Towards global dominance of invasive alien plants in freshwater ecosystems: the dawn of the Exocene?

The global and seemingly unstoppable spread of invasive alien plants emerges as one of the main topics of current science. This is due to the multiple repercussions of invasive plants on biodiversity and ecosystem functioning, plus huge consequences on human existence. In freshwaters, lots of invaders are extremely competitive by virtue of their idiosyncratic reproductive and adaptive strategies. As “inland islands”, freshwaters seem particularly prone to changes when invaded, such as reorganisation of food webs and biotic interactions. Together, these events are self-reinforcing, implying hardly reversible hysteric phenomena. The intention of this paper is to point out: invasion pathways, driving factors, invasion mechanisms, and noticeable effects mediated by plant invaders in freshwaters through an extensive knowledge review. The growing evidence suggests the dawn of a new epochal phase: a globally alien-dominated “bio-historical horizon”, tentatively called “Exocene”, where invaders play predominant roles that drive freshwaters functioning and successional unexpected stages. In the context of invasion science, Exocene reinforces the need for an ecosystem-based perspective to properly understand the implications of plant invaders in freshwaters. Seven challenging issues emerge to be addressed to better outline the global paths of biodiversity and functioning between biomes when faced with biological invasion

Linking vegetation patterns, wetlands conservation, and ecosystem services provision: From publication to application

Natural wetlands emerge as the best sites to preserve the diversity of aquatic and riparian vegetation; however, especially in the lowlands, pristine wetlands and aquatic ecosystems have almost completely disappeared through land reclamation and agricultural development. Actions are needed, therefore, to maintain and recreate a wide network of wetlands able to preserve adequate levels of vegetation diversity. Focusing on a complex wetland system located in an overexploited plain, the article entitled ‘The importance of being natural in a human-altered riverscape: Role of wetland type in supporting habitat heterogeneity and the functional diversity of vegetation’, published in 2016 in Aquatic Conservation: Marine and Freshwater Ecosystems (AQC) explored the role of wetland origin and hydrology as the main drivers of physical and vegetation functional diversity, following a hierarchical sampling approach. The main results reinforced the key contribution of natural sites in maintaining vegetation diversity in heavily impaired riverine contexts, suggesting a direct effect of the interannual and seasonal dynamics of water-level variations in the observed vegetation patterns. The article offered an important contribution to our knowledge of vegetation patterns in wetlands, partly attributed to the innovative functional, hierarchical approach applied which is able to guarantee reliable data on the distribution patterns of physical heterogeneity and wetland vegetation. The findings of the article have been applied and adopted in a series of technical handbooks designed, inter alia, to support the monitoring programmes of habitats of community interest or vegetation of relevance for aquatic biodiversity conservation. In addition, this article has helped to raise awareness of the essential roles played by wetlands in agricultural landscapes and has emphasized the need for a better synergy between the European Habitats Directive and the Water Framework Directive. Several ecological recovery projects have been funded in line with the results described in the AQC article

A longitudinal snapshot of pioneer plant patterns along lowland temperate rivers

Overexploitation of water, loss of dynamism and current climate change cause river drought and baseflow events that are globally increasingly intense and prolonged. This strongly affects riverine plant communities, especially those of seasonally exposed sediments. However, few data are available concerning this key component of river biodiversity currently. Furthermore, the role of alien species is expected to be important in riverbeds since fluvial corridors have an extraordinary susceptibility to be invaded and act as primary routes for invader dispersal. This study provides a systematic investigation of the longitudinal patterns of pioneer plants—in terms of richness and cover—in three mid-size rivers (Ticino, Adda and Oglio rivers) in northern Italy, from 15 different sampling sites. The alien plants dominated the pioneer richness (52 out of 91 species, 57% of the total richness found), whereas plant cover followed less-defined patterns. Along the upstream–downstream gradient, native plants showed a steep decline (−69% in richness and −52% in cover rates on average), only partially offset by the local (extremely variable) changes in alien taxa (−41% on average, in the range of −2.7 to +4.1 species per plot and with cover rates varying form −10 to +1,018%). The magnitude and sign of the detected trends strongly reflected individual river identity. Further investigations are required to better assess the role of hydromorphology in driving the riverine pioneer plant assemblages

The importance of being petioled: leaf traits and resource-use strategies in Nuphar lutea

Intraspecific trait variability (ITV) can be considerably high and reveal plant local adaptation. The aim of this study is to investigate ITV of leaf traits (including petioles) in a rooted floating-leaved macrophyte (Nuphar lutea) at the local scale. We expected to see changes in resource-use strategies in relation to water and sediment properties. 96 leaves were sampled in a hyper-eutrophic shallow lake in central Italy, together with environmental parameters. Results highlight the influence of water depth and sediments in modulating environmental conditions and thus leaf traits. Leaf area and fresh and dry weight increased with water depth, a relation that catches the construction costs of petioles. The negative relation found between specific petiole area and conductivity suggests a stress imposed by high nutrient availability which triggers unfavorable conditions for N. lutea. We demonstrated the relevance of petiole traits for a rooted macrophyte. Petiole aerenchyma or photosynthetic extra surfaces fundamentally contribute to fit the environment, opening new questions for the functional investigation of macrophytes. Excluding petioles in the assessment of leaf traits reduces the possibility to understand the ecological/adaptive processes of nympheids. New data are urgently needed to increase the number of data and species to be analyzed

Pioneer annual vegetation of gravel-bed rivers: First insights on environmental drivers from three Apennine streams

A huge knowledge gap exists on riverine pioneer vegetation. Despite its relevance in regulating the C metabolism at the catchment scale, and the triggering role in shrubs and trees establishment along riverbanks, little data is available on its environmental determinants. Indeed, most existing knowledge in this field refers to woody species or aquatic macrophytes neglecting the ecosystem relevance of ephemeral herbaceous vegetation. Focusing on three gravel bed rivers located in northern Italy (Baganza, Nure and Parma streams), the present study is aimed to evaluate the riverine ephemeral plant richness, considering both native and alien taxa, and the role of hydrogeomorphological disturbance and sediment quality in the observed richness patterns. At higher disturbance rates (e.g., larger river sizes), our data indicates a progressive decrease in overall plant richness, but also an increase in the coverage-abundance rates mainly due to alien species. This evidence confirms that variations in hydrology imply changes in pioneer plant species richness at in-stream periodically exposed sediments. More attention must be given to the vulnerability of pioneer vegetation to climate change and direct human impacts to fully understand the functioning of lotic ecosystems, especially the non-perennial ones

Functional traits in macrophyte studies: Current trends and future research agenda

The use of functional traits (FTs) can provide quantitative information to explain macrophyte ecology more effectively than traditional taxonomic-based methods. This research aims to elucidate the trait-based approaches used in recent macrophyte studies to outline their applications, shortcomings, and future challenges. A systematic literature review focused on macrophytes and FTs was carried out on Scopus database (last accessed May 2020). The latest 520 papers published from 2010 to 2020, which represent 70 % of the whole literature selected since 1969, were carefully screened. Reviewed studies mainly investigated: 1) the role of FTs in shaping communities; 2) the responses of macrophytes to environmental gradients; 3) the application of FTs in monitoring anthropic pressures; and 4) the reasons for success of invasive species. Studied areas were concentrated in Europe (41 %) and Asia (32 %), overlooking other important biodiversity hotspots, and only 6.2 % of the world macrophytes species were investigated in dedicated single species studies. The FTs most commonly used include leaf economic and morphological traits, and we noticed a lack of attention on root traits and in general on spatial traits patterns, as well as a relatively poor understanding of how FTs mediate biotic interactions. High-throughput techniques, such as remote sensing, allow to map fine-scale variability of selected traits within and across systems, helping to clarify multiple links of FTs with ecological drivers and processes. We advise to promote investigations on root traits, and to push forward the integration of multiple approaches to better clarify the role of macrophytes at multiple scales

Contribution to the knowledge of Mediterranean wetland vegetation: Lemnetea and Potamogetonetea classes in Western Sicily

The freshwater aquatic vegetation of the Lemnetea and Potamogetonetea classes in Western Sicily was surveyed and analyzed. 85 lakes and small pools were investigated collecting 147 phytosociological unpublished relev\ue9s to integrate the very scarce available data (only 3 relev\ue9s). By applying statistical analyses on abundances data and on the bases of physiognomy and dominant species, two alliances (Lemnion minoris and Stratiotion) and four different coenoses have been identify for the Lemnetea class; while 11 associations, assigned to two orders (Potamogetonetalia pectinati and Callitricho hamulatae-Ranunculetalia aquatilis) and four alliances (Potamogetonion, Nymphaeion albae, Ranunculion aquatilis and Ranunculion omiophyllo-hederacei) have been recognized for the Potamogetonetea class. A new association (Junco bufonii-Ranunculetum omiophylli ass. nov.) and a new subassociation (Ranunculetum peltati ranunculetosum rionii subass. nov.) have been proposed, whereas other six vegetation units were found to be new for the study area (Potamogetono-Ceratophylletum submersi, Potamogetonetum pusilli, Ranunculetum aquatilis, Ranunculetum peltati and Ranunculetum rionii, Lemno-Callitrichetum obtusangulae). For all the coenoses recognized, new insights on the floristic composition, syntaxonomy, synphiognomy, synecology and synchorology are reported, offering a reasoned overview of the aquatic vegetation of the western sector of the main Mediterranean island

In vitro propagation of isoëtes sabatina (Isoetaceae): A key conservation challenge for a critically endangered quillwort

Isoëtes sabatina is an aquatic quillwort endemic to Italy. It is one of the rarest quillworts in Europe, and is critically endangered due to restricted range and to the continuous decline of both population and habitat quality. This study aims to develop an optimized protocol to reproduce and grow I. sabatina sporelings. Mature and immature megaspores were mixed with mature microspores to evaluate the influence of the developmental stage on germination and sporeling development. Two substrates, distilled water and water-agar medium, were tested for germination and sporeling emergence, and three substrates, sand, lake sediment and water-agar, were tested for transplants. A high percentage of megaspore germination (a total of 79.1%) was obtained in both substrates, higher for mature than immature spores. A total of 351 sporelings were produced in distilled water and water-agar cultures, with similar percentages (64.5% and 69.6%, respectively). The development stage of the megaspores affected both germination and sporeling development. Sporeling emergence showed significantly higher percentages in mature megaspores than immature ones (69.6% vs. 11.6%, respectively), with 85% of germinated spores developing sporelings. Only transplants over water-agar medium were successful. This protocol could be useful for the propagation of sporelings as the key step towards the planning of in situ actions to save this Mediterranean quillwort from extinction

Algal biomass and macroinvertebrate dynamics in intermittent braided rivers: new perspectives from instream pools

Perennial streams and rivers are now largely subjected to fragmentation and lentification processes due to flow reduction, which creates a number of lateral habitats with different degrees of hydrological connectivity. These habitats have environmental conditions and biotic interactions that can be far divergent than those of main channel habitats. However, they remain largely unexplored, especially in temperate regions. We here focused on studying algal dynamics and their interactions with aquatic invertebrates across mesohabitats (i.e., main channel, secondary channel, pools) in streambeds under both normal and low flow conditions. We selected four watercourses in the Po Plain (northern Italy), where we detected the main dynamics and drivers of algal and invertebrate communities by applying mixed effect modelling. A clear algal growth trend was detected in summer, and was similar for all mesohabitats, but with temporal decoupling and doubled values in pools. Mesohabitat and time were central factors in driving benthic algae dynamics that, in turn, negatively affected aquatic invertebrates. Hydrology and algae seemed to have a mutually reinforcing effect on macroinvertebrates by reducing almost all the investigated metrics. By considering future projections on further regime shifts in lotic systems, loss of biodiversity driven by algal blooms could become a major concern, and also for potential cascade impacts on other biotic compartments of river networks