Classification of the riparian vegetation of the montane and subalpine zones in western Colorado

A classification of the riparian vegetation of part of western Colorado was developed by cluster analysis and ordination of 115 samples of relatively undisturbed vegetation. The classification of plant associations contains five montane riparian forests, three subalpine riparian forests, four lower subalpine willow carrs, three upper subalpine willow carrs, and one subalpine wetland. Riparian ecosystems are widely recognized as important; yet in many areas their composition and structure have never been studied. Information on the composition and structure of riparian vegetation is essential if land managers are to be able to evaluate both the effects of their practices on riparian vegetation and their priorities for conservation. Particularly important for these purposes are data from relatively undisturbed examples of riparian systems. The purpose of this paper is to present the results of a plant association level classification of relatively undisturbed examples of the riparian vegetation of a part of western Colorado. The study area (Fig. 1) is a 300 X 50-km part of the southern Rocky Mountains. The western boundary approximates the low-elevation limit of riparian vegetation dominated by Populus angustifolia and Picea pungens . The eastern study area boundary is the Continental Divide. The study focuses on larger streams from about 1,850 m in elevation to about 3,650 m (treeline). Although Populus angustifolia extends along the Rocky Mountains from Mexico to southern Canada, Picea pungens occurs more spottily over a less extensive range (Little 1971). Thus, regions having montane forests similar to those in the study area may include only eastern Idaho and western Wyoming southward to northern New Mexico, parts of eastern Utah, and possibly northern Arizona. Similar regional limits may apply to the Abies lasiocarpa-Picea engelmannii riparian forests of the study area, as well as to many of the Salix -dominated wetlands (carrs). Si ce European settlement, the riparian vegetation of the study area has been significantly altered. Most of the major streams have dams or water diversions. Gravel extractions and gold mining have altered many reaches. Upstream mining has introduced sediment into many stream channels. Livestock grazing of riparian vegetation is the most pervasive source of alteration and has resulted in widespread replacement of native plants by exotics.