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1
R. Roy Johnson
2 and Charles H. Lowe
3
Abstract.--The peculiarly western development of riparian ecology in North America is examined. Gradients in riparian systems are discussed with regard to transriparian and intrariparian continua, including xeroriparian communities.
Consistent with the facL that riparianlands are technically wetlands, Aquatic, Riparian, and Terrestrial systems harbor peculiarly obligate species structured into distinctive biotic communities throughout all of North America.
EASTERN AND WESTERN RIPARIAN ECOSYSTEMS
IN NORTH AMERICA
Riparian ecology, one of the newest of scientific disciplines, originated in the southwestern
United States where wetland scientists remain few.
There were several reasons for the establishment of riparian ecology in the Southwest. It was a natural development initiated by naturalists attracted to the singular distinctiveness and richness of
Southwest riparian woodlands and forests--woodlands and forests correctly reminiscent of the winterdeciduous hardwood forests of the eastern United
States. Water is a much more pressing problem for natural communities in the West than for those in most of the eastern United States. Eastern deciduous hardwood forests are commonly supported by 35 to 50 inches of annual precipitation. Most of the
West receives less than 20 inches while pan evaporation rates commonly exceed 100 inches annually.
Evaporation and runoff rates are lower in the East where vegetation holds the moisture while forming a blanket over the soil, partially protecting it from wind and sun. In eastern uplands the competition is primarily for space and light above ground, while in the arid West competition is primarily below ground for water.
In the East, the natural communities of both the riparian bottomlands and surrounding uplands commonly consist largely of deciduous hardwood forests. Even though the species composition of lPaper presented at the First North American
Riparian Conference, Tucson, Arizona, April 16-18,
1985.
2R. Roy Johnson is Sr. Research Scientist with the National Park Service and Adjunct Professor of
Renewable Natural Resources, University of Arizona,
Tucson, Ariz.
3Charles H. Lowe is Professor of Ecology and
Evolutionary Biology, University of Arizona, Tucson,
Ariz. these riparian communities may differ greatly from upland communities, their physiognomies are not as strongly contrasting; often there is little visual contrast between the two types. But in most of the arid and semiarid West, luxuriant riparian deciduous hardwood forests and microphyll woodlands are linear landscape communities framed sharply by contrasting desertland, scrubland, woodland, and forestland of the immediately surrounding uplands. The relative inconspicuousness of eastern riparian communities contributed to the fact that significantly fewer riparian studies happened in North America before, versus after, the late 1960's.
The late establishment of riparian ecology is due partly to the complexity of riparian ecosystems.
In the ecotone formed by the land-water interface, one finds a textbook example of the edge effect
(Leopold 1933). This was first discussed for riparian ecosystems by Johnson (1979a) and Odum (1979).
A reticulum of interlaced species interactions and ecological processes forms a continuum between the wettest deepwater habitat and the driest upland habitat. "Until recently, structure, composition, and function of the riparian zone had received little consideration in ecosystem level research, because this zone forms the interface between scientific disciplines as well as ecosystem components"
(Swanson et al. 1982). In addition to the intricacies of the land-water interface, the diversity of western ecosystems is greatly increased by extreme contrasts in soil, slope, exposure, latitude and elevation--from near 200 feet below sea level in
Death Valley to more than 14,000 feet on Mount
Whitney.
The role these factors play in speciation on gradients in the West is discussed by Hall (1959) who notes that the southwestern United States contains more intraspecific diversity (subspecies) in mammals than any comparable continental area in the world. Within these oft-discussed environmental gradients there are two important riparian gradients
--a trans riparian continuum and an intrariparian continuum (see below). While the importance of ecotones and edge effect associated with riparian
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habitats is clear, the role of transriparian and intrariparian gradients in biotic diversity and speciation have not been previously addressed and are still poorly understood.
Little Riparian Interest--
Easte~n Ecologists on Western Landscapes
It was in this complex, diverse, and unfamiliar western ecological setting that earlier eastern ecologists conducted important studies in natural community structure and distribution.
Classical ecology in North America in the sense of
Clements, Daubenmire, Merriam, and others, and much of modern field ecology was, until recently, treated largely under theory and bias developed in and for non-arid regions. C. Hart Merriam, the first North American biologist to generate a general classification system for natural communities in
North America, traveled from Washington, D.C. to
Flagstaff, Arizona where he developed the Life Zone
System (Merriam lS90, lS9S). Merriam and one of his assistants, Vernon Bailey, recorded detailed observations on the volcanic-cindered slopes of the
San Francisco Peaks and the surrounding Kaibab Limestone surface of the Colorado Plateau where there is no permanent surface water. In the recent preparation of a book on the birds of Grand Canyon,
Arizona (Brown et al., in press) a major gap was found in the records of these eastern biologists-that for the riparian habitat of the Colorado River which they had reached by descending into the Grand
Canyon. The contribution of this riverine riparian habitat to avian density in this case is enormous.
In the 1960's two well-known eastern ecologists, Robert Whittaker and William Niering (1965,
1965) conducted extensive ecological investigations on the vegetation of the Santa Catalina Mountains near Tucson, Arizona. Curiously, within these otherwise excellent studies we are provided with little more information about natural riparian communities in Arizona than that provided by Merriam
SO years earlier. Whittaker and Niering differentiated eight upland vegetational community types between J,OOO and 9,500 feet elevation. Also on this Catalina gradient there is a paralleling riparian complex of distinctive riparian vegetational types that were dismissed with the single designation "Canyon Wood1cmd." It is worth noting again that eastern ecologists who were centrally interested in wetlands and moisture gradients did not recognize, or did not at least treat as important, the most obvious moisture gradient of all--the transriparian gradient, from the aquatic habitat of the streamway across the riparian zone into the adjacent uplands. This edaphic moisture regime--the moisture gradient associated with the aquatic to upland continuum--is a major factor controlling the biological diversity and structural complexity of riparian ecosystems in the Southwest.
Much Riparian Interest--
Western Users on Western Landscapes
The lack of interest in or understanding of riparian ecosystems by early investigators in the
Southwest was matched by an opposite zeal in the public sector for using riparian areas for grazing, agriculture, and urban development. "Most of the surviving river-bottom habitat has been cleared, leveled, and converted to farmlands, ... Perhaps nowhere else in Arizona have the changes been more dramatic" (Phillips et a1. 1964). "Moreover, i t is as are all ecological 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 water-users" (Lowe 1964).
Today, two decades later and fueled by the
National Environmental Policy Act (NEPA) of 1969, at least five basic national programs are concerned with the land-water interface in the United States: coastal wetlands, inland wetlands, floodplains, instream flow, and riparian protection and management.
Numerous resource management agencies have developed riparian policy and in-house programs. Other programs, e.g., floodplain management, coincidentally touch on riparian lands. One program, instream flow, continues to emphasize sportfishing while riparian areas and their nonconsumptive recreational and wildlife values are badly in need of instream flow considerations. Four of the five programs have at least one governmental agency with a legal mandate for their administration. And, although numerous state and federal agencies have explicit or de facto riparian management policies, no coordinated-Uational program has given riparian ecosystems the same protection as the other four. Without the needed legal mandate many types of riparian communities of native plants and animals will continue to deteriorate as some of the most endangered of all North
American biotas.
GRADIENTS--THE CONTINUUM IN THE RIPARIAN ZONE
The etymology of the word riparian has been recently discussed by Johnson (1979b) and Warner
(1984) as well as its common origin with the terms
"river" and "riverine" (Johnson et al. 1984a). And as the words themselves have varied, evolving through time, so have the interpretations of their meanings.
Regardless of how broadly or narrowly one defines the term, riparian areas are functionally wetlands since they are supportcd by inflowing water, be it perennial, intermittent, or ephemeral. The ecological concept of riparian community is properly expressed by the functional difference rather than by a strictly physical or biological definition. "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" (Lowe 1964).
The transriparian continu.um extends from the water in a stream or lake into the surrounding upland.
In moving along this continuum one transverses, sequentially, several aquatic and terrestrial habitat types from deep water through semiaquatic, riparian, semiriparian, and finally into upland ecosystems, in that order (fig. 1). Soil moisture forms a continuum from complete saturation on the bottom of the lake or stream to little measurable moisture at the
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Fraxi nus in common with biotic communities of the surrounding upland (Johnson et al. 1984b). However, even though these shared species may be preferential or facultative xeroriparian species, the washside individuals are commonly more robust and/or occur in greater densities than on the adjacent uplands. It is for this natural situation that Campbell and Green (1968) coined the term "pseudoriparian." Although no longer a useful term, as explained by Johnson et al. (1984a), it helped to draw attention to the issue during the
"phreatophyte period" (1960-1975) when riparian vegetation in general was viewed as "water-wasters" by some scientists as well as by the water salvage and reclamation interests, and some resource management agencies.
TRANSRIPARIAN PROFILE (SOUTHERN ARIZONA)
UPLAND AQUATIC
100r----o
RIPARIAN SEMI-
RIPARIAN
UPLAND
100
% DISTANCE FROM STREAMBED
Figure l.--Schematic of the transriparian continuum on the moisture gradient from aquatic through riparian into the adjacent upland community. top of an adjacent rocky hill. The intrariparian continuum may be physically present in a single system, such as the Colorado River Basin. Here one can (or could before the damming of the system) travel from the hydroriparian estuarine system at the delta of the Colorado River on the Gulf of
California, upstream along the hydro riparian riverine systems to its confluence with the Gila and from there to the mesoriparian habitats of the intermittent Rio San Pedro, finally terminating in the dry, desert xeroriparian habitats of the ephemeral streamcourses feeding into the San Pedro. In sharp contrast, some intrariparian systems located in the more mesic eastern U.S. and Pacific Northwest are completely hydroriparian with perennial water from the source to the mouth of the river system. Conversely, some important drainage systems, especially in Sonora and Baja California,
Mexico are entirely or essentially ephemeral from their origin to the Pacific or Gulf of California.
Conceptually, the entire spectrum of the North
American intrariparian continuum begins with, or varies from, something like the Florida Everglades hydro riparian system, where most species of the biotic community differ from those of the ecosystems of adjacent upland evergreen broadleaf or pine forests. From such wet extreme the continental continuum progresses uphill along a decreasing moisture gradient to the upper end of a dry riparian ecosystem along a dry desert water course; here the first order wash or arroyo may support very few species of plants and animals that are not shared
XERORIPARIAN HABITATS VERSUS UPLANDS
From a functional or biological standpoint,
"wetland" is a comparative term contrasting with
"dryland" or the more commonly used "upland." Most wetland ecologists, classifiers, mappers, et al. have developed their respective schemes in the more mesic East and have based them on soil moisture or water regimes (Cowardin et al. 1979, Johnson et al.
1984a). The concept of the wet riparian "big five"
--cottonwood (Populus fremontii), willow (Salix species), sycamore (Platanus wrightii), ash (Fraxinus pennsylvanica velutina) , and walnut (JuglallS major)--finds application more or less throughout the sub-Mogollon Southwest (Lowe 1961). Minckley and Brown (1982) "updated" the list by adding alder
(Alnus oblongifolia), in a wet riparian photograph of the "big six," immediately followed with another photo of the "big 6 minus one." In addition to alder one could add other species of similarly limited distribution. The "big five" are winter-deciduous hardwood trees with similarly wide ecological distributions.
Using the same criteria for xeroriparian systems in the sub-Mogollon desert region finds the dry riparian "big four"--mesquite (Prosopis species), catclawacacia (Acacia gregii), ironwood (Olneya tesota), and blue paloverde (Cercidium floridum).
It is difficult enough for an eastern biologist to understand how these four subtropical legumes can be called the dry riparian "big four." Then, when it is explained that sahuaros in extreme southwestern
Arizona (annual precipitation approximately 3 inches, pan evaporation 100 inches) become ultimately riparian plants, it tends to stretch the credulity of even the most learned wetland biologist (see Lowe
1964).
Upon investigating the complex ecological processes associated with the sahuaro in regions where it is an obligate xeroriparian plant, one finds that i t is, indeed, functionally a riparian species of utmost ecological importance. It is often the only large tree along these desert washes where the dry riparian "big four" are often shrub form or small trees. Numerous bird species of the region are largely or entirely dependent on the sahuaro for nesting. Red-tailed Hawks (Buteo jamaicensis), Great
Horned Owls (Bubo virginianus), and Roadrunners (Geococcyx califo~nus) nest in sahuaro crotches while the Gila Woodpecker (Melanerpes uropygialis) and gilded race of the Common Flicker (Colaptes auratus
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mearnsi) excavate cavities for nesting. The abandoned nest cavities made by these two species in turn are used by several secondary cavity nesters including the American Kestrel (Falco sparverius), Western Screech Owl (Otus kennicottii), Elf
Owl (Micrathene whitneyi), extremely rare Ferruginous Pygmy-Owl (Glaucidium brasilianum) , Purple
Martin (Progne subis), Brown-crested Flycatcher
(Myiarchus tyrannulus), Ash-throated Flycatcher
(~. cinerascens), and occasionally other species, such as Lucy's Warbler (Vermivora luciae). In wet riparian habitats these birds use cottonwoods and other large riparian trees, but in these extremely arid xeroriparian situations the sahuaro is the only plant large enough to provide nest sites for all of these species.
Available moisture for a plant or animal on a yearly or seasonal basis is several times as great in the Everglades as in a desert arroyo.
Everglades river and a pine upland.
SUMMARY
The mid-century development of North American riparian ecology happened where riparian ecosystems are most strikingly apparent in the landscape climax pattern--in the North American Southwest. In the
West, riparian systems are wetland systems exhibiting diverse continua from wet to dry and are consequently of great interest to western field investigators in diverse pursuits. In discussing this peculiarly western development in North American field ecology, we examine some ecological concepts as they pertain to riparian ecosystems. In contrast to the situation in western North America, many riparian communities in the eastern United States are relatively less conspicuous landscape features and have been inconsistently treated,sometimes as wetlands and at other times ignored. It is not unexpected when eastern wetland workers show relatively little interest in distinguishing riparian systems and their diverse communities.
However, the small percentage of shared floral and faunal species between these xeroriparian sahuaro communities with adjacent desertscrub upland communities may provide as much or more biological contrast as that found when comparing shared species between an
While we obviously agree with the current consensus among western workers that riparianlands are technically wetlands, we see no compelling reason to stress unduly the reality nor argue the point.
Rather, we would stress that riparian systems are intermediate in ecological and evolutionary position between the purely aquatic and purely terrestrial systems and overlap both of them in varying degrees.
Undeniably the three--Aquatic, Riparian, Terrestrial-are distinctive natural systems that harbor peculiarly obligate species structured into distinctive biotic communities across all of North America.
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