LONG‐TERM VEGETATION DYNAMICS AFTER HIGH‐DENSITY SEEDLING ESTABLISHMENT: IMPLICATIONS FOR RIPARIAN RESTORATION AND MANAGEMENT

Human disturbances have contributed to the deterioration of many western US rivers in the past century. Cottonwood‐willow communities, present historically along the Colorado River, protect watersheds and provide wildlife habitat, but are now among the most threatened forests. As a result, restoration efforts have increased to re‐establish and maintain cottonwood‐willow stands. While successful establishment has been observed using multiple strategies with varying investments, few projects are evaluated to quantify efficacy and determine long‐term sustainability. We monitored a seeded cottonwood‐willow site over a five‐year period beginning in 2007, with particular interest in how density affected vegetation diversity and stand structure over time. Fremont cottonwood (Populus fremontii) and volunteer tamarisk (Tamarix ramosissma) were the only abundant riparian trees at the site after one year. P. fremontii, compared to T. ramosissma, had higher growth rates, lower mortality, and dominated overstory and total cover each year. Vegetation diversity decreased from 2007–2009, but was similar from 2009–2011 as a result of decreased herbaceous and increased shrub species richness. Diversity was highest in the lowest density class (1‐12 stems/m²), but similar among all other classes (13–24, 25–42, 43+). High initial woody species densities resulted in single‐stemmed trees with depressed terminal and radial growths. Allometry, relating height to DBH at different densities, could prove to be an important tool for long‐term restoration management and studying habitat suitability. Understanding long‐term trends at densely‐planted or seeded sites can benefit restoration managers who aim to establish specific community structure and vegetation diversity for wildlife habitat. Copyright © 2012 John Wiley & Sons, Ltd.     

Rodent use of anthropogenic and ‘natural’ desert riparian habitat,lower Colorado River,Arizona

The role of native trees, Fremont cottonwood (Populus fremontii) and Goodding willow (Salix gooddingii), in structuring the riparian small mammal assemblage on rivers in the American desert southwest is unclear. It is unknown, for example, whether these trees directly or indirectly provide the food or shelter necessary for the presence of any species. Because of the rapid and widespread decline of gallery forest, due in part to river regulation, the retention of remnant stands and replacement of lost stands are major regional conservation issues. To elucidate small mammal–forest linkages, we compared patterns of macrohabitat use among terrestrial small mammals at two rehabilitated and one unmanipulated alluvial floodplain site along the highly regulated lower Colorado River. We also compared current patterns to the Colorado River faunal associations Joseph Grinnell documented in 1910, prior to significant flow regulation. We used grid‐based, capture‐mark‐recapture techniques at two revegetation sites, each a mosaic of six distinct macrohabitats, including planted cottonwood/willow, to associate species with specific macrohabitats. We also trapped a ‘reference’ grid containing naturally regenerating cottonwood and willow at a site on the lower Bill Williams River floodplain. Despite very poor development of cottonwood plantings at one of the revegetated sites, each supported at least nine species and harbored all seven species that Grinnell associated with areas flooded nearly every year. The set of species Grinnell associated with cottonwood/willow stands (Peromyscus maniculatus, Reithrodontomys megalotis, and Sigmodon arizonae) was trapped at both revegetation sites but entirely absent at the reference site. The Bill Williams site may be inaccessible to Sigmodon, but the absence of the other two species is probably a consequence of differences in floodplain structure and functioning among the study sites as well as between the Bill Williams site and historic Colorado River riparian areas. Our data suggest the richness of the native lower Colorado River riparian small mammal assemblage is unrelated to the presence or absence of cottonwood/willow trees, but does depend in part upon the presence or absence of dense herbaceous vegetation. Resource managers attempting to rehabilitate degraded desert riverine ecosystems need to consider understory as well as overstory plant species in revegetation efforts. Copyright © 1999 John Wiley & Sons, Ltd.     

USE OF NATIVE AND NONNATIVE NEST PLANTS BY RIPARIAN‐NESTING BIRDS ALONG TWO STREAMS IN NEW MEXICO

Nonnative plant invasions are a management concern, particularly in riparian forests, but little is known about mechanisms through which they influence vertebrate communities. In the American Southwest, native trees such as cottonwood (Populus spp.) are thought to provide better habitat for breeding birds than nonnative plants, which are more tolerant of human‐altered conditions. To evaluate effects of riparian forest composition on riparian‐nesting birds, we examined nest plant use along two rivers in New Mexico that differed in abundance of nonnative vegetation. Of the nests we observed, 49% along the Middle Rio Grande were constructed in nonnative plants, compared with 4% along the Gila River. Birds in the canopy and cavity‐nesting guilds constructed less than 5% of their nests in nonnative plants along either river. At the Middle Rio Grande, birds in the subcanopy/shrub guild constructed 67% of their nests in nonnative plants. Despite the relatively low availability of cottonwoods, they were used by greater numbers of species than any other woody plant at either river. Riparian obligates and species of conservation concern in the canopy and cavity guilds were especially dependent on cottonwood and Arizona sycamore (Platanus wrightii). Our results show that, although nonnative trees and shrubs support large numbers of nests for certain birds, cottonwoods and other large native trees are disproportionately important to riparian bird communities. Copyright © 2013 John Wiley & Sons, Ltd.     

Responses of obligate versus facultative riparian shrubs following river damming

Riparian or streamside woodlands include obligate riparian trees and shrubs (obligates) that are restricted to streamside zones, and facultative riparian species that are abundant in, but not restricted to the riparian areas. Due to their distinctive life history requirements, it may be predicted that the ecological specialist obligates would be more vulnerable than the facultative generalists to impacts from river damming and flow regulation. We tested this along the Snake River through Hells Canyon, USA, where two native riparian shrubs dominate: the obligate sandbar willow (Salix exigua), and the facultative, netleaf hackberry (Celtis reticulata). We assessed riparian conditions over the past century by comparing ground‐level and aerial photographs taken after 1907 and in the 1950s in advance of three dams, versus recent conditions. These comparisons revealed three changes downstream from the dams: (1) the depletion of surface sands and sandbars and (2) reductions in sandbar willow versus (3) the proliferation of hackberry in dense bands above the typical high‐water line. The willow decline probably resulted from the depletion of sand following sediment trapping by the reservoirs, combined with changes in the seasonal water flow pattern. The increase in hackberry may have resulted from a beneficial ‘irrigation effect’ of daily water releases for power generation during the summer. The opposing responses reflect the plants' differing life histories and may partially resolve impacts of river regulation on alluvial sediments versus the instream flow pattern. We consider other riparian studies that suggest that obligates such as cottonwoods (Populus angustifolia, P. deltoides and P. fremontii) are highly vulnerable to river regulation, while facultative trees and shrubs such as trembling aspen (Populus tremuloides), wolf‐willow (Elaeagnus commutata) and velvet mesquite (Prosopis velutina) are more resilient. These results suggest that conservation of riparian woodlands should emphasize the ecological specialist obligates, while facultative species may be less vulnerable to river regulation. Copyright © 2009 John Wiley & Sons, Ltd.     

Putting down roots: Afforestation and bank cohesion of Icelandic Rivers

Riparian vegetation is widely recognized as a critical component of functioning fluvial systems. Human pressures on woody vegetation including riparian areas have had lasting effects, especially at high latitude. In Iceland, prior to human settlement, native downy birch woodlands covered approximately 15%–40% of the land area compared to 1%–2% today. Afforestation efforts include planting seedlings, protecting native forest remnants, and acquiring land areas as national forests. The planted and protected nature of vegetation along rivers within forests provides a unique opportunity to evaluate the various taxa within riparian zones and the channel stabilizing characteristics of the vegetation used in afforestation. We investigated bank properties, sediment textures, and root characteristics within riparian zones along four rivers in forests in Iceland. Bank sediment textures are dominantly sandy loam overlying coarser textures. Undercut banks are common because of erosion of the less cohesive subsurface layer. Quantitative root data indicate that the woody taxa have greater root densities, rooting depths, and more complex root structures than forbs or graminoids. The native downy birch has the highest root densities, with <1 mm roots most abundant. Modeling of added bank cohesion indicates that willow provides up to six times and birch up to four times more added cohesion to the coarse sediment textures comprising stream banks compared to no vegetation. We conclude that planting and protecting the native birch and willow helps to reduce bank erosion, especially where long-term grazing exclusion can be maintained.     

Temporal and Spatial Variation in Riparian Vegetation and Floodplain Aquifers on the Regulated Dolores River,Southwest Colorado,USA

The importance of flow variability and floodplain water table recharge for the establishment and long‐term survival of riparian vegetation has been well‐documented. However, temporal and spatial variation in floodplain aquifers has received less attention, although native species can have narrow tolerances for groundwater decline. Our observations of decreased cottonwood cover on floodplains and increased willow cover on river banks since dam completion on the Dolores River led to comparisons between three long‐term study sites above and below McPhee Dam. We summarize 5 years (2010–2014) of shallow groundwater well data from transects of three wells per site. Vegetation cover data were collected from quadrats and line‐intercept transects. In the willow zone, groundwater well levels mirror in‐channel flows and rarely drop below 0.6 m from ground surface. Willow cover and stem counts on point bars are higher at dammed sites. Wells in the cottonwood zone indicate that alluvial recharge happens only during prolonged peak discharge during spring snowmelt or dam release. Years with no dam spill reduced connectivity between surface flows and groundwater, and groundwater depth dropped to between 2 and >2.5 m. Long‐term data below the dam indicate that canopy cover of the dominant cottonwoods has declined over time (48% in 1995, 19% in 2003), especially in the wake of severe drought. Mature cottonwood cover is significantly higher at the undammed site (p = 0.025). Our results indicate that floodplain habitats below dams exist under artificially extreme drought and inform how biologically diverse riparian systems will be impacted by a drying climate. Copyright © 2016 John Wiley & Sons, Ltd.     

Aerial photographic analysis of channel narrowing and vegetation expansion in Canyon De Chelly National Monument,Arizona, USA, 1935–2004

Aerial photographs from the past 70 years show narrowing of channels in Canyon de Chelly National Monument, which coincided with the establishment and expansion of woody riparian vegetation, primarily non‐native tamarisk (Tamarix ssp.) and Russian olive (Elaeagnus angustifolia) and native cottonwood (Populus spp.). Rectified air photo sets were used to map the extent of woody riparian vegetation cover in the canyon bottom as well as the channel planform geometry for 6 years: 1935, 1964, 1975, 1981, 1989 and 2004. In 1935, vegetation covered less than 1 per cent of the canyon bottom and the channel was braided and 91 m wide on average. By 2004, the channel in the upper 75% of the canyon had narrowed to a single thread an average of 6.5 m wide and was lined by riparian vegetation, with vegetation covering up to 45% of the canyon bottom in some reaches. A relative timing index was developed that incorporates information from both channel and vegetation area expansion or contraction into a single variable. Vegetation establishment lagged channel narrowing in the upper reaches of the canyons, but narrowing and vegetation establishment occurred simultaneously in the middle and lower reaches. Narrowing progressed downstream through time, and the lowermost reaches remained braided and unincised in 2006. We interpret these patterns to suggest that riparian vegetation is responding to and interacting with changes in the channel morphology of Canyon de Chelly rather than driving channel change. Copyright © 2010 John Wiley & Sons, Ltd.     

Regulation leads to increases in riparian vegetation,but not direct allochthonous inputs,along the Colorado River in Grand Canyon,Arizona

Dams and associated river regulation have led to the expansion of riparian vegetation, especially nonnative species, along downstream ecosystems. Nonnative saltcedar is one of the dominant riparian plants along virtually every major river system in the arid western United States, but allochthonous inputs have never been quantified along a segment of a large river that is dominated by saltcedar. We developed a novel method for estimating direct allochthonous inputs along the 387 km‐long reach of the Colorado River downstream of Glen Canyon Dam that utilized a GIS vegetation map developed from aerial photographs, empirical and literature‐derived litter production data for the dominant vegetation types, and virtual shorelines of annual peak discharge (566 m³ s^?1 stage elevation). Using this method, we estimate that direct allochthonous inputs from riparian vegetation for the entire reach studied total 186 metric tons year^?1, which represents mean inputs of 470 gAFDM m^?1 year^?1 of shoreline or 5.17 gAFDM m^?2 year^?1 of river surface. These values are comparable to allochthonous inputs for other large rivers and systems that also have sparse riparian vegetation. Nonnative saltcedar represents a significant component of annual allochthonous inputs (36% of total direct inputs) in the Colorado River. We also estimated direct allochthonous inputs for 46.8 km of the Colorado River prior to closure of Glen Canyon Dam using a vegetation map that was developed from historical photographs. Regulation has led to significant increases in riparian vegetation (270–319% increase in cover, depending on stage elevation), but annual allochthonous inputs appear unaffected by regulation because of the lower flood peaks on the post‐dam river. Published in 2010 by John Wiley & Sons, Ltd.     

The role of riparian vegetation and woody debris in the development of macroinvertebrate assemblages in streams

Riparian vegetation development and macroinvertebrate assemblages were studied in 16 streams formed between 35 and 230 years ago, following glacial recession in Glacier Bay National Park, southeast Alaska. Riparian vegetation established most rapidly in streams where flow variation in downstream reaches was buffered by a lake. Riparian vegetation development was positively correlated with lower bank stability, but was independent of stream age. Roots and branches of riparian vegetation trailing into streams (trailing riparian habitat—TRH) were shown to be an important habitat for a number of macroinvertebrate taxa. In young and unstable streams, TRH was colonized mainly by Plecoptera whereas in more stable lake‐influenced streams Simuliidae dominated. Significant coarse woody debris (CWD) accumulations were not observed until after approximately 130 years of stream development had occurred when certain channel features, such as gravel bars, were stabilized by dead wood. Where dead wood was present, opportunistic wood taxa were abundant, even in the younger streams. However, a xylophagous species, Polypedilum fallax, was not recorded until streams were over 100 years old. Two‐way indicator species analysis (TWINSPAN) using presence/absence of macroinvertebrate taxa on TRH, initially divided streams into lake and non‐lake systems, but subsequent divisions were consistent with differences in stream age. TWINSPAN of macroinvertebrate assemblages on dead wood again highlighted differences in stream age. Canonical correspondence analysis indicated that bed stability and stream age were the most important environmental variables influencing macroinvertebrate distribution on TRH. Trailing riparian habitat was most abundant in moderately unstable streams where it facilitates invertebrate colonization. CWD contributes markedly to channel stabilization, provides habitat for invertebrate xylophages, and confers additional habitat complexity. Maximum levels of CWD are predicted to occur in non‐lake streams after approximately 300 years, but at least a further 100 years will be required in stable streams below lakes where dead wood entrainment is not enhanced by flooding, channel migration and bank undercutting. A conceptual model summarizing the role of TRH and CWD on stream development in Glacier Bay is presented. Copyright © 2005 John Wiley & Sons, Ltd.     

ECOSYSTEM ENGINEERS MODULATE EXOTIC INVASIONS IN RIPARIAN PLANT COMMUNITIES BY MODIFYING HYDROGEOMORPHIC CONNECTIVITY

Patterns of native and exotic plant species richness and cover were examined in relation with ecosystem engineer effects of pioneer vegetation within the Mediterranean gravel bed river Tech, South France. The floristic composition was characterized according to two distinct vegetation types corresponding to two habitats with contrasted conditions: (i) open and exposed alluvial bars dominated by herbaceous communities; and (ii) islands and river margins disconnected from annual hydrogeomorphic disturbances and covered by woody vegetation. A significant positive correlation between exotic and native plant species richness and cover was observed for both vegetation types. However, significant differences in native and exotic species richness and cover were found between these two vegetation types. Higher values of total species richness and Shannon diversity were attained within the herbaceous vegetation type than within the woody type. These differences are most likely related to changes in local exposure to hydrogeomorphic disturbances driven by woody engineer plant species and to vegetation succession. A lower exotic species cover within the woody vegetation type than within the herbaceous type suggested an increase of resistance to invasion by exotic species during the biogeomorphic succession. The engineer effects of woody vegetation through landform construction resulted in a decrease of alpha (α) diversity at the patch scale but, in parallel, caused an increase in gamma (γ) diversity at the scale of the studied river segment. Our study corroborates recent investigations that support the theory of biotic acceptance of exotic species by native species at the local scale (generally <10 m²) within heterogeneous and disturbed environments. Furthermore, we suggest that in riparian contexts such as the River Tech exotic species trapp sediment at the same time as native species and thus contribute to the increase in ecosystem resistance during the biogeomorphic succession. Copyright © 2012 John Wiley & Sons, Ltd.     

Abundance of Invasive,Non‐Native Riparian Herbs in Relation to River Regulation

River regulation is associated with vegetation encroachment and invasions of some non‐native species in the semi‐arid west. Shifts in the abundance of native and non‐native woody riparian species are an interplay of regulation, life history traits and an array of flow and physical environmental variables. We sought to compare plant densities and per cent cover of several invasive species over two time periods in a paired river study, contrasting three different degrees of regulation along reaches of the Green and Yampa rivers in Colorado and Utah, USA. We censused patches of non‐native plants and recorded per cent cover in quadrats along 171 river km. The upper Green (10.1 patches ha^?1) had the highest invasive plant patch density followed by the lower Green (4.4 per ha) and the Yampa (3.3 per ha). Invasive species were present in 23%, 19% and 4% of sample quadrats, and an average of 0.28, 0.22 and 0.04 invasive species detected per square metre was recorded along the upper Green, lower Green and Yampa Rivers, respectively. Most species had significantly (p ≤ 0.02) higher percent cover on the upper Green than either or both the lower Green and the Yampa River. Whereas the less regulated river reaches maintain lower densities of invasive species than the most regulated reach, long‐term persistence of this pattern is still in question as some species patches showed notable increases on the Yampa and lower Green Rivers from 2002–2005 to 2010–2011. Although invasion is enhanced by flow regulation, life history traits of some species suggest invasion is likely, regardless of flow regulation. Published 2015. This article is a U.S. Government work and is in the public domain in the USA     

Instream flows and the decline of riparian cottonwoods along the Yakima River,Washington, USA

Under pre‐settlement conditions the Yakima River in Washington state, USA was characterized by multiple channels, complex aquifers and extensive riparian cottonwood forests. Subsequent implementation of headwater dams to supply irrigation water has altered river and floodplain processes critical to the cottonwoods and associated riparian vegetation. In this study, we analysed hydrology and floodplain forests and especially the dominant black cottonwoods (Populus trichocarpa) along sequential reaches of the Yakima River. Elevations were surveyed and vegetation inventoried along cross‐sectional belt transects, and cottonwood tree ring interpretations investigated historic associations between river hydrology and cottonwood establishment and growth. We analysed hydrographs relative to the apparent episodes of cottonwood recruitment and applied a quantitative model for seedling colonization that required: (1) floods, disturbance flows to produce barren nursery sites, and subsequent flows for seedling (2) establishment and (3) survival. In contrast to earlier conditions, flow patterns after the 1960s have generally been unfavourable for cottonwood recruitment although some cottonwood colonization has occurred in association with physical disturbance from gravel mining. With recent flow regimes, regulated flows along upper reaches maintain the river near bank‐full throughout the growing season, thus inundating suitable seedling recruitment sites. Downstream, irrigation withdrawals reduce the river stage, resulting in seedling establishment at low elevations that are lethally scoured by subsequent high flows. These regulated flow regimes have not hindered growth of established trees, but have reduced the recruitment of cottonwoods, and particularly disfavoured females, thus altering sex ratios and producing skewed cottonwood population age and gender structures. The cottonwood decline has also been associated with other changes in riparian plant community composition, including the encroachment of invasive weeds. Based on this ecohydrologic analysis we discuss flow adjustments that could rejuvenate cottonwood forests along the Yakima River. Copyright © 2007 John Wiley & Sons, Ltd.     

Selective use and spatial distribution of native and non‐native fish in wetland habitats

Wetland habitats are crucial for many fish species as spawning, feeding or nursery areas, but the major factors that govern their use by fish are poorly identified. In the present study, we aim to investigate the selective use and the spatial distribution of native and non‐native fish species in different types of wetland habitats (grasslands and reed beds) in a large freshwater marsh (North Western France). The selective use was measured by comparing the community that uses wetland habitats to the total community of the marsh (sampled in the permanent aquatic habitats (canals) during the low water period). The spatial distribution was studied by analyzing the presence probability of fish in wetland habitats as a function of the distance from adjacent canals. All sampled wetland habitats were occupied by fish, and the fish community in wetland habitats was dominated by three native (Abramis brama, Scardinius erythrophthalmus and Anguilla anguilla) and three non‐native (Ameiurus melas, Gambusia holbrooki and Lepomis gibbosus) species. Species richness and total fish abundance differed between canals and wetland habitats as a consequence of a variable propensity to use wetland habitats by native (avoidance and preference) and non‐native (no preference) species. Non‐native species were also more abundant in reed beds than in grassland while no differences were observed for native species. Universally, the presence probability of fish always decreased in wetland habitats as the distance from the canals increased and only a narrow area, close to canals (50–80 m), was well used by fish. However, non‐native fish species used over greater distances in reed beds than in grasslands while no differences were observed for native species. Variable interpretations related to species tolerance, reproductive guilds or diet are proposed to understand the mechanisms that might explain the widespread success of non‐native species in this spatially varying environment. Copyright © 2008 John Wiley & Sons, Ltd.     

Restoration of riparian vegetation on a mountain river degraded by historical mining and grazing

Riparian ecosystems in montane areas have been degraded by mining, streamflow alterations, and livestock grazing. Restoration of ecological and economic functions, especially in high-elevation watersheds that supply water to lower elevation urban and agriculture areas is of high priority. We investigated the response of riparian vegetation and bank stability following channel treatments and riparian habitat restoration along a segment of the upper Arkansas River south of Leadville, Colorado. The study area has been historically degraded by heavy-metal mining and is designated a U.S. Superfund site. Additionally, trans-basin water diversions and livestock grazing have contributed to channel widening and altered vegetation composition and cover. We used a before-after-control impact study design in four reaches with varied contamination and grazing history to assess restoration success. Before restoration, streambanks were dominated by graminoids and total vegetation cover varied among reaches with willow cover less than 16% in three reaches. Post-restoration, changes in total vegetation cover fell short of projected goals, but willow cover was greater than 20% in all study reaches. The increase in woody cover likely contributed to reduced erosion and vegetation encroachment post-restoration. Differences in functional group cover among reaches persisted post-restoration and may be attributed to soil contamination levels and low willow seed rain and dispersal. These results highlight the importance of setting realistic restoration goals based on elevation and past land use. We recommend further remediation of fluvial tailings with low vegetation cover and continued monitoring of willow height and cover to determine if further restoration activities are needed.     

Implications of flood pulse restoration for populus regeneration on the upper Missouri River

We developed a mass balance flow model to reconstruct unregulated daily peak flows in the National Wild and Scenic reach of the Missouri River, Montana. Results indicated that although the observed frequency of large peak flows has not changed in the post‐dam period, their magnitude has been reduced from 40 to 50% as a consequence of flow regulation. Reductions in the magnitude of these flows should reduce the expected frequency of large flood‐pulses over a longer time‐scale. Results of a two‐dimensional hydraulic model indicated that limited cottonwood (Populus deltoides subsp. Monilifera) recruitment occurs at relatively small peak discharges, but to maximize establishment of cottonwoods in the Wild and Scenic reach, a threshold of 1850 m³/s would be necessary at the Virgelle gauge. Floods of this magnitude or greater lead to establishment of cottonwood seedlings above the zone of frequent ice‐drive disturbance. Restoring the frequency, magnitude, duration and timing of these flood pulses would benefit important natural resource values including riparian cottonwood forests and native fish species in the upper Missouri River basin. However, efforts to naturalize flow must be made in the context of a water management system that was authorized and constructed for the primary purposes of flood control, power generation and irrigation. Using the synthesized flow model and flood damage curves, we examined six scenarios for delivering flows ≥1850 m³/s to the Wild and Scenic reach. Whereas some scenarios appeared to be politically and economically infeasible, our analysis suggested that there is enough operational flexibility in the system to restore more natural flood pulses without greatly compromising other values. Published in 2002 by John Wiley & Sons, Ltd.     

Vegetation development and restoration potential of drained reservoirs following dam removal in Wisconsin

Dam removal is potentially a powerful tool for river and riparian restoration. However, long‐term studies on the fate of former reservoirs do not exist, limiting assessment of the utility of dam removal as a means of riparian restoration. We took advantage of the decades‐long legacy of dam removals in Wisconsin to determine human uses of drained reservoirs and to evaluate vegetation establishment and species replacement at these sites. More than half of the 30 dam removal sites in southern Wisconsin over the past 47 years were used as commercial areas, parks and agricultural land, and active riparian restoration occurred on only two sites. For the 13 sites that were allowed to revegetate on their own, plants established in the first growing season and cover was very high at all sites in 2001. Species diversity and frequency (defined as percentage of sampled quadrats where a species is present) of trees were positively correlated with time since removal. No relationship existed between site age and frequencies of other growth forms, nor were there significant relationships between site age and the number or frequency of introduced species. However, mean frequency of introduced species was 75% per site and several sites were dominated by the introduced grass Phalaris arundinacea. Frequency of P. arundinacea was negatively correlated with number of native forbs, and lowest species diversity occurred on sites dominated by P. arundinacea. Ordination analyses revealed substantial site‐to‐site variation in vegetation that was weakly associated with gradients of site location, age, area, and soil phosphorus. Thus, temporal vegetation dynamics following dam removal were site‐specific. Rapid revegetation demonstrates the potential of these sites for riparian restoration. However, if dam removal is used as a means of restoring native riparian communities, then approaches must be tailored to individual sites and will need to focus on techniques to minimize establishment of aggressive invading species. Copyright © 2006 John Wiley & Sons, Ltd.     

Novel plant communities limit the effects of a managed flood to restore riparian forests along a large regulated river

Dam releases used to create downstream flows that mimic historic floods in timing, peak magnitude and recession rate are touted as key tools for restoring riparian vegetation on large regulated rivers. We analysed a flood on the 5th‐order Green River below Flaming Gorge Dam, Colorado, in a broad alluvial valley where Fremont cottonwood riparian forests have senesced and little recruitment has occurred since dam completion in 1962. The stable post dam flow regime triggered the development of novel riparian communities with dense herbaceous plant cover. We monitored cottonwood recruitment on landforms inundated by a managed flood equal in magnitude and timing to the average pre‐dam flood. To understand the potential for using managed floods as a riparian restoration tool, we implemented a controlled and replicated experiment to test the effects of artificially modified ground layer vegetation on cottonwood seedling establishment. Treatments to remove herbaceous vegetation and create bare ground included herbicide application (H), ploughing (P), and herbicide plus ploughing (H + P). Treatment improved seedling establishment. Initial seedling densities on treated areas were as much as 1200% higher than on neighbouring control (C) areas, but varied over three orders of magnitude among the five locations where manipulations were replicated. Only two replicates showed the expected seedling density rank of (H + P) > P > H> C. Few seedlings established in control plots and none survived 1 year. Seedling density was strongly affected by seed rain density. Herbivory affected growth and survivorship of recruits, and few survived nine growing seasons. Our results suggest that the novel plant communities are ecologically and geomorphically resistant to change. Managed flooding alone, using flows equal to the pre‐dam mean annual peak flood, is an ineffective riparian restoration tool where such ecosystem states are present and floods cannot create new habitat for seedling establishment. This problem significantly limits long‐term river and riparian management options. Copyright © 2010 John Wiley & Sons, Ltd.     

Simulated recruitment of riparian trees and shrubs under natural and regulated flow regimes on the Wisconsin River,USA

Models that link ecological responses to hydrologic changes are important for assessing the effects of flow regulation on aquatic and riparian ecosystems. Based on the Recruitment Box Model, a graphical model used to prescribe environmental flows for cottonwood (Populus spp.) recruitment, we designed a simulation model to represent the influence of river flow dynamics on seedling recruitment of riparian pioneer woody plants. The model simulates the influence of temporal patterns of river stage on dispersal, germination, initial recruitment and over‐winter survival of first‐year seedlings of riparian pioneer shrubs and trees. We used the model to simulate seedling recruitment patterns for five species (Acer saccharinum, Betula nigra, Populus deltoides, Salix nigra and Salix exigua) on the Wisconsin River (Wisconsin, USA) under three flow scenarios: historic (1935–2002), simulated natural (1915–1975) and simulated regulated flows (1915–1975). Simulation results agreed well with field‐observed relative differences among years (1997–2000) in seedling densities for the five focal species. Simulated successful recruitment years were highly synchronous among species, but species differed in their sensitivity to flows at different times during the growing season, consistent with among‐species differences in seed dispersal timing. Comparison of simulated natural and regulated flows for 1915–1975 showed that flow regulation decreased monthly flow variability, increased late summer to winter baseflow and reduced the magnitude of spring peaks. Simulated recruitment and over‐winter survival of tree seedlings of all species was enhanced under the regulated flow scenario, likely due to increased summer baseflow and reductions in peak flood magnitude. Our analyses show the utility of extending the Recruitment Box Model to include multiple species of riparian shrubs and trees, and the effects of post‐colonization flows on their recruitment success. However, some key functional relationships between flow patterns and woody seedling demography (e.g. shear stress thresholds for seedling mortality) have not been adequately quantified and merit further study. Copyright © 2006 John Wiley & Sons, Ltd.