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548 GREGOR T. AUBLE ET AL. Ecological Applications Vol. 4, No.3 Hydric-Mesic ELEOCHARIS Mesic-Xeric EQUISETUM FIG. 4. Schematic of cover type classification. Moving-Average alternative simulates the moderating effect that could be expected from additional upstream reservoir storage. Flow duration curves were construct- ed for each hydrologic regime by calculating the cu- mulative flow frequency distribution for the respective sets of daily discharge values. REsULTS Existing vegetation Eighty-three vascular plant species were observed in the 133 plots. Vegetation was essentially herbaceous except for scattered, short (< 2 m) individuals of Acer negundo, Tamarix ramosissima, and Salix exigua and a discontinuous fringe of mature Acer negundo and Tamarix ramosissima along the base of the canyon wall. Most of the plants recorded in the study reach are widely distributed species typical of low-elevation riparian zones in Colorado (Weber 1987, Kittel and Lederer 1993). The first two divisions of TWINSPAN grouped the plots into three cover types (Fig. 4): Heterotheca, Eq- uisetum, and Eleocharis. The distribution of plots among cover types is illustrated by the Detrended Cor- respondence Analysis ordination in Fig. 5. These cover 400 300 . + . N ' .. + (fl 200 . .~. ~ + + 'x · -. Bel> + + .... · .. 'kxl 01 co V '.oi~o ++ <( + () 100 . . 0 '0 0 . Eleocharis & 0 o Equisetum 0 0 0 + Heterotheca -100 -100 0 1 00 200 300 400 500 600 DCA axis 1 FIG. 5. Detrended Correspondence analysis (DCA) ordi- nation of plots, by cover type, for the first two DCA axes. Eigenvalues for Axis I and Axis 2 were 0.56 and 0.27, re- spectively. types, defined solely on the basis of species-occurrence data, occupied clearly different positions on the in- undation-duration gradient (Fig. 6). In addition, a Kruskal-Wallis test (SAS Institute 1987) showed that the cover types were significantly different in inunda- tion duration, vegetative cover, and soil particle size (P < .000 I), demonstrating that floristic differences are strongly related to environmental gradients. A final cover type, Open Water, was added to account for the river channel and off-channel pools, which were un- sampled areas, devoid of emergent vegetation, and in- undated at the time of sampling. Based on stage-dis- charge information, this cover type was assigned an inundation duration of > 99%. The Heterotheca cover type consisted of xeric grasses and herbs dominated by Bromus tectorum, Heterothe- ca villosa, and Sporobolus cryptandrus. The plants most strongly associated with this cover type were largely upland species (Table 1, Reed 1988). Vegetative cover was sparse, usually 0-25%. The substrate was primarily sands, cobbles, and boulders. This cover type was con- fined to the upper portions of the Upstream, River Right bar (Fig. 2). This was the driest cover type (Fig. 6); its inundation duration was generally < 2%. The Equisetum cover type consisted of mesic to xeric herbs and grasses dominated by Equisetum hyemale, Poa compressa, Agrostis stolonifera, Muhlenbergia ra- cemosa, and Euthamia occidentalis. Vegetative cover was mostly between 5 and 75%. The substrate was primarily organic matter and sands; however, some plots were dominated by boulders. This cover type was typically found on middle- and upper-elevation gravel and cobble bars. Generally, this was a relatively dry and infrequently inundated cover type (Fig. 6). Inun- dation durations ranged from "'" 2 to 60%, with most plots in the range of 2-20%. The Eleocharis cover type consisted of mesic herbs and grasses dominated by Agrostis stolonifera, Eu- thamia occidentalis, Eleocharis palustris, Phalaris arundinacea, Poa palustris, Epilobium ciliatum, and Poa compressa. Species with a high probability of oc- curring in wetlands (Reed 1988) were associated with this cover type (Table 1 ). Vegetative cover was highly variable, ranging from 2S to 100%. However, most