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Summary: Monitoring the condition of returning vegetation over time is essential for effectively managing riparian vegetation. Rapid riparian assessment methods are standard tools for capturing a suite of variables relevant to river health and are accessible to a range of users with different levels of experience. Monitoring efforts are particularly important for rivers undergoing degrading impacts and subsequent management interventions. In coastal catchments of New South Wales, Australia, vegetation is returning to rivers that were historically cleared for agriculture and are now experiencing geomorphic recovery. We present a rapid riparian assessment method for rivers undergoing vegetative and geomorphic recovery in this region. It addresses a gap in available rapid assessments by providing a methodology that can be used at sites with no historical analogue. Our assessment method adapts and extends a widely used method called the Rapid Assessment of Riparian Condition (RARC) to capture the quality of returning riparian vegetation along recovering rivers (rec), including native species diversity and vegetation structure (hence the name RARCrec). The RARCrec incorporates four sub‐indices – vegetation cover, native species richness, proportion of native to exotic species, and features of vegetation health – to produce an overall vegetation condition score that classifies sites as poor, moderate, or good condition. Scores for each sub‐index are assessed for the vegetation strata (groundcover, graminoids, midstorey, vines, and canopy) on the landform units of river bars, benches, and floodplains. The RARCrec has been designed to be sufficiently general for use in the main Vegetation Formations in coastal New South Wales and has been trialled at 36 sites across this region. Assessments can be conducted within half a day by two technically competent users and can be used to monitor vegetation condition over time to inform where to prioritise implementation of management activities. Field methodology and data sheets, including a scoring system, are provided to support the implementation of the RARCrec. [ABSTRACT FROM AUTHOR]

Extensive riparian vegetation clearance occurred in many rivers of southeastern Australia throughout the 19th and 20th century, post colonisation. With improvements in river management practices, coincident with a period of minimal flooding, vegetation recovery of riparian corridors has been occurring since the 1980s in all coastal catchments of New South Wales (NSW). However, the catchment‐by‐catchment spatial pattern and temporal trends, trajectories and rates of vegetative recovery remain unknown. We reconstruct a 70‐year time series of change in riparian vegetation coverage for all rivers in coastal catchments of NSW, utilising historical aerial imagery, Landsat and Sentinel data and a manual digitisation and machine learning method. We quantify and statistically analyse decadal trends in woody and non‐woody vegetation coverage along 19 485 km of stream length in 20 catchments. Across the region, there has been, on average, a 40% increase in vegetation coverage along riparian corridors since the 1950s, with some catchments increasing towards a current day coverage of between 60% and 80%. However, the trends vary by subregion and by catchment. An early logarithmic increase occurred up to 2004 on the North coast, and a recent and accelerated exponential increase since 1995 has occurred on the South coast. After a steep initial increase, riparian corridor vegetation coverage has fluctuated on the Central coast since about 1995. We used these linear, exponential and logarithmic regressions to undertake preliminary forecasts of possible future timeframes of riparian vegetation change, based on several climate, management and natural disaster disturbance scenarios. If the current trend continues, riparian vegetation coverage can reach 70% across the region by 2055. However, if management practices change or catastrophic disturbance events such as fire and flood occur, increases in riparian vegetation coverage could be set back by up to 30 to 40 years. [ABSTRACT FROM AUTHOR]

Natural flood management (NFM), a nature‐based solution to flood mitigation where hydrological and biophysical processes are harnessed to reduce flow velocity, erosive energy and flood risk, is an emerging global theme of water and river management. The catastrophic 2021 and 2022 floods in eastern Australia are used to assess the hydrological properties of discrete events and to start an investigation of whether widespread changes in flood hydrology are occurring. We find that most coastal rivers in New South Wales (NSW) had a noticeable decrease in flood wave celerity (increase in flood travel time) when the 2021 and 2022 floods are contrasted with equivalent floods since the 1970s and that several also exhibit increases in flood peak attenuation. For some rivers, there is a coincident trend between these changing flood properties, riparian vegetation regrowth (regreening) and geomorphic recovery over the last +30 years. These may be the first signals that passive riparian management is counteracting some of the more severe hydrological effects of floods on these rivers and that some degree of NFM is possible. Lessons from this work are that there is an immediate opportunity to implement large‐scale NFM in coastal catchments of NSW, but this will only occur if we re‐examine current flood mitigation and adaption strategies and recognise and prioritise nature‐based solutions that include space‐to‐flood and corridors of river recovery, as essential parts of the modern flood mitigation toolkit. [ABSTRACT FROM AUTHOR]

Despite research into the dynamics of seed transport in fluvial systems, few consider how far seeds will travel, and how far from local or upstream seed sources passive regeneration can occur. We experimentally test the seed floating time of 60 plant species (50 native and 10 exotic) commonly found in riparian corridors of southeastern Australia. Around 50% of species had seeds that floated for 2 days or less, and for most species' (90%) all seeds had sunk within one week. Seeds of native species tended to sink more quickly than exotic, with 64% of native species' seeds floating for less than 2 days. In contrast, most exotic species (80%) floated for longer than 2 days, and 40% had seeds still floating after one week. This suggests that exotic species are good floaters and likely to travel long distances, making them excellent hydrochores. Finally, we applied the findings to a real case study, Wollombi Brook, NSW, Australia. We combined the findings from the seed floating experiment with low flow hydrology calculations to map the potential travel distance of seeds from known extant vegetation sources. We show that maximum seed travel distance per day could be up to 21 km. Thus, species that float for a week could potentially travel almost 150 km downstream before sinking or being deposited. We discuss how local versus upstream seed sources, and hydrochory, could be utilised in passive revegetation and weed management of riparian corridors. [ABSTRACT FROM AUTHOR]

Passive riparian revegetation techniques are important tools in river rehabilitation. However, the utility of the sediment seed bank as a passive riparian regeneration option is poorly understood. After modelling a range of flows for field‐surveyed cross sections, a glasshouse seedling emergence experiment was undertaken to compare the effects of simulated flow duration on seedling emergence of desirable riparian species that occur on benches of the gravel‐bedded Hunter River near Muswellbrook, New South Wales. The duration of inundation did have an effect (although not always significant) on seed germination for most species. The most successful simulated flood conditions differed among species, with the control treatment (no inundation) resulting in the most germination for only two of the 10 species examined. These findings suggest that although environmental flows for the sole purpose of stimulating the riparian seed bank in order to facilitate regeneration of desirable riparian species would be largely ineffective and complicated by differing inundation responses among species, seed bank stimulation using environmental flows could be a value‐added benefit from flows allocated for other purposes. As such, consideration of flow duration is worthy of inclusion in environmental flow allocation planning. [ABSTRACT FROM AUTHOR]

Abstract: Human disturbance induces significant geomorphic changes to river systems. In eastern Australia, land‐use practices such as clearance of forests and riparian vegetation, and removal of wood from channels in the 19th and 20th centuries induced widespread geomorphic impacts. However, since the 1970s, there has been a noticeable shift in the geomorphic condition of many rivers in eastern New South Wales (NSW). This transition to a recovery trajectory reflects a reduction in land‐use pressures and improved farming practices on the one hand and adoption of recovery enhancement approaches to river conservation and rehabilitation by management authorities on the other. Monitoring and tracking changes in condition by state government agencies involves identifying when geomorphic recovery is occurring so that decision‐support frameworks can determine whether river management is required, where, when, and how much to intervene to enhance river recovery and when to opt‐out of management because the system requires little (or no) intervention. Here, we document procedures that are used to manage process‐based, recovery enhancement in NSW. Following a brief review of river change in eastern NSW since European settlement, we outline an approach to identify and measure key geomorphic indicators of river recovery for different river types. We use case studies to demonstrate examples of geomorphic river recovery, prior to showing how state government agencies in NSW are monitoring trajectories and rates of recovery. Broader implications of this ‘good news story’ are discussed. [ABSTRACT FROM AUTHOR]

The ability to survive and thrive in repeatedly waterlogged soils is characteristic of plants adapted to riparian habitats. Rising atmospheric CO2 has the potential to fundamentally alter plant responses to waterlogging by altering gas exchange rates and stoichiometry, modifying growth, and shifting resource-economic trade-offs to favor different ecological strategies. While plant responses to waterlogging and elevated CO2 individually are relatively well characterized, few studies have asked how the effects of waterlogging might be mediated by atmospheric CO2 concentration. We investigated interactive effects of elevated (550 ppm) atmospheric CO2 and waterlogging on gas exchange, biomass accumulation and allocation, and functional traits for juveniles of three woody riparian tree species. In particular, we were interested in whether elevated CO2 mitigated growth reduction under waterlogging, and whether this response was sustained following a refractory 'recovery' period during which soils were re-aerated. We found species-specific effects of atmospheric CO2 concentration and waterlogging status on growth, gas exchange, and functional traits between species, and no evidence for a general effect of elevated CO2 in mediating plant responses to flooding. For one specie, Casuarina cunninghamiana, elevated CO2 substantially increased growth, but this effect was entirely removed by waterlogging, and there was no recovery following a refractory period. Differential responses to combined waterlogging and elevated CO2 among species may result in compositional changes to riparian plant communities and associated changes in ecosystem functioning. [ABSTRACT FROM AUTHOR]

Riparian ecosystems are biophysically complex and highly diverse taxonomically, structurally and functionally. While many environmental factors determine the structure and function of riparian vegetation communities, hydrology is thought to be the 'master variable'. Flooding and variability in water availability are known to be key drivers of taxonomic diversity, but their influence on the functional trait diversity of riparian vegetation communities remains largely unexplored., We collected data on species abundance, quantitative plant functional traits and hydrology from 15 sites distributed across south-eastern Australia to address the following questions: (a) Is functional trait diversity related to the frequency and magnitude of flooding disturbance? (b) Is functional trait diversity related to variability in seasonal water availability within the riparian zone?, We confirm that metrics describing both flooding disturbance and patterns of water availability exhibit strong relationships with functional trait diversity in riparian vegetation communities of south-eastern Australia. Our key finding is that functional trait diversity in these systems tends to be positively associated with variability in hydrological conditions and the intensity of rare, high-magnitude flooding events, rather than average patterns of flow., Our study highlights the importance of extreme flooding events and temporal patterns of water availability as determinants of diversity in riparian vegetation communities. These relationships may have significant consequences for plant communities experiencing alterations to hydrology caused by anthropogenic flow modification and the changing climate. [ABSTRACT FROM AUTHOR]

Wood density is a key plant functional trait which integrates the trade-offs characteristic to riparian plant ecological strategies. Although high-density wood is costly to construct, it confers mechanical stiffness to stems, increasing a plant's capacity to withstand flooding, and also enables increased tolerance to water stress. For riparian plants, fluctuations in soil moisture driven by surface hydrology should therefore be an important driver of variation in wood density., We asked the following questions in the study: (1) Does wood density increase with increasing frequency and magnitude of flood disturbance? (2) Does wood density increase with increasing unpredictability of water availability in the riparian zone? (3) Does dispersion of wood density peak at intermediate levels of hydrological disturbance?, We surveyed wood density of dominant species at 15 riparian sites along flow-gauged rivers across south-eastern Australia. Due to the broad range of hydrological variability associated with Australian river systems, this set of sites functions as a useful model for assessing the response of riparian plants to changing hydrological conditions., We found wood density varied strongly along a single axis of hydrological variability. This axis integrates flood intensity and frequency with metrics of hydrological unpredictability and can be conceptualized as a gradient of environmental harshness, with higher wood density associated with harsher conditions., Synthesis. Our study highlights the importance of hydrological conditions, particularly disturbance and environmental unpredictability, as determinants of ecological strategy in riparian plants. Large, rare flood events in particular appear to favour higher wood density strategies. This is likely to have significant ecological consequences for riparian plant communities in a south-east Australian context, as well as in other regions where increasing climatic variability and frequency of extreme events are hallmarks of climate change predictions. [ABSTRACT FROM AUTHOR]

Throughout eastern Australia, and much of the New World, river systems have undergone a series of hydro-geomorphic and vegetative changes following colonisation. Recent research has shown that river recovery is occurring, as demonstrated by improved geomorphic and vegetative condition, but there is limited information on the extent of returning riparian vegetation and its effect on roughness and flow hydrology. Using the Allyn River, New South Wales, Australia, as a case study, historical aerial photographs and satellite imagery were used to characterize the structural changes brought about by geomorphic recovery and quantify the amount of riparian vegetation cover that has returned to the system since the 1940s. Along the Allyn River, total riparian vegetation canopy cover has increased from 30% in 1940 to 38% in 1967 and 63% in 2016. Using space-for-time substitution and retrospective analysis, riparian vegetation roughness, calculated using a terrestrial laser scanner (TLS) gap fraction method, showed an increase in average Manning's n from 0.0156 in 1940 to 0.0194 in 1967 to 0.0372 in 2016 for flows that reach a 2 m stage height (approximately 1 in 2 yr flood average recurrence interval). As a result, flood wave travel time has extended by over 8 h (or 220%) for small to moderate within-channel flows, but further river recovery is required to affect major flow hydrology. This study highlights the outcomes of catchment-scale passive river recovery and provides improved methods for quantifying site and stage height specific vegetative roughness. The demonstrated influence of increased vegetation cover on flow hydrology and geomorphic condition holds significant promise for agencies, practitioners and land managers to achieve substantial river recovery outcomes using active and passive vegetation rehabilitation and river management. [Display omitted] • Geomorphic and vegetative recovery are occurring along the Allyn River, Australia. • A terrestrial laser scanner gap fraction method is used to quantify roughness changes. • Flood wave travel time has increased by over 8 h (or 220%) for small to moderate in-channel flows. • Further river recovery is required to affect major flood hydrology. • Passive river management can achieve substantial geomorphic recovery and flood mitigation. [ABSTRACT FROM AUTHOR]

Soil and sediment seed banks contribute to the diversity of riparian plant communities. In degraded river systems, seed banks represent an important regeneration niche that may contribute to restoration efforts through the establishment of vegetation. The vertical dimension of seed banks has been neglected in river research, despite its importance for the regeneration of vegetation after disturbances such as erosive floods., We sampled sediment at various depths within three geomorphological features: bars, benches and the floodplain, across four river reaches in the Wollombi subcatchment of New South Wales, Australia. A seedling emergence study was conducted to characterise the abundance and species richness of the germinable seed bank within these sediments. We hypothesised that the vertical distribution of seeds in bars and benches would show no clear pattern, but that bars would have lower propagule counts overall, due to their non-cohesive sediment and potential for frequent reworking by low-level flows. The floodplain seed bank, in contrast, would resemble that of terrestrial systems, with propagule abundance decreasing markedly with depth due to infrequent inundation and sediment reworking., In total, 9456 seedlings emerged, representing 131 different species (83 native and 47 exotic) from 47 families. Propagule abundance and species richness in bar and bench seed banks were highly variable with depth, with the greatest average propagule numbers found at 25-30 cm and 20-25 cm, respectively. In contrast, and as hypothesised, propagule abundance and species richness in the floodplain decreased significantly with depth. Propagule abundance was surprisingly variable in bars, with some displaying extremely high values and others containing no detectable seeds, although overall species richness was significantly lower than in benches and the floodplain., The vertical distribution of seeds in bars, benches and floodplains may be determined by the proportional influence of hydrochory (seed transport and deposition by water) during deposition events and seed losses, resulting from sediment reworking and erosion, set within the timescales over which they are formed and reworked. Bar seed banks are continually flushed by frequent inundation and reworking, especially at the surface, reducing seed deposition and burial. Abundant seed fall may be provided by local vegetation, however. Diverse seed banks in benches may form through alternating periods of hydrochoric seed deposition along with sediment, augmented during periods of exposure when propagules from the extant vegetation accumulate. Decreases in germinable propagule abundance and species richness with depth in the floodplain may reflect much slower rates of vertical accretion and seed losses due to mortality over time. Finally, we present some implications for the management of riparian vegetation and applications for river restoration. [ABSTRACT FROM AUTHOR]

Restoration of native vegetation often focuses on the canopy layer species, with the assumption that regeneration of the understory elements will occur as a consequence. The goal of this study was to assess the influence of canopy restoration on the composition and abundance of understory plant species assemblages along riparian margins in the Hunter Valley, NSW, Australia. We compared the floristic composition (richness, abundance, and diversity) of understory species between nonrevegetated (open) and canopy revegetated plots across five sites. A number of other factors that may also influence understory vegetation, including soil nutrients, proximity to main channel, and light availability, were also measured. We found that sites where the canopy had been restored had lower exotic species richness and abundance, as well as higher native species cover, but not native species richness, compared with open sites. Multivariate analysis of plots based on plant community composition showed that revegetated sites were associated with lower total species diversity, light availability, and exotic cover. This study has found that the restoration of the canopy layer does result in lower exotic species richness and cover, and higher native species cover and diversity in the understory, a desirable restoration outcome. Our results provide evidence that restoration of native canopy species may facilitate restoration of native understory species; however, other interventions to increase native species richness of the understory should also be considered as part of management practice. [ABSTRACT FROM AUTHOR]

In perennial stream settings, there is abundant literature confirming that riparian vegetation affects flood hydrology by attenuating the flood wave, enhancing deposition and reducing bank erosion. In contrast, relatively little is known about the effectiveness of riparian vegetation during floods in hydrologically-variable regions. The dominant channel form in these settings is often referred to as a ‘macrochannel’ or compound channel-in-channel which displays multiple inundation surfaces where it is often difficult to identify the active channel bank and bank top. This study uses the inundation pattern of recent flood events in the Lockyer Valley of South East Queensland (SEQ), Australia to present a framework which specifically considers the interaction between inundation frequency and trapping potential on a range of inundation surfaces. Using hydrological modelling and a consistent definition of floodplains and within-channel features, it outlines five key priority areas for the placement of riparian vegetation to alleviate common flood problems within the catchment. The highest priority for the placement of riparian vegetation to ameliorate the effects of small-moderate floods is on within-channel benches. For out-of-macrochannel flows, riparian vegetation is most effective on genetic floodplains which occupy the largest spatial extent within the valley. In particular, it identifies the need for, and benefits of, revegetation in spill out zones (SOZ) which occur where upstream channel capacity is larger and flow is funnelled at high velocity onto the floodplain downstream. This study highlights the importance of understanding the key geomorphic processes occurring within a catchment and developing effective catchment management plans to suit these conditions. [ABSTRACT FROM AUTHOR]