Riparian Plant Communities Research Articles

The closer the better? Relative influence of forest continuity and distance to water on the taxonomic and functional structure of riparian plant communities along headwater streams

AbstractQuestions(a) Is forest temporal continuity an important factor in structuring the riparian plant communities along small alluvial valleys? (b) To what extent can distance from the stream modulate the effect of forest continuity and provide a better understanding of the taxonomic and functional structure of these communities?LocationOuanne watershed, France.MethodsWe used a paired‐samples design, involving stands close to and far from the stream, in ancient or recent riparian forests, in which we studied the vegetation. We performed analyses of species richness and cover, as well as mean values of species preferences and functional traits, and measures of species composition, distinguishing between the understorey and overstorey.ResultsStrong differences in richness and mean values, as well as in composition, were found between ancient and recent forests. Ancient forests were characterized by species with a dominant stress‐tolerant strategy, whereas recent forests were dominated by hygrophilous and nitrophilous species with a dominant competitive strategy. Furthermore, we found that ancient and recent stands close to the stream encompassed a larger richness of species, than ancient and recent riparian stands far from the stream, a pattern mostly driven by the greater co‐occurrence of small ruderal species with rapid leaves turnover but also of ancient forest species.ConclusionsIn small riparian forests along headwater streams, we have shown that past human activities leave still detectable traces in contemporary plant communities. In addition, we pointed out that colonization by ancient indicator species was faster in recent forests closer to the stream. This underscores the need to protect remaining ancient riparian forests that are well connected to the stream, even if they are small, and suggests that bank reforestation as well as restoration of hydrological connectivity on rivers fragmented by artificial barriers may be an effective way to increase forest biodiversity in riparian zones.

Flood regime alters the abiotic correlates of riparian vegetation

AbstractQuestionsPredicting the influence of climate change on riparian plant communities improves management strategies. The sensitivity of riparian vegetation to climate and other abiotic factors depends on interactions between properties of the ecosystem, like flood regime, and plant characteristics. To explore these interactions, we addressed three questions: (a) does the composition and diversity of riparian vegetation vary with the flood regime; (b) do abiotic correlates of vegetation, including climate and groundwater, differ between sites that flood compared to locations that did not experience floods; and (c) which plant functional groups account for differential plant community sensitivity to abiotic factors between flood regimes?LocationMiddle Rio Grande Valley, New Mexico.MethodsWe used long‐term observations of plant community composition, groundwater depth, precipitation and interpolated temperature from 24 sites spanning 210 km of the Rio Grande riparian cottonwood–willow forest to explore the relative importance of climate and hydrologic correlates of riparian vegetation diversity and composition.ResultsRiparian plant diversity was higher at sites flooding compared to non‐flooding sites. Plant diversity positively tracked shallower groundwater depth at flooding sites, but was best predicted by intra‐annual groundwater variability at non‐flooding sites. Plant community composition correlated with groundwater depth and air temperature at all sites, but at non‐flooding sites, also with intra‐annual groundwater variability and precipitation. Relationships between native plant cover and potential environmental drivers diverged strongly between the two flood regimes; non‐native plant cover had only weak relationships with most environmental predictors.ConclusionsThe current flood regime of a site determined the climate and hydrologic factors that best predicted riparian plant community composition and diversity. Relationships between plant diversity or total cover and groundwater, temperature, precipitation, or groundwater variability can change in strength or direction depending on a site's flood history, highlighting the importance of flood regime to predicting the sensitivity of riparian woodlands to future environmental change.

Environment-driven changes in diversity of riparian plant communities along a mountain river.

The study of changes in species richness and composition along rivers has focused on large spatial scales. It has been ignored that in different sections of the river (high mountain area, middle zone, and mouth of the river) the specific environmental conditions can generate different longitudinal patterns of the species richness and composition. In this study, we determine whether species richness and composition of the riparian plant communities change along a mountain river and whether these changes are related to environmental variables. We expect an increase in species richness and turnover along the river, that the upstream communities would be a subset of the downstream communities, and that such would be related to edaphic and hydrologic conditions. To test this, we sampled three strata of the riparian vegetation (upper: individuals with <1 cm of ND, middle: individuals with >1 cm of ND, low: individuals with >1 m tall) in a set of 15 sites that we place along a mountain river. Additionally, we recorded topographic, hydrological, morphological, and soil variables. We performed correlation analyzes to determine whether changes in species richness and turnover were related to increased distance to the origin of the river. Also, we obtained the nestedness and evaluated the importance of environmental variables with GLM, LASSO regression, and CCA. With the increase in distance, the species richness decreases in the upper stratum, but not in the middle and the low stratum (although the highest values were observed near the origin of the river), the turnover increase in all strata and the upstream communities were not a subset of the downstream communities. The changes in species richness and composition were related to topographic (altitude), hydrological (flow), and edaphic (conductivity and pH) variables. Our results indicate that at small spatial scales the patterns of richness and composition differ from what has been found at larger spatial scales and that these patterns are associated with environmental changes in the strong altitude gradients of mountain rivers.

Changing climate requires shift from refugia to sanctuaries for floodplain forests

ContextRiparian areas are considered to undergo major alterations under changing climate, making floodplain habitats targets for conservation and landscape planning. Protected areas might provide sanctuaries especially for sessile riparian plant species, but these niches are not always persistent over time.ObjectivesWe investigate if plant species of floodplain forests are provided with suitable habitat within currently protected areas and if these refugia persist. A coupled-modelling approach is used to gain spatially explicit information on new areas for sanctuaries.MethodsWe use species distribution models to predict the niche of 12 Salicion albae and 7 Fraxinion floodplain forest species along rivers in Switzerland, under current, moderate and extreme climate change scenarios up to 80 years to the future (2100). The spread of plant species from current habitat to suitable future habitat is simulated using dispersal vectors and life history traits.ResultsSalicion albae species are more flexible under both climate change scenarios than Fraxinion species. The main limitation for the spread of species is their dispersal ability, as only a minority of the suitable cells is colonized during the simulation process. The predicted future presence within currently protected areas decreases under both climate change scenarios in the model.ConclusionsCurrent protected floodplains do not provide persistent refugia for the plants studied, but might still be of importance to other organisms. Planning of sanctuaries for riparian plant species and communities need to focus on connectivity along rivers to maintain viable source populations in dynamic riverine landscapes under changing climate.

Native Riparian Plant Species Dominate the Soil Seedbank of In-channel Geomorphic Features of a Regulated River.

Flow regulation impacts on riparian vegetation composition, often increasing the prevalence of exotic and terrestrial plant species. Environmental flows may benefit native riparian vegetation via the promotion of plant recruitment from riparian soil seedbanks, but this is dependent on an intact native seedbank. Thus, we assessed the composition of the soil seedbank of different riverine geomorphic features to determine its potential response to environmental flows. Soil seedbank samples were taken from channel bars, benches and floodplains at six sites along the Campaspe River, Australia, a heavily regulated river that receives environmental flows. These geomorphic features represent a gradient in elevation and thus flooding frequency from frequently flooded (bars) to infrequently flooded (floodplain). Seedbank samples were 'grown out' in a glasshouse, and seedlings identified and classified according to taxa, flood tolerance and origin (native or exotic). We identified 6515 seedlings across all geomorphic features and sites, with monocots most abundant. Soil seedbank composition varied between geomorphic features. Overall, seedling abundances were greater for in-channel features (bars and benches) than floodplains, but taxa richness did not vary likewise. Soil seedbanks of in-channel features were dominated by flood tolerant and native taxa, while flood intolerant and exotic taxa were generally associated with floodplains. The dominance of native flood tolerant taxa in the soil seedbanks of in-channel geomorphic features suggest these seedbanks can play an important role in the resilience of native riparian plant communities. Moreover, environmental flows are likely to play a positive role in maintaining native riparian plant communities given such conditions.

Correlation between Riparian Plant Communities and Plantpropagule Banks in Dicun Stream

Correlation between Riparian Plant Communities and Plantpropagule Banks in Dicun Stream

Riparian Plant Communities Remain Stable in Response to a Second Cycle of Tamarix Biocontrol Defoliation

Reduced abundance of non-native Tamarix shrubs in western U.S. riparian systems following biological control by a defoliating beetle has led to concerns that replacement plant communities could be dominated by other invasive species and/or not provide some of the ecosystem services that Tamarix was providing. In previous studies, Tamarix decline following biocontrol was accompanied by small increases in native and non-native herbaceous species, with variable responses of woody vegetation. However, none of these studies spanned periods longer than a decade since beetle release. This is an important caveat, given the cyclical nature of plant-herbivore interactions and potential lags in vegetation recovery. We report plant community response to an eight-year-long second cycle of Tamarix defoliation-refoliation in two reaches of the upper Colorado River in eastern Utah, 11–13 years after beetle arrival. Tamarix cover across sites initially declined an average of ca. 50% in response to the beetle, but then recovered. Changes in the associated plant community were small but supported common management goals, including a 47% average increase in cover of a native shrub (Salix exigua), and no secondary invasions by other non-native plants. We suggest that the effectiveness of biocontrol programs must be assessed case-by-case, and on a long-term basis.

Effects of disturbance and alien plants on the phylogenetic structure of riverine communities

AbstractQuestionsPhylogenetic analyses provide important insights in the study of biological invasions. Previous studies have shown contrasting effects of alien species on the phylogenetic structure of recipient communities. In this study, we focus on two riparian plant communities with contrasting natural disturbance regimes: riparian forests and river bar communities. We ask whether these communities differ in alpha diversity and degree of plant invasion. Further, are the phylogenetic diversity (PD) and structure of these habitats related to the level of plant invasion?LocationNorthern Spain.MethodsWe determined level of plant invasion in forest and river bar vegetation plots, and calculated PD using mean pairwise distance (MPD) and mean nearest taxon distance (MNTD). We applied null models to analyse the phylogenetic structure of plots, and we tested whether inclusion of alien plant species was associated with differences in observed phylogenetic structure.ResultsRiver bar plots experienced more invasion than forest plots, but the level of invasion was only related to the PD of native species in forests. Most plots had random phylogenetic structure, with a minority of plots tending to phylogenetic overdispersion in forests and to phylogenetic clustering in river bars. MPD increased with the inclusion of alien plant species in forest plots, suggesting phylogenetic overdispersion, while no such pattern was detected with MNTD. MPD increased slightly with increasing invasion in river bar plots, suggesting reduced clustering, while MNTD values decreased with increasing invasion, suggesting the opposite trend.ConclusionsInvasion by alien plants is differentially associated with phylogenetic structure in riparian habitats with different disturbance levels. Our results contrast with those of previous studies of plant invasion of riparian communities, which suggests geographic and ecological variation in the relationship between invasion and phylogenetic community structure in riparian systems. Research is needed to identify the causal factors underlying this variation.

Riparian plant species differ in sensitivity to both the mean and variance in groundwater stores

Abstract Aims Determining the ecological consequences of interactions between slow changes in long-term climate means and amplified variability in climate is an important research frontier in plant ecology. We combined the recent approach of climate sensitivity functions with a revised hydrological ‘bucket model’ to improve predictions on how plant species will respond to changes in the mean and variance of groundwater resources. Methods We leveraged spatiotemporal variation in long-term datasets of riparian vegetation cover and groundwater levels to build the first groundwater sensitivity functions for common plant species of dryland riparian corridors. Our results demonstrate the value of this approach to identifying which plant species will thrive (or fail) in an increasingly variable climate layered with declining groundwater stores. Important Findings Riparian plant species differed in sensitivity to both the mean and variance in groundwater levels. Rio Grande cottonwood (Populus deltoides ssp. wislizenii) cover was predicted to decline with greater inter-annual groundwater variance, while coyote willow (Salix exigua) and other native wetland species were predicted to benefit from greater year-to-year variance. No non-native species were sensitive to groundwater variance, but patterns for Russian olive (Elaeagnus angustifolia) predict declines under deeper mean groundwater tables. Warm air temperatures modulated groundwater sensitivity for cottonwood, which was more sensitive to variability in groundwater in years/sites with warmer maximum temperatures than in cool sites/periods. Cottonwood cover declined most with greater intra-annual coefficients of variation (CV) in groundwater, but was not significantly correlated with inter-annual CV, perhaps due to the short time series (16 years) relative to cottonwood lifespan. In contrast, non-native tamarisk (Tamarix chinensis) cover increased with both intra- and inter-annual CV in groundwater. Altogether, our results predict that changes in groundwater variability and mean will affect riparian plant communities through the differential sensitivities of individual plant species to mean versus variance in groundwater stores.

Diversity and succession of riparian plant communities along riverbanks bioengineered for erosion control: a case study in the foothills of the Alps and the Jura Mountains

Soil bioengineering for riverbank stabilization involves the use of living plant materials to treat unstable or eroding riverbanks. These near-natural structures may harbor a higher plant richness and vegetation cover compared to classical civil engineering structures such as ripraps, but little information exists on vegetation dynamics during secondary succession on stabilized riverbanks. We hypothesized that soil bioengineering, by means of active introduction of early successional Salix shrubs, can foster successional trajectories of riparian plant communities, unlike civil engineering. We sampled three types of riverbank stabilization structures: pure bioengineering structures, mixed structures (combining riprap and bioengineering techniques) and ripraps, across a 14-year sequence on 42 sites located along 23 different streams running through the foothills of the Alps and the Jura Mountains (France and Switzerland). We quantified species richness and density and compared the temporal patterns of four groups of species that normally appear sequentially in natural succession on riverbanks (ruderal, hygrophilous, shade-tolerant, competitive species), as well as non-native species. Plant community composition differed greatly between ripraps and the two types of bioengineered sites, and ligneous species typical of advanced successional stages (Cornus sanguinea, Corylus avellana) spontaneously established in the oldest bioengineered sites. In general, richness of total species was higher in stabilization structures using soil bioengineering (including mixed structures) than in riprapped sites. In particular, the number of shade-tolerant and competitive species in bioengineered sites was double that found at ripraps after 14 years. Yet, richness of shade-tolerant species increased over time only on purely bioengineered sites, and their density there was almost twice that in mixed structures. Neither the type of stabilization structure nor time explained the variability in richness and density of non-native species across sites. Our study showed that along streams running through foothills, where erosion processes are usually intense, vegetation of bioengineered riverbanks exhibits successional dynamics similar to those theoretically found in natural conditions. Bioengineering can therefore foster ecological processes while stabilizing eroding riverbanks along foothill streams, thus satisfying human needs for infrastructure protection with less impact on the riparian ecosystem than riprap structures.

Shifts in Riparian Plant Life Forms Following Flow Regulation

Flow regulation affects bordering riparian plant communities worldwide, but how different plant life forms are affected by river regulation still needs further research. In northern Sweden, we selected 10 rivers ranging from free-flowing to low, moderately, and highly regulated ones. In 94 reaches across those rivers, we evaluated the relative abundance of woody and herbaceous (i.e., graminoids and forbs) life forms, their species richness, and their relative presence. We also explored which, and to what extent, hydrological variables drove species assembly within each life form. The relative abundance and species richness of each life form decreased across river categories with increasing levels of regulation. This was particularly apparent in herbaceous life forms, and the most drastic decreases were observed in all life forms in moderately or highly regulated reaches. Additionally, when river regulation increased, the relative presence of many species from all life forms decreased. Unlike woody species, only a few new herbaceous species appeared in regulated reaches. A canonical correspondence analyses (CCA) revealed that a wide range of hydrological variables explained the occurrence of woody species, while fewer variables explained variation in the graminoid and forb life forms. We conclude that flow regulation and its intensity result into clear shifts in the relative abundance of different life forms, as well as in changes of within-group species richness and composition. Consequently, the modification of certain flow attributes in flow regulation schemes, as well as the intensity of these modifications, may alter the ratio between herbaceous and woody species, ultimately impacting the functions and benefits derived from each life form.

Studies on the threshold density of Phragmites australis plant concentration as a factor of hydraulic interactions in the riverbed

The paper features the results of numerical analyses and field survey concerning the density threshold for concentration of Phragmites australis plants in the aspect of its impact on hydraulical flow conditions in the river. Simulations were performed for a typical vegetation concentration mapped from representative natural rivebed of Sleza River (Lower Silesia/Poland). The numerical model was constructed, which was calibrated on the basis of field survey conducted with use of hydrometric current meter and an acoustic Doppler current profiler (ADCP). For given fixed spatial configuration (of the area) of analysed riparian plant community, in numerical model, the community density was altered by i.a increase of stems distribution density. Such boundary measure of plant concentration (density) was sought, that after it is exceeded, the flow through the plant community is negligibly small in comparison to dominant stream circumfluenting the plant „obstacle”. The analysis was also conducted on variable and dependent on vegetation density hydraulic interactions between the vegetation zone and water stream in river. Knowledge of boundary density for particular vegetation species in riparian zones can be used in ecological engineering, especially in proecological river regulation. The conservation practices can be performed in the riverbed that allow to keep vegetation communities, interfering only its density and by that there is an opportunity to control the riverbed processes.

Fine‐grained sediment deposition alters the response of plant CSR strategies on the gravel bars of a highly regulated river

AbstractAimsIn riverine ecosystems, variations in disturbance and stress intensities along the flood gradient shape vegetation community structure. However, human modifications to the flood regime may change the magnitude of disturbance and stress, leading to different species distribution patterns along environmental gradients. We aimed to study the effects of local‐ and broad‐scale environmental gradients on riparian plant CSR (Competitors–Stress tolerators–Ruderals) strategies along a highly regulated river.LocationRhône River, France.MethodsWe assessed how changes in elevation, fine sediment and temperature influenced CSR strategies of riparian plant communities, by studying variations in the community‐weighted means and the functional dispersion of the CSR values, on gravel bars along a 250 km north–south river corridor.ResultsSpecies with contrasting ecological strategies co‐occurred less on gravel bars subject to fine sediment deposition, leading to patterns of biotic homogenization along the flood gradients. Furthermore, a shift in CSR strategies from ruderal to stress‐tolerant communities occurred along the elevation gradient on bars with low proportions of fine sediments and from stress‐tolerant to ruderal communities along the elevation gradient on bars with high proportions of fine sediments. Regarding the climatic gradient, the mean value of ruderals increased with temperatures. Also, for stress tolerators, the dispersion value diverged between the northern and southern gravel bars when the proportion of fine sediments was low and converged when the proportion of fine sediments was high.ConclusionsOverbank fine sedimentation alleviates the intensity of stress along not only the elevation gradient but also the climatic gradient. By homogenizing the functional composition of communities, this process may have detrimental consequences for riparian plant diversity, especially of ruderal plants in the littoral zone. In regulated river systems, restoration measures should therefore promote an increase in the intensity of: (a) disturbance through the reactivation of bedload supply and transport; and (b) stress through increased flow dynamics.

Combined effects of biological control of an invasive shrub and fluvial processes on riparian vegetation dynamics

Plant community responses to biocontrol of invasive plants are understudied, despite the strong influence of the composition of replacement vegetation on ecosystem functions and services. We studied the vegetation response to a folivore beetle (Diorhabda genus, Coleoptera) that has been introduced along southwestern US river valleys to control the invasion of non-native shrubs in the genus Tamarix (Tamaricaceae). We collected detailed plant compositional and environmental data during four different surveys over 7 years (2010–2017), including two surveys prior to when substantial beetle-induced dieback occurred in summer 2012, along the lower Virgin River, Nevada. The study river was of special interest because it is one of only a few largely unregulated rivers in the region, and a large flood of 40-year return period occurred between the first and second surveys, allowing us to study the combined effects of fluvial processes, which typically drive riparian plant community assembly, and biocontrol. Vegetation trajectories differed as a function of the dominant geomorphological process. Tamarix cover declined an average of 75% and was replaced by the native shrub Pluchea sericea as the new dominant species in the floodplain, especially where sediment deposition predominated. Following deposition, and especially erosion, opportunistic native herbs, Tamarix seedlings, and noxious weeds colonized the understory layer but did not increase in cover over time. Stands of the native shrub Salix exigua, a desirable replacement species following Tamarix control, only increased slightly and remained subordinate in the floodplain. Overall, our results showed that, by successfully controlling the target non-native plant, a biocontrol agent can substantially modify the replacement plant communities in a riparian system, but that fluvial processes also strongly influence the resulting communities.

Sampling event dataset for ecological monitoring of riparian restoration effort in Colorado foothills.

BackgroundThe foothills and shortgrass prairie ecosystems of Colorado, United States, have undergone substantial and sustained anthropogenic habitat change over the past two centuries. Riparian systems have been dramatically altered by agriculture, hydrological engineering, urbanisation and the introduction of non-native invasive species. In 2016, Denver Botanic Gardens began a restoration effort of Deer Creek which seeks to modify the hydrology of the creek by mimicking the effects of beaver dams with artificial structures. The site, owned by the US Army Core of Engineers and managed by Denver Botanic Gardens, had been the subject of previous botanical surveys. With the initiation of the restoration project, permanent transects were established along the stream and are sampled for ground vegetation richness and abundance, canopy cover, soil and stream conditions and aquatic macroinvertebrate community makeup on an annual basis. To provide a means for tracking any post-intervention changes in the riparian ecosystem, this resource reports all recorded occurrences and measurements, along with methodologies and motivations from past and current surveys in the form of a sampling event dataset.New informationThe current project and past surveys document 382 plant taxa and 157 aquatic macroinvertebrate taxa. A total of 16304 occurrences and 7422 measurements are included in the resource. Occurrence and measurement data taken from transects provide a means to measure species abundance, ground cover and other biotic and abiotic characteristics relevant to assessing the effects of hydrological restoration on riparian plant communities.

Ecological Differentiation between Invasive and Native Species of the Genus Epilobium in Riparian Ecosystems Is Associated with Plant Functional Traits

The biomass allocation, leaf traits, and the CO2 assimilation rate in invasive Epilobium adenocaulon Hausskn and E. pseudorubescens A.K. Skvortsov and indigenous E. palustre L. were studied in the Middle Urals. E. adenocaulon had the maximum height, and E. palustre had the largest leaf canopy diameter. The most compact in linear sizes species E. pseudorubescens was characterized by the lowest biomass. The pattern of organ biomass allocation in invasive E. adenocaulon and E. pseudorubescens corresponded to plants with a ruderal strategy, which was expressed in a low proportion of roots and a large proportion of generative organs. Among the studied species, E. adenocaulon was characterized by the largest the stem mass ratio. The indigenous E. palustre had a large proportion of roots and leaves and a smaller proportion of generative organs, which indicated the stress-tolerant properties of this species. In addition to the pattern of biomass allocation, E. palustre differed from the invasive species in a large (30% on average) amount of chlorophyll and 1.2 times less carotenoid content, as well as low photosynthetic activity of chlorophyll. The rate of CO2 uptake per unit leaf area was the greatest in E. adenocaulon, which had thick leaves with a large leaf mass per area (LMA), exceeding that in species E. palustre and E. pseudorubescens with thin leaves with lower LMA by 1.8 and 1.6 times, respectively. A negative correlation between the CO2 assimilation rate per 1 g of dry weight and leaf density (LD) (r = –0.78, p < 0.001) predetermined the maximum value of the photosynthesis rate in E. pseudorubescens, which had the least dense leaves. The analysis of the general dataset of the functional traits indicated that the studied species of the Epilobium genus had different ecological strategies: E. palustre—S (stress tolerant), E. adenocaulon—CR (competitive-ruderal strategy), and E. pseudorubescens—R (ruderal species). It is concluded that invasive species do not have a direct negative effect on E. palustre, even being characterized by distinct ecological properties compared to native species. However, owing to the CR strategy, E. adenocaulon is able to invade riparian plant communities when they are affected by disturbing factors, which may lead to a decrease or to disappearance of the indigenous stress-tolerant species E. palustre and thus to its replacement in disturbed communities by the more competitive invasive species E. adenocaulon.

Ecological implications of the replacement of native plant species in riparian systems: unexpected effects of Reynoutria japonica Houtt. leaf litter

In European rivers, one of the most invasive of terrestrial plant species, Reynoutria japonica Houtt. (Polygonaceae), is frequently found at the water-land interface. In this study, ecological effects of native species replacement by this invasive species was explored. In particular, we examined changes in leaf litter composition, and the subsequent impacts of their decomposition on riparian and in-stream plant community growth. We compared leaf litter from two riparian species, native Rubus fruticosus L. agg. (Rosaceae) and its spontaneous substitute along river corridors, R. japonica. Effects were assessed through phytotoxicity tests on (1) a terrestrial model plant species, the lettuce Lactuca sativa var. capitata L. (Asteraceae) and (2) a typical semi-aquatic riparian plant species growing in the same riparian zone, the watercress Nasturtium officinale R.Br. (Brassicaceae). Seedling root length measurement permitted calculation of a root length reduction index. The experiment was performed over a six-month decomposition period, with samples taken at five time intervals. Phytotoxic abilities of R. japonica were inconsistent over the decomposition period, leading to breaks in the regulatory processes of the structuring native species, R. fruticosus. Decomposition of R. japonica leaf litter led to the unexpected emergence of successive competition and facilitation interactions. This study showed distinct phytotoxic impacts from R. japonica leaf litter decomposition, causing ecological effects that differed from that of native species. In a context where R. japonica progressively replaces native riparian species, such impacts need to be considered further as R. japonica phytotoxicity does not steadily prevent other species with offset phenologies to establish, including invasive species.

Aquatic and riparian ecosystem recovery from debris flows in two western Washington streams, USA.

An exceptionally powerful storm struck southwestern Washington in December 2007 causing large debris flows in two adjacent streams. The two affected streams had been studied prior to the storm, providing a rare opportunity to examine ecosystem recovery. We monitored the streams and their riparian zones for six years after the disturbances to determine whether recovery rates of biota, physical habitat, and water temperature differed, and if so, what factors affected resilience. Along both streams, the debris flows removed wide swaths of soil, rock, and coniferous riparian forests, widening the active channel and increasing solar exposure and summer water temperatures. Initially depauperate of vegetation, after four years red alder trees dominated the riparian plant communities. The warmer water, greater solar radiation, and unstable substrates likely contributed to variable benthic insect and tailed frog tadpole densities over time, although benthic insect communities became more similar after three years. The debris flows also decreased channel slopes and removed channel step barriers such that cutthroat trout were able to rapidly occupy habitats far upstream, but sculpins were slower to recolonize and both fish species exhibited some differences in recovery between the two streams. Crayfish were severely impacted by the debris flows; this may be due to attributes of their life history and the timing of the flows. Overall, we found that recolonizing aquatic species exhibited varying levels of resilience and recovery after the disturbances being related to the influence of physical habitat conditions, species dispersal ability, and the presence of nearby source populations.

Hydropeaking affects germination and establishment of riverbank vegetation.

Hydropeaking, defined as frequent and rapid variation in flow in regulated rivers with hydropower plants over a short period of time, usually sub-daily to weekly, alters hydraulic parameters such as water levels or flow velocity and exerts strong impacts on fluvial ecosystems. We evaluated the effects of hydropeaking on riverbank vegetation, specifically assessing the germination and establishment of seedlings and cuttings of plant species representing a variation in traits. We used seeds and seedlings and cuttings varying in size as phytometers, and transplanted them to riverbanks both above and below dams used for hydropower production in northern Sweden, selected to represent a gradient in hydropeaking intensity, and along a free-flowing reach. We also analyzed sub-daily water-level variables modified by hydropeaking to identify variables key in explaining the observed vegetation patterns. We found that plant responses to hydropeaking varied with species, with flood-intolerant species being the most strongly affected, as early as the germination stage. In contrast, seeds of flood-tolerant species managed to germinate and survive the early establishment phase, although strong erosive processes triggered by hydropeaking eventually caused most of them to fail. The fate of flood-intolerant species identifies germination as the most critical life-history stage. The depth and frequency of the inundation were the leading variables explaining plant responses, while the duration of shallow inundation explained little of the variation. The rise and fall rates of water levels were key in explaining variation in germination success. Based on the results, we propose restoration measures to enhance establishment of riparian plant communities while minimizing the impact on hydropower electricity production. Given the strong decrease in the germination of species intolerant to prolonged flooding with hydropeaking, planting of seedlings, preferably of large sizes, together with restrictions in the operation of the power plant during the establishment phase to enhance survival would be the best restoration option. Given the high probability of plant uprooting with hydropeaking, bank protection measures have the potential to increase riparian plant survival of all species, including flooding-tolerant species.

Influence of wildfire severity on geomorphic features and riparian vegetation of forested streams of the Sierra Nevada, California, USA

Fires are a common feature of many landscapes, with numerous and complex ecological consequences. In stream ecosystems, fire can strongly influence fluvial geomorphic characteristics and riparian vegetation, which are structural components of stream–riparian ecosystems that contribute to biodiversity and ecosystem function. However, the effects of fire severity on stream–riparian ecosystems in California’s Sierra Nevada region (USA) are not well described, yet critical for effectively informing fire management and policy. At 12 stream reaches paired by fire severity (one high-severity burned, one low-severity burned), no significant differences were found in riparian plant community cover and composition or stream geomorphic characteristics 2–15 years following wildfire. In addition, minimal changes in riparian vegetation and stream geomorphic properties were observed in the first summer following the extensive and severe Rim Fire. However, an upstream-to-downstream influence of multiple fire occurrences was observed over the previous 81 years within each catchment on stream geomorphic metrics, including sediment size, embeddedness and channel geometry, at our study reaches. The inconsistent effects of wildfire on stream–riparian vegetation and geomorphic characteristics over space and time may be related to time since fire and precipitation.