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Human modification of waterways has reduced flooding in many river systems, leading to the decline of riparian forests, which rely on flooding for their regeneration. Coppicing may help to promote the persistence of riparian trees by triggering resprouting and vegetative regeneration. The vigour of resprouting plants can vary with timing and height of coppicing and may depend on stored non-structural carbohydrate reserves like starch, the availability of which can vary seasonally. However, starch storage dynamics and the resprouting potential of broad-leafed evergreen riparian trees is not well understood. We coppiced two riparian tree species, Eucalyptus camphora and Melaleuca squarrosa, at two different times (autumn, spring) and at two different heights (0 cm and 90 cm). Over 52 weeks, we regularly quantified shoot growth and changes in the starch storage pool size, compared to uncoppiced control trees, in different tree organs (root and stem) and estimated the final shoot volume. The final shoot volume did not differ significantly between coppice treatments. Trees coppiced in autumn had a greater reliance on stored starch while they remained leafless (without shoots) over winter. Trees cut at 90 cm had more starch reserves due to remaining stems but also had higher biomass maintenance costs. Starch storage varied seasonally only in E. camphora, with starch concentrations in control trees increasing over winter and decreasing over summer. Although coppice timing and height affected use of stored starch, resprouting in our study species was not limited by starch availability - both species regenerated vegetatively to recover from physical disturbance. Thus, coppicing may be an efficient means to promote rejuvenation and persistence of tree species where site and tree condition are degraded and no longer support recruitment.

Riparian forest ecosystems are declining globally. Many floodplains no longer flood and thus cease to satisfy the hydrologic requirements for riparian tree maintenance and regeneration. To promote woody riparian plant recruitment where flood regimes have been altered by flow regulation, effective approaches to restoration need to be developed. We implemented a landscape-scale experiment in a remnant, temperate floodplain forest. By constructing two weirs within channelized reaches of a stream, we redirected flows into networks of historic distributary channels, which facilitated widespread floodplain inundation. Using a control-reference-impact study design, we assessed the establishment and growth of planted seedlings of three woody species (Eucalyptus camphora, Leptospermum lanigerum and Melaleuca squarrosa) over 13 months in response to flooding achieved by floodplain reconnection. Planted seedlings had higher height and diameter growth rates at both induced (19–29 cm, 1 mm) and naturally flooded (34–44 cm, 3–5 mm) than at non-flooded (4–10 cm, −5 to −3 mm) sites. However, survival rates and temporal growth patterns differed between species according to variation in flood duration and soil moisture, illustrating the different hydrological requirements of the coexisting species. This highlights that variable flooding and drying patterns are essential to create recruitment niches for different riparian plant species and shows the importance of river-floodplain connectivity for providing adequate flooding regimes. Our study demonstrates the suitability of two complementary restoration approaches – restoring hydrology and active revegetation – for promoting the regeneration of riparian forests.

Much research effort has been invested in understanding ecological impacts of invasive alien species (IAS) across ecosystems and taxonomic groups, but empirical studies about economic effects lack synthesis. Using a comprehensive global database, we determine patterns and trends in economic costs of aquatic IAS by examining: (i) the distribution of these costs across taxa, geographic regions and cost types; (ii) the temporal dynamics of global costs; and (iii) knowledge gaps, especially compared to terrestrial IAS. Based on the costs recorded from the existing literature, the global cost of aquatic IAS conservatively summed to US$345 billion, with the majority attributed to invertebrates (62%), followed by vertebrates (28%), then plants (6%). The largest costs were reported in North America (48%) and Asia (13%), and were principally a result of resource damages (74%); only 6% of recorded costs were from management. The magnitude and number of reported costs were highest in the United States of America and for semi-aquatic taxa. Many countries and known aquatic alien species had no reported costs, especially in Africa and Asia. Accordingly, a network analysis revealed limited connectivity among countries, indicating disparate cost reporting. Aquatic IAS costs have increased in recent decades by several orders of magnitude, reaching at least US$23 billion in 2020. Costs are likely considerably underrepresented compared to terrestrial IAS; only 5% of reported costs were from aquatic species, despite 26% of known invaders being aquatic. Additionally, only 1% of aquatic invasion costs were from marine species. Costs of aquatic IAS are thus substantial, but likely underreported. Costs have increased over time and are expected to continue rising with future invasions. We urge increased and improved cost reporting by managers, practitioners and researchers to reduce knowledge gaps. Few costs are proactive investments; increased management spending is urgently needed to prev

Riparian forest loss and degradation due to river-floodplain disconnection is a global problem. Prospects for the maintenance and recruitment of riparian trees via restored flooding can be uncertain, in part due to competition from understorey vegetation and limited availability of tree propagules. In a field-based trial, we assessed the response of a keystone riparian tree to restored flooding, reduced competition and seed addition. We built diversion weirs to reconnect floodplains supporting riparian forest. Using a multiple control-intervention-reference study design with two restored flooding (intervention) sites, two naturally-engaged (reference) and two dry (control) sites, we assessed seedling recruitment of the dominant tree, Eucalyptus camphora, with and without clearance of understorey vegetation and with and without seed addition. We also assessed the growth of extant trees using dendrometers and seedfall using funnel traps at all six sites. Our weirs resulted in extensive flooding of the adjacent floodplain, while control sites remained dry. Flooding increased seed germination, seedling establishment, tree growth and seedfall. However, seed germination benefits were only realised where understorey vegetation was cleared and seed was added. Seedling establishment was also limited by dry summer conditions, except where flooding duration was longest (∼6 months cf. < 3 months). Restored flooding via river-floodplain reconnection is likely to promote the rehabilitation of riparian forests degraded by flow regulation or stream modification through benefits at multiple tree life-history stages. However, widespread tree recruitment may require complementary works that reduce competition via clearing of understorey vegetation and alleviate seed limitation by direct seeding or planting.