Vegetation and water
The floristic-vegetational results of the four spring-wells show signs of environmental degradation but also valuable elements that are common to all four study areas at various level. The high number and/or coverage of ruderal therophytes of Stellarietea mediae class living in the spring-wells banks/edges, and even more in the corn fields, is a clear sign of disturbance as suggested by the low EIM value (far lower than the maximum value of 9 related to a mature wood free from exotic species) (Fig. 4). Such disturbance is caused by human activities, more precisely it is due to the intensive management of the surrounding fields and to the presence of urban areas around the considered spring-wells. These wetlands are deeply influenced by the adjacent highly disturbed areas as corn fields where IM and EIM values are extremely low (Fig. 4). Similar values were found for other corn fields of Po Valley (Giupponi et al. 2013) and are due to the presence of annual/short lived species (mainly cosmopolitan or exotic species) of the initial stages of the plant succession. These species can reach the banks because spring-wells are patchy and narrow (not extended in width) in the agricultural and urban landscape that characterizes a great part of Po Valley (Bischetti et al. 2012). The low EIM value of banks vegetation is also determined by the presence of the most diffused trees naturalized in Po Valley, especially on the edges of fields and roads and in little wooded spots as Robinia pseudoacacia and Ailanthus altissima (Del Favero 2002; Celesti-Grapow et al. 2010). White mulberry (Morus alba) can be added to these. Even though being an exotic plant, it can be considered an added value if present in these environments. White mulberry is in fact a Moraceae native to Eastern Asia probably introduced in Europe in the XII century (archeophyte) for silkworm farming (Bombyx mori) and grown in Italy from the XV Century (Bof 2017). At the end of the Nineteenth century Italy was the main producer of silk in Europe (Bof 2017), in Brescia, for example, during 1825, 26.800 tons of cocoons were produced (Fappani and Maffeis 1994; Bof 2017). Then, from the beginning of the Twentieth century, when synthetic textile fibres were introduced and importing silk from China became cheaper, the silkworms breeding in Italy and consequently the cultivation of the white mulberry narrowed gradually and almost totally disappeared in many Italian areas (Bof 2017). The presence of mulberry in all the spring-wells under study is therefore an element of historical and cultural value as a memory of agricultural activities, landscapes and agro-ecosystems of Po Valley almost totally disappeared today. IM and EIM, as formulated by their authors (Taffetani and Rismondo 2009; Rismondo et al. 2011; Giupponi et al. 2015), do not consider the historical-cultural value that some exotic species (as white mulberry) can have if present in determined environments (agro-ecosystems). The limit of these indexes is to consider every exotic species as a degradation and disturbance element, aspect that could be overcome integrating these indexes or creating new ones able to consider the historical-cultural value of these kind of exotic species in the plant communities where they are present.
Beyond the ruderal spontaneous species and the exotic invasive/naturalized species that naturally colonized the banks of the spring-wells, ornamental (exotic) species planted out of controls by local inhabitants (oral communication) were detected. These species, although not invasive, contribute to the degradation of the spring-wells either from an ecological (they lower EIM values) and physiognomic/landscaping point of view and their presence suggests the necessity of involving the local community in outreach/formative activities (Schild 2016). This would empower people awareness on the value and biological, scenic, and cultural importance of spring-wells and to know the basic concepts of nature conservation.
Together with the ruderal and exotic species, the floristic analysis allowed the identification of native species typical of Po Valley wetlands, as the ones of Lemnetea minoris, Potametea pectinatii, Phragmito australis-Magnocaricetea elatae and Salici purpureae-Populetea nigrae phytosociological classes (Table S1). These species present in areas I and II, represent the biological-naturalistic valuable element of the spring-well analysed. This applies particularly for spring-well A that is the richest in wetland species some of which found exclusively (such as: Carex riparia and Lythrum salicaria) and/or that have noteworthy coverage (such as: Poa palustris, Phalaroides arundinacea and Populus nigra) on the banks/edges of this specific spring-well. Spring-well A could be then employed as “source” area where to collect plant material (seeds or other propagation organs) to employ in possible re-introduction/re-population or ecological restoration initiatives of this or nearby spring-wells banks/edges (“sink” areas).
The riverbed of the spring-wells (zone I), although it has much less species comparing to banks/edges, does not present exotic species, and in fact its EIM and IM values are higher comparing to the ones of banks/edges vegetation (Fig. 4). Such value is however far from the maximum EIM value (9) for the reason that in zone I, a series of biotic and abiotic disturbances more or less recent (among which the constant presence of running water and the excavation and cleaning initiatives of the riverbed and spring-wells) hamper the establishment of a soil layer able to host species of the mature forest typical of Po Valley. In zone I, additionally to the absence of exotic species, there are some species uncommon/rare in Lombardy and in general in Italy as Callitriche obtusangula. From the analysis of the chemical-phyisical features of spring-wells (Table 3) and from CCA (Fig. 5) it might seem that Callitriche obtusangula needs eutrophic waters with a high nitrate, phosphates, and organic substance concentration as in spring-well B in which Callitriche obtusanguala has interesting values of coverage and algae are abundant (Table S1; Fig. 1). Conversely, in waters poorer in phosphates, nitrate and organic substance, Callitriche obtusanguala is less abundant (as well as algae) till being totally absent in spring-well C, that is the one with the least eutrophic water and that presents the minor number of species (3) in the riverbed (Table S1). The presence of Callitriche obtusangula in polluted waters is explained by being a nitrophilous and salt dependent species (“ss”, Landolt et al. 2010), according to Landolt et al. (2010) and Ellenberg and Leuschner (2010). Hence its presence, associated to algal uncontrolled growth, it is to consider an indicator of slow running waters particularly rich in nutrients/pollutants (eutrophic waters). Equisetum telmateia abundance in the riverbed and riverbanks of spring-well C could be instead due to minor nutrient requirements comparing to Callitriche obtusangula and to the fact that this species is not salt tolerant (Ellenberg and Leuschner (2010); Landolt et al. 2010). This only partially explains the results of the research, since spring-well C waters have low nitrate, orthophosphate and organic substance content, but a high content in nitrite (0.29 ppm). So, tailor made studies would be necessary to understand if the presence of nitrite in the water (and/or in the soil) can be or not a factor able to favour Equisetum telmateia or if this species is instead favoured by the low concentration of nitrate, phosphate and organic substance.
Considering this research results, some management suggestions to increase the naturalistic and cultural-historical value of the analysed spring-wells or others in similar conditions are provided below.
The relevant presence of exotic/ruderal species inhabiting land to the disadvantage of native and typical species of Po Valley wetlands is one of the main problems to solve. Fig. 6 shows an operational scheme to remove ornamental/exotic trees and shrubs replacing them with native woody species typical of hygrophilous woods of Po Valley. Removing ornamental/exotic shrubs and trees (excepting the oldest Morus alba with an historical-cultural value) must be carried out gradually over time (van Wilgen et al. 2000), starting from shrubs (Fig. 6). Removing the highest ornamental/exotic trees or the ones with the most developed foliage could in fact create clearings favourable to the development of new exotic naturalized plants as Robinia pseudoacacia and Ailanthus altissima that are heliophilous invasive species (Landolt et al. 2010) found in the spring-wells and in general in a great part of Po Valley (Del Favero 2002; Pignatti 2017; Fogliata et al. 2021). Thus, removing exotic/ornamental shrubs and trees, letting the highest trees, would reduce exotic invasive species growth and allow the implantation of native plants less heliophilous as Cornus sanguinea, Acer campestre, Sambucus nigra, Corylus avellana, Alnus glutinosa, Salix alba, Populus nigra and Carpinus betulus. These essences should be planted before the vegetative restart (fall, and/or late winter) considering their needs concerning the soil humidity. In fact, some cited trees/shrubs (Alnus incana, Salix alba and Populus nigra) are particular of alluvial forests and need a waterlogged soil (always or periodically) while others are in optimal growth conditions in soils not soaked with water. For this reason, planting the most hygrophilous species in the lower part or spring-wells riverbank (little over the water level) and the others in the higher part (1-2 m over the water level) would be advisable. Besides the cited species (present at least in one of the analysed spring-wells), planting the common oak (Quercus robur) would be worthwhile. This oak species is in fact increasingly rare in Po Valley even if it is, together with hornbeam (Carpinus betulus), one of the tree species representing the “current potential vegetation” (Biondi 2011) of Po Valley (Del Favero 2002; Verde et al. 2010). Once the young native trees/shrubs have taken seeds and after they created a dense shrubland, it will be possible to eliminate the highest exotic trees and the ones dangerous for people and goods (Fig. 6). Passing the years, the young native trees and shrubs will go on growing and expanding, thus improving the floristic and ecological features of the spring-wells vegetation. At this stage the management operations of trees/shrubs would be limited to the removal of dead/unstable plants (Fig. 6) and to the vegetation destruction after landslides on the riverbanks (often very steep, Table S1) in extraordinary circumstances. The intervention with low-impact soil stabilization works as soil bioengineering based on plants (or parts thereof) as building materials in combination with dead materials (such as stones, steel, iron, timber, etc.) would be advisable (Bischetti et al. 2014).
While the control of exotic/ornamental trees/shrubs on the riverbanks represents an operation relatively easy, the removal of ruderal and exotic herbaceous species could be much more difficult and expensive (van Wilgen et al. 2000). A great part of the exotic weeds found in the spring-wells are heliophilous species (as for example: Phytolacca americana, Erigeron canadensis and Ambrosia artemisiifolia) recurrent in Po Valley (Celesti-Grapow 2010; Pignatti 2017). Therefore, once built a dense shrubland (see above) and limiting the quantity of light in the herbaceous layer of the vegetation, the most heliophilous exotic and annual ruderals (therophytes) should reduce and the least heliophilous could be removed by qualified technicians (van Wilgen et al. 2000). Sowing or planting young plants of herbaceous species could be planned to encourage the establishment of wetlands. In this case a more structured environmental restoration project involving specialized centres/university could be envisioned. Sowing the native wetlands species of Po Valley could be then an option. However, finding this material in Italy is regrettably difficult (if not impossible), as Italy, comparing to other European countries, has very few producers of native seeds. It is sufficient to consider that in Italy there are only three producers (“Seme Nostrum”, “Centro Flora Autoctona” and “Flora Conservation”) (Giupponi and Leoni 2020) while in Germany there are 12 (Prasse et al. 2010) as well as they are more numerous in Switzerland (SKEW 2009). The shortage of native seeds producers in Italy is due principally to the lack of regulatory instruments to govern the use of autochthonous seeds in restoration work (De Vitis et al. 2017), issue to sort out as soon as possible to favour and improve the restoration of wetlands vegetation (Giupponi et al 2017b, 2019).
For what concerns the riverbeds of the spring-wells (zone I), given the absence of exotic species, possible interventions of re-introduction/re-colonization are the only actions to consider. In this zone, changes of the physical-chemical characteristics of the water and removing/upset the mud layer of the riverbed where hydrophytes as Callitriche obtusangula can root are to avoid. While the mechanic disturbance of the spring-well riverbeds is an action relatively easy to control as it is mainly attributable to human cleaning intervention, controlling the chemical-physical water and mud features is more difficult as they depend on both climatic and land use factors (Kløve et al. 2014; Balestrini et al. 2021). In the case study the high nitrogen content (in particular ammonium and nitrite), phosphates and organic substance is reasonably ascribable to the use of frequently employed intensive agriculture products (as ammonium nitrate) or possible industrial/urban discharges. Considering the difficulties of depurating groundwaters (Balestrini et al. 2008, 2018), this issue could be at least reduced identifying and stopping possible uncontrolled spills of discharge water in the spring-wells or nearby areas, and/or limiting the use of polluting chemicals in the fields adjacent the study areas. This last action could be facilitated by instituting one or more protected areas (as local park of supra-municipal interest – PLIS) able to protect spring-wells creating also buffer areas where only low-input agricultural techniques (as organic agriculture) can be employed. If land managers want to start the depuration and/or protect spring-well groundwaters against pollution in accordance with the European directive (EC 2006), the trade-off showed in this research will be to take into consideration: polluted/eutrophic waters favour the growth (and conservation) of Callitriche obtusangula while oligotrophic waters limit this rare species. In this case, a situation of compromise would seem ideal, meaning waters moderately eutrophic (with nutrients/pollutants values possibly under the legal threshold) that however allow the growth of Callitriche obtusangula. Finally, for what concerns the water temperature at the spring, the higher values comparing to other research works (Cavagnis and Orsini 1992), could be due to global warming (Kløve et al., 2014), aspect that is extremely difficult to manage in particular at a local scale.