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.     

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.     

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.     

Effects of stream flow intermittency on riparian vegetation of a semiarid region river (San Pedro River,Arizona)

The San Pedro River in the southwestern United States retains a natural flood regime and has several reaches with perennial stream flow and shallow ground water. However, much of the river flows intermittently. Urbanization‐linked declines in regional ground‐water levels have raised concerns over the future status of the riverine ecosystem in some parts of the river, while restoration‐linked decreases in agricultural ground‐water pumping are expected to increase stream flows in other parts. This study describes the response of the streamside herbaceous vegetation to changes in stream flow permanence. During the early summer dry season, streamside herbaceous cover and species richness declined continuously across spatial gradients of flow permanence, and composition shifted from hydric to mesic species at sites with more intermittent flow. Hydrologic threshold values were evident for one plant functional group: Schoenoplectus acutus, Juncus torreyi, and other hydric riparian plants declined sharply in cover with loss of perennial stream flow. In contrast, cover of mesic riparian perennials (including Cynodon dactylon, an introduced species) increased at sites with intermittent flow. Patterns of hydric and mesic riparian annuals varied by season: in the early summer dry season their cover declined continuously as flow became more intermittent, while in the late summer wet season their cover increased as the flow became more intermittent. Periodic drought at the intermittent sites may increase opportunities for establishment of these annuals during the monsoonal flood season. During the late summer flood season, stream flow was present at most sites, and fewer vegetation traits were correlated with flow permanence; cover and richness were correlated with other environmental factors including site elevation and substrate nitrate level and particle size. Although perennial‐flow and intermittent‐flow sites support different streamside plant communities, all of the plant functional groups are abundant at perennial‐flow sites when viewing the ecosystem at broader spatial and temporal scales: mesic riparian perennials are common in the floodplain zone adjacent to the river channel and late‐summer hydric and mesic annuals are periodically abundant after large floods. Copyright © 2005 John Wiley & Sons, Ltd.     

Riparian Vegetation Communities of the American Pacific Northwest are Tied to Multi‐Scale Environmental Filters

Riparia surrounding low‐order streams are dynamic environments that often support distinct biodiversity. Because of their connection to nearby uplands, riparian vegetation communities at these streams respond to many environmental filters—climatic, physical, chemical or biotic factors—that restrict what species can occur at a given location from within larger regional species pools. In this study, we examined how environmental filters originating at the landscape, watershed and reach scales correspond to riparian plant community composition across the interior Columbia and upper Missouri River basins, USA. We correlated riparian vegetation to environmental filters, identified unique communities and partitioned the variance within riparian vegetation data among filters originating at different scales. Riparian vegetation composition was strongly correlated to landscape‐scale filters including elevation, precipitation and temperature. Watershed‐scale filters such as grazing and reach filters indicative of fluvial setting were also correlated to vegetation composition, often differentiating communities with similar landscape settings. We identified 10 distinct vegetation communities. Forested communities occurred at higher elevation, moderate gradient reaches with high mean annual precipitation. Shrub–forb systems corresponded to fluvial and watershed disturbances and occurred within climates that could preclude forest establishment. Meadows corresponded to high water tables and/or high grazing activity. Variance partitioning showed that landscape‐scale filters explained the most variance within vegetation communities. Global change will alter many of the environmental filters that drive vegetation. Vegetation change may occur rapidly if local filters (e.g. fluvial process) change rapidly or may occur more slowly if larger‐order filters (e.g. climate) change slowly and without influencing local hydrogeomorphic filters. By identifying filter–vegetation relationships at large spatial scales, hypotheses can be constructed on how riparian vegetation communities may change under future environmental conditions. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.     

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.     

An evaluation of different forest cover geospatial data for riparian shading and river temperature modelling

Riparian tree planting is increasingly being used as a strategy to shade river corridors and offset the impact of climate change on river temperature. Because the circumstances under which tree planting generates the greatest impact are still largely unknown, researchers are increasingly using process‐based models to simulate the impacts of tree planting (or felling) on river temperature. However, the high‐resolution data on existing riparian tree cover needed to parameterise these models can be difficult to obtain, especially in data‐sparse areas. In this paper, we compare the performance of a river temperature model parameterised with a range of different tree cover datasets, to assess whether tree cover data extracted from readily available GIS databases or coarser (i.e., 2–5 m) digital elevation products are able to generate river temperature simulations approaching the accuracy of higher resolution structure from motion (SfM) or LiDAR. Our results show that model performance for simulations incorporating these data is generally degraded in relation to LiDAR/SfM inputs and that tree cover data from “alternative” sources can lead to unexpected temperature model outcomes. We subsequently use our model to simulate the addition/removal of riparian tree cover from alongside the river channel. Simulations indicate that the vast majority of the “shading effect” is generated by tree cover within the 5‐m zone immediately adjacent to the river channel, a key finding with regards to developing efficient riparian tree planting strategies. These results further emphasise the importance of incorporating the highest possible resolution tree cover data when running tree planting/clearcutting scenario simulations.     

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.     

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.     

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.     

Defining and Testing Targets for the Recovery of Tropical Streams Based on Macroinvertebrate Communities and Abiotic Conditions

Here, we set target values to measure the ecological improvement of streams, based on invertebrate communities, riparian vegetation, instream habitat conditions and water chemistry. The study area is a large tropical catchment (Rio das Velhas, Minas Gerais, Brazil) affected by pastures, mining areas and a large urbanized area but also includes natural protected areas. Two stream types were found in the catchment, based on stream size, elevation, climate and geology with significantly different macroinvertebrate communities. In spite of a marked wet/dry seasons' climatic pattern, that does not result in the segregation of communities. Four classes of global degradation (IV—bad to I—good condition) were defined based on the available abiotic information, corresponding to a gradient in structure and biotic metrics of macroinvertebrate communities, matching the current knowledge on taxa sensitivity to pollution and general disturbance. Class I corresponds to target conditions to be achieved under restoration programmes. Using this approach, we were able to detect an improvement of abiotic conditions in four urban streams that benefited from enhancement measures in 2007–2008. However, invertebrate communities improved clearly in only one site (biotic metrics and community structure). Our study highlighted that good water quality alone is not enough and that only the combined effect of water quality, riparian vegetation and instream habitat condition enhancement resulted in the improvement of invertebrate communities. An important limiting factor for macroinvertebrate communities' recovery may be the distance to source populations. We concluded that the combined use of biological and abiotic target values for measuring the recovery of streams is needed to fully achieve an ecological restoration. This approach can also be valuable in the regular monitoring of streams to assess stream degradation. Target values based on other biological elements, such as fishes and algae, and functional processes could also contribute to define more global and realistic goals. Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

Changes to herbaceous plant communities on a regulated desert river

Riparian plant communities are shaped by gradients of disturbance intensity and frequency and resource availability. Reservoir operation can alter the composition and abundance of riparian vegetation by changing the flood regime and by trapping fine sediments and associated nutrients within the reservoir system. We examined differences in herbaceous species richness, abundance and composition in Populus‐Salix stands along an unregulated and regulated reach of a river in semi‐arid Arizona, contrasted flood inundation frequency and edaphic conditions (soil moisture, nutrients and texture) between the reaches, and interpreted the vegetation differences in light of observed differences in environmental conditions. Flooding frequency was similar between reaches, but the proportion of fine textured soils in the unregulated reach was nearly double that of the regulated reach and soil nutrient levels were up to three times higher in the unregulated reach. Herbaceous cover and richness were consistently lower in the regulated reach, with between‐reach differences greatest during dry seasons. These patterns suggest that an edaphic‐based change in resource availability is the principal pathway by which river damming is altering herbaceous vegetation in this system. Our results demonstrate that sediment transport within riparian corridors is important for maintenance of herbaceous communities and that restoration of flow regimes alone may be insufficient to restore herbaceous flora on some regulated reaches. Copyright © 2008 John Wiley & Sons, Ltd.     

Application and assessment of a dynamic riparian vegetation model to predict the spatial distribution of vegetation in two Japanese river systems

Riparian ecosystems which are highly sensitive for hydrological processes are being altered worldwide, largely because of anthropogenic activities that include the construction of dams, the afforestation of river basins and the mining of river gravel. During last 60 years, many of the Japanese rivers have been severely forested, and the gravel habitats that were dominant before the 1950s no longer exist. The restoration of these habitats to their former condition to conserve the diversity associated with undisturbed gravel habitats is a growing concern. Therefore, the influences that alter these riparian habitats must be recognized, and the development of a management tool to predict possible changes and evaluate causes and effects is emerged. In this context the numerical models used to describe the riparian vegetation dynamics is important. Therefore, the objective of this study was to evaluate the application of a recently developed dynamic riparian model (DRIPVEM) under Japanese river conditions to predict the spatial distribution of riparian vegetation. The model simulation was conducted for two Japanese river reaches, and the compatibility of the model was verified with observations on the distribution of vegetation. Based on these visual observations, the simulated results showed reasonable agreement with the observations. Furthermore, the compatibility of the model was evaluated with the help of a GIS tool. The model compatibility was greater than 70% for the simulation of both tree and herb distributions; however, for the bare areas, the compatibility was approximately 50%. The accuracy of the model was greater than 0.85 and based on the Kappa coefficient of agreement, the performance of the model was moderate to substantial for the conditions of the two river systems. Although mimicking complex ecosystem processes is a challenging task for any type of dynamic model; the DRIPVEM has great potential to predict the distribution of riparian vegetation.     

The effect of the shortage of gravel sediment in midstream river channels on riparian vegetation cover

The vegetation cover in the steep, gravelly reaches of rivers was originally low. However, significant vegetation colonization can currently be observed in these riparian areas, and understanding the cause of this colonization is important for management. The objective of this study was to assess the effect of sediment deposition and erosion on vegetation colonization in gravel bed rivers. The delay in colonization by herbs and trees after sediment deposition or erosion was investigated using aerial photos of 6 Japanese rivers. A field study was conducted before and after a large flood at depositional and erosional locations. The colonization of vegetation after flushing was substantially delayed at sites where gravels were deposited compared with that at erosional sites, and it was faster in sandy reaches compared with gravelly reaches. Little tree colonization was observed at the depositional sites of gravelly sediments, whereas at erosional sites, new shoots sprouted from the collapsed live trees in the following spring, achieving a rapid increase in tree density. The nutrient and moisture contents of the sediment were significantly higher at the erosional sites. Gravels are deposited after washing and being segregated from fine sediment during floods. This coarse‐sediment layer is low in moisture and nutrients compared with the erosional sites, at which the underlying sediments are exposed by flooding. However, moisture and nutrients are the primary requirements for vegetation colonization. Therefore, the reduction in gravelly sediments due to gravel mining, river regulation, and modification of river basins can have a substantial effect on vegetation colonization.     

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.     

水电开发影响下河岸带植被结构和生物量分布

为了解水电开发不同阶段对河岸带植被的影响差异,在澜沧江中下游小湾电站、漫湾库区、漫湾坝下、大朝山库区、大朝山坝下、景洪电站河岸带设置6个样带进行植被结构指标和生物量空间分布特征调查分析。结果表明:澜沧江中下游河岸带植被以乔木为主体,乔木层结构指标以已建库区和坝下样带较高,灌木层和草本层结构指标以在建电站和坝下样带较高;已建库区、坝下和在建电站样带乔木层和群落生物量的最高值分别出现在距离河岸100,50 m和200 m处;乔木层生物量以库区样带最高,灌木层生物量以坝下样带最高,草本层生物量以在建电站样带最高;库区和坝下样带群落生物量大致相当,高于在建电站样带;漫湾库区及其坝下乔木层结构指标以及群落生物量相对变化幅度小于大朝山库区及其坝下样带,小湾样带小于景洪样带。研究区植被结构和生物量的分布格局反映了库区植被恢复程度的差异和电站施工影响强度的差异。     

红柳根系对塔里木河岸坡土体起动规律影响研究

为了研究红柳根系对塔里木河岸坡土体起动的影响规律,对不同植被根系体积密度及不同的根系分布方式下的根-土复合体进行起动特性试验,并分析植被根系影响下起动切应力变化规律,得到了根系影响下河岸土体起动切应力与植被根系体积密度及土体干密度的定量关系式。试验结果表明:不同根系放置形式土体抗水流冲刷的由大到小分别为相交放置,竖直放置,倾斜放置。当有红柳根系覆盖河岸土体时,土体的起动切应力得到了明显提高。     

Sediment storage and shallow groundwater response to beaver dam analogues in the Colorado Front Range,USA

Enthusiasm for using beaver dam analogues (BDAs) to restore incised channels and riparian corridors has been increasing. BDAs are expected to create a similar channel response to natural beaver dams by causing channel bed aggradation and overbank flow, which subsequently raise water tables and support vegetation growth. However, lack of funding for monitoring projects post‐restoration has limited research on whether BDAs actually cause expected channel change in the Front Range and elsewhere. Geomorphic and hydrologic response to BDAs was monitored in two watersheds 1 year post‐restoration. BDAs were studied at Fish Creek, a steep mountainous catchment, and Campbell Creek, a lower gradient piedmont catchment from May to October 2018. At each restoration site, the upstream‐ and downstream‐most BDAs were chosen for intensive study in comparison with unrestored reference reaches. Monitoring focused on quantifying sediment volumes in BDA ponds and recording changes to stream stage and riparian groundwater. Despite differences in physical basin characteristics, BDA pools at both sites stored similar volumes of sediment and stored more sediment than reference pools. Sediment storage is positively correlated to BDA height and pool surface area. However, BDAs did not have a significant influence on shallow groundwater. The lack of groundwater response proximal to BDAs could indicate that local watershed factors have a stronger influence on groundwater response than restoration design 1 year post‐restoration. Systematic, long‐term studies of channel and floodplain response to BDAs are needed to better understand how BDAs will influence geomorphology and hydrology.     

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.