Response Of Riparian Vegetation Research Articles

Responses of Typical Riparian Vegetation to Annual Variation of River Flow in a Semi-Arid Climate Region: Case Study of China’s Xiliao River

Responses of Typical Riparian Vegetation to Annual Variation of River Flow in a Semi-Arid Climate Region: Case Study of China’s Xiliao River

Mapping and Assessing Riparian Vegetation Response to Drought along the Buffalo River Catchment in the Eastern Cape Province, South Africa

The increasing drought frequency poses a significant threat to global and regional river systems and ecosystem functioning, especially in the complex topographical Buffalo River catchment area of the Eastern Cape Province, South Africa. This study explored the impact of drought on riparian vegetation dynamics using the Normalize Difference Vegetation Index (NDVI), Transformed Difference Vegetation Index (TDVI) and Modified Normalized Difference Water Index (MNDWI) from satellite-derived Landsat data from 1990 to 2020. The least-squares linear regression and Pearson’s correlation coefficient were used to evaluate the long-term drought in riparian vegetation cover and the role of precipitation and streamflow. The correlation results revealed a moderate positive correlation (r = 0.77) between precipitation and streamflow with a significant p-value of 0.04 suggesting consequences on riparian vegetation health. Concurrent with the precipitation, the vegetation trends showed that precipitation increased insignificantly with less of an influence while the reverse was the case with the streamflow in the long term. The results show that the NDVI and TDVI were significant indices for detecting water-stressed vegetation in river catchment dynamics. Much of these changes were reflected for MNDWI in dry areas with a higher accuracy (87.47%) and dense vegetation in the upper catchment areas. The standardized precipitation index (SPI) revealed the inter-annual and inter-seasonal variations in drought-stressed years between 1991–1996, 2000–2004, 2009–2010, 2015, and 2018–2019, while 2020 exhibited slight sensitivity to drought. The findings of this study underscore the need for heightened efforts on catchment-scale drought awareness for policy development, programs, and practices towards ecosystem-based adaptation.

Impacts of the Three Gorges Dam on riparian vegetation in the Yangtze River Basin under climate change

As the largest hydroelectric project in the world, the Three Gorges Dam (TGD) is expected to have significant environmental and ecological impacts on riparian vegetation in the Yangtze River Basin (YRB). However, existing studies have mainly focused on small segments of the YRB. In addition, few studies have quantified the responses of riparian vegetation to both climatic factors and dam construction. In this study, we investigated riparian vegetation dynamics over the entire YRB before, during, and after the construction of TGD from 1982 to 2015 using the normalized difference vegetation index (NDVI). Furthermore, the effects of climatic factors and dam construction on riparian vegetation were quantitatively analyzed using path analysis. The results demonstrate that the YRB has experienced a generally greening trend after TGD construction. The impacts of climate change on riparian vegetation have exhibited notable spatial heterogeneity and temperature is the main climatic factor that affects riparian vegetation growth. Moreover, TGD becomes the major contributor to riparian vegetation dynamics in the YRB after TGD construction. TGD has not only directly enhanced riparian vegetation but also indirectly affected riparian vegetation by regulating the microclimate. This study highlights the significance of anthropogenic interference when evaluating the relationships between riparian vegetation and climatic factors, providing useful insights for the effective management and conservation of large-scale riparian ecosystems.

Determining the response of riparian vegetation and river morphology to drought using Google Earth Engine and machine learning

Riparian vegetation composition and channel morphology are susceptible to long-term alterations caused by external stressors, including climate-change-induced droughts and engineered infrastructures. The objectives of this study were to (1) quantify trends in riparian vegetation and channel/floodplain morphology over large spatial (∼290 km) and temporal scales (∼30 years) and (2) investigate the relationships between hydroclimatic drivers and changes in riparian vegetation and channel morphology. We implemented a random forest classifier via a machine learning technique in Google Earth Engine. The study area was a 290 km reach of the Rio Grande located in New Mexico, USA. We used the combination of remotely sensed data and products (e.g., Landsat imagery, Normalized Difference Vegetation Index (NDVI), and land cover) to characterize vegetation, vegetation cover changes, and river morphology shifts from 1984 to 2020. The trend analysis revealed increased vegetated areas and NDVI (0.0004/yr) during long-term drought. The channel experienced a reduction in width associated with vegetation encroachment and the formation of stable vegetated islands. The streamflow hydrograph characteristics were positively correlated with vegetation cover and channel morphology. Our study contributes novel insights into the long-term riparian ecosystem dynamics under drought stress, informing drought impact mitigation and ecosystem management in arid and semi-arid regions.

Resilience of riparian vegetation productivity to early 21st century drought in northern California, USA

AbstractDrought and intensive land use can interact as stressors on riparian vegetation, especially along rivers flowing through seasonally dry landscapes. Knowledge of past riparian vegetation response to drought and land use change can provide land managers with a better understanding of changes induced by upstream management actions, climate change, and chronic stressors. To investigate the response of riparian vegetation productivity to drought and land use, we developed a 21‐year time series (2000–2020) of growing season vegetation dynamics using near‐infrared reflectance of vegetation (NIRV) derived from satellite data across 30 watershed subbasins that drain into the San Francisco Bay Delta in central California, USA. We observed a strong response of riparian vegetation to drought, but rapid recovery and very few long‐term declines in productivity. At a local level, vegetation communities' response to drought and post‐drought productivity dynamics were highly variable across biophysical settings and land use gradients. Most of the riparian areas with long‐term declines in NIRV were located in the lower elevation Coast Range on the western side of the study area where there is little to no water engineering or agricultural irrigation runoff to subsidize riparian vegetation. Riparian areas with the greatest long‐term increase were along rivers draining the higher elevation Sierra Nevada range to the east. Our results suggest that river systems with a high proportion of water originating as snowmelt may be more buffered against long‐term drought‐driven declines in productivity than those dependent exclusively on winter rainfall. The long‐term increase in NIRV in the vast majority of riparian areas within our study area may also have been driven in part by increasing atmospheric CO2 concentrations, which have been shown to increase plant water use efficiency.

Riparian vegetation response amid variable climate conditions across the Upper Gila River watershed: informing Tribal restoration priorities

Riparian systems across the Southwest United States are extremely valuable for the human and ecological communities that engage with them. However, they have experienced substantial changes and stresses over the past century, including non-native vegetation expansion, vegetation die-offs, and increased fire activity. Vegetation management approaches, such as ecological restoration, may address some of these stressors as well as reduce the risk of future impacts. We apply remote sensing to inform restoration priorities along the upper Gila River within the San Carlos Apache Reservation and Upper Gila River watershed. First, we develop a spatially and temporally explicit trend analysis across three observed climate periods (1985–1993, 1993–2014, 2014–2021) using the Landsat-derived indices to quantify changes in riparian vegetation conditions. These maps can be used to identify areas potentially more at risk for degradation. Second, we analyze changes in riparian vegetation within a climate framework to better understand trends and the potential effect of climate change. Vegetation greenness has largely increased throughout the watershed despite intensifying drought conditions across our study period, though areas within the lower watershed have shown increased stress and higher rates of wildfire and other disturbances over the past 5-years. Nevertheless, small-scale restoration activities appear to show improving vegetation conditions, suggesting efficacy of these restoration activities. Results from this study may be integrated with restoration objectives to develop a restoration plan that will help riparian vegetation communities adapt to change.

Response of riparian vegetation to short- and long-term hydrologic variation.

Increasing demand for river water now conflicts with an increasing desire to maintain riparian ecosystems. Efficiently managing river flows for riparian vegetation requires an understanding of the time scale of flow effects, but this information is limited by the absence of long-term studies of vegetation change in response to flow variation. To investigate the influence of short- and long-term flow variability and dam operation on riparian vegetation, we determined the occurrence of 107 plant species in 133 permanent plots of known inundating discharge along the Gunnison River in Colorado on five different occasions between 1990 and 2013. Individual species moved up and down the gradient of inundating discharge coincident with increases and decreases in mean annual flow, and the correlations between flow and species occurrence were strongest when flows were weighted by time before vegetation sampling with a median half-life of 1.5 years. Some tall, rhizomatous, perennial species, however, responded to flows on a longer time scale. Logistic regression of species occurrence showed a significant relation with inundation duration for 70 out of 107 species. Plot species richness and total vegetative cover decreased in association with desiccation at low inundation durations and with fluvial disturbance at high inundation durations. Within-plot similarity in species occurrence between years decreased strongly with increasing inundation duration. Moderate inundation durations were dominated by tall, rhizomatous, perennial herbs, including invasive Phalaris arundinacea (reed canary grass). Over the 23-year study period, species richness declined, and the proportion of rhizomatous perennials increased, consistent with the hypothesis that decreases in flow peaks and increases in low flows caused by flow regulation have decreased establishment opportunities for disturbance-dependent species. In summary, annual-scale changes in vegetation were strongly influenced by flow variation, and decadal-scale changes were influenced by decreases in fluvial disturbance from upstream flow regulation beginning decades prior to the onset of this study.

Ecosystem services (ES) provided by ditches in a desert agricultural valley

Agricultural ditches can be an efficient method for removing nutrients in wetlands and rivers. In this paper, we document that additionally, ditches offer other ecosystem services (ES) such as providing bird habitat and sediment retention. A typical ditch reach of 6 km with variable width between 3 and 80 m and water depth from 0.06 and 0.5 m, was selected as a monitoring site among 1492 km of a drain network in the agricultural Mexicali Valley located in the Colorado River Delta. Nitrogen and phosphorus concentration and discharge flow rate determinations were obtained monthly during a year, at three stations along the ditch, dividing the reach into two segments. Total nutrient removal efficiency of 39%, along the monitoring year was observed. Additionally, nutrient mass flow had a significant winter-spring seasonal increment associated with crop irrigation, up four times for total nitrogen, nearly 2.5 times for nitrogen-based nutrients, and five times for PO4. Monthly estimations of normalized difference vegetation index (NDVI) from Sentinel images were obtained to evaluate the response of riparian vegetation to high flow discharge seasonal fluctuations, from 0.072 to 1.219 m3 s−1, and nutrient mass flow changes. No correlation among them was found, as riparian species are tolerant to high water levels and nutrient concentration fluctuation. Downstream changes in water electrical conductivity from 6.06 to 8.45 mS cm−1 along the resulted in the shift from Typha domingensis to Phragmatis australis which has a higher salinity tolerance. An intensive bird census showed ditch vegetation provided suitable habitat and refuge to 29 different bird species, including five common birds of the flooded remnant areas of the Colorado River Delta, as well as migratory birds. By using slope criteria of drainage network, 47% of it has a high and moderated potential of sediment retention. These ES are fundamental in semiarid regions because of the intense land-use changes from riparian and desert ecosystems to irrigation lands, as well as water being diverted from natural streams to irrigation channels. This study highlights the importance of considering agricultural ditches as helpful, not only as a natural water treatment of agricultural pollutants, but also as providers of resident and migratory bird habitat and vegetation diversity and erosion regulation by sediment retention on desert agricultural valleys.

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

AbstractRiparian 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.

Field evidence of riparian vegetation response to groundwater levels in a gravel‐bed river

AbstractVegetation establishment, growth and succession in riparian ecosystems are linked to river flow dynamics and groundwater table fluctuations. This is especially true in Alpine gravel‐bed rivers with wide floodplains, geomorphically active floods and a strong river‐aquifer exchange. The role of short‐term groundwater fluctuations is not always clear in these ecosystems, as it is assumed that phreatophytic vegetation close to rivers is adapted to such conditions. Here, we provide data evidence of riparian plant response to short‐term groundwater table fluctuations in a braided gravel‐bed river (Maggia). We used indirect physiological variables for photosynthesis and transpiration—stomatal conductance gs and daily variation in stem diameter ΔDd—which we measured at six mature riparian trees of the Salicaceae family at two sites with different mean depths to groundwater during two growing seasons. The data demonstrate that (a) short‐term variation of the groundwater table affects riparian vegetation—at the site with deeper groundwater, the water table depth was the best predictor of gs variability, while at the site with shallower groundwater, temperature and vapour pressure deficit (VPD) were the best predictors of ΔDd variability; (b) instantaneous stomatal conductance is related to VPD, but conditioned by groundwater levels, with higher stomatal conductance for the same radiative input and VPD when the water table was higher for all trees; and (c) local microclimate measured at tree locations had a stronger predictive power for gs than valley scale climate, suggesting local climate controls on vegetated stands on gravel bars. Our results provide evidence of riparian trees undertaking physiological adjustments to transpiration in response to groundwater stage, depending on their riparian floodplain setting.

Remote sensing depicts riparian vegetation responses to water stress in a humid Atlantic region

Riparian areas in the Cantabrian Atlantic ecoregion (northwest Portugal) play a key role in soil formation and conservation, regulation of nutrient and water cycle, creation of landscape aesthetic value and the preservation of biodiversity. The maintenance of their ecological integrity is crucial given the ever increase in multiple anthropogenic (water demand and agriculture) and climatic pressures (droughts and extreme events). We developed a transferable remote sensing approach, taking advantage of the latest freely available technologies (Sentinel-2 and Copernicus Land products), to detect intra-annual and inter-annual changes in riparian vegetation productivity at the river basin scale related to water stress. This study has used the normalized difference vegetation index (NDVI) to investigate riparian vegetation productivity dynamics on three different vegetation types (coniferous, broadleaved and grassland) over the past 5 years (2015–2019). Our results indicated that inter-annual seasonality differed between drier (2017) and wetter (2016) years. We found that intra-annual dynamics of NDVI were influenced by the longitudinal river zonation. Our model ranked first (r2m = 0.73) showed that the productivity of riparian vegetation during the dry season was positively influenced by annual rainfall and by the type of riparian vegetation. The emergent long lags between climatic variation and riparian plant productivity provides opportunities to forecast early warnings of climatically-driven impacts. In addition, the different average productivity levels among vegetation types should be considered when assessing climatic impacts on riparian vegetation. Future applications of Sentinel 2 products could seek to distinguish riparian areas that are likely to be more vulnerable to changes in the annual water balance from those that are more resistant under longer-term changes in climate.

From satellites to frogs: Quantifying ecohydrological change, drought mitigation, and population demography in desert meadows

Increasing frequency and severity of droughts have motivated natural resource managers to mitigate harmful ecological and hydrological effects of drought, but drought mitigation is an emerging science and evaluating its effectiveness is difficult. We examined ecohydrological responses of drought mitigation actions aimed at conserving populations of the Columbia spotted frog (Rana luteiventris) in a semi-arid valley in Nevada, USA. Abundance of this rare frog had declined precipitously after multiple droughts. Mitigation included excavating ponds to increase available surface water and installing earthen dams to raise water tables. We assessed responses of riparian vegetation to mitigation using a 30-year time series of satellite-derived Normalized Difference Vegetation Index (NDVI) and gridded weather data. We then analyzed a 23-year mark-recapture dataset to evaluate the effects of drought mitigation and NDVI on the probability of frog survival and rates of recruitment. After accounting for interannual precipitation variability, we found that NDVI increased significantly from before to after drought mitigation, suggesting that mitigation influenced the hydrology and vegetation of the meadows. Frog survival increased with NDVI, but mitigation had a stronger effect than NDVI suggesting that excavated mitigation ponds were particularly important for frog survival during drought. In contrast, frog recruitment was associated with NDVI more than mitigation, but only in meadows where NDVI was dependent on precipitation. At meadows with available groundwater, recruitment was associated with mitigation ponds. These findings suggest that mitigation ponds are critical for juvenile frogs to recruit into the adult population, but recruitment can also be increased by raising water tables in meadows lacking groundwater sources. Lagged recruitment (i.e., effects on larvae and juveniles) was negatively associated with NDVI. This study illustrates the ecohydrological complexity of drought mitigation and demonstrates novel ways to assess the effectiveness of drought mitigation using time series of readily available satellite imagery and organismal data.

Unnatural flooding alters the functional diversity of riparian vegetation of the Three Gorges Reservoir

Abstract The response of riparian vegetation to flow regulation has been a research focus for decades. Several studies have shed light on the effects of flow stabilisation on riparian woody species, but other life forms exposed to intensified inundation have been overlooked. Furthermore, studies from a functional perspective are scarce. We evaluated the functional response of riparian vegetation along the shores of the Three Gorges Reservoir on the Yangtze River in China to unnaturally long annual flooding (>7 months) after the first year of filling. We aimed to answer the following: (1) can we derive well‐defined flow‐response guilds from the riparian zones of the Yangtze River? and (2) which plant traits and guilds are favoured or disfavoured by the unnaturally long flooding environment? Woody and herbaceous species were inventoried in 12 reaches along the shorelines of the Three Gorges Reservoir and another 12 reaches along the free‐flowing Yangtze River. We performed a cluster analysis to derive riparian guilds using abundance data (projective coverage) from 40 riparian plant species and 13 responsive traits. Structural composition and functional diversity of the unnaturally and naturally flooded riparian vegetation were compared. Unnaturally long flooding substantially reduced species richness, but it did not change the riparian vegetation cover. This novel flooding reduced functional diversity, mostly owing to the loss of stress‐tolerant woody species and competitive perennial herbs. However, competitive annual herbs and flood‐tolerant riparian herbs, as the most abundant functional guilds, were favoured even under such long‐term hypoxic conditions. These guilds under regulation revealed a high functional resilience to prolonged flooding along the upstream reaches of the Yangtze River. Flooding tolerance and the capacity to synchronise germination and growth with short‐exposure periods underlie the plant species changes. Our findings are useful for anticipating the effects of long‐lasting inundation on riparian areas triggered by flow regulation or warmer climates. The functional perspective lends confidence that our conclusions can be generalised to other geographical regions despite not sharing the same species pool. Finally, the plant species that showed a high flooding tolerance should be considered for the restoration of riparian areas affected by the Three Gorges Reservoir.

Drought Sensitivity and Trends of Riparian Vegetation Vigor in Nevada, USA (1985–2018)

Dryland riparian areas are under increasing stress due to expanding human water demands and a warming climate. Quantifying responses of dryland riparian vegetation to these pressures is complicated by high climatic variability, which can create strong, transient changes in vegetation vigor that could mask other disturbance events. In this study, we utilize a 34-year archive of Landsat satellite data to (1) quantify the strength and timescales of vegetation responses to interannual variability in drought status and (2) isolate and remove this influence to assess resultant trends in vegetation vigor for riparian areas across the state of Nevada, the driest state in the USA. Correlations between annual late-summer Normalized Difference Vegetation Index (NDVI) and the Standardized Precipitation–Evapotranspiration Index (SPEI) were calculated across a range of time periods (varying timing and durations) for all riparian pixels within each of the 45 ecoregions, and the variability of these values across the study area is shown. We then applied a novel drought adjustment method that used the strongest SPEI–NDVI timescale relationships for each ecoregion to remove the influence of interannual drought status. Our key result is a 30 m resolution map of drought-adjusted riparian NDVI trends (1985–2018). We highlight and describe locations where impacts of invasive species biocontrol, mine water management, agriculture, changing water levels, and fire are readily visualized with our results. We found more negatively trending riparian areas in association with wide valley bottoms, low-intensity agricultural land uses, and private land ownerships and more positive trends in association with narrow drainages, public lands, and surrounding perennial water bodies (an indication of declining water levels allowing increased vegetative cover). The drought-adjusted NDVI improved the statistical significance of trend estimates, thereby improving the ability to detect such changes. Results from this study provide insight into the strength and timescales of riparian vegetation responses to drought and can provide important information for managing riparian areas within the study area. The novel approach to drought adjustment is readily transferrable to other regions.

Short‐term geomorphological and riparian vegetation responses to a 40‐year flood on a braided, dryland river

AbstractIn December 2010, a 40‐year flood occurred in the lower Virgin River (SE Nevada, southwestern United States), a braided river segment with riparian vegetation largely dominated by invasive shrubs in the genus Tamarix. We assessed geomorphological and vegetation responses to this large magnitude disturbance event by comparing pre‐ and post‐flood remote sensing and field survey data in four river reaches. Analyses of orthophotos and light detection and ranging‐derived topography showed that both the active channel area and channel width increased between 80% and 258%, representing an increase from 13% to 30% of the total river corridor area. Erosion predominated in the outer bends of the enlarged channel and deposition in the pre‐flood channel, causing local avulsions of the low‐flow channel. Field‐based topographic data recorded before and after the flood in 385 plots also showed that deposition occurred in parts of the floodplain that were not eroded. Two thirds of woody vegetation cover (mainly dominated by Tamarix, with some native shrub Pluchea sericea) was lost in areas that eroded (~20% of the river corridor). In the remaining ~80% of stable river corridor (aggrading or no change in elevation) Tamarix remained dominant. Following erosion, but also where sediment deposition predominated under the Tamarix canopy, the most common colonizing vegetation in the understory was comprised of annual plants, especially Salsola tragus. Our study supported previous studies describing large floods in braided rivers; we documented the first phase of a cycle of channel widening and increase in vegetation heterogeneity that is commonly followed by narrowing and vegetation homogenization.

Functional responses of aquatic and riparian vegetation to hydrogeomorphic restoration of channelized lowland streams and their valleys

Abstract Streams and riparian zones are highly heterogeneous ecosystems. Their high biodiversity is promoted by variable flow velocities and water depths, strong hydrological gradients and disturbance regimes. However, human interventions like damming and channelization have degraded these ecosystems world‐wide. And, although restoration efforts have increased in the past decades, ecological improvement is lagging. We assessed vegetation development in channelized lowland stream valleys in the Netherlands, combining innovative restoration measures to the stream and stream valleys. This “stream valley restoration” entailed construction of narrower and shallower channels to increase flow velocities during base discharges, meandering of the watercourse to increase flow and depth heterogeneity and excavation of banks to create wide v‐shaped stream valleys. We evaluated the effects on functional aspects of the developing in‐stream and riparian vegetation by comparing restored stream reaches to nearby unrestored reaches. The reduced channel dimensions led to higher flow velocities, which, through interaction with meandering, triggered a higher variability in flow and depth. Combined with enlargement of the floodplain, this promoted flooding in stream valleys and created wider environmental gradients. Plant diversity strongly increased in the floodplain area, the land–water interface and the shallow water habitat at the channel margins, but decreased in the central parts of stream channels. There, higher flow velocities led to more typically lotic (running water) in‐stream plant communities, indicated by a sharp decrease in floating‐leaved species and an increase in trailing species. Riparian vegetation showed a higher beta‐diversity across the wider valley slopes of restored reaches, with more wetland species in areas with water‐tables between 0.0 and −0.6 m, and more upland species as well. Synthesis and applications. This study demonstrates that the combination of strongly reduced channel dimensions, remeandering and widening of riparian zones, is effective in restoring in‐stream and riparian habitat heterogeneity. The restoration efforts lead to distinct immediate increases in total and beta‐diversity of many typical stream and riparian plant species. Overall, this stresses the importance of applying restoration measures to both streams and stream valleys simultaneously, considering them as a single landscape unit.

Livestock management, beaver, and climate influences on riparian vegetation in a semi-arid landscape.

Riparian and aquatic habitats support biodiversity and key environmental processes in semi-arid and arid landscapes, but stressors such as conventional livestock grazing, wildfire, and drought can degrade their condition. To enhance habitat for fish and wildlife and increase resiliency in these critical areas, land managers in the interior western United States increasingly use alternative grazing strategies, beaver management, or beaver dam surrogates as low-effort, low-expense restoration approaches. In this study we used historical archives of satellite and aerial imagery spanning three decades to characterize riparian vegetation productivity and document beaver dam occurrences, then evaluated vegetation productivity relative to land management associated with livestock grazing and beaver dam densities while accounting for climate and wildfire. After controlling for stream characteristics such as stream size, elevation, and stream slope, we demonstrate a positive response of riparian area vegetation to conservation-oriented grazing approaches and livestock exclosures, extensive beaver dam development, increased precipitation, and lack of wildfire. We show that livestock management which emphasizes riparian recovery objectives can be an important precursor to beaver activity and describe 11–39% increases in floodplain vegetation productivity where conservation-oriented grazing approaches or livestock exclosures and high beaver activity occur together on low-gradient sites. Land management decisions can therefore potentially confer resiliency to riparian areas under changing and variable climate conditions–the increased vegetation productivity resulting from conservation-oriented grazing or exclosures and high amounts of beaver activity at our sites is the equivalent to moving conventionally-grazed, low-gradient sites without beaver up at least 250 m in elevation or increasing water year precipitation by at least 250 mm.

Response of Riparian Vegetation, Instream Habitat, and Aquatic Biota to Riparian Grazing Exclosures

AbstractImproper riparian grazing can alter riparian vegetation and reduce streambank stability, negatively impacting aquatic habitat and biota. We evaluated differences in riparian and instream habitat, benthic macroinvertebrates, and fish inside versus outside seven riparian exclosures constructed from 1982 to 2005 in Idaho. The normalized difference vegetation index from Landsat imagery (1985 to 2015) showed significant increases in riparian vegetation productivity after some but not all exclosures were constructed. Field data collected in 2015 showed woody riparian vegetation to be more abundant, streambanks less altered, and stream channels narrower inside versus outside exclosures, as we expected. However, instream habitat, a benthic macroinvertebrate index, fish species richness, and adult salmonid density showed no response. Higher age‐0 salmonid densities were the only aquatic organism response observed. Broader watershed‐scale factors likely limited strong and consistent responses by aquatic biota to exclosures. We conclude that riparian exclosures have localized effects on stream systems, such as increased riparian vegetation, reduced streambank alteration, and improved channel stability, but may not always influence aquatic biota that commonly reflect larger watershed‐scale processes and conditions. Therefore, well‐maintained riparian exclosures should be used to eliminate livestock use in localized areas of stream corridors that are sensitive to disturbance. To improve stream health at the larger watershed scale that is relevant to most aquatic biota, exclosures should only be one of several grazing management tools implemented simultaneously to reduce the impacts of improper livestock grazing.

Evaluating the effects of check dams on channel geometry, bed sediment size and riparian vegetation in Mediterranean mountain torrents

In mountain streams possible negative impacts of check dams on soil, water and riparian vegetation due to check dam installation can be noticed. In spite of the ample literature on the qualitative effects of engineering works on channel hydrology, morphology, sedimentary effects and riparian vegetation characteristics, quantitative evaluations of the changes induced by check dams on headwater characteristics are rare.In order to fill this gap, this study has evaluated the effects of check dams located in headwaters of Calabria (Southern Italy) on hydrological and geomorphological processes and on the response of riparian vegetation to these actions. The analysis has compared physical and vegetation indicators in transects identified around check dams (upstream and downstream) and far from their direct influence (control transects).Check dams were found to influence significantly unit discharge, surface and subsurface sediments (both upstream and downstream), channel shape and transverse distribution of riparian vegetation (upstream) as well as cover and structure of riparian complexes (downstream). The actions of the structures on torrent longitudinal slope and biodiversity of vegetation were less significant. The differences on bed profile slope were significant only between upstream and downstream transects. The results of the Agglomerative Hierarchical Cluster analysis confirmed the substantial similarity between upstream and control transects, thus highlighting that the construction of check dams, needed to mitigate the hydro-geological risks, has not strongly influenced the torrent functioning and ecology before check dam construction. Moreover, simple and quantitative linkages between torrent hydraulics, geomorphology and vegetation characteristics exist in the analysed headwaters; these relationships among physical adjustments of channels and most of the resulting characteristics of the riparian vegetation are specific for the transect locations with respect of check dams. Conversely, the biodiversity of the riparian vegetation basically eludes any quantitative relations with the physical and other vegetal characteristics of the torrent transects.

Modelling feedbacks between geomorphological and riparian vegetation responses under climate change in a Mediterranean context

AbstractClimate change is expected to alter temperatures and precipitation patterns, affecting river flows and hence riparian corridors. In this context we have explored the potential evolution of riparian corridors under a dryness gradient of flow regimes associated with climate change in a Mediterranean river. We have applied an advanced bio‐hydromorphodynamic model incorporating interactions between hydro‐morphodynamics and vegetation. Five scenarios, representing drier conditions and more extreme events, and an additional reference scenario without climate change, have been designed and extended until the year 2100. The vegetation model assesses colonization, growth and mortality of Salicaceae species. We analysed the lower course of the Curueño River, a free flowing gravel bed river (NW Spain), as a representative case study of the Mediterranean region.Modelling results reveal that climate change will affect both channel morphology and riparian vegetation in terms of cover, age distribution and mortality. Reciprocal interactions between flow conditions and riparian species as bio‐engineers are predicted to promote channel narrowing, which becomes more pronounced as dryness increases. Reductions in seedling cover and increases in sapling and mature forest cover are predicted for all climate change scenarios compared with the reference scenario, and the suitable area for vegetation development declines and shifts towards lower floodplain elevations. Climate change also leads to younger vegetation becoming more subject to uprooting and flooding. The predicted reduction in suitable establishment areas and the narrowing of vegetated belts threatens the persistence of the current riparian community. This study highlights the usefulness of advanced bio‐hydromorphodynamic modelling for assessing climate change effects on fluvial landscapes. It also illustrates the need to consider climate change in river management to identify appropriate adaptation measures for riparian ecosystems. Copyright © 2018 John Wiley & Sons, Ltd.