Functional traits: the pathways to riverine plant resistance in times of hydropeaking

Abstract

Hydropeaking, which refers to rising or falling discharges caused by the turning on or off of hydro-turbines to generate electricity, is a topic of growing interest due to its impact on fluvial ecosystems. To date, most hydropeaking studies have focused on the impact of peak fluctuations on invertebrate and fish communities, but little attention has been paid to its impact on riverine plants and how functional traits may make them resistant to hydropeaking. We performed a review to assess how a set of 32 plant functional traits can be expressed in riverine plant species, and found evidence of how such expression affects their capacity to cope with common sources of hydropeaking disturbance (i.e., inundation, fast water drawdown, and rapid water fluctuations linked to up-ramping and down-ramping hydropeaking operations). The information here presented can simplify the evaluation of impacts on riverine plant communities worldwide, as well as to detect suitable species to successfully restore rivers affected by hydropower production.

Background

Hydropeaking (HP), which refers to repeated daily or sub-daily sudden water releases downstream of a hydropower plant in response to daily peaks of electricity demand (e.g., Cushman 1985; Morrison and Smokorowski 2000; Costa et al. 2019), is considered a flexible method of producing electricity because it can generate power within minutes. Unfortunately, it is also one of the most ecologically harmful modes of hydropower generation since the river ecosystem’s downstream biotic and abiotic elements suffer from extreme flow events that deviate greatly from the free-flowing natural regime (Smokorowski 2022). The range of flow alterations resulting from HP depends highly on the operation scheme of the hydropower plant (i.e., magnitude, rate of change, frequency, duration, and timing of HP operations; Harby and Noack 2013), but four distinct operational phases are commonly distinguished: baseflow discharge (no electricity production), increasing discharge or up-ramping (start of the turbines), continuous high peak discharge (peak energy demand), and decreasing discharge or down-ramping (shutdown of the turbines) (e.g., Bruder et al. 2016; Tonolla et al. 2017). The succession of multiple daily hydropeaks (with their respective operational phases) introduces a significant number of artificial flow events altering the ecological integrity of the river downstream of hydropower stations (e.g., Bunn and Arthington 2002).

Like other organisms, riverine plants in HP systems are exposed to frequent inundations, fast water drawdowns, and short-term fluctuations in water flow and water levels, but also extended periods of drought linked to periods of low energy supply and along bypassed river reaches (Stella et al. 2010; Bejarano et al. 2020; Baladrón et al. 2022). Novel hydrology caused by HP has no natural correspondence in freshwater systems, and hence few species have adaptations to all its aspects (Bejarano et al. 2018). Although riverine plants have mechanisms and adaptations to cope with peak flows, drought periods, fast water currents, and rapid water-level changes (see Kawecki and Ebert 2004), the frequent and abrupt variations in hydraulic parameters imposed by HP may exceed plant tolerance (Bejarano et al. 2018), ultimately affecting germination, growth and long-term survival, inducing changes in plants´ morphology and physiology (Baladrón et al. 2022), and modifying long‐established patterns of local adaptation.

Climate change is putting pressure on governments to reduce reliance on fossil fuels. In this regard, hydropower can play a dual role in the transition toward a low-carbon energy future (Jager et al. 2022). First, it can support the grid when wind and solar are not available. In addition, hydropower has flexibility similar to that provided by natural gas and may constitute a key renewable source in the electricity portfolio (Shan et al. 2020). Therefore, given that HP is unavoidable (Smokorowski 2022), it is critical to understand the adaptive capacity of riverine plants to the various environmental stresses resulting from it to ensure proper river management. The study of plant functional traits can serve this purpose, as they are behind the mechanisms that determine whether species manage to adapt to environmental change and stress (Kearney et al. 2010; Pollock et al. 2012; Regos et al. 2019).

Functional traits are defined as morphological, anatomical, biochemical, physiological, and phenological features or mechanisms of resilience, with the capacity to impact fitness via their effects on growth, reproduction, and survival of individuals (McGill et al. 2006; Violle et al. 2007; Mouillot et al. 2013; Nock et al. 2016). The use of plant functional traits to investigate the effects of human disturbance at multiple field scales (from plant communities to ecosystem-level functions; e.g., Suding et al. 2008) is gaining momentum nowadays (e.g., Garnier et al. 2004; Miedema Brown and Anand 2022). The adaptive capacity of plants to the surrounding environment depends on functional trait expression (e.g., Pollock et al. 2012), meaning that only species adopting particular trait values or modalities have the opportunity to become abundant under certain environmental conditions (Tang et al. 2022). The values or modalities taken by each trait are called attributes (see Violle et al. 2007). Independently of their taxonomic identity, plants sharing identical attributes are expected to respond similarly to environmental stressors (Lavorel et al. 1997; Merritt et al. 2010; Stromberg and Merritt 2015) and will display similar degrees of tolerance to them (see Valladares et al. 2007). Environmental filters, such as fluvial disturbances, can exclude species lacking suitable attributes from entering and remaining in a community, consequently shaping its structure (Venn et al. 2011). In this respect, the novel hydrology imposed by HP regimes may filter out species from functional groups without appropriate trait attributes to cope with flooding, frequent water-level fluctuations, and drought (i.e., intolerant plants). Conversely, species with attributes suitable to deal with the new fluvial environment (i.e., easily dispersed, flexible, flood-tolerant, and amphibious plants; Keddy 1992; Mouillot et al. 2013; Bejarano et al. 2018) may successfully germinate, grow, and persist.

Identifying functional trait attributes capable to confer plants advantages against common HP disturbances may help determine which riverine communities are vulnerable to HP and, conversely, which ones may reasonably withstand it without risk of suffering severe degradation (Baladrón et al. 2022). Additionally, analyzing the impact of HP on plant species from a trait-based perspective is more informative than predicting their response according to “ecological affinities” (i.e., data that describe the general response of a species to an environmental variable, habitat, or resource (e.g., drought or flooding tolerance, soil moisture); see Palmquist et al. 2017). Trait-based approaches facilitate the generalization of patterns and transfer across systems since traits may be shared among different floristic regions (Díaz et al. 1998; Bejarano et al. 2016), whereas “ecological affinities” approaches often integrate multiple traits, obscuring linkages among environmental variables and hampering efforts to avoid circularity in cause and effect (Verberk et al. 2013; Palmquist et al. 2017).

Previous trait database and website projects, including EcoFlora (Fitter and Peat 1994), BiolFlor (Klotz et al. 2002), BIOPOP (Poschlod et al. 2003), LEDA (Kleyer et al. 2008), PLANTS (USDA 2010), SID (SER, INSR, RBGK 2023), GLOPNET (Reich et al. 2009), MARIWENN (Baralotto et al. 2011), BROT (Paula and Pausas 2013), FLOWBASE (Aguiar et al. 2013), TRY (Boenisch and Kattge 2014), and eHALOPH (Flowers et al. 2015), have gathered data on biological traits and ecological affinities of thousands of common plant species throughout the world (Bejarano et al. 2016; Palmquist et al. 2017). Likewise, there is an extensive literature published on how plants benefit from functional traits to withstand the effects of fluvial disturbances and water availability gradients in a variety of freshwater ecosystems (Tables 1, 2, 3).

Table 1 Selected reviews/syntheses on functional traits that protect plants from the effects of fluvial disturbances

Table 2 Selected reviews/syntheses on functional traits that protect plants from the effects of fluvial disturbances

Table 3 Selected reviews/syntheses on functional traits that protect plants from the effects of fluvial disturbances

However, to date, no specific studies have been conducted to compile information on traits capable of improving the fitness of riverine plant species exposed to common HP disturbances. The goal of this study is to synthesize and bring together information from published sources on functional plant traits scattered in the literature which, if well selected and analyzed from an HP perspective, can help to advance science and management in the field of HP.

Selection of relevant traits in the HP context

Common HP disturbances affecting riverine vegetation

We selected the most relevant hydrological alterations linked to HP (i.e., inundation, water drawdown and water fluctuations) likely impairing plants' performance (e.g., germination, plant survival and growth). The main consequences of HP on plant performance were retrieved from Bejarano et al. (2018) and summarized in Table 4.

Table 4 Main consequences resulting from HP affecting plant performance (Baladrón et al. 2023; adapted from Bejarano et al. 2018)

Functional trait selection

We selected a list of 32 functional traits (19 morpho-anatomical and 13 physiological) which, directly or indirectly, have been extensively documented as capable to adapt plants to the stress imposed by submergence, soil waterlogging, mechanical forces exerted by large flow discharges, and drought. The selected pool of traits is therefore important with respect to the hydrological alterations and derived changes in the river environment (Table 4) resulting from HP.

A review of the TRY database (Kattge et al. 2011; Boenisch and Kattge 2014) and of multiple published papers in the field of plant traits was performed to find functional traits of interest in the field of HP. Consulted papers showed a wide variety of attribute descriptions, and focused on particular or several traits. Deciding on the number of functional traits to include in the study was a back-and-forth process in which multiple considerations were made. Some trait-based approaches rely on the smallest number of traits to explain plants’ responses to environmental conditions (principle of parsimony, e.g., Bernhardt-Römermann et al. 2008), whereas other approaches encourage to consider as many traits as possible in the analysis (e.g., Cornelissen et al. 2003). We chose to include multiple traits a priori intercorrelated and functionally redundant (i.e., traits syndrome; e.g., Bontemps et al. 2017). For instance, thick leaves, woody stems, and the presence of trichomes in plants' surfaces appear together as coordinated suites of traits to help plants prevent excessive evapotranspiration (Crang et al. 2018) during dry periods. Since these traits are functionally correlated, we could select one of them and disregard the other two. However, recent studies have highlighted that simplifying trait dimensionality can obscure overall patterns of plant adaptation because individual traits often contribute in a combined way to plant tolerance in different environments (Bruelheide et al. 2018; Yin et al. 2018). Given that HP creates complex and substantially different environments in the river within short-time scales (i.e., inundation and desiccation may occur within periods of a few hours or days), it is possible that some traits framed within a given trait syndrome may provide resistance only against a specific HP component. For instance, thick leaves, woody stems, and trichomes protect plants against drought, but not against the mechanical forces that occur during up-ramping HP stages, or against the inundation events resulting from water storage. Therefore, we believe it is key to incorporate multiple traits presumably relevant from an HP standpoint, even those functionally redundant.

Additionally, trait selection is constrained by the amount of information available in the literature (Gayraud et al. 2003). Traits may be soft or hard. A variety of soft traits (i.e., morphological traits; e.g., plant and shoot growth-form, presence of woody tissues, leaf cuticle thickness, leaf mas per leaf area) and hard traits (i.e., physiological traits; e.g., CO₂ concentrating mechanisms, antioxidants generation, stomatal control aperture) provide plants different abilities to live in a range of magnitudes, frequencies, and durations of extreme flow events (Bejarano et al. 2016), including those imposed by peaking hydropower plants. Soft traits are relatively easy and quick to quantify, but also less directly related to a given plant function (Hodgson et al. 1999; Cornelissen et al. 2003). Conversely, hard traits usually are more accurate indicators of plant responses (e.g., Lavorel and Garnier 2002; Cornelissen et al. 2003) than soft traits, but also time-consuming and difficult to measure. As a result, the amount of information available on soft traits was significantly higher than that published on hard traits, explaining the higher number of morphological traits included in our study.

After having a definitive list of relevant traits, we defined modalities (i.e., trait attributes under which plant species can be grouped; Violle et al. 2007). Subsequently, we evaluated the degree of protection that each trait attribute may offer against HP disturbances. Generally, an attribute that confers an advantage against one disturbance triggered by HP may at the same time confer a disadvantage against a contrasting HP disturbance. For instance, deep-rooting systems in plants may contribute to capture water during dry conditions associated to HP drawdown (e.g., Bielak et al. 2014), but they will not be of help to avoid root hypoxia (e.g., Kaelke and Dawson 2003) in waterlogged soils resulting from frequent flow rise. Following this rationale, trait attributes were reclassed into three HP categories: "resistant”, “partially resistant” and “vulnerable” (see Baladrón et al. 2023). We used the term “resistant” for attributes that may help plants to either avoid or tolerate HP disturbances (i.e., avoidance refers to traits that help plants to prevent the deleterious effects of adverse conditions, whereas tolerance consists in traits that enable plants to endure adverse conditions; Fitter and Hay 2002; Puijalon et al. 2011). On the contrary, the term “vulnerable” refers to attributes that may not offer plants the capacity to successfully avoid neither tolerate the negative effects of HP (Baladrón et al. 2023).

Traits were of binary nature when a functional property was present or absent (e.g., presence of chloroplasts in epidermis can adopt the categories “yes” or “no”), or adopted a set of categorical values e.g., stomatal control adopted three categories: (1) non-efficient control of stomatal aperture, (2) moderately efficient control, and (3) highly efficient control. The final list of selected traits, as well as the attributes adopted by each trait, is summarized in Table 5.

Table 5 Trait names, sections where they are described, and attributes corresponding to each trait

Riverine vegetation-hydropeaking response traits

Trait–disturbance interactions, as well as the degree of resistance (or vulnerability) that trait attributes may confer against HP are described from Sects. "Plant growth-form (PGF)" to "Mycorrhizal symbioses (MS)", and summarized in Table 6.

Table 6 Summary of resistance (green), partial-resistance (orange) and vulnerability (red) conferred by each trait category described in Table 5 to flooding (F), water stress (WS) and water fluctuations (WF)

Plant growth-form (PGF)

Plants can be grouped into life form or growth-form classes based on similarities in structure. PGF describes the plant form based on the place of the plant's growth-point (bud) during seasons with adverse (cold or dry) conditions (Raunkiær 1934). Plant growth-forms can be divided into nine categories: (1) nano-phanerophyte; (2) micro-phanerophyte; (3) meso-phanerophyte; (4) macro-phanerophyte; (5) chamaephytes; (6) hemicryptophytes and cryptophytes; (7) hydrophytes; (8) helophytes; and (9) geophytes. Hydrophytes can live in water or on substrates at least periodically anaerobic due to excess water (Raunkiær 1934), but will not tolerate exposure to drought. Helophytes can root in mud and waterlogged soils, but are less specialized to water than hydrophytes (Raunkiær 1934). This suggests that helophytes may withstand sedimentation of fine materials during down-ramping stages and frequent soil moisture disturbances derived from HP rapid water fluctuations, but will not cope with long-lasting submergence as hydrophytes may do. Geophytes can protect buds under the soil from desiccation and the mechanical stress induced by drag and lift forces derived from rapid water fluctuations (Raunkiær 1934). Phanerophytes (i.e., large shrubs and trees) present overwintering (perennating) structures (i.e., buds, meristems, and leaves) located high above the ground, and therefore can avoid a flood from reaching them (Raunkiær 1934). Chamaephytes, hemicryptophytes, some cryptophytes and small phanerophytes may minimize the risk of breakage by flow mechanical forces by reducing plant frontal area (Puijalon et al. 2011) but, as opposed to tall plants (i.e., phanerophytes), their structures will be close to the ground, and therefore affected even by hydropeaks of reduced water release. Geophytes have buds positioned deep in the soil (Lubbe and Henry 2020), and this may confer an advantage to desiccation periods associated to low flow release.

Shoot growth-form (SGF)

This trait describes the canopy structure of shoots (Kleyer 1995). Shoot growth-forms can be (1) submerged, attached to the substrate; (2) free-floating plants; (3) floating leaves, attached to the substrate; (4) emergent, attached to the substrate (amphibious); (5) stem prostrate; (6) stem ascending to prostrate; and (7) stem erect.

The diffusion of gases in water is about 10⁴-fold slower than in air and, therefore, plant submergence during HP events may difficult the access to gases (i.e., oxygen and carbon dioxide) for their metabolism (Voesenek et al. 2006). Aquatic plants efficiently maintain photosynthesis rates and aerobic respiration under flooding with different adaptations to overcome oxygen and carbon dioxide deficiencies (Björn et al. 2022). Submerged plants may enhance plant ventilation via passive molecular diffusion processes (Colmer 2003), promote aeration of rhizosphere via buried leaves (Winkel et al. 2011), and perform direct exchange of gases between plant tissues and water (Mitsch and Gosselink 2007).

Free-floating and plants with floating leaves can improve O₂ supply to respiring tissues via thermo-osmotic gas transport systems (e.g., thermal transpiration; Schröder 1989; Björn et al. 2022). Solar warming and cooling by transpiration of the upper leaf surface in these plants generate a temperature gradient within the leaf, inducing a flow of pressurized gas (Grosse et al. 1991; Armstrong and Armstrong 2005; Richards et al. 2012); the air enters through the stomata of young leaves that have just reached the water surface, streams through the channels of long petioles, through rhizomes and roots, and back to the external air through older leaves (Björn et al. 2022). Additionally, stomata in floating leaves and free-floating plants are found on the upper surface exposed to the air (Crang et al. 2018) favoring CO₂ uptake. Also, floating leaves contain air spaces that give the plant buoyancy allowing the leaves to float on the water surface. As in most aquatic species, ventilation is enabled by an extended system of air canals and intercellular spaces called aerenchyma (Justin and Armstrong 1987; Jackson and Armstrong 1999) that provide a diffusional pathway for oxygen transport to the roots (Crang et al. 2018). In addition to the atmosphere, gases in aerenchyma can originate from the rhizosphere or plant metabolism (Lambers and Oliveira 2019). All the aforementioned attributes help plants with floating leaves to resist flooding events derived from hydropeaks. Oxygen generated in the leaves of emergent plants during photosynthesis is transported to the submerged stem and roots, maintaining respiration when exposed to low environmental oxygen concentrations (Crang et al. 2018). In addition to aerenchyma, suction via old broken stems (Venturi-effect; e.g., Armstrong et al. 1992), air films on leaves when submerged (e.g., Armstrong and Armstrong 2014), water-repellent leaf surfaces; large air spaces inside leaves and roots, tissues remarkably porous, and barriers in roots to prevent radial O₂ loss from roots (Groot et al. 2005; Colmer et al. 2006; Herzog et al. 2018) are also special features allowing emergent plants to thrive in oxygen-deficient environments (Björn et al. 2022), similar to those imposed by HP submergence.

In contrast to aquatics, aerenchyma in terrestrial plants (i.e., rigid, non-floating, erect forms) has in general a reduced ventilation capacity, and therefore flooding will usually compromise their survival (see Björn et al. 2022). In aquatic species, intercellular spaces associated with aerenchyma contribute up to 60% of the leaf volume (Laan et al. 1989), while in non-aquatic species leaf volumes range from 2–7% (Larcher 2003). On the other hand, trees with large erect forms ensure that some parts of the plant will remain unscathed during submergence (Bejarano et al. 2018), which might help counterbalance their rather limited capacity to supply O₂ via ventilation.

While being good at resisting stress imposed by flooding conditions, aquatic plants (especially floating and submerged plants) usually present low tolerance to above-water conditions (Havens et al. 2004). This is in contrast to non-floating plants (e.g., erect stems or stems ascending to prostrate forms), which usually present stems of high density, and therefore more capable of reducing evapotranspiration and tolerate water stress.

Additionally, non-rigid, non-erect structures are less flexible and less resistant to mechanical disturbance during submergence and stages of rapid water fluctuations compared to large, erect shoot growth forms (Bejarano et al. 2018). Prostrate forms minimize the risk of breakage by mechanical forces by reducing plant frontal area (e.g., small growth forms; Puijalon et al. 2011), but will be more prone to be buried and experience mud coating. Conversely, erect stems will experience less mud coating, but their larger plant frontal area increases the risk of breakage.

Woodiness (WD)

Woodiness is a soft trait which is a surrogate of the stem tissue density and flexibility. Three categories have been defined: (1) non-woody; (2) semi-woody; and (3) woody species. Non-woody plants have flexible organs and the ability to reconfigure shape with increasing flow velocity (e.g., through flattening and the alignment of shoots in the flow direction; Kouwen and Fathi-Moghadam 2000), therefore avoiding physical injury derived from HP forces (Bejarano et al. 2018). In contrast, denser branches can be a mechanical liability because they are less flexible and cannot adopt streamline shapes under flow, which increases the likelihood of breakage (Vogel 1988; Kouwen and Fathi-Moghadam 2000). The higher density of woody tissues, on the other hand, prevents xylem embolism derived from drought. Xylem embolism occurs when gaseous bubbles are aspirated into xylem conduits (e.g., Delzon et al. 2010), a phenomenon which becomes more likely during dry periods (Tyree and Sperry 1989; Nardini et al. 2011). Under drought conditions, bending stresses occur in the common wall between an embolized and a water-filled conduit generating a negative pressure that may cause conduit collapse (Hacke et al. 2001), ultimately reducing plant hydraulic conductance (Nardini and Pitt 1999; Nardini et al. 2011). In extreme cases, the reduction of hydraulic conductance may lead to complete failure of water transport and plant death (e.g., Brodribb and Cochard 2009). The thicker (or denser) the double wall between xylem conduits, the greater the reinforcement against collapse from bending (Young 1989; Hacke et al. 2001). For this reason, woody plants, with greater internal loads on the xylem conduit walls than non-woody plants, are likely to be more resistant to HP drought events.

Leaf cuticle thickness (LCT)

The cuticle, a protective barrier of aerial surfaces of plants, acts as a diffusion barrier and, therefore, influences the diffusion of multiple molecules, including water, O₂ and CO₂ (e.g., Chen et al. 2011a, b). Leaf cuticle thickness (LCT) varies more than 100 times across species (Onoda et al. 2012), although most species present a LCT comprised between 1 and 10 μm (Riederer and Muller 2006). To our knowledge, there is no clear threshold defined in the scientific literature to accurately differentiate plant leaves based on their cuticle thickness. Scientific publications usually describe leaves with thicker cuticle layers as “thick”, “leathery” and/or “waxy”. On the contrary, leaves with rather thin cuticle layers are described as “thin”, “flat” and/or “broad” (e.g., Furlow 1979; Jermy et al. 1982; Hultén and Fries 1986; Tutin et al. 2001; López Gonzalez 2001; Clayton et al. 2006; Chytrý et al. 2021). Such adjectives may provide guidance to group species under two broad categories: (1) thin leaf cuticles (e.g., Betula pubescens, Filipendula ulmaria, Viola palustris, Agrostis sp., Rosa sp.; Baladrón et al. 2023) and (2) thick leaf cuticles (e.g., Alnus incana, Pinus sylvestris, Ledum palustre, Salix sp.; Baladrón et al. 2023). Thinner leaves facilitate underwater gas exchange (e.g., Maberly and Madsen 2002) and therefore photosynthesis (e.g., Pedersen et al. 2013) during hydropeaks. Maintenance of photosynthesis is key in sustaining both internal oxygen concentrations and carbohydrate contents in submerged plants (Mommer and Visser 2005), and it facilitates aerobic respiration (under submergence, anaerobic glycolysis can substitute aerobic respiration, but it is inefficient and toxic end products accumulate; Parolin 2012). On the other hand, plants with thicker cuticle layers enable plants to minimize water losses under drought conditions (Yul Yoo et al. 2009; Onoda et al. 2012; Arya et al. 2021), including those derived from periods of low water release for hydropower production. Thick cuticles constitute a barrier to water movement because leaves’ lipophilic barriers are very hydrophobic, which difficults water movement out of the leave (e.g., Lambers and Oliveira 2019). Additionally, thicker cuticles can increase mechanical resistance (Onoda et al. 2012), potentially helping plants to cope with rapid water flow fluctuations linked to HP operations.

Leaf shape (LSH)

Plant leaf shape plays important roles in photosynthetic capacity, water use (Givnish 1979; Nicotra et al. 2011) and physiological tolerances to abiotic stresses (Demmings et al. 2019), including that imposed by extreme hydrological events derived from HP. Leaf shape can be divided into five general categories: (1) dissected leaves; (2) leaves finely divided into cylindrical pinnae; (3) finger-like leaves; (4) needle-like leaves; (5) others (e.g., egg-shaped, oblong, elliptical, obovate). Dissected leaves present a relatively thinner boundary layer that reduces diffusional resistance resulting in a greater gain of carbon, which is potentially beneficial for enhancing photosynthesis during HP submergence (Gurevitch and Schuepp 1990; Nicotra et al. 2007). Likewise, leaves finely divided into cylindrical pinnae allow for a greater and more uniform surface for the diffusion of carbon dioxide (Crang et al. 2018). Finger-like leaves also help to circulate dissolved gases (Crang et al. 2018), hence facilitating the plant´s photosynthetic activity during prolonged submergence of aerial parts (Bejarano et al. 2018). Needle-like leaves are suited to dry environments (Du et al. 2020) and therefore can cope with HP drawdown periods. Simple leaves may not confer any clear advantage against HP disturbance.

Leaf size (LSI)

Leaf size is a key plant structural trait associated with plant growth, light interception, and photosynthetic efficiency (e.g., Rouphael et al. 2010). Raunkiær (1934) proposed the following classes of plant leaves based on their size: leptophyll (< 25 mm²); nanophyll (25–225 mm²); microphyll (225–2025 mm²); mesophyll (2025–18,225 mm²); macrophyll (18,225–164,025 mm²); megaphyll (> 164,025 mm²). Later authors have modified the classes (e.g., microphyll (< 2025 mm²); notophyll (2025–4500 mm²); mesophyll (> 4500 mm²); Webb 1959) and have sometimes used leaf length instead of leaf area (nanophyll (< 2.5 cm); microphyll (2.5–7.6 cm); notophyll (7.6–12.7 cm); mesophyll (> 12.7 cm); Boland et al. 2006). Whitten et al. (1996), Ingrouille (2012) and van der Maarel and Franklin (2012) have also provided definitions for leaf size categories. According to these classifications, three broad categories have been defined for leaf size: (1) large (> 4500 mm²); (2) medium (4500–2000 mm²); and (3) small (< 2000 mm²). Generally, smaller leaves are advantageous in dry environments, while large leaves are so in cooler and lower irradiance environments (e.g., Tozer et al. 2015). Large leaves maximize light interception (e.g., Atwell et al. 1999), and therefore may help plants to maintain photosynthetic rates under presumably low irradiance levels and cooler temperatures resulting from HP submergence. Conversely, during prolonged HP drawdowns, plants may potentially benefit from having small leaves since these will reduce evapotranspiration under soil moisture deficits and water scarcity characterizing the no electricity production phases (e.g., Bejarano et al. 2018).

Leaf consistency (LCON)

Leaves can be categorized by referring to the hardness of the leaves (Shugart and Woodward 2011). Four general leaf consistency categories can be established: (1) succulent, ligneous (with woody photosynthetic parts); (2) papery (malacophyllous, herbaceous leaves); (3) between papery and coriaceous (hard and stiff, partially sclerophyllous leaves); and (4) coriaceous (thick, leathery, sclerophyllous leaves). Succulent leaves have specialized water-storing tissues to survive under water shortages (e.g., Eggli and Nyffeler 2009; Ogburn and Edwards 2010). Coriaceous leaves are more resistant to drought than malacophylls (soft, thin, papery leaves) because cells in sclerophyllous leaves have thick and rigid walls. This characteristic prevents cell collapse caused by negative turgor pressures (contraction of the cell wall) developing in them under water stress (e.g., Oertli et al. 1990), and therefore helping the plant to cope with HP drawdown periods. Additionally, coriaceous leaves usually exhibit a high tensile strength (Wang et al. 2010) compared to more papery leaves. An advantage of papery and flexible leaves is that they float above the water where gas exchange is optimum, which is advantageous during phases of up-ramping and continuous high peak discharge.

Leaf anatomy (LAN)

This trait classifies leaves according to their water relations, as follows: (1) helomorphic (i.e., leaves with well-developed aeration tissue (e.g., aerenchyma) as an adaptation to oxygen deficiency in swampy soils); (2) hygromorphic (i.e., leaves that depend on a large supply of moisture or grow partly or completely submerged in water); (3) mesomorphic (i.e., leaves that require abundant available soil water and a humid atmosphere); (4) xeromorphic (i.e., leaves adapted to dry habitats); and (5) Kranz leaf anatomy (leaves with specialized structures to perform C₄ photosynthesis, an adaptive feature for plants to cope with water stress (see Grigore et al. 2012; Crang et al. 2018).

Hygromorphic and helomorphic leaves are adapted to maximize the diffusion of CO₂ from the water into the leaves for photosynthesis by means of ventilation systems (e.g., aerenchyma structures). Proper ventilation provides buoyancy to leaves and maintains them out of the water where gas exchange is faster. It also enables a diffusional pathway from emergent leaves to roots for oxygen transportation (Sculthorpe 1967; Crang et al. 2018). Hygromorphic and helomorphic leaves present characteristics that are key to cope with the slow gas exchange rates (Crang et al. 2018) presumably occurring under HP submergence. Some of these leaf characteristics are the presence epidermal cells with a single-layered mesophyll (or even lack of the entire mesophyll), finger-like shapes (or leaves presenting fine divisions into cylindrical pinnae), and stomata located on the leaves’ upper surface (i.e., adaxial position, very common in floating leaves).

Xeromorphic and helomorphic leaves can minimize water losses under HP drawdown and water stress periods by adopting a number of anatomical adaptations, including: thick cuticles, dense covering of trichomes, reduced number of stomata, deeply sunken stomata (which allows reducing the diffusion gradient nearby the stomate area), leaf epidermis with thick lignified secondary walls, and the presence of epistomatal cavities (i.e., depression over guard cells of stomata) filled with deposits of wax particles. Some xeromorphic and helomorphic leaves also have the ability to roll exposing the thick, waterproof cuticle, and creates a humid space in the middle of the rolled leaf (Taiz et al. 2015; Crang et al. 2018). Xeromorphic leaves may also present needle (pines), scale-like (cedars, cypress), or flattened (spruces and firs) shapes to reduce the surface-to-volume ratio and likewise transpiration (Crang et al. 2018). Mesomorphic leaves are adapted to habitats intermediate between hydrophytic and xerophytic.

Leaf mass per leaf area (LMA)

This trait refers to the ratio between leaf dry mass and leaf area (e.g., Poorter et al. 2009). This morphological trait is highly correlated with leaf and whole-plant processes (de la Riva et al. 2016), including plant’s maximum photosynthetic rates (e.g., Wright et al. 2004) and species’ potential growth rate (e.g., Ruíz-Robleto and Villar 2005). In general, (1) aquatic plants may have the lowest LMA, followed by (2) ferns; (3) herbs/grasses; (4) deciduous shrubs and trees; (5) evergreen shrubs and trees; and (6) succulents, which usually present the highest LMA (Poorter et al. 2009; de la Riva et al. 2016).

Metabolic cost production of each single leaf in low LMA plants is low compared to high LMA leaves. A “low-cost” strategy is associated with leaves with short life spans and fast turnover rates (i.e., leaves that may not age in excess and display a high photosynthetic capacity during their entire lifespan; see Westoby et al. 2002). The production of short-lasting, low LMA leaves could be an advantageous strategy for plants under submergence, drag forces, and water stress derived from HP because the ratio of resource investment to photosynthetic function will be likely maximized; in relation to its photosynthetic activity, a leaf with a short lifespan will most likely have been amortized by the time it is damaged or dragged by high flow velocities derived from HP. In addition, low LMA leaves have more light-capture area deployed per mass (Reich et al. 1997), higher leaf nitrogen concentrations (Field and Mooney 1986; Lambers et al. 1998), and shorter diffusion paths from stomata to chloroplasts (Parkhurst 1994). All these characteristics may facilitate photosynthesis under light attenuation and slow gas exchange conditions likely occurring during HP submergence. On the other hand, high LMA, usually related to cuticle thickening, allows leaves to cope better with soil moisture deficits usually triggered by HP drawdown.

Rooting depth (RDE)

Rooting depth refers to the distribution of roots over sequential soil layers ranging from the soil surface to deeper horizons (Mueller et al. 2013; Ravenek et al. 2014; Freschet et al. 2021), and it is considered a trait of drought avoidance (Kashiwagi et al. 2005; Padilla and Pugnaire 2007).

Shallow and spreading rooting systems may help plants avoid deeper anoxic soil layers, and hence root hypoxia (Hosie 1969; Pedersen et al. 2021). This rooting pattern might be beneficial for plants to deal with waterlogged riverbanks derived from increasing and maximal water-level phases of HP. Conversely, deep-rooting systems contribute to an upward transport of water and nutrient from deeper soil layers (Bielak et al. 2014). Even a small number of roots deep in the soil profile may enable plants to capture water from deeper soil layers (Caldwell et al. 1998; Jackson and Armstrong 1999), which can be extremely relevant for plants to successfully cope with dry conditions associated to HP drawdown periods.

But in addition to giving access to permanent water, deep and wide root systems may provide mechanical stand stabilization (Bielak et al. 2014) to cope with drag forces derived from HP, therefore maximizing resistance to uprooting (see Bejarano et al. 2018).

Riverine species variation in mean rooting depth is extremely ample, ranging from 0.5 m (e.g., Carex) to more than 10 m (Acacia) (Fan et al. 2017). Three rooting depth categories have been established according to the mean rooting depths (MRD) reported by Fan et al. (2017) for different plant genera: (1) shallow-rooted systems (i.e., MRD < 1.5 m; e.g., Carex, Fagus, Picea, Salix, Festuca, Abies, Stipa); (2) moderately deep-rooted systems (1.5 m < MRD < 3 m; e.g., Populus, Juniperus, Andropogon, Acer, Pinus) and (3) deep-rooted systems (MRD > 3 m; e.g., Ulmus, Quercus, Tamarix, Eucalyptus, Acacia).

Root morphology (RMO)

Root morphology is a trait tightly related to the root system architecture and provides spatial and structural information on the root system (e.g., Kalliokoski 2011), including its capacity to support plant growth via water and nutrient absorption, and to provide anchorage to the plant (e.g., Tripathi et al. 2001). Types of root morphology can be classified into three groups: (1) lateral/fibrous systems; (2) lateral and taproot; and (3) taproot.

Taproots are characterized by the presence of a primary or dominant root that grows deep into the soil while developing lateral roots from the main root. On the contrary, fibrous root systems move more horizontally than vertically, and may not penetrate deeper into the soil (e.g., Mauseth 2009). Therefore, taproots may anchor the plant more tightly and firmly to the soil than fibrous roots (see Nair 2010), hence preventing plant uprooting during HP rapid water fluctuations. Taproots may also facilitate water uptake from deep soil layers under baseflow periods (see Brunner et al. 2015) as opposed to fibrous roots, which are considered surface feeders as they do not penetrate deep into the soil (e.g., Mauseth 2009). Conversely, fibrous roots may expose plants less to deeper anoxic soil layers than taproots, and hence to root hypoxia. In this regard, fibrous roots might help plants to cope with waterlogged soils resulting from HP operations.

Shape reconfiguration (SHRE)

Some riverine plants are flexible and bend under high flow velocities. This reconfiguration can reduce drag forces acting on the plant (Vogel 1994; O’Hare et al. 2007), both by reducing the frontal area and by creating a more streamlined shape (Nikora 2010; Zhang and Nepf 2020).

In general terms, riverine plants can be categorized into the following groups: (1) plants presenting flexible, streamlined leaves and/or flapping leaves, as well as the elastic deformation of stems in response to current; (2) plants in which elastic deformation of stems likely occurring in response to current, but streamlined/flapping leaves are absent; and (3) plants with strong and rigid stems and leaves that may experience plastic deformation under strong water currents, and therefore at risk of experiencing damage or breakage (see Nikora 2010). Plants belonging to category (1) will passively follow the flow (Nikora 2010) adopting a ‘flexible and extensible’ biomechanical strategy (Koehl 1979), which may significantly reduce injuries by drag forces derived from HP. Flapping leaves under high flow motion may also improve photosynthetic activity due to a decrease in self-shading and an increase in gas exchange (Koehl and Alberte 1988; Nikora 2010), which may benefit growth on plants during maximal HP water-level phases involving turbulent and fast water currents.

Suberin barriers (SB)

Suberin is a structural lipophilic polyester of fatty acids, glycerol and some aromatics found in cell walls of plant tissues (Serra and Geldner 2022). The presence of suberin gives rise to radial O₂-loss barriers (i.e., apoplastic barriers in root outer cortex/exodermis; Manzur et al. 2015), which help plants tolerate anoxic conditions by preventing oxygen diffusion outward to the soil. Heavily suberized root tissues limit radial oxygen loss (ROL) from the root to the rhizosphere, supporting root growth in oxygen-depleted soils under flooded conditions (Armstrong et al. 2000; De Simone 2003). The content of suberin explains, in part, ROL rates from roots of riverine plants, and it is strongly species-dependent (Chen et al. 2022). For instance, the roots of wetland monocotyledonous species such as Juncus effusus and Carex acuta show very low ROL values (< 5 ng cm⁻² min⁻¹; measured 5 cm away from the root tip), whereas the dicotyledonous species Caltha palustris, Ranunculus sceleratus and Rumex palustris have roots relatively permeable to oxygen loss (ROL ranging between 20–60 ng cm⁻² min⁻¹; measured 5 cm away from the root tip) (Laan et al. 1989; Visser et al. 2000). In general, low ROL rates are linked to the presence of non-permeable, ‘strong’ apoplastic barriers, whereas higher rates indicate a lower concentration of suberin and lignin in the roots, and therefore the presence of a ‘partial’ apoplastic barrier (Visser et al. 2000). ROL values higher than 60 ng cm⁻² min⁻¹ indicate significant oxygen losses from the roots to the soil, and such values would correspond to species intolerant to flooding.

Taking the above into consideration, we define three categories: (1) presence of strong barriers (i.e., species presenting low ROL values, similar to those described for Juncus effusus and Carex acuta); (2) presence of partial barriers (i.e., plants showing ROL values similar to those described for Caltha palustris, Ranunculus sceleratus and Rumex palustris); and (3) absence of barriers aimed at preventing ROL from roots (i.e., species presenting ROL values superior to 60 ng cm⁻² min⁻¹).

The stronger the barrier is, the more limited the oxygen loss through the root will be and, therefore, the more likely it is for the plant to withstand submergence and riverbank inundation derived from hydropeaks.

Below-ground organs (BGOs) and root mass allocation

Below-ground organs (i.e., rhizomes, tubers, stolons, buds and bulbs) constitute carbohydrates stockings and provide some plants with the ability to grow clonally and regenerate after disturbance (Klimešová et al. 2018), which may potentially have positive effects on plant survival under peak flow events derived from HP. Accumulation of carbohydrates on below-ground organs of perennial herbs and shrubs may likely help them to restore the above-ground biomass (Klimešová and Klimeš 2007; Clarke et al. 2013) damaged by HP mechanical forces. Species relying on below-ground organs to propagate will also have an advantage under HP regimes because a large portion of their structures will remain hidden on the ground, hence minimizing exposure to drag and lift forces and to scouring derived from HP rapid water-level fluctuations (Bejarano et al. 2018). Below-ground organs are also involved on a number of strategies and mechanisms that should help avoid or minimize damages on plants exposed to HP, including the geotropic growth, the adoption of multi-stemmed growth forms, or the presence of contractile roots under the soil surface (Pütz 2002, 2006). Finally, thigmomorphogenetic responses (Goodman and Ennos 1997; Mickovski and Ennos 2003) might be a key adaptive mechanism to cope with HP stress. Specifically, the application of horizontal forces on the stem of a plant during up-ramping and peak flow HP stages may induce changes in the allocation of below-ground root biomass, or even change the mechanical properties of roots, contributing to increasing the plant´s stability and hence to avoid uprooting during the hydropeaks.

Plant species without any of the characteristics and mechanisms described above will potentially be much more vulnerable to HP disturbance than those that do have them.

Plant height (PHE)

Tall plants can avoid flood and HP drag forces from reaching green parts and meristems in the canopy, where most photosynthesis happens (e.g., Lambers et al. 2008). In general, trees will have their green parts and meristems more protected from HP disturbances than shrubs, and shrubs slightly more protected than herbaceous plants (i.e., grasses and forbs).

Leaf venation network (LVN)

Leaf venation is the distribution and arrangement pattern of a leaf vein system (Li et al. 2013a, b). Plant resistance to flooding, drought, and rapid water-level fluctuations might vary depending on LVN’s architectural features, including the distribution, density, diameter, length, and loopiness of veins (i.e., vein topology), vein tapering, distance between veins, and structures supporting LVN’s functioning (i.e., accessory transport elements).

LVNs might be classified under three broad groups: (1) LVNs making plants vulnerable to HP (i.e., non-hierarchical, non-reticulate LVNs; craspedodromous leaves, low vein length per unit area (VLA), large vein diameters (VD), absence of vein tapering, presence of bundle sheath extensions (BSEs) and accessory transport elements); (2) LVNs conferring plants partial-resistance to HP (i.e., evidence of at least one characteristic described in (1)); and (3) LVNs conferring plant resistance to HP disturbance: hierarchical, reticulate LVNs; brochidodromous leaves, high VLA, small VD, low venation density, and presence of vein tapering.

Reticulate LVNs tend to increase the length of all vein paths (Price and Weitz 2014), including that of major veins. It is believed that major veins act as water transport superhighways (Sack and Scoffoni 2013) enabling the plant to tolerate hydraulic system disruptions caused by drought. Therefore, reticulate systems will likely help plants to maintain water supply under water stress conditions, and hence to successfully cope with drought events resulting from HP (Brodribb et al. 2016).

Vein topology may also play a role under HP disturbance. Brochidodromous leaves present leaf vein topologies that display loops in second order veins. This topology may prevent the propagation of embolisms that reduce conductance under low water potential (Brodribb et al. 2016). Therefore, the presence of loops in the network (Katifori et al. 2010) may help to tolerate better hydraulic system disruptions resulting from vein mechanical damage or drought, as opposed to craspedodromous leaves, in which second order veins run to the leaf´s margin (Sack and Scoffoni 2013). Eucamptodromous leaves (i.e., mix of brochidodromous and craspedodromous patterns) would provide partial resistance to HP disturbance.

Length, diameters and density of LVNs can help plants to deal with submergence and drought derived from HP events. High VLA allows higher rates of gas exchange per unit leaf area (Sack and Scoffoni 2013) and may provide benefits for biomechanical support. Smaller conduits are also less vulnerable to drought (Sack and Scoffoni 2013), while veins with larger diameters provide additional mechanical support to leaves (Niklas 1992; Méndez-Alonzo et al. 2013) exposed to HP drag forces. Additionally, large mid-veins allow flexural bending along the midrib reducing mechanical load (e.g., Cooley et al. 2004), and potentially making leaves more resistant to drag forces. Finally, low LVNs densities decelerate water flow within the leaf and, consequently, reduce evaporation under conditions of moisture deficiency (Migalina et al. 2010) such as those occurring during HP baseflow stages.

Vein tapering (i.e., veins’ cross-sections narrow along their length, from first to second order veins) can make plants more resistant to drought since they reduce the spread of embolism bubbles (under drought stress, capillary forces that prevent air from bursting into the xylem are exceeded, causing a bubble to invade the water-filled lumen that blocks the xylem conduit in the leaf network). Susceptibility to embolism increases proportionally with the size of veins, with minor veins being the last to lose function (Brodribb et al. 2016).

Accessory transport and support tissues (e.g., elements such as sclereids, transfusion tracheids, and idioblasts) can act as water storage to buffer cell water potentials from transiently high transpiration rates likely occurring during HP drawdown events. Support tissues may also increase the mechanical strength of leaves by providing resistance to shrinkage under dehydration conditions (see Sack and Scoffoni 2013).

BSEs may allow stomata to respond more quickly to changes in xylem water potential, which should be beneficial when soil moisture (and therefore leaf hydraulic conductance) changes rapidly (Sack and Scoffoni 2013), as it may occur in riverbanks affected by HP. BSEs may also play a role in guiding light inside the leaf favoring higher photosynthetic rates under the low light intensities likely occurring under HP submergence. Additionally, BSEs may increase the mechanical strength of the leaf, especially when fibers or scleroids are present.

Presence of chloroplasts in epidermis (PCHE)

Submerged aquatic plant species cope with submergence owing to a suite of traits that favor gas exchange, such as leaves with epidermal chloroplasts (Maberly and Madsen 2002; Bailey-Serres and Voesenek 2008). The transfer of chloroplasts to the outer epidermal layer of submerged and floating plants reduces the distance for CO₂ transport from the medium toward the carboxylation centers and improves the illumination conditions in submerged leaves (e.g., Pedersen et al. 2013). Conversely, in terrestrial plants (e.g., riparian trees), the epidermis exerts a protective function, and the epidermal chloroplasts are poorly developed in most species (Ronzhina and P’yankov 2001). These characteristics do not favor gas exchange under HP submergence. We have considered PCHE a trait of binary nature, and therefore plants are classified as those with chloroplasts in epidermis (1) “present” or (2) “absent”.

Location of stomata (LSTO)

Leaf stomata (i.e., small pores responsible for gas exchange in vascular plants) can be present (1) on the upper surface of the leaf (stomata on adaxial position, or epistomatic); (2) on the lower surface of the leaf (stomata on abaxial position, or hypostomatic); or (3) on both sides of the leaf (amphistomatic).

Aquatic plants have stomata present in adaxial position, which helps to reduce the contact of stomata with the water and maintain them in contact with air, where gas exchange is faster (Maberly and Madsen 2002; Lambers et al. 2008). Riverine plants can experience episodes of complete submergence in HP rivers, and therefore facilitation of gas exchange via adaxial stomata might be critical for them to sustain photosynthetic activity and below-ground organs respiration (Pedersen et al. 2013). On the other hand, abaxial stomata are more sensitive than adaxial stomata to water-stress signals. Externally applied ABA or Ca²⁺ has much less of an effect on the regulation of adaxial stomatal movement compared with their effects on abaxial stomata (Wang 1998). A higher sensitivity to water-stress signals might help to close stomata guard cells faster during HP drought episodes, hence favoring a better control of plant water losses. Amphistomaty may optimize both photosynthetic rates and plant respiration under HP submergence, and control water losses under HP drought. Additionally, there is evidence that amphistomaty may have evolved in response to increasing aridity during the Tertiary, as it is considered to be a highly xeromorphic trait (e.g., Parkhurst 1978); therefore, amphistomatic stomata will likely help plants to cope with water scarcity during HP baseflows.

Presence of trichomes (PTRI)

Trichomes (i.e., appendages present on the surface of many plants that originate from the outer epidermal cell tissue of the plant; e.g., Johnson 1975) protect plants from being damaged by drought stress, and the density and types of trichomes are related to plant stress resistance (Sandquist and Ehleringer 2003; Kenzo et al. 2008). Based on trichomes characteristics, we propose a classification of plants structured in three broad categories: (1) plants lacking hairs or trichomes; with a smooth surface (i.e., glabrous or glabrate); (2) plants with a “moderate” trichome density (i.e., leaves with sparse, soft hairs, puberulent, pulverulent, setose, stellate, hoary leaves, velvety plant tissues); and (3) plants with a high trichome density (i.e., leaves with quite hairy leaves; covered in coarse, stiff hairs, bristly hairs, woolly hairs (hirsute, hispid, lanate, tomentose, velutinous, pilose, downy leaves)). It is expected that plants belonging to category (3) will be more protected from HP drought episodes than plants classified under category (2), and those in category (2) may in turn resist drought conditions better than plants grouped under category (1).

Leaf persistence (LPER)

LPER describes whether species lose their foliage over the year and when it happens. Leaf persistence can be divided into three general categories: (1) hibernal (i.e., leaves which arise in fall, persist through the winter, and die back in the heat and dryness of summer); (2) vernal and/or aestival (i.e., green from early spring to the end of the summer, or just during the warm period, then usually decaying); and (3) evergreen leaves (i.e., leaves that persist throughout the year and last at least two years) (see Grime et al. 1988).

Evergreen leaves, usually tougher and endowed with thicker mechanical tissues than deciduous leaves (Givnish 2002), make a more conservative use of water and may tolerate better drought conditions linked to HP drawdown. Evergreens might also make a more efficient use of nutrients compared to deciduous leaves. During leaf senescence, plants recover nutrients (e.g., N, P, K) and leaf proteins from chloroplasts before the leaf is shed by the end of the abscission process, and the fraction of nutrients not recovered must be replaced when new leaves are produced (Chabot and Hicks 1982). Unexpected weather events such as late summer droughts or heavy fall rainstorms may disrupt the abscission process by accelerating or modifying the timing of abscission, ultimately affecting nutrient recapture from leaves (see Crang et al. 2018). In a similar fashion, hydrological alterations derived from HP (i.e., drought, rapid and short-term water-level fluctuations, and inundation) may potentially disrupt the leaf abscission process. During drought, leaves are shed to conserve water and avoid damages to plant central organs, which are more costly than peripheral organs for plants to replace (Tyree et al. 1993; Wolfe et al. 2016). During inundation events, as a result of slow gas diffusion underwater, submerged plant tissues rapidly accumulate ethylene (e.g., Hartman et al. 2019), a gaseous hormone that triggers a number of flood-adaptive responses (Jackson 2008), including leaf abscission (Denny 1924). Mechanical forces associated to rapid water-level fluctuations triggered by HP operations can cause breakage and removal of plant biomass (Bejarano et al. 2020), including leaves. In short, unexpected drought, submergence and rapid water-level fluctuations linked to HP may limit the recovery of nutrients from leaves since these might be shed or mechanically removed before the nutrient recovery takes place. Given that the cost of leaf construction in deciduous plants is higher than in evergreens, disruptions in nutrient recapturing implies higher metabolic losses for deciduous species and therefore, under HP disturbance, evergreens might have an advantage over deciduous plants.

Whether deciduous hibernal plants are more vulnerable to HP than vernal or aestival species (or vice versa) may depend on annual patterns of electricity demand. In principle, peaks of hydropower demand may dictate to which extent leaf nutrient recapture is disrupted. For instance, in Northern Europe, nutrient recovery in hibernal plants might be more disrupted by HP than in aestival species, because higher electricity demands will likely occur in winter, when hibernal species´ peak foliage occurs. Conversely, in Southern Europe, especially during summer periods of record-breaking temperatures, it may occur the opposite; aestival species might be more vulnerable since peak foliage may coincide with periods of increased hydropower production for building cooling.

Resprouting ability (RA)

Resprouting is the capacity of some plants to survive and regenerate vegetatively (e.g., Clarke et al. 2013) and permits recovery from disturbance by means of dormant buds that consume below-ground reserves even when all above-ground biomass has been removed (e.g., Bond and Midgley 2001; Paula and Pausas 2006). We established three categories to classify plants according to their resprouting ability: (1) resprouters (i.e., the plant is efficient at resprouting, or it has the following characteristics: deep-root system, presence of underground storage organs (e.g., rhizomes) and non-sclerophylly characters; see Paula and Pausas 2006; see Clarke et al. 2013); (2) relative potential to resprout (i.e., the species presents some of the characteristics described in (1)); and (3) non-resprouters (i.e., none of the characteristics described in (1) are present in the species).

In theory, resprouters will be capable to regenerate biomass damages derived from anoxia periods, breakage, burial, mud coating and desiccation resulting from HP (see Bejarano et al. 2018). However, although resprouting favors plant regrowth (Pate et al. 1990; Schwilk and Ackerly 2005), and this should help plants to thrive under HP, this strategy may also present downsides. For instance, resprouters tend to be deep-rooted species, whereas non-resprouters are usually shallow-rooted (e.g., Bell 2001; Paula and Pausas 2006). In this regard, resprouters might be more exposed than non-resprouters to the negative effects of frequent waterlogging resulting from HP. Likewise, resprouters usually have lower drought resistance than non-resprouters at leaf level because they have lower water use efficiency and lower leaf mass per area (i.e., lower sclerophylly). These characteristics may constitute a disadvantage to cope with desiccation derived from HP baseflow periods.

Considering all the above, it is likely that resprouters may present full resistance to certain elements of HP (e.g., mechanical damages derived from drag and lift forces), but only partial resistance to others (e.g., submergence, waterlogging, and desiccation).

CO₂ concentrating mechanisms (CO₂-CM)

Some plants have evolved ATP-dependent mechanisms to concentrate CO₂ around Rubisco (i.e., active CO₂ concentrating mechanisms) as well as other concentration strategies (i.e., CO₂-recycling, generation of suberin barriers; Lambers and Oliveira 2019) that may be key for plant survival in riparian areas affected by HP operations.

Plants preferentially take up the lighter isotope of carbon ¹²C during photosynthesis and ¹³C accumulates (Fogel and Cifuentes 1993; Evans 2001). Submergence and drought resulting from HP may constrain CO₂ supply within plants' leaves, ultimately forcing plants to fix ¹³C during photosynthesis instead of ¹²C, which implies a higher metabolic cost for plants (Farquhar and Richards 1984). Switching from ¹²C to ¹³C fixation might potentially translate into lower plant morphological performance (e.g., reduced leaf, stem, and root biomass) and limited energy readily available to maintain plant internal processes (Baladrón et al. 2022).

Physical and physiological concentrating mechanisms (CO₂-CMs) help to increase and optimize the available pool of CO₂ within the plants’ leaves. These mechanisms may potentially have an impact in reducing energy expenses linked to photosynthesis (Farquhar and Richards 1984; Lambers and Oliveira 2019) under limited gas diffusion conditions likely occurring during HP submergence. CO₂-CMs can be divided as “soft” or “hard”. Common soft mechanisms are suberin barriers (i.e., apoplastic barriers in root outer cortex/exodermis to prevent oxygen diffusion outward to the soil; Manzur et al. 2015) and stem recycling photosynthesis (i.e., a syndrome in which chlorophyllous cells in the cortex of shrub and tree species refix a portion of the CO₂ respired by the underlying tissues or carried into the stem segment by the transpiration stream; Avila et al. 2014; Cernusak and Cheesman 2015; Wittmann and Pfanz 2018). Common hard mechanisms are C₄, C₃-C₄ and C₂ (glycine (Gly) shuttling) photosynthetic pathways (see Sage et al. 2014). As opposed to C₃ photosynthesis (which undergoes the standard mechanism of the Calvin cycle), C₄ plants increase photosynthetic efficiency via compartmentation of CO₂ assimilation and fixation by increasing PEP carboxylase activity (e.g., Schuler et al. 2016) in the mesophyll cells and limiting Rubisco only to bundle sheath chloroplasts (Sage et al. 2014); during the dark period, CO₂ is fixed in the form of HCO₃⁻ by PEPC and stored in large vacuoles in the form of malate (Poschenrieder et al. 2018). The CO₂ for fixation with Rubisco is obtained by decarboxylation of malate during the following day-light period (Lüttge 2004; Poschenrieder et al. 2018). The C₃-C₄ photosynthesis are common to species that are intermediates between C₃ and C₄ photosynthesis (Mercado and Studer 2022). These plants have intermediate leaf anatomies that contain bundle sheath cells that are less distinct and developed than the C₄ plants (e.g., Moore et al. 1995). Although probably less efficient than C₄ physiology, C₃-C₄ might help plants to partially cope with limited CO₂ supply associated to HP submergence. In C₂ photosynthesis, glycine (Gly) produced in photorespiration diffuses from the mesophyll to sheath cells for metabolism, and photorespiratory CO₂ is re-assimilated; this mechanism elevates CO₂ levels in the bundle sheath two to three times that of the mesophyll cells, thereby improving Rubisco efficiency (Sage and Khoshravesh 2016) and the chances of plant survival under HP submergence.

Considering all the above, we have established the following trait categories: (1) absence of traits to concentrate CO₂ (i.e., plants that perform photosynthesis using the C₃ photosynthetic pathway, lacking CO₂-CMs); (2) presence of a "soft" CO₂-CM; (3) species equipped with more than one “soft" CO₂-CM (i.e., presence of suberin barriers and stem recycling photosynthesis); and (4) presence of "hard" CO₂-CMs.

Antioxidant mechanisms (AM)

Antioxidant mechanisms allow the removal of reactive oxygen species (ROS) that may occur during phases of anaerobic energy production (Blokhina et al. 2003; Turkan 2018). ROS act as secondary messengers when plants are exposed to environmental stress (e.g., drought or flooding), but they also induce plant cell damages (i.e., degradation of biomolecules like pigments, proteins, lipids, carbohydrates, DNA) and, ultimately, may cause plant cellular death (Das and Roychoudhury 2014; Bhatla and Lal 2018). Under flooding, ATP is produced via glycolysis; this metabolic pathway also triggers the production of acetaldehyde, a potent toxin that enhances the formation of ROS (Lambers and Oliveira 2019). There is also evidence that ROS in plants can increase as a response to drought (e.g., Noctor et al. 2014) and trigger different types of cellular damage, including the oxidation of protein sulfhydryl groups, photosystem damage, enzyme inhibition, lipid peroxidation, free fatty acid accumulation in membranes and activation of programmed cell death (McKersie 1991; Sharma and Dubey 2005; Sharma et al. 2012). Therefore, it is key for plants to eliminate ROS excess derived from exposure to anoxia and drought episodes associated to HP operations.

Plants have defense systems (i.e., non-enzymatic and enzymatic antioxidant mechanisms) implicated in lowering the amount of ROS and overcome their oxidative damage, as well as in maintaining redox homeostasis (Bhatla and Lal 2018; Das and Roychoudhury 2014). Other mechanisms of some plants (e.g., Salix martiana) involve building up a several millimeter tick oxygenated layer around the roots that help eliminate phytotoxins via radial oxygen loss (ROL) (see Haase et al. 2003). In other species, toxic products are transported upward and excreted (e.g., through branch lenticels and leaf stomata; Kenefick 1962; Chirkova and Gutman 1972; Hook 1984; or through well-oxygenated aerenchyma structures; e.g., Haase et al. 2003).

According to what is described above, five general antioxidant mechanisms categories can be established: (1) enzymatic systems; (2) non-enzymatic systems; (3) transportation of toxic products by excretion; (4) presence of enzymatic, non-enzymatic or excretion systems, but of limited efficacy to remove ROS; and (5) absence of antioxidant mechanisms.

Presence of coleoptile (PCOL)

Coleoptiles (i.e., the cylindrical, soda straw-like first structure that emerges from graminoids; e.g., Parodi 1987) allow grasses to bend toward the light via phototropism, a mechanism that may potentially help riverine plants to adapt their growth and development under changing environments (Estelle 1996; Holland et al. 2009). Plants in fluvial habitats affected by changing flow regimes associated to HP operations can potentially benefit from this mechanism because it can help the plant to grow towards the surface and “escape” from light-attenuated environments resulting from submergence, where photosynthesis might be difficult. Phototropism is triggered by light perception, followed by the synthesis and translocation of signal molecules (i.e., auxins) in the tip of photo-stimulated grass coleoptiles. Differential accumulation of auxin in response to tropic stimulation, and the activity of auxin transport and auxin receptor proteins in plants (see Liscum et al. 2014) might in turn potentially translate into differences in phototropism performance between plant species, therefore affecting its capacity to escape from submergence under HP.

In addition, the coleoptile constitutes a tube within which the cotyledon grows upward to the surface, potentially offering graminoids some degree of protection against physical injury and biomass loss provoked by drag and lift forces occurring during phases of rapid rise in discharge. The coleoptile may also protect the cotyledon from desiccation resulting from HP baseflow periods.

We have considered PCOL a trait of binary nature, and therefore plants will be classified under the categories (1) “present” or (2) “absent”.

Seed germination (SG)

Seed germination under HP is reduced by constant soil waterlogging and riverbank erosion resulting from highly fluctuating waters, as well as by soil desiccation resulting from fast drawdowns (Stella et al. 2010; Sarneel et al. 2014; Bejarano et al. 2020). The risk of desiccation during HP phases of low water levels is especially high in arid regions characterized by long drought periods (Bejarano et al. 2018).

Plants that germinate on a yearly basis may present a high vulnerability to HP disturbances, as germination is required every year and seed exposure will be maximum. On the contrary, perennial plants complete a single life cycle in three or more growing seasons, and hence particularly sensitive life stages are not faced every year (Bejarano et al. 2018).

In addition, the availability of light for riverine plants under HP submergence may be limited to short time windows comprised between the end of a hydropeak (i.e., down-ramping stage) and the beginning of a new hydropeak (i.e., up-ramping stage). Plants capable of germinating under "fleeting time windows" of sunlight (see Kettenring et al. 2006) between two hydropeaks may have an advantage over species that need "long-lived gaps" of multiple days or weeks (‘low-risk’ germination strategy; Kettenring et al. 2006). We foresee that (1) perennial plants might display the highest tolerance to HP, followed by (2) biennials, (3) annuals capable to germinate under short "fleeting time windows" of sunlight, and (4) annual plants adopting a ‘low-risk’ germination-based strategy. Plants under category 4 may likely experience the greatest difficulties to germinate under HP submergence.

Stomatal control (STOC)

Stomatal control determines the water use efficiency (WUE) of a plant by optimizing water lost against carbon gained (Cowan and Farquhar 1977; Yang et al. 2021). The effectiveness of stomatal control depends on the regulation of the stomatal aperture (i.e., speed and ‘tightness’ of closure). Fast opening and closing allow optimizing CO₂ influx into the leave and H₂O losses. But in addition, the faster the stomatal closure is, the most effective photosynthetic induction will be (e.g., Urban et al. 2007), which in turn will determine an effective utilization of fluctuating irradiance (Kaiser et al. 2015; Morales and Kaiser 2020), a phenomenon likely occurring under flooding and water fluctuations derived from HP (i.e., "fleeting time windows" of sunlight; see Kettenring et al. 2006).

Plants can be classified under three stomatal control categories (see Shtein et al. 2017; Haworth et al. 2011): (1) Type I (i.e., kidney-shaped stomata with passive stomata control), which corresponds to the least efficient stomatal control, usually present in ferns and gymnosperms; (2) Type II (i.e., kidney-shaped stomata with an active stomatal control), which corresponds to plants with moderately efficient control, commonly found in angiosperms; and (3) Type III (i.e., dumbbell-shaped stomata with an active stomata control) are “morphologically innovative” stomata highly efficient at opening and closing, and are common in grasses (Poaceae). This ‘graminoid’ morphology is associated with faster stomatal movements leading to more efficient gas exchange in changing environments (e.g., Shtein et al. 2017), for instance, riverine areas affected by episodic submergence and drought, constant waterlogging and frequent water-level variations derived from HP.

Non-structural carbohydrates and flooding acclimation responses (NSC)

The amount of non-structural carbohydrates (NSC, i.e., starch) in plant tissues is the result of carbon gain (photosynthesis) minus consumption (growth and maintenance respiration) (Chapin et al. 1990; Li et al. 2013a, b). Plant energy metabolism under HP submergence stress can likely be maintained with the degradation of NSC reserves, which is used by some plants (e.g., rice) to produce energy for the maintenance of basic metabolic processes (Jackson and Ram 2003; Parolin 2009), including plant respiration in anaerobic tissues (Perata et al. 1992; Colmer and Greenway 2005). This is known as a quiescence strategy (e.g., Bailey-Serres and Voesenek 2008; Voesenek and Bailey-Serres 2015), where plants do not elongate shoots under flooding to minimize carbohydrate consumption. Instead, quiescent plants use their reserves to maintain underwater metabolism and to regrow after stress (see Manzur et al. 2009; Zhao et al. 2021). Mobilization of NSC reserves differ greatly among plant species (e.g., Crawford 1992), allowing them to survive underwater from several days (e.g., Juncus effusus) to nearly 3 months (e.g., Scirpus maritimus) (Manzur et al. 2009; see Crawford 1992). Therefore, the larger the NSC reserves are, the more chances of survival during complete HP submergence. A second flooding-induced acclimation response is known as the escape strategy (Bailey-Serres and Voesenek 2008; Zhao et al. 2021) useful under relatively shallow submergence. To facilitate the escape strategy, starch reserves can be mobilized so the plant can rapidly extend their petioles or stems to allow leaves to reach the water surface to aerate the remainder of the plant (Colmer and Voesenek 2009; Nagai et al. 2010; Zhao et al. 2021). In theory, the higher is the capacity of the plant to mobilize starch granules, the more soluble sugar available will be as an energy source for the plant to elongate and “escape” from HP submergence (see Striker et al. 2008). Operational HP regimes generating deep and long-lasting flooding in riverbanks may select against species relying on escape strategies because elongation growth competes with maintenance processes involved in plant survival during complete submergence (Ram et al. 2002). Under deep and long-lasting HP flooding, elongation growth may not be sufficient for shoots to regain contact with the air, and if that were the case, high carbohydrate consumption will likely lead to an energy deficit, severe tissue damage, and mortality (see Pierik et al. 2009; Chen et al. 2011a, b; 2019).

Based on the aforementioned, four categories can be established to classify plants based on their NSC reserves and flooding acclimatation responses: (1) plants using carbohydrates to enable a quiescence strategy and with large carbohydrate reserves (species that will likely tolerate deep and long-lasting flooding); (2) plants adopting a quiescence strategy with limited carbohydrate reserves (species likely capable to thrive under deep (but short-lasting) HP submergence; (3) plants adopting an escape strategy and with large carbohydrate reserves (these plants will likely tolerate prolonged (but relatively shallow) HP submergence); and (4) plants without the capacity to adopt flooding-induced acclimation responses (i.e., quiescence or escape) and without large starch reserves.

Plant ventilation systems (PVS)

Ventilation mechanisms such as aerenchyma (i.e., extended system of air canals and intercellular spaces aimed at enabling ventilation in roots and plant rhizomes anchored in water-saturated soils; e.g., Björn et al. 2022), heat pumps (i.e., convections, or ‘internal winds’, generated across stomata that drive gasses from the atmosphere via young natant leaves, petioles to roots and back, via older leaves to the atmosphere; Dacey and Klug 1982; Armstrong and Armstrong 2011), pneumatophores, knee roots, stilt roots (i.e., roots of plants that grow vertically above the water level to get oxygen for respiration; e.g., McKee 1993; Pallardy 2008), formation of lenticels (i.e., large cracks on the surface of stems and roots which facilitates O₂ entry into the aerenchyma; Jackson et al. 2009), and snorkeling (e.g., Rogers 2021), are crucial for plants to thrive in water-saturated soils (Björn et al. 2022) such as those located in riverbanks of HP rivers. Based on their capacity to maintain optimal conditions for photosynthesis and respiration under oxygen-deficient environments, plants may be classified under three categories: (1) those presenting aerenchyma; (2) those with alternative ventilation systems (usually less efficient than aerenchyma structures); and (3) those without aerenchyma or any other alternative ventilation mechanism.

Aerenchyma enables a diffusional pathway from emergent leaves to roots to transport oxygen, nitrogen, and various metabolically generated gases such as CO₂ and ethylene (Parolin 2012) under flooded and oxygen-deficient conditions (e.g., Steffens and Rasmussen 2016). In addition, it provides buoyancy to floating-leaves maintaining them out of the water where gas exchange is faster. Different aerenchyma types arise from the combination of four general root structural types. The shape of these root types resembles a bicycle wheel (graminaceous), a spider web (cyperaceous), a honeycomb (Rumex) and a non-organized structure with irregular aerenchyma lacunae (Apium) (e.g., Justin and Armstrong 1987; Striker et al. 2007).

Alternative ventilation systems may also facilitate O₂ entry from the atmosphere into the plant (see Jackson et al. 2009), but they might be slightly less efficient than aerenchyma. Plants with reduced aerenchyma production and without alternative ventilation mechanisms to facilitate oxygen diffusion between shoots and roots will likely be vulnerable to soil waterlogging and submergence derived from HP.

Presence of adventitious roots (ADV)

Adventitious roots grow from any non-root tissue (Esau 1953; Lovell and White 1986) and can be produced in response to stress conditions (Bannoud and Bellini 2021), such as flooding. Emergent adventitious roots grow in the oxygenated layer at the surface of the flood-water table facilitating the entry of oxygen into the root and the stem by the shortest possible pathway (e.g., Parolin 2012), therefore helping plants to avoid anoxia under submergence (e.g., Steffens and Rasmussen 2016). While under experimental conditions with stable water levels most species are able to develop adventitious roots, it is rare to find such ability in field plants, probably because their adaptive value with rapidly changing water levels is to question (Parolin et al. 2004). Regarding HP, this means that anoxia avoidance via adventitious roots might be achieved by plants located upstream the HP dam, where surface water levels may not rise rapidly. Downstream HP dams surface water levels will rapidly rise during every hydropeak and, in such case, adventitious roots may not constitute a strong advantage, especially when HP water-level fluctuations are high and frequent. Nonetheless, while adventitious roots may not offer strong protection to anoxia under rapidly changing water levels, they may help plants tolerate burial effects resulting from frequent and fast water fluctuations (Bejarano et al. 2018); if high sedimentation rates occur during down-ramping HP stages, adventitious roots can replace the function of the ordinary root system, which often dies under several decimetres of sediment (Parolin 2012).

Adventitious roots are produced vigorously and rapidly in some species, but much less readily in others, and plants can be grouped according to their ease of rooting (Jackson 1985). Herbaceous plants with prostate stems such as Veronica filiformis and V. persica (Harris and Lovell 1980), species presenting rhizomes or stolons (e.g., Agropyron repens (Palmer and Sagar 1963) and Ranunculus repens (Ginzo and Lovell 1973)), and climbers are often vigorous adventitious root formers (Jackson 1985). On the contrary, woody plants, especially older individuals, may present limited adventitious root production (e.g., Haissig 1974).

Based on the aforementioned, we have considered ADV a trait of binary nature, and plants are classified under the categories (1) “well-developed” adventitious roots (i.e., herbaceous plants, species with rhizomes or stolons, climbers) or (2) “poorly developed” adventitious roots (i.e., woody plants).

Chloroplasts movement under changing light intensities (CHMOV)

Plants can deal with changing light intensities through light-induced chloroplast movements (Wada et al. 2003). Under low light intensities, such as those likely received by plants under HP submergence, chloroplasts move toward light-irradiated area (i.e., the “accumulation response”) and situate along the cellular edges perpendicular to the incident light to optimize absorption, ensuring the capture of the maximum amount of light (Zurzycki 1955), and enhancing leaf photosynthesis and biomass production (Gotoh et al. 2018). Two phototropins (phot1 and phot2; e.g., Liscum et al. 2014) drive chloroplast movement during the “accumulation response”. By contrast, under high-intensity light, which may potentially affect aquatic plants during baseflow periods, phot2 activation causes chloroplasts to move away from the irradiated edge of the cell in a so-called “avoidance response” to prevent photodamage (e.g., Kasahara et al. 2002). Leaves of most shade-grown plants tend to exhibit stronger chloroplast movement than sun-grown plants (Davis et al. 2011; Higa and Wada 2016; Gotoh et al. 2018) and therefore should be more efficient at enhancing photosynthesis under HP submergence, and preventing photodamage under baseflow periods.

According to the aforementioned, two categories can be established to classify plants based on CHMOV: (1) shade-grown plants (strong chloroplast movement); and (2) sun-grown plants (weak chloroplast movement).

Corticular photosynthesis (COPH)

Chlorophyll-containing bark tissue can be found in stems and branches of trees, shrubs and bushes. Several authors have demonstrated that the bark chlorenchyme in woody trees is able to photosynthetically reduce the flux of respiratory CO₂ to the atmosphere, a process that has been termed "CO₂-refixation" or alternatively "corticular photosynthesis" (Foote and Schaedle 1976; Berveiller et al. 2007; Ávila et al. 2014; Wittmann and Pfanz 2018). The prerequisites necessary for a working reductive CO₂ assimilation metabolism (e.g., an effective chloroplast structure, enzymatic equipment, nutrients, water, light and carbon dioxide) are present in sufficient amounts and quantities within the chlorenchymal bark tissues of trees (Pfanz et al. 2002). Corticular photosynthesis improves plant-scale water-use efficiency due to the low water loss from the stems compared with the losses associated with foliar photosynthesis (Cernusak and Hutley 2011; Avila et al. 2014; Tarvainen et al. 2017). This is beneficial for trees to cope with water shortages linked to dry HP periods. Additionally, corticular photosynthesis may provide means for maintaining hydraulic function during drought (Vandegehuchte et al. 2015; Bloemen et al. 2016). Likewise, corticular photosynthesis may help avoiding oxygen deficiency in plants (Pfanz et al. 2002; Wittmann and Pfanz 2018) exposed to repeated HP inundations. In addition, leaf photosynthesis might be lost or severely impaired when mechanical forces resulting from fast HP currents break or damage plants' leaves. In such cases, corticular photosynthesis might help the plant to maintain a favorable carbon balance (see Ivanov et al. 2006) until it recovers the aerial elements lost.

We have considered COPH a trait of binary nature, and therefore plants will be classified under the categories (1) “present” or (2) “absent”.

Mycorrhizal symbioses (MS)

Mycorrhizal symbioses refer to the association between fungi and plants (e.g., Kirk et al. 2001). The roots of over 90% of all plant species establish these relationships with fungal taxa (Bonfante and Genre 2010). Mycorrhizas can be broadly classified as ectomycorrhizas (ECMs), when the fungus surrounds the root tip and colonizes the root intercellular spaces, and endomycorrhizas (e.g., ericoid (ERs) and arbuscular mycorrhizas (AMs)), when the fungus develops inside the root cells (Smith and Read 2008; Bonfante and Genre 2010). Four categories have been established for this trait: (1) presence of AM; (2) presence of ER; (3) presence of ECM; and (4) absence of mycorrhizal symbioses.

AMs (the commonest mycorrhizas) are associated with roots of about 80% of plant species, and facilitate host plants to grow vigorously under stressful conditions by mediating a series of complex communication events between the plant and the fungus leading to enhanced photosynthetic rate (Birhane et al. 2012), as well as increased water uptake (Begum et al. 2019); this means that AM can help the plant to cope with photosynthetic and gas exchange limitations that may occur during HP flooding events, as well as to withstand desiccation derived from drawdown periods, and waterlogging resulting from frequent surface water-level fluctuations.

ERs are mycorrhizas of Erica (heather), Calluna (ling) and Vaccinium (bilberry), that is, plants that endure moorlands and similar environments where consecutive waterlogging and desiccation stages may occur. Therefore, ER may likely help plants to cope with the repeated wetting and drying cycles affecting soil riverbanks in HP systems.

ECMs are the most advanced symbiotic association between higher plants and fungi, involving the majority of forest trees (Moore 2011), and including common riparian species such as Alnus spp., Betula spp. and Salix spp. (Iversen et al. 2014; Weemstra et al. 2016; Chen et al. 2020). ECM symbioses may improve the water status of trees under drought conditions, through an increased absorbing surface, enhanced hydraulic conductivity at the soil–root interface, and hormonal and nutritional effects modifying stomatal regulation (Breda et al. 2006; Brunner et al. 2015). Some ECM fungi have a high resilience to drying and, therefore, plants that establish associations may increase their tolerance to drought conditions (Jarvis et al. 2013) derived from baseflow HP periods.

Main findings, challenges and further research

Plant functional traits can help predict species resistance to fluvial alterations derived from HP, and hence to better understand the fate of local community structure, biodiversity, and ecosystem function in rivers affected by the use of this hydropower generation mode. This study provides new ideas and hypotheses on how plants may respond to HP, and brings together literature items (e.g., empirical or theoretical) that were previously disconnected. Our effort was to compile data available from multiple literature sources on plant functional traits, but here synthesized and interpreted to facilitate its use in assessing the resistance of riverine plants to common HP disturbances (i.e., flooding, water stress, and water fluctuations).

The establishment of categories for each functional trait has proven to be a challenging task. The type and number of categories defined for the traits here presented are constrained by the information and data available in the original sources. For some traits there is little data available, while for others the published studies may be contradictory, making it necessary to establish categories based on expert judgement. Before using the information here presented, it is wise to consider if the categories as provided here are suitable for the questions asked (see Tyler et al. 2021), or if they should better be adjusted or modified. For instance, in our work, three categories were defined to classify plants based on their resprouting ability: (1) resprouter species; (2) species with relative potential to resprout; and (3) non-resprouters. If for research purposes clear cut differences between plant species want to be avoided, and rather need to classify species under a “continuous resprouting spectrum”, we may have to switch from qualitative to quantitative-based categories (e.g., measure species resprouting ability based on the number, volume, length and/or diameter of resprouts). Further improvements in the definition of trait categories will benefit any future attempt to rank riverine species based on their resistance to HP disturbance.

Our work addresses a combination of physiological (hard) and morpho-anatomical (soft) traits (Hodgson et al. 1999; Zakharova et al. 2019), and the theoretical importance of each of them in helping species to grow and survive under HP. In general, the availability of data on physiological traits in the literature is substantially lower than for morphological traits. As evidenced by Bejarano et al. (2016), traditionally, information on plant traits has focused on morphological characteristics, which were merged into local floras and field guides (e.g., Linnaeus 1745; British Ecological Society 1941), and much less attention has been paid to physiological traits. This fact represents a problem because physiological traits are better predictors of species habitat affinities than morpho-anatomical traits (see Belluau and Shipley 2018).

Processes such as the biosynthesis, metabolism, transport, and signal transduction of plant hormones (i.e., key regulators of plant growth and development such as auxin, cytokinin, brassinolide, gibberellin, ethylene, abscisic acid, jasmonate, and salicylic acid; e.g., Li et al. 2017; Bhatla and Lal 2018) involved in plant adaption to drought, waterlogging and flooding stress have been progressively studied (Jia et al. 2021) in plants. Studies on signaling pathways linked to plant hydrological stress have been conducted in the past using species of commercial or scientific interest (e.g., Arabidopsis thaliana; Yeung et al. 2018), but few or none in wild plants, including those growing in riparian corridors. The lack of these type of experimental studies on riverine plants complicates their grouping based on their greater or lesser capacity to trigger physiological processes potentially critical to withstand HP. Hence, it still is necessary to further clarify how physiological mechanisms operate in most riverine plants and, furthermore, how they may provide them an advantage in HP environments.

New experiments aimed at understanding stress signaling pathways and hormone synthesis involved in riverine plant resistance to fluvial disturbances would be helpful in order to incorporate new physiological traits into our analysis framework, and to determine connections between functional categories and their theoretical capacity to confer plant resistance to HP. Similarly, numerical datasets from such experiments would allow shifting from qualitative trait categories to quantitative-based classification systems.

Due to the reasons stated above, physiological traits of interest had to be discarded from our trait set due to a lack of information in the scientific literature either to define functional trait categories, or to accurately classify riverine species under any specific trait category. Shoot growth rate is an example of an understudied trait that had to be discarded from this study. Plants that grow fast may avoid submergence or at least stay closer to water surface to avoid light attenuation, potentially facilitating photosynthesis under HP submergence. Plants adopting the so-called low-O₂ escape strategy (Bailey-Serres and Voesenek 2008; Voesenek and Bailey-Serres 2015) can rapidly produce elongated shoots that emerge out of the water functioning as ‘snorkels’. This strategy can help plants to restore gas exchange (Voesenek and Sasidharan 2013; Sasidharan et al. 2018) when exposed to shallow HP inundation, and to avoid erosion, burial, and mud coating derived from fast water currents resulting from hydropeaks. However, with the exception of some species for which submergence‐induced shoot growth responses have been studied (e.g., elongation in Rumex palustris (Voesenek et al. 2003), there is a lack of information on riverine plants’ shoot elongation responses, and therefore this trait cannot be currently used for HP assessment purposes.

Groups of traits intercorrelated (e.g., He et al. 2020), functionally redundant, and probably common, across sets of species within riverine communities should be kept in mind when assessing the resistance of species to HP. Traits strongly correlated are usually part of major plant adaptive strategies (Li et al. 2022), and can be considered as part of a single spectrum (He et al. 2020). Therefore, having information of part of the traits within a spectrum may allow inferring the attributes of the remaining traits for which information is not available.

Some of the traits presented in this work are strongly correlated. For instance, thick leaf cuticles, dense covering of trichomes, high leaf mass per area, and reduced number of stomata in leaves of xerophytic plants belong to the leaf economic spectrum (LES; Reich et al. 1997; Wright et al. 2004), and all of them allow plants to reduce water losses under drought conditions (Crang et al. 2018). Likewise, the formation of aerenchyma, increased gas exchange through reduced leaf thickness, and chloroplasts that lie directed toward the epidermis are part of the low-oxygen escape syndrome (LOES), a strategy adopted by some plant species to avoid submergence (Bailey-Serres and Voesenek 2008).

Principal components analysis (PCA) and Pearson’s correlation coefficients can be used to identify associations among plant traits (e.g., Bontemps et al. 2017; Fyllas et al. 2020). The application of these methods can provide a better understanding of trait intercorrelation, and improve trait-based index calculations aimed at determining riverine plant resistance to HP.

The database here presented can be expanded to cover all possible traits related to hydrological and hydraulic processes triggered by HP. Future additions to it may include a better understanding of trait–disturbance interactions, as well as additional advantages not reported in this work that may likewise confer plants resistance to HP disturbance (Baladrón et al. 2023). Future initiatives can also build on our work integrating information on traits and species (i.e., filling “(multi-) species by (multi) traits” matrices to rank species based on their differential tolerance to HP).

Conclusions

Despite its limitations, this study is unique in that it comprehensively provides the rationale supporting the theoretical resistance of riverine species according to the presence/absence of specific functional traits, or the expression of a given trait attribute (i.e., value or modality taken by each trait). Using trait-based groups of species instead of individual species may simplify the evaluation of HP impacts on riverine plant communities worldwide and guarantee transferability of results across regions. Additionally, restoration of river margins suffering from HP impacts involving revegetation may benefit from our study. Revegetation prevents the establishment of invasive plant species, reduces the leakage of nutrients and sediments from riparian areas, and may increase in-stream habitats and stabilize water temperature and provide cover for fish and macroinvertebrate communities (Jowett et al. 2009; Peng et al. 2014). Consequently, correctly selected species for revegetation purposes may improve the chances of restoration success (Jowett et al. 2009; Peng et al. 2014).