DRIVERS OF RIPARIAN TREE INVASION ON A DESERT STREAM

Understanding mechanisms of exotic species' invasions is essential to managing riparian landscapes throughout the world. In the southwestern USA, the two most dominant invaders of riparian habitats are the exotic tree species tamarisk (Tamarix ramosissima, Tamarix chinensis, and their hybrids) and Russian olive (Elaeagnus angustifolia). These plants were introduced around 1900, and their success may be facilitated by river regulation, river channel changes, and precipitation patterns. We hypothesized that riparian invasion in Canyon de Chelly, Arizona, was initiated by a change point event such as plantings, dam construction, or channel incision and that establishment near a change point was tied to flood events. We aged tamarisk, Russian olive, and native cottonwood trees from study sites in Canyon de Chelly and used tree ring analysis to determine the year of establishment and the elevation of the germination point relative to the channel. We used Bayesian Poisson regression and information theoretics to identify change points and precipitation variables driving annual tree establishment. We found that most tamarisk and Russian olive trees established in the late 1980s, and most cottonwoods established in 1930‐1950 and 1980‐2000. Regression models indicated that change points occurred in 1983 for Russian olive and 1988 for tamarisk, and precipitation was important for establishment. Although plantings and river regulation probably played a role in tree invasion, our results suggest that these species required precipitation and stream channel change for widespread establishment in Canyon de Chelly. The factors driving riparian invasions may not be those often associated with degraded rivers, such as altered hydrographs and land management changes, thus requiring analyses of the full range of ecological and physical processes. Copyright © 2012 John Wiley & Sons, Ltd.     

Equilibrium response of riparian vegetation to flow regulation in the Platte river,Nebraska

The Platte River in central Nebraska responded to water development by rapid channel narrowing and expansion of native riparian woodland. Woodland expanded most rapidly in the 1930s and 1950s; open channel and woodland area stabilized in the 1960s and have remained stable for most reaches into the mid-1990s, despite relatively low flows and infrequent peak flows in the past decade. Open channel area may have been maintained or increased under recent lower flows because of increased erodibility of the floodplain as it has aggraded, developed vertical banks and as its woodland vegetation has become older, sparser and less protective of banks. One section of the Platte River, near Grand Island, has disequilibrated in the past decade by undergoing a 10% loss of channel area. The reach occurs below an area where vegetation has been removed to increase open channel area for migrating whooping and sandhill cranes and other water birds. Vegetation clearing may have liberated excess sediment, locally aggraded the channel and stimulated tree and shrub recruitment. This management practice needs to be examined before it is used more widely in the Platte River. © 1997 John Wiley & Sons, Ltd.     

Reptile community responses to native and non‐native riparian forests and disturbance along two rivers in Arizona

Aridland riparian forests are undergoing compositional changes in vegetation and wildlife communities due to altered hydrology. As flows have been modified, woody vegetation has shifted from native‐tree dominated to non‐native and shrub encroached habitats. Squamate vertebrates such as lizards and snakes are important food web links in riparian ecosystems of the Sonoran Desert. However, little is known about how these communities might respond as riparian forests transition from native tree dominated habitats to open xeroriparian woodlands. We used pitfall arrays deployed across three types of riparian forest to document reptile community patterns, measure vegetation, and produce species‐habitat models. Riparian forests differed on the basis of habitat composition and physiognomy. Two types, cottonwood‐willow (Populus‐Salix) and mesquite (Prosopis) stands, were characterized by high woody species richness. The third type, non‐native saltcedar (Tamarix) stands, had high densities of woody debris and greater canopy coverage. Results show that lizards were common and abundances greatest in cottonwood‐willow, especially for arboreal species. Species‐habitat models for three of five lizard species indicated a negative association to saltcedar‐invaded habitat and no species appeared to select saltcedar‐dominated habitat. Mesquite was an intermediate habitat between upland and riparian, and supports high species diversity. A wildfire in the cottonwood‐willow forest disproportionately affected abundance of ground‐foraging whiptail (Aspidoscelis) lizards; whereas, abundance of arboreal spiny (Sceloporus) species was unchanged. Expected drivers from climate and water use could transition cottonwood forests to other woody‐dominated types. Our results suggest that mesquite woodlands would provide higher quality habitat for riparian reptiles compared to non‐native saltcedar stands.     

Entrainment of riparian gravel and cobbles in an alluvial reach of a regulated canyon river

Many canyon rivers have channels and riparian zones composed of alluvial materials and these reaches, dominated by fluvial processes, are sensitive to alterations in streamflow regime. Prior to reservoir construction in the mid‐1960s, banks and bars in alluvial reaches of the Gunnison River in the Black Canyon National Monument, Colorado, USA, periodically were reworked and cleared of riparian vegetation by mainstem floods. Recent interest in maintaining near‐natural conditions in the Black Canyon using reservoir releases has created a need to estimate sediment‐entraining discharges for a variety of geomorphic surfaces composed of sediment ranging in size from gravel to small boulders. Sediment entrainment potential was studied at eight cross‐sections in an alluvial reach of the Gunnison River in the Black Canyon in 1994 and 1995. A one‐dimensional water‐surface profile model was used to estimate water‐surface elevations, flow depths, and hydraulic conditions on selected alluvial surfaces for discharges ranging from 57 to 570 m³/s. Onsite observations before and after a flood of 270 m³/s confirmed sediment entrainment on several surfaces inundated by the flood. Selective entrainment of all but the largest particle sizes on the surface occurred at some locations. Physical evidence of sediment entrainment, or absence of sediment entrainment, on inundated surfaces generally was consistent with critical shear stresses estimated with a dimensionless critical shear stress of 0.030. Sediment‐entrainment potential over a range of discharges was summarized by the ratio of the local boundary shear stress to the critical shear stress for d₅₀, given hydraulic geometry and sediment‐size characteristics. Differing entrainment potential for similar geomorphic surfaces indicates that estimation of minimum streamflow requirements based on sediment mobility is site‐specific and that there is no unique streamflow that will initiate movement of d₅₀ at every geomorphically similar location in the Black Canyon. Copyright © 2000 John Wiley & Sons, Ltd.     

Quantifying the effect of riparian forest versus agricultural vegetation on river meander migration rates,central Sacramento River,California, USA

Riparian forest vegetation is widely believed to protect riverbanks from erosion, but few studies have quantified the effect of riparian vegetation removal on rates of river channel migration. Measured historical changes in a river channel centreline, combined with mapped changes in floodplain vegetation, provide an opportunity to test how riparian vegetation cover affects the erodibility of riverbanks. We analysed meander migration patterns from 1896 to 1997 for the central reach of the Sacramento River between Red Bluff and Colusa, using channel planform and vegetation cover data compiled from maps and aerial photography. We used a numerical model of meander migration to back‐calculate local values for bank erodibility (i.e. the susceptibility of bank materials to erosion via lateral channel migration, normalized for variations in near‐bank flow velocities due to channel curvature). A comparison of migration rates for approximately 50 years before and after the construction of Shasta dam suggests that bank migration rates and erodibility increased roughly 50%, despite significant flow regulation, as riparian floodplains were progressively converted to agriculture. A comparison of migration rates and bank erodibilities between 1949 and 1997, for reaches bordered by riparian forest versus agriculture, shows that agricultural floodplains are 80 to 150% more erodible than riparian forest floodplains. An improved understanding of the effect of floodplain vegetation on river channel migration will aid efforts to predict future patterns of meander migration for different river management and restoration scenarios. Copyright © 2004 John Wiley & Sons, Ltd.     

Increasing River Flow Expands Riparian Habitat: Influences of Flow Augmentation on Channel Form,Riparian Vegetation and Birds Along the Little Bow River,Alberta

With river regulation, water withdrawal is common, reducing instream flows. The opposite alteration, flow augmentation, is less common and could reveal a mechanistic coordination between flow regime, channel form, and riparian ecosystems. The Little Bow River, a naturally intermittent prairie stream in Southern Alberta, has experienced flow augmentation since the late 1890s, and the Little Bow/Highwood Project of 2004 enabled a tripling of diversion flows from 2.9 to 8.5 m³/s. We investigated the subsequent responses by assessing the channel form and riparian vegetation based on aerial photographs taken in 2000 versus 2010, and riparian birds were assessed between 2005 and 2013 to investigate associations with riparian vegetation. Following recent flow augmentation, the mean channel width increased from 12.2 to 13.5 m, while sinuosity was relatively unchanged. Streamside zones with true willows (especially Salix exigua and Salix bebbiana) increased from 7 to 11% of the river corridor, and the facultative riparian wolf willow (Elaeagnus commutata) zones increased from 16 to 20%, while grassy zones decreased from 64 to 52%. Avian species richness and Shannon–Wiener index increased, while species evenness was relatively unaltered, suggesting an increase of rarer bird species in response to the increased habitat structure and diversity following the expansion of riparian shrubs and woodland. This study revealed responses to the recent flow augmentation over the first decade of implementation, and alterations following flow augmentation would likely continue for decades until the river and riparian zones adjust to the new flow regime. Copyright © 2016 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.     

Mortality of riparian box elder from sediment mobilization and extended inundation

To explore how high flows limit the streamward extent of riparian vegetation we quantified the effects of sediment mobilization and extended inundation on box elder (Acer negundo) saplings along the cobble‐bed Gunnison River in Black Canyon of the Gunnison National Monument, Colorado, USA. We counted and aged box elders in 144 plots of 37.2 m², and combined a hydraulic model with the hydrologic record to determine the maximum shear stress and number of growing‐season days inundated for each plot in each year of the record. We quantified the effects of the two mortality factors by calculating the extreme values survived during the lifetime of trees sampled in 1994 and by recounting box elders in the plots following a high flow in 1995. Both mortality factors can be modeled as threshold functions; box elders are killed either by inundation for more than 85 days during the growing season or by shear stress that exceeds the critical value for mobilization of the underlying sediment particles. Construction of upstream reservoirs in the 1960s and 1970s reduced the proportion of the canyon bottom annually cleared of box elders by high flows. Furthermore, because the dams decreased the magnitude of high flows more than their duration, flow regulation has decreased the importance of sediment mobilization relative to extended inundation. We use the threshold functions and cross‐section data to develop a response surface predicting the proportion of the canyon bottom cleared at any combination of flow magnitude and duration. This response surface allows vegetation removal to be incorporated into quantitative multi‐objective water management decisions. Copyright © 1999 John Wiley & Sons, Ltd.     

Post‐flooding distribution and characteristics of large woody debris piles along the semi‐arid Sabie River,South Africa

The formation of large woody debris (LWD) piles has a profound impact on channel patterns and riparian succession in temperate rivers. The opportunity to study LWD along the Sabie River, a river in the semi‐arid region of Kruger National Park, South Africa, arose in February 2000 after a significant flood (c. 100‐year return interval) removed a large proportion of the fully mature riparian forest and other plant communities. Much of the uprooted vegetation was deposited as LWD piles (woody vegetation accumulations deposited on the ground > 0.1 m³) throughout the riparian and upland zones. In this article we describe the spatial distribution patterns of LWD as related to geomorphic channel type and flood frequency zone, and assess pile composition characteristics six months after the flood. Within the areas surveyed there were 68 LWD piles per hectare, the median size of LWD piles was 4.6 m³ but pile sizes (by volume) varied widely. Pool/rapid geomorphic channel types had the highest density of LWD piles (79 ha^?1) and the largest piles (by volume) were in the bedrock anastomosing channels (mean = 124 m³). Piles were larger in the seasonal and ephemeral flood frequency zones (mean = 54 m³ and 55 m³) than piles in the active zone (c. 2 m³). The patterns of distribution and volume of LWD will affect the subsequent development of vegetation communities as debris piles form a mosaic of patches of surviving organisms and propagules that can strongly influence the initial trajectory of succession. The amount, distribution, and subsequent decomposition of LWD are different from that reported for temperate rivers, suggesting that the role of LWD may be different on non‐floodplain rivers such as the Sabie in semi‐arid South Africa. Copyright © 2004 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.     

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.     

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.     

A method for assessing channelization effects on riparian vegetation in a Mediterranean environment

This paper proposes a methodology to assess the effects of artificial embankments on riparian vegetation. It then tests the methodology on a mountain torrent system in Calabria, Southern Italy. The method delimits homogeneous reaches within which a sample of embanked and control (unembanked) sites is identified for study. Transects, subdivided into sample areas, are located in embanked and control sub‐reaches. At these transects, vegetation parameters (number of species, canopy cover of each species and vegetation layer, height of each vegetation layer, vegetation type, biological forms and ecological groups) are surveyed. Two new parameters (global canopy cover and weighted canopy height) are also proposed to give synthetic information on the global development of vegetation. A simple hydraulic index (cA^b/W) is used to explore differences in riparian vegetation both along the river and according to local narrowing caused by embanking. Application of this methodology to a torrent system (locally called ‘fiumara’) illustrates a clear influence of concrete embankments in the upper and middle torrent reaches. It demonstrates that torrent narrowing induces: a decrease in the number of vegetation types with a tendency to confine the types to generally lower evolution levels and with a less marked trend between the thalweg and embankment; an increase in annual species and decrease in perennial species; and also a lower global canopy cover with, sometimes, a reduction in the weighted canopy height (WCH). This example illustrates that the methodology provides useful information concerning the impact of existing control works, which can aid the design of new works and can inform environmentally sensitive restoration of Mediterranean water courses. Copyright © 2007 John Wiley & Sons, Ltd.     

Spatial Variation of Woody Riparian Vegetation across the Riverbank Topographic Gradient in Mediterranean Rivers: Species and Growth Categories

The composition and structure of riparian vegetation are linked to the natural hydrological variability and variation of environmental parameters in several spatial scales. The objective of this study is to determine the relationship between the spatial distribution of the woody riparian vegetation and the variation in the riverbank topographic gradient, verifying whether this variation was significant between species and/or growth categories. Specifically, our research examined the location of the woody species with respect to the thalweg along two reaches of the Jarama River in Central Spain. The positioning variables of each individual and distance and elevation above the thalweg were evaluated for four growth categories using statistical analysis. This study revealed that the positioning of the species along reaches is not random and differs with the species and growth categories. In addition, groups of species were specified in the different growth categories using similar positioning patterns with respect to the thalweg. Examples of similar groups of species in a specifically growth category were as follows: Alnus glutinosa–Salix alba–Salix fragilis for one reach and Alnus glutinosa–Populus nigra–Salix alba–Salix salviifolia for the other one. Topographical preference ranges of the riparian species and groups of the Jarama River were also obtained. The integration of data relative to the distribution of species along the topographic gradient can be very useful in identifying species with a preference for specific locations and can also contribute to the success of the measures adopted to restore these frequently highly degraded environments. Copyright © 2015 John Wiley & Sons, Ltd.     

Modelling effects of discharge on habitat quality and dispersal of juvenile humpback chub (Gila cypha) in the Colorado River,Grand Canyon

A two‐dimensional hydrodynamic model was applied to seven study reaches in the Colorado River within Grand Canyon to examine how operation of Glen Canyon Dam has affected availability of suitable shoreline habitat and dispersal of juvenile humpback chub (Gila cypha). Suitable shoreline habitat typically declined with increasing discharges above 226–425 m³/ s, although the response varied among modelled reaches and was strongly dependent on local morphology. The area of suitable shoreline habitat over cover types that are preferred by juvenile humpback chub, however, stayed constant, and in some reaches, actually increased with discharge. In general, changes in discharge caused by impoundment tended to decrease availability of suitable shoreline habitat from September to February, but increased habitat availability in spring (May–June). Hourly variation in discharge from Glen Canyon Dam substantially reduced the amount of persistent shoreline habitat at all reaches. Changes in suitable shoreline habitat with discharge were shown to potentially bias historical catch per unit effort indices of native fish abundance up to fourfold. Physical retention of randomly placed particles simulating the movement of juvenile humpback chub in the study reaches tended to decline with increasing discharge, but the pattern varied considerably due to differences in the local morphology among reaches and the type of swimming behaviour modelled. Implications of these results to current hypotheses about the effects of Glen Canyon Dam on juvenile humpback chub survival in the mainstem Colorado River are discussed. Copyright © 2004 John Wiley & Sons, Ltd.     

Soil seed banks of degraded riparian zones in southeastern Australia and their potential contribution to the restoration of understorey vegetation

Although soil seed banks are understood to be integral to the vegetation dynamics and restoration of many ecosystems, little is known of their role in riparian zones. In this study, we investigated soil seed banks of riparian zones of contrasting condition in an agricultural landscape and evaluated their potential to influence riparian restoration. We examined the composition and structure of germinable soil seed banks along lateral gradients from stream channels in both cleared and wooded riparian zones of three lowland creeks within the Goulburn Broken catchment in temperate southeastern Australia. Environmental correlates of soil seed bank characteristics and similarity to extant vegetation were also examined. We found an abundant and species‐rich soil seed bank mostly comprising propagules of perennial rushes and sedges and annual and perennial grasses with many species of annual forbs. While the majority of identifiable germinants and species were native, exotic species were common at all locations. Soil seed bank composition was relatively homogeneous among streams and along lateral gradients from the channel. Riparian condition (i.e. cleared or wooded), however, had a strong influence on soil seed bank composition and structure with cleared reaches containing more species, more germinable annual grasses and higher total numbers of germinable seeds. Soil seed bank composition was correlated with site openness suggesting that extant vegetation structure plays an important role in soil seed bank dynamics. Recruitment from the in situ soil seed bank will help restore only some components of the riparian plant community and may hinder restoration by introducing undesirable species. Copyright © 2008 John Wiley & Sons, Ltd.     

POTENTIAL FOR VEGETATION‐BASED RIVER MANAGEMENT IN DRYLAND,SALINE CATCHMENTS

The approaches used to manage rivers have been developed and adapted to many different problems and settings. Because of their relatively low cost, vegetation‐based approaches implemented at the reach, landholder and catchment scales have become the foundation for river management in most landscapes. In many dryland agricultural catchments, secondary (anthropogenic) salinity caused by clearing native vegetation has resulted in rising saline groundwater, streamflow salinity values that exceed seawater and severe the degradation of riparian vegetation communities. The potential effectiveness of vegetation‐based strategies in these landscapes remains largely unknown, yet these strategies are still widely pursued. This study initially investigated the patterns of vegetation recovery and recolonization following a large flood in a saline river that disturbed the system. A conceptual model was developed to describe spatial patterns of where different vegetation types had regrown and recovered in relation to controls exerted by streamflow salinity, surface texture characteristics, topography and reach morphology. Using this model, vegetation‐based river management options for different reaches were developed, and their potential effectiveness in stabilizing reaches was investigated using a 1‐D hydraulic modelling approach. This study finds that in a dryland catchment with high stream salinity (20 000–93 000 mg L^?1), there is still a strong potential for successful vegetation‐based management, but only in selected reaches. Results showed that changes in stream power and channel velocity were not associated with the areas of most severe vegetation degradation. Rather, there is a complex interplay between channel morphology, channel slope and places of potential vegetation growth within a reach. This paper outlines an approach to evaluate the potential success of vegetation‐based river management in saline landscapes. It identifies the need to prioritize investment based on the following: an understanding of factors controlling revegetation potential, the likely impact of revegetation in mitigating adverse channel changes and the proximity of reaches to high‐value infrastructure and biodiversity assets. Copyright © 2011 John Wiley & Sons, Ltd.     

GEOMORPHOLOGICAL CONTROLS ON VEGETATION RESPONSES TO FLOW ALTERATIONS IN A MEDITERRANEAN STREAM (CENTRAL‐WESTERN SPAIN)

The expected recovery of the natural conditions of large regulated rivers over the distance downstream from a dam is limited by relative tributary size according to the Serial Discontinuity Concept; however, geomorphology may also influence the recovery process. We examined the woody vegetation of the riparian zone in seven river segments distributed along the regulated reach of the Tiétar River in central‐western Spain, which flows through two distinct geomorphic templates. Whereas the annual runoff has decreased by 30% on average along the entire studied reach following the construction of the Rosarito Dam and the initiation of field irrigation in the region, the magnitude and frequency of the peak flows decreased by 30% immediately downstream from the dam but recovered the natural values with the distance downstream. We evaluated the recovery patterns toward the natural riparian conditions by comparing woody species composition, diversity and distribution of vegetation patches established prior to and after dam completion. Our results did not indicate a recovery gradient of any of the analysed vegetation attributes downstream from the dam. Instead, we found that the difference in the slope of the stream channel and banks, the width of the valley and the size of substratum particles among the surveyed patches were factors that significantly mediated dam and tributary effects on vegetation and influenced the degree of vegetation recovery. Hence, the maintenance of the intensity of the flow alteration scheme by the numerous water withdrawals and the low tributary contributions, coupled with differential geomorphological characteristics along the reach, overwhelmed the natural tendency for the river to restore its natural conditions with distance downstream. Improving water management and, particularly, restoring endangered riparian ecosystems require a detailed understanding of existing and potential woody species behaviour across the geomorphological settings of rivers. Copyright © 2012 John Wiley & Sons, Ltd.     

Changes in riparian vegetation communities below a large dam in a monsoonal region: Futase Dam,Japan

This paper examines the effects of a dam reservoir flood control on riparian plant communities in an Asian monsoon area subjected to large summer floods under natural conditions. We compared riparian plant communities downstream of the Futase Dam located on the Arakawa River, with communities on the Nakatsu River, a tributary of the Arakawa River with no dam. Historical changes in spatial distribution of riparian vegetation communities were analysed using aerial photographs. Stand ages and flood history were also examined to analyse the likelihood of tree erosion in scenarios with and without flood control. Comparison of the two reaches shows that the reduced flood peaks below the Futase Dam led to the expansion of riparian forest area and an increase in species diversity. Reduction in the flood peaks below the dam means that the toppling moment of the trees is no longer exceeded. The result is that the riparian forest below the dam is able to develop into later successional stages. Copyright © 2004 John Wiley & Sons, Ltd.