Our Livinii Rcsinirces — Riparian EcosvaWms 



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Although most riparian plants are adapted to 

 flooding, the frequency, liming, and duratiim of 

 floods may be highly altered on regulated 

 stream reaches. Fire appears to have increased 

 in importance relative to flooding as a form of 

 disturbance affecting regulated southwestern 

 rivers, including the Colorado. Colorado River 

 Cottonwood and willow canopy cover decreased 

 only slightly following tire, but burned cotton- 

 wood-willow stands had significantly greater 

 cover of both anowweed and tamarisk (P < 

 0.005). Efficiency in water uptake, transport, 

 and use are among the mechanisms responsible 

 for superior post-fire recovery of halophytic 

 shrubs compared with trees native to the 

 Colorado River ecosystem (Busch and Smith 

 1993). 



As the result of ecosystem change over the 

 last century, cottonwoods have become rare 

 along the lower Colorado River, and most 

 remaining stands are dominated by senescent 

 (i.e., in decline) individuals (Fig. 4). Although a 

 senescent segment was also a substantial por- 

 tion of the willow population, this species is still 

 relatively abundant in stands classified as cot- 

 tonwood-willow habitat. Even so, salt-tolerant 

 or water stress-tolerant shrubs such as tamarisk 

 and arrowweed now dominate these habitats. 



Similar to tamarisk, the non-native Russian 

 olive is a shrubby tree that has become natural- 

 ized throughout the western United States 

 (Olson and Knopf 1986), forming extensive 

 stands in some areas (Knopf and Olson 1984; 

 Brown 1990), particularly where historical river 

 flow patterns have been altered by water devel- 

 opment, such as along the Platte River in 

 Nebraska (Currier 1982) and the Bighorn River 

 in Wyoming ( Akashi 1988). Such conversion of 

 riparian vegetation from native to non-native 

 species may have profound wildlife manage- 

 ment implications. Bird species richness and 

 density, for example, are higher in native ripar- 

 ian vegetation than in habitats dominated by 

 tamarisk or Russian-olive (Knopf and Olson 

 1984; Brown 1990; Rosenberg et al. 1991). 



Future 



The health of natural riparian ecosystems is 

 linked to the periodic occurrence of flood flows, 

 associated channel dynamics, and the preserva- 

 tion of base flows capable of sustaining high 

 floodplain water tables. The establishment of 

 native riparian vegetation is diminished when 

 the frequency and magnitude of peak river 

 flows are reduced. Water uptake and water-use 

 patterns indicate that native trees are replaced 

 by non-native species in riparian ecosystems 

 where streamflows are highly modified. 

 Although riparian ecosystems are most directly 

 affected by altered streamflow, additional fac- 



tors threaten their integrity, including ground- 

 water pumping (Stromberg et al. 1992), grazing 

 (Armoin- et al. 1991), timber harvest and land 

 clearing (Brinson et al. 1981), and fire (Busch 

 and Smith 1993). Studies are under way to eval- 

 uate whether exotic plants will encroach further 

 into riparian ecosystems, given conditions pre- 

 dicted under global climate change scenarios. 



References 



Akashi, Y. 1988. Ripanan vegetation dynamics along the 

 Bighorn River. Wyoming. M.S. thesis. University of 

 Wyoming. Laramie. 245 pp 



Anderson. B.W., and R.D. Ohmart. 1984. Vegetation com- 

 munity type maps, lower Colorado River. U.S. Bureau 

 of Reelaination. Boulder City. NV. 59 pp. 



Armour. C.L.. D.F. Duff, and W. Elmore. 1991. The effects 

 of livestock grazing on riparian and stream ecosystems. 

 Fisheries 16:7-1 1. 



Bradley. C.E.. and D.G. Smith. 1986. Plains Cottonwood 

 recruitment and survival on a prairie meandering river 

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Brinson. M.M. 1990. Riverine forests. Pages 87-141 in A.E. 

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 1981. Riparian ecosystems: their ecology and status. 

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 155 pp. 



Brown. C.R. 1990. Avian use of native and exotic riparian 

 habitats on the Snake River, Idaho. M.S. thesis, Colorado 

 State University. Fort Collins. 60 pp. 



Busch, D.E., N.L. Ingraham, and S.D. Smith, 1992, Water 

 uptake in woody riparian phreatophytes of the south- 

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Busch. D,E.. and S.D, Smith. 1993. Effects of fire on water 

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Busch, D.E.. and S.D. Smith. 1995. Mechanisms associated 

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Currier. P,J. 1982, The floodplain vegetation of the Platte 

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Everitt, B.L. 1968. Use of the Cottonwood in an investiga- 

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Gregory. S,V., FJ. Swanson, W.A. McKee, and K.W. 

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Hobbs, R.J. 1989. The nature and effects of disturbance rel- 

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Hobbs. R.J.. and L.F Huenneke. 1992. Disturbance, diver- 

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Jackson, J., J.T. Ball, and M.R. Rose, 1990. Assessment of 

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Johnson, W.C. 1992. Dams and riparian forests: case study 

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Fig. 4. Relative plant canopy 

 cover and relative frequency per- 

 centages for woody riparian 

 species of the lower Colorado 

 River floodplain. Cottonwood and 

 willow populations are grouped 

 into juvenile (J), adult (A), and 

 senescent (S) age segments. 



