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550 GREGOR T. AUBLE ET AL. Ecological Applications Vol. 4, No.3 TABLE 1. Percentage of plots in each cover type containing selected species. Species with 10 or more total occurrences in the 133 plots are listed in order of their score on DCA (Detrended Correspondence Analysis) Axis 1. Wetland indicator status is from Reed (1988). Cover type Equisetum % occurrence Veronica anagallis-aquatica OBL 11 22 0 0 Ranunculus cymbalaria OBL 19 35 3 0 Epilobium ciliatum FAC 30 59 1 0 Plantago major FAC 27 51 3 0 Trifolium repens FACU 21 39 3 0 Hordeumjubatum FAC 10 18 1 0 Poa palustris FACW 34 59 7 0 Eleocharis palustris OBL 41 71 9 0 Melilotus alba FACU 14 20 6 0 Mentha arvensis FACW 12 12 9 0 Conyza canadensis UPL 27 49 4 0 Phalaris arundinacea OBL 51 71 23 0 Equisetum arvense FAC 16 18 10 0 Acer negundo FACW 38 55 14 7 Lactuca serriola FACU 10 12 6 0 Euthamia occidentalis OBL 99 96 75 0 Agrostis stolonifera FACW 103 100 77 7 Agropyron sp. 22 6 28 0 Aster hesperius OBL 12 0 17 0 Carex lanuginosa OBL 57 35 57 7 Apocynum sp. 30 22 28 0 Cirsium arvense FACU 24 16 23 0 Solidago sparsijlora UPL 13 22 1 7 Verbena bracteata FACU 16 22 6 7 Muhlenbergia racemosa FACU 66 57 54 7 Juncus balticus FACW 22 8 26 0 Verbascum thapsus UPL 18 31 1 13 Carex nebrascensis OBL 21 6 25 7 Poa compressa FACU 97 63 90 27 Equisetum hyemale FACW 75 18 91 20 Artemisia ludoviciana FACU 46 55 22 27 Euphorbia serpyllifolia UPL 10 10 6 7 Equisetum laevigatum FACW 60 24 61 40 Chrysothamnus linifolius UPL 10 18 0 7 Bromus tectorum UPL 25 2 16 87 Heterotheca villosa UPL 20 6 7 80 Sporobolus cryptandrus FACU 14 0 4 73 * OBL = Obligate, >99% occurrence in wetlands; FACW = Facultative Wet, 67-99% occurrence in wetlands; FAC = Facultative, 34-66% occurrence in wetlands; FACU = Facultative Upland, 1-33% occurrence in wetlands; UPL = Upland, < 1 % occurrence in wetlands. Species Wetland indicator status* sonably requested from hydrologic engineers. Our di- rect-gradient prediction approach employs many ofthe same conceptual elements, computational procedures, and field methods as the Instream Flow Incremental Methodology (Bovee 1982, Harris et al. 1985), which is widely used to relate instream flow to fish habitat. Relationships between vegetation and inundation duration could be formulated at either the species or cover-type level. Species tend to respond individual- istically to environmental change, and this is the most appropriate level for understanding details of a tem- porally and spatially complex response. However, an overall synthesis is difficult when a large number of species are present. For this reason, we grouped the species into distinct cover types that become the basic units of vegetation description and change. The model could also be used to predict changes in abundance of individual species. Total occurrences E1eocharis Heterotheca Finally, the basic output ofthe direct gradient model is a statement about the new, quasi-equilibrium veg- etation associated with a new flow regime. This is a simplification of complex spatial and temporal re- sponse potentials. However, it has the strong advantage of being a single result, rather than a family of response curves. An alternative, the dynamic simulation model, naturally produces a family of output sequences in re- sponse to sequences of hydrologic input (e.g., Pearlstine et al. 1985). While these models are potentially more accurate and precise, their use dictates another com- plicated step to synthesize a family of possible vege- tation responses. Vegetation response has to be rep- resented simply enough to allow its inclusion as one of many variables (e.g., fish habitat, water supply, hy- dropower) considered in a water management decision. Furthermore, dynamic simulation models require more