Riparian Ecosystems 



Overview 



The strict definition of 

 riparian is "streanibank," 

 but riparian ecosystems are often broadly defined 

 to include riverine tloodplains. In the broad 

 sense, the riparian zone is both a transition and 

 interface between riverine and upland systems. 

 Functionally and structurally, riparian areas are 

 different from surrounding uplands because of 

 proximity to a water course. In the eastern United 

 States, the upland landscape is generally moist 

 enough to support woody vegetation while the 

 often extensive bottomland forests comprise only 

 those plants able to tolerate flooding and exces- 

 sive moisture. In much of the West, areas near 

 water courses are often the only places with suf- 

 ficient moisture for trees. Thus, western riparian 

 ecosystems are often relatively narrow ribbons of 

 trees in a generally unforested landscape. 



We lack good estimates of the status or histor- 

 ical changes in area for riparian ecosystems of the 

 West as a whole, although we do know that they 

 have always represented a very small fraction of 

 the land area because of their dependence on 

 water in a dry region. Their importance stems 

 from the unique features that they provide, repre- 

 senting desirable habitat for a variety of species. 

 Many of the same features that make these sys- 

 tems relatively rare and important also make 

 them relatively sensitive. 



Western riparian systems have been massive- 



ly altered in the last 200 years; the history of 

 development in the West is to a large extent one 

 of water development. As the articles in this sec- 

 tion illustrate, it is hard to make a hydrologic 

 change without also altering the associated ripar- 

 ian ecosystem. Busch and Scott (this section) 

 show how hydrologic changes can influence the 

 long-term species composition by altering soil 

 salinity and changing the nature of disturbances 

 that create opportunities for regeneration. Some 

 changes described in Roelle and Hagenbuck's 

 article (this section) on the Middle Rio Grande 

 are relatively straightforward: riparian vegetation 

 is inundated by a reservoir or a channel narrows 

 with lower streamflow. Other effects of hydro- 

 logic alteration are more complex and may be 

 played out over many decades. As the authors 

 note, the absence of a change in net area may 

 mask dramatic shifts in the location of different 

 vegetation types. 



Although hydrology is the dominant factor 

 shaping these ecosystems, it is not the only one. 

 In all the riparian systems described in the fol- 

 lowing articles, invasions of non-native plants 

 have changed the composition of the communi- 

 ties and the way the systems will likely respond 

 in the future. Timber clearing, overgrazing by 

 livestock, agricultural conversion, and urban 

 growth are other important causes of change in 

 these ecosystems. 



■ -iilsSaiS ^'»5t-'>J',Ji*5ii«Kr*r^'?>' 



Science Editor 



Austin K. Andrews 



text by 



Gregor T. Auble 



National Biological 



Service 



Midcontinent Ecological 



Science Center 

 Fort Collins, CO 80525 



