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Priority effects, where species that arrive first influence later arriving species, are often considered in terms of seed arrival time. However, the timing of seedling emergence may play a more critical role, as this defines when plants start interacting. Further, initial seed density may also be important, allowing early-arriving species with low initial seed density to overcome seed limitation, while also potentially allowing late-arriving high-density species to overcome the disadvantage of arriving late. In this large-scale, multi-site field experiment, we manipulated species arrival and emergence timing by sowing fast- and slow-germinating meadow species in various arrival orders and seed densities across two climatically contrasting sites in Sweden. Our findings demonstrate that germination speed modulates the strength and direction of priority effects: fast-germinating species were less affected by both early- and late arrival. Conversely, slow-germinating species were disadvantaged by late arrival and benefited significantly from early arrival, particularly at the more productive, northern site with shorter growing seasons. Contrary to expectations, initial sowing density had limited and inconsistent effects on priority effect outcomes. These results highlight that emergence timing, not just seed arrival, is a key aspect of priority effects, influencing plant competition and community structure. Furthermore, the context dependency across sites emphasizes the importance of environmental conditions in modulating priority effects, with implications for predicting vegetation dynamics under climate change.

Reservoir shorelines of regulated rivers, created by over 2.8 million dams worldwide, have experienced substantial biodiversity loss, particularly in plant communities. While actively introducing local riparian plants is a common restoration strategy, existing approaches often lack applicability and transferability across river basins. To address this, we propose a guild-based restoration framework that classifies plant species used for restoration into guilds based on shared functional traits and adaptive strategies. This approach allows for evaluating restoration outcomes across guilds along environmental gradients, optimizing restoration design.

We tested this framework along the shorelines of the Three Gorges Reservoir (TGR), characterized by steep environmental gradients. Plant guilds were identified based on shared functional traits, focusing on hydrological and geomorphological associations. The effectiveness of guild introductions was assessed across gradients of submergence intensity, topography and substrate properties by comparing outcomes to those of spontaneous colonization.

Results showed that under intermediate and high submergence intensity, active plant introduction and spontaneous colonization yielded similar restoration outcomes, with short-clonal flood-tolerant herbs naturally dominating. At low submergence intensity, the introduction of flood-tolerant woody plants increased functional diversity, whereas tall-clonal flood-tolerant herbs reduced diversity due to competitive exclusion. Actively introduced plants did not suppress invasive species. Unexpectedly, under intermediate submergence intensity, the introduction was associated with increased invasive plant presence.

Synthesis and applications: Our study validates the guild-based framework as an effective approach for shoreline vegetation restoration in regulated rivers. We show that guild identity and submergence intensity jointly shape restoration outcomes, offering insights for optimizing active plant introduction strategies in the TGR and similar reservoirs in the upper Yangtze River. Furthermore, this framework enhances the transferability of restoration practices by offering a functionally driven approach to species selection across river basins.

Over two-thirds of global rivers are subjected to flow regulation. Although it is widely recognized that flow regulation can adversely affect riparian vegetation—a critical component of river ecosystems—the specific roles of various drivers remain poorly understood. To address this gap, we conducted a broad-scale meta-analysis, aiming to elucidate how different factors mediate the adverse impacts of flow regulation on riparian vegetation. This meta-analysis encompassed 59 papers, spanning 278 dams constructed on 146 rivers. We extracted data on four key indices of riparian vegetation: species richness and abundance of all riparian species, and those indices exclusively for non-native species. Indices were compared between regulated and free-flowing or pre-damming rivers to quantify the impact of flow regulation. Our meta-analysis revealed a moderate but significant reduction in the richness and abundance of all riparian species under flow regulation, coupled with a strong increase in the abundance of non-native species. Riparian vegetation in arid and continental climate regions experienced stronger negative impacts than those in tropical and temperate climates. Furthermore, the adverse effects on riparian vegetation were more pronounced downstream of dams than upstream. Considering climate region, study identity, and relative position to the dam as random variables, it became evident that years since flow regulation emerged as the most important factor influencing species richness. Over time, richness gradually recovered from initially low levels. However, this recovery was slowed by increasing flow regulation intensity (percentage of annual runoff stored). Additionally, the impact was more evident in larger rivers. To support regulated river management, we recommend prioritizing the protection of riparian vegetation in arid and continental climates, with emphasis on areas downstream of dams, limiting flow regulation intensity, particularly in larger rivers, and monitoring non-native species to prevent disproportionate spread.

This work examines the relationships between flow restoration, natural flow regimes and riparian vegetation utilizing a case study in northern Sweden. The riparian zone is one of the most species-rich ecosystems and forms an important link between aquatic and terrestrial systems. The integrity of the riparian zone and its vegetation is harmed by flow alteration in numerous rivers, which calls for enhancing riparian management and ecological mitigation measures. Here, we take an ecohydraulic approach, combining a hydraulic model and data on inundation tolerance of riparian vegetation to evaluate the effects of reintroducing seasonal flow variation in a bypassed reach with minimum discharge. The results show that implementing the seasonal flow variation is projected to benefit riparian vegetation by extending the riparian zone and to lead to the development of distinct vegetation belts similar to riparian vegetation along free-flowing rivers. Additional simulations demonstrated that a further increase in riparian area could be achieved by increasing the magnitude of the minimum flow release. While the method assumes the riparian vegetation to be in equilibrium with the flow regime, continued monitoring is needed to assess how fast the riparian vegetation adjusts to new flow conditions.

Hydropeaking causes unnatural flow and water level fluctuations in the riparian zone. Biological processes in the riparian zone, such as nutrient cycling, are affected by changes in temperature, oxygen, and moisture, which are all to some extent influenced by the water level. Here, we studied the relationship between hydropeaking and decomposition of organic matter in the riparian zone. Specifically, we measured the decomposition rate (k) and decomposition endpoint (the stabilization factor, S), and examined associations between k, S, hydropeaking, and different environmental variables. Decomposition was estimated using the Tea Bag Index at 33 sites in Swedish rivers and streams with different degrees of hydropeaking. Rivers and streams were sampled in the summer of 2020 (N = 26) and autumn of 2023 (N = 16), with nine sites sampled on both occasions. In summer 2020, k was negatively related to high hydropeaking intensity, but there was no effect of hydropeaking on S. In autumn 2023, there was no effect of hydropeaking on k or S. Of the other environmental variables tested (substrate grain size, temperature, and length of growing season), the growing season had a positive relationship with S in autumn 2023. Our findings indicate that reducing hydropeaking during summer, when biological activity is high, may be beneficial for maintaining riparian function.

Aim: In riparian zones along rivers, plant demography is shaped by hydrologic disturbances, the dendritic structure of the river networks, and asymmetric gene flow due to the prevalence of unidirectional dispersal by hydrochory. Downstream-biased dispersal may lead to the accumulation of genetic diversity in populations situated lower within the catchment area—a phenomenon referred to as ‘downstream increase in intraspecific genetic diversity’ (DIGD). Our study aimed to test if the presence of this pattern in riparian plants depends on the species traits, sampling design and ecosystem integrity.

Location: Riparian zones along rivers worldwide.

Time Period: 1978–2023.

Major Taxa Studied: Vascular plants.

Methods: We conducted meta-analysis of population genetic studies on riparian plants to identify the factors linked to the occurrence of the DIGD pattern. We modelled the correlation between position along rivers and population genetic diversity using a dataset consisting of variables extracted from the studies, supplemented by data from plant trait databases.

Results: We found no evidence for a general trend in plant genetic diversity along rivers, but species traits and environmental factors partially explained the patterns. A downstream increase in genetic diversity was more likely to be found in species capable of hydrochoric dispersal and along the unmodified rivers which maintain habitat continuity.

Main Conclusions: Our study highlights that different patterns of genetic diversity can be linked to species traits or different levels of habitat fragmentation. Population genetic studies of riparian plants have frequently investigated patterns of genetic diversity in remnant populations in degraded riparian habitats. Although such investigations are important, more population studies of common plants in well-preserved riparian zones are needed, as these can help understanding the general mechanisms that control natural population dynamics of plant species.

Hydropower production leads to losses of species and ecosystem types in rivers. A major knowledge gap is represented by bypassed reaches where all or most discharge is diverted to hydropower stations. Bypassed reaches may have some flow, derived from tributaries and dam leakage, along with minimum flow. In Sweden, mandates for minimum discharge are lacking for most bypassed reaches, and when present, generally amount to only a few percent of mean annual discharge. We asked how fish communities in bypassed reaches differ from comparable non-regulated reference reaches, and whether minimum discharge can help sustain fish faunas typical of non-regulated rivers. We assembled a unique data set of 2693 samples collected via electrofishing by wading from 166 bypassed reaches and 247 free-flowing reaches spread across Sweden. To quantify flow-related community changes, we compared the relative dominance of rheophilic (preference for high flow conditions) versus limnophilic (preference for slow-flowing to stagnant conditions) fish species, fish density and fish species richness between bypassed and free-flowing reaches and investigated the effects of minimum discharge on community structure. Bypassed reaches had lower rheophilic dominance and fish density, but slightly higher fish species richness than comparable free-flowing reaches. Minimum discharge had positive effects on rheophilic dominance and fish density but not fish species richness. For relatively low values of minimum discharge, small changes in flow corresponded to large changes in the predicted values of fish density (up to about 0.5 m3/s) and rheophilic dominance (up to 2 m3/s), after which curves flattened. When minimum discharge was expressed as a percentage of mean annual discharge, rheophilic dominance and fish density increased almost linearly with increasing discharge, but the effects on fish density were dependent on river size. Synthesis and applications. We conclude that small increases in minimum discharge can be particularly beneficial to the restoration of rheophilic community structure, especially at low levels of minimum flow, even though the area of lotic habitat has been reduced to a fraction of natural conditions. Implementation of minimum discharge in bypassed reaches can thus be an effective rehabilitation measure, providing ecological benefits helping to meet environmental objectives.

Priority effects, the effects of early-arriving species on late-arriving species, are caused by niche preemption and/or niche modification. The strength of priority effects can be determined by the extent of niche preemption and/or modification by the early-arriving species; however, the strength of priority effects may also be influenced by the late-arriving species, as some species may be better adapted to deal with niche preemption and/or modification. Therefore, some combinations of species will likely lead to stronger priority effects than others. We tested priority effects for all pairwise combinations of 15 plant species, including grasses, legumes, and nonleguminous forbs, by comparing simultaneous and sequential arrival orders in a 10-week-long, controlled, pot experiment. We did this by using the competitive effect and response framework, quantifying the ability to suppress a neighbor as the competitive effect and the ability to tolerate a neighbor as the competitive response. We found that when arriving simultaneously, species that caused strong competitive effects also had weaker competitive responses. When arriving sequentially, species that caused strong priority effects when arriving early also had weaker responses to priority effects when arriving late. Among plant functional groups, legumes had the weakest response to priority effects. We also measured plant functional traits related to the plant economic spectrum, which were combined into a principal components analysis (PCA) where the first axis represented a conservative-to-acquisitive trait gradient. Using the PCA species scores, we showed that both the traits of the focal and the neighboring species determined the outcome of competition. Trait dissimilarities between the focal and neighboring species were more important when species arrived sequentially than when species arrived simultaneously. Specifically, priority effects only became weaker when the late-arriving species was more acquisitive than the early-arriving species. Together, our findings show that traits and specifically the interaction of traits between species are more important in determining competition outcomes when species arrive sequentially (i.e., with priority effects present) than when arriving simultaneously.

Key message: Seed germination responses to variation in temperature and light differed among six dry forest species, results that will inform ecological restoration and climate change adaptation projects.

Abstract: In dry forests, where opportunities for plant establishment occur in a narrow window of opportunity, seeds must respond to cues to germinate when conditions for growth are suitable. Knowledge of the strategies and adaptations of seeds to the seasonal dry-forest ecosystems, being under constant threat, is needed to guide restoration and management actions in the face of climate change. We investigated the effects of scarification, temperature and light in germination percentage, germination time and synchrony of six woody Fabaceae species. The species have ecological potential for restoration and are of cultural or economic importance for the local people in the Tehuacán-Cuicatlán Valley, Mexico. We carried out a multifactorial germination experiment with five temperatures, two light regimes and two scarification conditions for Mimosa luisana, M. polyantha, M. adenantheroides, M. lactiflua, Acaciella angustissima and Vachellia constricta. All germinated in a wide range of temperatures (10–40 °C), and mechanical scarification highly increased the germination percentage. Higher temperature increased and speeded up germination in dark conditions for most of the species, but they exist heterogeneous responses in their germination synchrony. Studied species had high germination percentages in warm temperatures, but their recruitment in nature might be negatively affected by warmer and drier conditions, and by the loss of shade and seed dispersers due to deforestation and changes in land use. It is crucial to study not just germination percentage and time but also other aspects of the germination process such as the germination synchrony, since it might reveal useful information for management actions.

Hydropower is central to renewable electricity systems, but degrades ecosystems, calling for environmental flow schemes to enhance the ecological status of river systems. Environmental flow assessments need to account for climate change, since climate-driven changes in runoff affect both hydropower operation and riverine ecosystems. Here, we quantify expected changes in hydropower production and environmental benefits of introducing environmental flows in a large regulated river system in northern Sweden in a future climate. Compared with the hydrology of 1981–2010, runoff is projected to increase with climatic conditions projected for 2040, leading to a 2.2 % increase in hydropower production with present rules for turbine and reservoir operation. Implementing environmental flows will result in lower hydropower production losses in with the 2040 climate than at present: Introducing restrictions against zero flow events, discharge to technical fishways and bypassed reaches throughout the year (with seasonal flow variation), as well as having more natural water-level variation in all run-of-river impoundments, would reduce annual hydropower production with 3.5 % with present conditions, and by 3.3 % in 2040. At the same time, the net effect of higher runoff and introducing environmental flows means that the annual hydropower production in the 2040 climate would be only 0.8 % lower compared to 1981–2010. In all scenarios, reservoir filling degree in 2040 was projected to increase compared to scenarios for 1981–2010, and flow requirements were met for both environmental flows and hydropower production over an 83-year scenario-based time series. This study demonstrates the feasibility of introducing environmental flow actions in Sweden, and other regions where increases in runoff are projected, with sustained hydropower production, having large benefits for riverine biodiversity and enhancing resilience of riverine ecosystems to climate change. For this to be successful, collaboration among stakeholders in riverine management is needed.