Introduction Beyer 2002 Hart and Poff 2002 Graf 2003 Hart et al. 2002 Pohl 2002 Stanley and Doyle 2003 Poff et al. 1997 Kondolf 1997 Lenhart 2003 Nilsson et al. 2005 Bednarek 2001 Shafroth et al. 2002 Keddy and Rznicek 1982 1986 Hill et al. 1998 Nilsson and Keddy 1988 Nilsson et al. 1997 Friedman et al. (1996) Populus deltoides monilifera P. deltoides monilifera Mahoney and Rood 1998 Rood et al. 2005 Bednarek 2001 Stanley and Doyle 2002 Hart et al. 2002 Lenhart 2000 Orr and Stanley 2006 Wohl and Cenderilli 2000 Zuellig et al. 2002 Study Site Fenneman 1931 Hansen et al. 1978 Marr (1961) Pinus ponderosa Marr 1961 Cercocarpus montanus Rhus trilobata (Rhus aromatica Great Plains Flora Association 1986 Pinus ponderosa Peet (1981) Pinus ponderosa Cercocarpus montanus Rhus trilobata Yucca glauca Stipa comata Bromus tectorum Helianthus pumilus Sporobolus cryptandrus, Bouteloua hirsuta Verbascum thapsus Peet 1981 Agropyron smithii, Andropogon scoparius, Bouteloua curtipendula, B. gracilis, Bromus tectorum Stipa comata Hansen and Dahl 1957 Salix irrorata, Betula occidentalis, Populus deltoides monilifera Salix amygdaloides Marr 1961 8 3 8 3 U.S. Bureau of Reclamation 2005 Methods Field Sampling We sampled vegetation along 13 transects that began 5 m into upland vegetation (above the reservoir’s high water mark of 1654.6 m above sea level) and extended down slope (perpendicular to the shoreline) to an elevation of 1621.5 m asl. Below 1621.5 m asl, the reservoir begins to separate into distinct pools with different water surface elevations. Transects were located randomly along the entire length of the shoreline, with the exception of the following excluded areas: small, shallow coves on the west side of the reservoir; the four dams; and 200 m on either side of each dam. 2 Great Plains Flora Association 1986 Weber 1990 1 2 Fig. 1 Reservoir water surface elevation and drawdown zones. Vegetation analysis is limited to four distinct elevation zones: UPL was never inundated; TOP was first exposed in 1999 and not subsequently; MID was first exposed in 2000 and not subsequently; BTM was first exposed in 2000 and was subsequently re-flooded and re-exposed in both 2001 and 2002  Fig. 2 01 01 01 Populus deltoides monilifera 01 Melilotus 01  Data Analysis Plant characteristics and communities Plant characteristics are summarized by elevational zone and sampling year with each combination denoted by the zone (BTM, MID, TOP, and UPL) subscripted by the last two digits of the year of sampling (2001 and 2002). The resulting eight zone-year combinations are grouped into classes based on the number of years since the zone was last exposed or drawn down (1, 2, 3, 4 years and Upland). There was no preexisting, rooted vegetation in any of these zones prior to their first exposure and no apparent survival of vegetation in the BTM zone between the two successive years it was drawn down. McGregor et al. (1986) USDA-NRCS (2004) Reed 1988 USDA-NRCS 2004 Tiner 1999 SAS 2003 McCune and Grace 2002 Inouye (1998) McCune and Meford 1999 McCune and Grace 2002 Cottonwood seedling establishment a priori Mahoney and Rood’s (1998) Populus deltoides monilifera Segelquist et al. (1993) Populus deltoides monilifera Results Plant Characteristics and Communities 02 02 3 3 01 02 01 02 01 02 01 02 Tiner 1999 02 3 Fig. 3 th th  Patterns in both species richness and cover were complicated by differences among sampling years in precipitation and among zones in substrate and sampled area. The second year of sampling, 2002, was drier than 2001, which contributed to lower total 2002 cover in the upland zone and lower numbers of identifiable species. Bromis japonicus B. tectorum Melilotus Melilotus alba M. officinalis 4 01 02 01 02 01 02 01 Fig. 4 2 Inouye 1998 McCune and Grace 2002  P 4 01 02 1 Table 1 McCune and Grace 2002 Years exposed Zone year Relativized Sorenson distance (% dissimilarity) Years exposed 1 2 3 4 Upland 01 02 01 02 01 02 01 02 1 01 — 15 58 80 87 94 99 99 02 15 — 65 79 88 94 99 99 2 01 58 65 — 51 58 79 98 98 3 02 80 79 51 — 58 79 98 97 01 87 88 58 58 — 50 91 92 4 02 94 94 79 54 50 — 84 83 Upland 01 99 99 98 97 91 84 — 19 02 99 99 98 97 92 83 19 — 2 Chenopodium glaucum Panicum capillare Rorippa curvipes Table 2 Relative cover of selected species by elevational zone and time exposed. Relative cover is calculated at the transect level for each zone-year combination and then averaged across transects. All species with cover ranks among the top four in any zone-year combination are included. Dashes indicate absence Species Characteristics Relative Cover (%) by Years Exposed and Sampled Zone Duration Nativity Wetland indicator 1 2 3 4 Upland 01 02 01 02 01 02 01 02 Amaranthus albus A N FACU 2.2 7.1 3.7 2.6 — — — — Ambrosia tomentosa P N UPL <0.1 <0.1 4.6 9.3 — — — — Bromus inermis P E UPL — <0.1 <0.1 <0.1 — 0.3 13.0 11.0 Bromus japonicus - B. tectorum A E UPL — — <0.1 0.2 1.7 4.3 24.7 18.7 Cercocarpus montanus P N UPL — — — — 0.6 2.3 10.4 17.1 Chenopodium glaucum A E FACW 57.8 56.1 15.8 0.4 — — — — Cirsium arvense P E FACU 0.2 0.1 1.8 13.3 0.8 3.5 1.0 0.7 Ericameria nauseosa P N UPL — — <0.1 0.2 10.3 9.2 1.7 1.8 Lactuca serriola A E FAC — — 2.0 2.9 12.0 11.7 — — Melilotus A/B E FACU 0.2 0.1 9.5 5.9 15.1 0.7 0.5 — Panicum capillare A N FAC 10.4 10.3 26.2 7.2 8.0 1.2 — — Populus deltoides monilifera P N FAC 4.4 1.8 2.4 3.5 0.4 0.5 — — Rhus aromatica P N UPL — — — — — — 13.2 9.2 Rorippa curvipes A/B/P E FACW 5.3 0.9 <0.1 — — — —  Salsola collina A E UPL — — 0.9 15.1 2.9 13.3 — 0.3 Suckleya suckleyana A N FACW 3.5 7.2 0.1 0.2 — — — — Verbascum thapsus B E UPL <0.1 <0.1 1.3 2.5 11.1 11.7 — — Verbena bracteata A/P N UPL 0.2 0.2 9.6 10.3 9.9 4.0 <0.1 — A = annual, B = biennial, P = perennial, N = native, E = exotic, W = wetland, FACW = facultative wetland, FAC = facultative, FACU = facultative upland, UPL = upland Cirsium arvense Ericameria nauseosa Lactuca serriola Salsola collina Verbascum thapsus Verbena bracteata Bromus japonicus B. tectorum Bromus inermis Cercocarpus montanus Rhus aromatica 2 Cottonwood Seedling Establishment Populus deltoides monilifera 2 Salix amygdaloides S. exigua Populus angustifolia P. tremuloides Tamarix ramossima Populus deltoides monilifera 5 6 Populus deltoides monilifera Fig. 5 Populus deltoides monilifera  Fig. 6 Populus deltoides monilifera  Discussion Nilsson et al. (1997) Orr and Stanley (2006) Lenhart (2000) Shafroth et al. 2002 Lenhart (2000) 1 2 The initial colonizing community of a bare surface might be expected to have higher fractions of short-lived and perhaps non-native species than later vegetation on the site. At the Horsetooth site, the fraction of perennials steadily increased through four years of exposure, whereas the fraction of native species was below that of the surrounding uplands and did not change appreciably or consistently. Substrate-adjusted mean values for total cover were lower than the upland and appeared to decline somewhat in the second through fourth years of exposure. A possible explanation for these patterns in cover and richness is that the mesic conditions in the first year of exposure produce a reasonably high plant cover composed heavily of annual species. This is followed by a period of reorganization, replacing annuals with deeper rooted perennials. Species richness peaks during this transition, whereas total cover tends to drop in the drier conditions of years two through four following exposure and only gradually increases to the ultimately higher cover of a slowly developing perennial community present on the dry upland sites. Mahoney and Rood (1998) Rood et al. 2005 Fenner et al. 1984 Mahoney and Rood 1991 Segelquist et al. 1993 Amlin and Rood 2002 Rood et al. 1998 Auble and Scott 1998 Shafroth et al. 1998 Rood and Mahoney 2000 Rood et al. 2005 Populus deltoides monilifera Mahoney and Rood 1998 P. deltoides monilifera 5 6 Mahoney and Rood 1991 Mahoney and Rood (1998) Implications for Dam Removal Decisions Poff and Hart 2002 Shafroth et al. 2002 Hart and Poff 2002 There are two primary ways such a first approximation could be misleading. The first is that the post-dam topography and substrate characteristics may have been altered substantially by the period of inundation. This is especially true if the dam has accumulated substantial quantities of trapped sediment that have altered topography, stored nutrients or contaminants, or altered organic content or particle size distributions. Our results showed a significant effect of substrate size proportions on total vegetation cover; however, Horsetooth Reservoir was not accumulating substantial sediment because of the diversion source of most of the impounded water. P. deltoides monilifera Menges and Waller 1983 Grime 2001 Orr and Stanley 2006 Lenhart 2000 Stanley and Doyle (2003) Lenhart (2000) Utrica dioica Leersia oryzoides