Biological invasions, meaning colonization of alien species to areas outside of their home range, are a major cause of biodiversity loss and functional and structural alteration of ecosystems worldwide (Seebens et al. 2017). Among the environments most susceptible to biological invasions, freshwater ecosystems occupy a critical place for their intrinsic vulnerability and because they are often subject to anthropogenic disturbance, which generally favors the spread of alien plants (Shea 2002; Anufriieva and Shadrin 2018; Dimitrakopoulos et al. 2022); indeed, freshwater ecosystems exhibit the highest number of invasive alien plant species (Lazzaro et al. 2020).

Invasive alien plants are adaptable species, with high reproductive capacity, that often outcompete native species, causing deterioration of local biodiversity and alteration of plant communities in colonized habitats (Pyšek et al. 2012; Gigante et al. 2018; Viciani et al. 2020). Although awareness of the impacts exerted by these species on invaded habitats has increased in the last decade (e.g., Ceschin et al. 2020; Lazzaro et al. 2020; Kerns et al. 2021), studies regarding the ecological traits that make some alien aquatic plants highly competitive and invasive in Europe are still quite scarce (Lazzaro et al. 2020; Viciani et al. 2020). Therefore, filling this knowledge gap should be one of the priorities in scientific research to fully understand the ecological traits of invasive species, including both their strengths and possible limits, comprehension of which becomes crucial for their better management and control.

A highly invasive aquatic plant in several European countries, recently including Italy, is Ludwigia hexapetala (Hook. & Arn.) Zardini, H.Y. Gu and P.H. Raven (Onagraceae), which is native to South America (Wagner et al. 2007; Liu et al. 2017). Ludwigia hexapetala was first reported in Italy in 1934 in the northern regions (Di Pietro et al. 2007), where it probably arrived as a consequence of its expansion throughout Europe due to its use as an ornamental plant (Dandelot et al. 2008). It has since spread in more recent times to the central parts of the Italian peninsula, invading major waterbodies, such as the volcanic Lake Bracciano (Azzella and Iberite 2010; Buono et al. 2019). Ludwigia hexapetala can colonize still or slow-flowing waters, marshy wetlands and banks of lakes, rivers and canals (Dandelot et al. 2005; Rolon et al. 2008; EPPO 2011; Thouvenot et al. 2013b). It can grow in both aquatic and terrestrial habitats, showing remarkable morphological plasticity, evidenced by possessing two different morphotypes (Billet et al. 2018; Thiébaut et al. 2018): an aquatic one and a terrestrial one. The aquatic morphotype is characterized by round leaves grouped in rosettes at the stem apex, and leaves of varying shape, ranging from elliptic to oblanceolate, along the stems below the water surface. The terrestrial morphotype is characterized by elongated vertical flowering stems and mostly lanceolate leaves, although leaf shape in both forms is highly plastic in response to life stage and local conditions (Thouvenot et al. 2013b; Hussner et al. 2016). Ludwigia hexapetala also produces three types of roots: traditional branched roots, which grow downward, anchoring the plant to the sediment; spongy pneumatophores, which grow upwards, that act as specialized structures for tissue aeration (Ellmore 1981); adventitious roots produced at floating stem nodes that acquire nutrients from the water column and facilitate propagules dispersal and establishment (Gérard et al. 2014; Hussner et al. 2016; Billet et al. 2018; Skaer Thomason et al. 2018b). These morphological adaptations, combined with high growth rate, photosynthetic efficiency (Thouvenot et al. 2013b), and the ability to produce allelopathic substances that hinder other plant species (Dandelot et al. 2008; Thiébaut et al. 2018), denote the clear competitive nature of this alien species (Billet et al. 2018; Genitoni et al. 2020). The phenotypic plasticity of L. hexapetala would also highlight the broad ecology of the species, another key determinant factor in the success of many alien species in invaded areas (Richards et al. 2006; Davidson et al. 2011; El-Barougy et al. 2021). However, it is still unclear whether such ecological breadth of many invasive alien species may concern all ecological parameters or only some (Palacio-López and Gianoli 2011; Zhang et al. 2022). Therefore, a detailed investigation of the ecological requirements of invasive alien species, such as L. hexapetala, becomes essential to identify both the optimal environmental conditions that favour their colonization and expansion, and any limiting conditions that might control its growth.

The aim of the present study has been to define the main environmental parameters that influence the colonization and abundance of L. hexapetala by collecting field data in aquatic habitats in north-central Italy recently invaded by this alien species. Particular attention was paid to the environmental characteristics of aquatic habitats, which represent the first “front” of invasion of this species, in the early colonization stage of a new area. Detecting the environmental parameters that most influence the growth of L. hexapetala becomes crucial, both to identify the sites most at-risk of invasion in which to initiate timely monitoring actions for the species, and to be able to develop better management and control actions for the alien species in sites that have already been invaded.

As for the quantitative chemical and physical water data, the first two PCA axes were chosen, together explaining over 55% of the total variance (Fig. 2). The first axis (PC1) was negatively correlated with conductivity and nitrogen content (ammonia and nitrate concentration), while the second axis (PC2) was negatively correlated with temperature and phosphate concentration, and positively correlated with water depth and dissolved oxygen. The post-hoc test for L. hexapetala cover was significant (p < 0.01), indicating a correlation between L. hexapetala cover and ordination analysis results. In particular, L. hexapetala cover was significantly negatively correlated with both the first and second axes (see Suppl. material 3: table S3), indicating that the alien species cover increased with conductivity, ammonia, nitrates and phosphates, while it decreased with increasing values of dissolved oxygen and depth.

In ordination analysis of environmental qualitative parameters, the first two axes explained about 50% of the total variance (Fig. 3). The first axis (PC1) was mostly negatively correlated with a silty substrate and high to mid-high disturbance level, while the second axis (PC2) was correlated with partial-null water transparency and sandy substrate. The post-hoc test revealed a significant correlation between L. hexapetala cover and these ordination results (p < 0.05), with the species cover highly negatively correlated with the first axis (see Suppl. material 4: table S4), thus increasing significantly with high disturbance levels and silty substrate.

Water chemical and physical data collected in all sampled sites (see Suppl. material 5: table S5) were used to further explore the relationship between the cover of the alien species and each environmental parameter analysed (Fig. 2). Specifically, L. hexapetala was found in waters with different levels of oxygenation, although showing higher cover in oxygen-poor waters (oxygen concentration < 7 mg/l; Fig. 4a). The species showed the highest cover in waters with average temperatures of about 27°C (Fig. 4b) and pH values ranging from 7.7 to 9.0 (Fig. 4c). It was also found in waters with a wide range of conductivity values, although the average conductivity was very high (> 800 μS/cm; Fig. 4d). Regarding water nutrient levels, L. hexapetala showed high coverage in both highly nutrient-poor and moderately nutrient-rich waters (Fig. 4e–h). The species was present almost exclusively in shallow waters (< 50 cm; Fig. 4i).

The analysis of variance (ANOVA) on environmental qualitative parameters pointed out that L. hexapetala cover was not significantly correlated with water transparency (p > 0.05), since the species showed the same cover values in both transparent and turbid waters (Fig. 5a). Regarding site disturbance level, the species cover was significantly different between sites with various disturbance conditions (p < 0.001). Specifically, most of the L. hexapetala populations were found in sites with a “high” or “mid/high” disturbance level (Fig. 5b).

We found that certain environmental conditions influence the growth of L. hexapetala, with shallow and poorly oxygenated waters favouring high abundance of the species. In particular, at investigated sites, L. hexapetala showed a marked preference for shallow waters (i.e., less than 50 cm depth) where its aquatic morphotype, characterized by short vertical stems, can better root on the bottom and emerge at the surface. However, established populations of the species have been found in its native range (Fortney et al. 2004), and in some European sites that have been invaded for longer than those investigated here (Lambert et al. 2010), even in waters up to 3 m deep. In investigated waterbodies, where the species is in an early stage of invasion, it does not seem to be able to grow where water depth exceeds 50 cm, except in one case, where it was found in water 1 m deep. The fact that the colonization of L. hexapetala in the study area appears to be constrained to the presence of shallow water, suggests that water depth may represent a limiting environmental factor for this species in an early colonization stage, while it does not in the case of longer established populations. Some experimental studies also show that L. hexapetala growth rate does not appear to depend on water level (Hussner 2010), meaning that this species behaviour could change as the invasion progresses. Nevertheless, this environmental parameter should be taken into account in the management of waterbodies potentially susceptible to its invasion. Shallow waters warm up quickly during the summer, especially in the Mediterranean area in which the sampled waterbodies fall and where in fact recorded water temperature was relatively high. However, it has been observed in literature that L. hexapetala, which is native to temperate to subtropical regions, tolerates a wide range of thermal conditions, even resisting much lower temperatures (Thouvenot et al. 2013a). This underlines the wide ecological tolerance of this species for different climatic conditions. Another environmental driving force explaining the abundance of L. hexapetala is the concentration of dissolved oxygen in the water. Our data indicate that L. hexapetala grows well in oxygenated water, although populations of L. hexapetala with the highest percentage cover have mainly been found in poorly oxygenated waters; this underlines the high tolerance of this alien species to a variety of oxygenation conditions, including those that are generally limiting for most other aquatic plants. This tolerance is related to the ability of the species, similarly to the congeneric L. peploides (Rejmánková 1992), to produce pneumatophores that increase aeration of plant tissues submerged in oxygen-poor waters (Gérard et al. 2014). Thus, it is evident that under such limiting conditions, L. hexapetala successfully outcompetes the other aquatic plants by producing extensive, often monospecific populations. It should be considered that the reduction of dissolved water oxygen could be the consequence of the presence of L. hexapetala; indeed, this species often forms dense floating mats on the water surface that limit oxygen exchange at the air-water interface and reduce the concentration of dissolved oxygen in water (Dandelot et al. 2005; Thouvenot et al. 2013a; Pelella et al. 2023b). Moreover, due to pneumatophores (Ellmore 1981; Armitage et al. 2013; Hoch et al. 2015), this species is also particularly efficient in absorbing large amounts of oxygen from the water (Dandelot et al. 2005; Pelella et al. 2023a,b), further limiting its availability to other plants.

Taking the other water parameters into consideration, in the study area L. hexapetala showed a tendency to prefer alkaline and moderately ion-rich waters, although it has been found in waters with a wide range of conductivity. It should be noted that the positive relationship between L. hexapetala cover and water conductivity could also be a consequence of the presence of the alien species rather than a driving factor, since L. hexapetala has been found to increase water conductivity (Pelella et al. 2023a). In addition, it produces dense populations in both very nutrient-poor and moderately nutrient-rich waters, growing in different trophic water conditions. These results are consistent with Grewell et al. (2016), and with those of Matrat et al. (2006) who showed that aquatic species of the genus Ludwigia, particularly L. grandiflora (Michx.) Greuter & Burdet and L. peploides, grow in a wide range of conditions in terms of nutrient availability. In any case, among the various nutrients measured in this study, phosphates were found to be the most relevant in affecting the growth of L. hexapetala, as in most cases the densest populations were found in phosphate-rich waters. This would confirm what was reported by Skaer Thomason et al. (2018a), according to whom aqueous phosphorous is an important environmental parameter that favours the abundance of L. hexapetala.

According to our data, L. hexapetala grows mainly in sites with a substrate characterized by fine grain size (i.e., silt and sand), which allows for better rooting of the aquatic morphotype on the substrate. In addition, the alien species cover did not vary significantly in different water transparency conditions, underlining its tolerance for a wide variety of conditions. Furthermore, it is noteworthy that the percentage cover of this species increased significantly in sites with a high anthropogenic disturbance level. This supports the idea that L. hexapetala, as a typical invasive alien plant, can successfully and opportunistically colonise disturbed, altered, and unstable sites, which are known in literature to be more susceptible to biological invasions (Schröter et al. 2005; Kowarik 2008; Meyer et al. 2021).

Based on the results that emerged from this field study, pioneer L. hexapetala populations were able to grow in different environmental conditions. This wide ecological breadth is later confirmed in established populations, when L. hexapetala can produce extensive stands in both oxygenated and poorly oxygenated waters, in clear and turbid waters, in light and shaded conditions, in oligo-mesotrophic and eutrophic waters, although its growth increases with nutrient enrichment regardless of light regime (Hussner et al. 2010; Lambert et al. 2010; Thouvenot 2013a; Grewell et al. 2016). In any case, despite this large ecological breadth, some environmental conditions seem to have favoured the establishment and colonization of L. hexapetala in investigated sites. Specifically, the species showed the highest cover in anthropically disturbed sites with shallow, warm, poorly oxygenated, and alkaline waters, moderately rich in minerals and nutrients. It should be emphasized that water depth would seem to be among the few environmental parameters capable of limiting the establishment, and thus the invasion of this species, since it has been found in waters that are always shallow and, in any case, never deeper than 1 meter. To attain an even more complete picture of the ecology of L. hexapetala, further analyses would be needed to also assess the ecological parameters that most influence the spread of the terrestrial morphotype of the species along the banks of invaded waterbodies. It would also be interesting to come back to the investigated sites in the following years, monitoring the invasion trend, as well as analysing more environmental parameters that were not included in this study, such as water flow, water level fluctuation and other hydrological variables. Often, data regarding the early invasion stages of alien plant species are not available; therefore, the strength of this study lies in its provision of ecological data that pertains to such stages. This can provide new insights when compared to studies concerning long-established populations in other invaded countries. Consequently, the more limiting environmental conditions for the initial establishment of L. hexapetala, as documented in this one-year study, should be taken into account when formulating timely and effective management plans for its eradication in the invaded areas, before the species can establish well developed, stabilized populations. Although L. hexapetala is a relatively recent invader in Italy, its rapid spread in the investigated Italian waterbodies is alarming and should draw the attention of both scientific researchers and local environmental managers for the purpose of its containment.

The authors have no specific funding to report.

Conceptualization: E.P., S.C.; Y; Methodology: E.P., S.C.; Formal analysis: E.P.; Investigation: E.P., F.M., B.Q., S.C.; Resources: S.C.; Data Curation E.P., B.Q.; Writing - Original draft E.P., B.Q; Writing - Review and Editing E.P., F.M., S.C.; Visualization E.P., S.C.; Supervision: S.C.; Project administration: S.C.; Funding Acquisition: S.C.