability to grow in different zones.

The classification scheme we present here is a refinement of an earlier system used by Johnson etal . (1977) composed of three categories: obligate, preferential, and non-riparian. This new, four-category system can be applied to plants or animals. By conducting a strip census or random count of individuals in a region and determining the percentage found in riparian zones compared to nonriparian zones, the species can be assigned to one of the four categories. Reed (in press) has developed a five-category system for wetland plants which would be more definitive, but is too sophisticated for our current level of information.

Perennial, Intermittent, and Ephemeral Riparian Zones

We have recently developed a new, simple, and inclusive classification system for riparian zones (Johnson et al . in press a). By combining standardized terms we have suggested: 1) Perennial Riparian; 2) Intermittent Riparian; and 3) Ephemeral Riparian for categorizing bank systems on streams, lakes, and tidewater areas. These three new riparian types may be applied to zones, vegetation-types, plant or animal communities, or ecosystems. These categories are not substitutes for various terms which have been used for plant species or vegetation communities in the past, including "desert riparian," "stream riparian" (Austin and Bradley 1971), "pseudoriparian," or "semi-riparian." Our three categories refer to zones occurring along and supported by perennial (permanent), intermittent, or ephemeral watercourses, lakes, or tidewaters.