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<br />Ecology of riparian systems
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<br />Riparian communities
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<br />Streams often are heterotrophic commumtIes, dependent on allochthonous
<br />leaves and woody debris (Hynes 1975; Meehan et al. 1977). However, in the
<br />xeric environments of the American Southwest the streamside canopy is limited.
<br />Allochthonous material from deciduous broadleaf trees and shrubs may be an
<br />important energy source in some headwater streams (cf. Stanford & Ward
<br />1984), but lotic environments in the Colorado Basin tend to be autotrophic (cf.
<br />Minshall 1978), Primary productivity is largely controlled by nutrient renewal
<br />from seasonal spates, especially in the desert streams of the Lower Basin (Busch
<br />& Fisher 1981; Fisher et al. 1982), Nonetheless, the riparian habitats of the
<br />Colorado River are strongly associ;;tted with fluvial processes, and are principal
<br />components of the ecosystem.
<br />As most of the Colorado flows through semi-arid and desert lands, the
<br />riparian community may be the only significant vegetation in some areas. The
<br />riparian zone varies from a few metres in dry canyons to a kilometre or more
<br />on the floodplain of the lower mainstem, where extensive marshes occur (cr.
<br />Brown et aI, 1977; Minckley 1973). The zone is vital as a source of particulate
<br />organic matter for the river community, and as cover and a forage area for many
<br />birds (Carothers et al. 1974; Anderson & Ohmart 1977; Stevens et al. 1977),
<br />herpetofauna (Carothers & Johnson 1983), rodents (Anderson et at. 1977) and
<br />other wildlife (Brinson et al. 1981). At higher elevations these habitats are a
<br />wintering area for mule deer (Odocoileus hemionus) and elk (Cervus canadensis),
<br />Thus, although limited in total area, the longitudinally and locally variable
<br />stream bank vegetation (with zones around parts of the reservoirs) is an import-
<br />ant part of the biophysiography of the American Southwest.
<br />Riparian communities along the smaller streams are difficult to classify,
<br />because higher elevation climax associations may intergrade with streamside
<br />vegetation on broad floodplains, or riparian species may merge with woodlands
<br />on the sides of steep canyons, However, there are several generalised classes of
<br />plant communities (Stanford & Ward, pers, obs,; Lowe 1964; Brown et aI, 1977):
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<br />Montane deciduous scrub
<br />Above 2000 m, riparian communities in the Upper Basin and upper Gila River
<br />are dominated by broadleaf species usually near or overhanging the channel.
<br />These include willows (Salix spp.), alder (Alnus tenuifolia), dogwood (Caml/s
<br />stolanifera), birch (Betula occidentalis), elder berry (Sambucus glauca), Rocky
<br />Mountain maple (Acer glabrum) and, in local areas, hawthorn (Crataegl/s
<br />erythropoda), In autumn, leaves from these plants are a major energy source for
<br />the stream communities. The association intergrades with pinyon (Pinus edl/lis).
<br />juniper (Juniperus spp.), sagebrush (Artemisia tridentata), scrub oak (Quercl/\
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<br />gambelli) and ponderosa pine (Pinus ponderosa) at lower, drier elevations,
<br />or with aspen (Populus tremuloides), spruce (Picea engelmannii, P. pungens)
<br />and fir (Abies lasiocarpa) in the more mesic areas (2700 m to the timberline
<br />at 3700 m). Strictly alpine willow species (e.g. Salix antiplasta) and small
<br />annual forbs (e.g. Ranunculus spp.) and sedges (Carex spp.) grow along the
<br />alpine streams.
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<br />Temperate deciduous forest
<br />At about 2000 m a climax community of cottonwood (Populus spp.) and willows
<br />intergrades with the montane scrub. Cottonwood-willow associations dominate
<br />along the large tributaries and the Colorado mainstem from foothills to delta,
<br />In the Upper Basin, willows (Salix caudata, S. amygdaloides and others) form
<br />hedgerows along the channels, and galleries of cottonwoods (P. wislizeni, P.
<br />augustifolia) occur on more stable areas. The cottonwoods often are dense
<br />stands of trees more than 200 years old, and contribute a large leaf fall to the
<br />rivers. In drier reaches of the Upper Basin the cottonwood-willow fringe may
<br />be sparse and interspersed with grease wood (Sarcobatus vermiculatus) and salt
<br />bush (Atriplex nutallit), typical of lower elevations on the Colorado Plateau. In
<br />the Lower Basin similar dominants occur, but with Salix gooddingii, S. bonplan-
<br />diana and other willows in association with the cottonwoods Populus fremantii
<br />and P. augustifolia. Some temperate deciduous communities in the interior of
<br />Arizona (Gila drainage) are mixed stands of sycamore (Platanus wrighti), ash
<br />(Fraxinus pennsylvanica velutina), boxelder (Acer negundo) and walnut (Juglans
<br />major), with the cottonwood-willow complexes. These are generally at middle
<br />altitudes (l000-2000 m), and intergrade with montane scrub. Stands exist also
<br />as hanging gardens along springbrooks which cascade over incisions in the walls
<br />of the Lower Basin canyons. Of course, this community is distinct from the
<br />similarly-named association in the New England area.
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<br />Subtropical deciduous woodland
<br />This association is in the lower Gila drainage, where floodplain groundwaters
<br />at tributary confluences support mesquite (Prosopis juliflora velutina) bosques.
<br />These intergrade with Sonoran Desert plants along the San Pedro, Santa Maria,
<br />Verde and middle Gila rivers.
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<br />Subtropical evergreen forest
<br />Relictual stands of California fan palm (Washingtonia filifera), of Miocene or
<br />Pliocene origin, occur at salt springs along ephemeral streams in the Sonoran
<br />Desert.
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<br />Colorado River marshes
<br />Various wetlands occur, with plant associations reflecting the alkalinity and
<br />availabili~y of water. In more xeric, saline systems saltgrass (Distichilis stricta}
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