CLASSIFICATION OF THE RIPARIAN VEGETATION ALONG A 6-KM REACH OF THE ANIMAS RIVER, SOUTHTWESTERN COLORADO

Riparian ecosystems are important components of landscapes, particular ly because of their role in biodiversity. A first step in using a ''co arse-filter'' approach to riparian biodiversity conservation is to det ermine the kinds of riparian ecosystems. These ecosystems vary substan tially in plant species composition along a single river reach, as wel l as between rivers, and yet the river-reach scale has received little attention. We sampled the vascular plant composition of 67 contiguous patches of riparian vegetation along a reach of the Animas River, in southwestern Colorado's San Juan Mountains, that is relatively undistu rbed by human land uses. Using cluster analysis and detrended correspo ndence analysis, we identified eight riparian community types along th e reach. Using a new technique, we combined overstory size-class data and understory cover data to identify community types. The eight commu nity types, which are in part the products of past floods, are spatial ly arranged along the reach in relation to variation in valley morphol ogy tributary location, and geomorphic landforms. These eight communit y types do not necessarily represent successional stages of a single p otential vegetation type. This study at the river-reach scale suggests that sampling and analysis, as well as conservation, may need to be t uned to the scale of patchiness produced by flood disturbances in the riverine landscape, since vegetation varies significantly at this scal e.