SIZE-CLASS STRUCTURE OF CONTIGUOUS RIPARIAN WOODLANDS ALONG A ROCKY MOUNTAIN RIVER

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Abstract
Trees in 7 diameter-classes, in 57 contiguous 0.1 ha plots, were tallied along a 10 km reach of the Animas River, southwestern Colorado. The age, height above water, and elevation for each plot, as well as the valley width and aspect at each plot location were determined. Detrended correspondence analysis (DCA) and two-way indicator species analysis (TWINSPAN) were then used to analyze these data. Age of the stand and its height above the water were significantly correlated with the first DCA axis, while only height above the water was significantly correlated with the second axis. Populus angustifolia and Pinus ponderosa have seedlings and saplings in stands 60 years old. Picea pungens and Pseudotsuga menziesii have seedlings and saplings in stands of all ages. A strict successional interpretation of the overall time-trends fails, because “succession” changed after 1927-1931. Current structure has been influenced by climatic and hydrologic effects on tree regeneration, by floods, and possibly by the unusual particle size of 1927 flood deposits. Vegetation structure on this 10 km river reach may be inherently unstable, because it is determined more by the timing and character of recent large-scale disturbances than by a repeating successional process. [Key words: Vegetation, Vegetation Dynamics, Forest Size-Class Structure, Riparian woodlands, Colorado.]