The vanished Alnus-dominated forests along the Tyrrhenian coast

Catena 182 (2019) 104136

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The vanished Alnus-dominated forests along the Tyrrhenian coast ⁎

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Donatella Magri , Alessandra Celant, Federico Di Rita Department of Environmental Biology, Sapienza University, Piazzale Aldo Moro, 5, Roma, Italy

A R T I C LE I N FO

A B S T R A C T

Keywords: Alnus Vitis Pollen Plant macrofossils Holocene Coastal floodplains

The development and decline of alder floodplain forests and alder carrs along the Tyrrhenian coasts, in relation to sea level changes, geomorphological processes, human activity, and climate change are presented and discussed. A number of 22 pollen records, complemented by Alnus macrofossil data, document the presence of widespread alder populations in the coastal Tyrrhenian floodplains throughout the Holocene, although with different density from one site to the other, mostly depending on local hydrological conditions. The role of climate changes in the dynamics of floodplain forests appears uncertain. In the last two centuries, major reclamation works disrupted this natural vegetation to obtain fertile plains that are now exploited for agricultural purposes, industrial activities, urban areas and related infrastructures. Only a few remnants of the original alder forests are preserved by international conservation conventions, as biodiversity reservoirs of severely endangered habitats. However, the vanished alder forests, which proved to be able to rapidly recover several times through the Holocene, may still have some potential to be restored.

1. Introduction An increased interest in palaeoenvironmental studies on coastal sites in the Central Mediterranean region (Di Rita and Magri, 2012; Antonioli et al., 2017) has provided new insights into the past extent of wet environments that are currently reduced and threatened with disappearance. While there is much attention to the sensitivity of these environments to the ongoing and foreseen climate changes (Di Paola et al., 2012; Cazenave and Le Cozannet, 2014), still little is known of their long-term development, resilience and vulnerability in the Mediterranean, largely because most coastal palaeoenvironmental records are chronologically limited to the middle and late Holocene and are affected by discontinuous sediment accumulation. Along the Tyrrhenian Sea, especially in Tuscany, Latium and Campania, several wide coastal plains are found (Fig. 1), formed by the recent geomorphological evolution of Neogene to Quaternary basins that were filled with alluvial, coastal and shallow-marine deposits (Santangelo et al., 2017). They are generally characterized by welldeveloped dune systems, formed during late Quaternary sea-level changes, and by inland wetlands. Most of these wetlands disappeared during the last century, following extensive reclamation works. The original wetlands and floodplain forests have been almost completely obliterated and their ranges are currently very fragmented and reduced in size, being confined in narrow stands behind the dunes and in the interdunal wet environments (Lucchese and Pignatti, 1990), and

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persisting only as relict communities in the most inaccessible or unexploited areas (Cutini et al., 2010). The few remnants left in their natural state (Fig. 2) are extremely valuable representatives of the original ecological complexity and biodiversity richness and are often protected by international conventions (e.g., Convention on Wetlands, called the Ramsar Convention). In particular, both floodplain forests and alder carrs are of great concern for the development of biodiversity conservation and management strategies in coastal regions (RodríguezGonzález et al., 2014). The long-term history of these habitats has been poorly investigated in the Mediterranean regions. It is known that Alnus-dominated forests have been present in the alluvial and marshland ecosystems of many regions of Europe since the early Holocene, with a major expansion in the Atlantic period (Douda et al., 2014, 2016; Giesecke et al., 2017; Giesecke and Brewer, 2018), but the history of coastal alder woodlands has never been the subject of a specific study in the central Mediterranean Basin, although several pollen records reveal the vegetation history along the Tyrrhenian coasts since time periods when the human impact was negligible (Fig. 1). The present study reviews the evolution of alder-dominated environments profiled by 22 coastal pollen records from Tyrrhenian Italy, Sardinia and Corsica, complemented by macroremains of Alnus from natural and archaeological archives, with the aim of: - defining times and modes of alder forest development and decline

Corresponding author. E-mail address: [email protected] (D. Magri).

https://doi.org/10.1016/j.catena.2019.104136 Received 25 March 2019; Received in revised form 14 June 2019; Accepted 19 June 2019 0341-8162/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Map of the pollen (red dots) and macrofossil records (yellow dots) of Alnus along the Tyrrhenian plains: 1. Lake Massaciuccoli (Colombaroli et al., 2007); 2. Versilia Plain (Bellini et al., 2009); 3. Arno M1 (Amorosi et al., 2009); 4. Portus Pisanus (Kaniewski et al., 2018); 5. Ombrone (Biserni and van Geel, 2005); 6. Lingua d'Oca-Interporto (Di Rita et al., 2010); 7. Portus (Pepe et al., 2013); 8. Ostia C5 (Bellotti et al., 2011); 9. Fiume Morto (Pepe et al., 2016); 10. Mezzaluna 3 (Eisner et al., 1986); 11. Femmina Morta 197 (Doorenbosch and Field, 2019); 12. Vendicio (Aiello et al., 2007); 13. Minturno (Bellotti et al., 2016); 14. Golfo di Gaeta (Di Rita et al., 2018b); 15. Lago Patria (Di Rita et al., 2018c); 16. Neapolis harbour (Russo Ermolli et al., 2014); 17. Lago d'Averno (Grüger and Thulin, 1998); 18. C106 (Russo Ermolli and di Pasquale, 2002; Di Donato et al., 2008); 19. Sant'Eufemia Plain (Russo Ermolli et al., 2018); 20. Sa Curcurica (Beffa et al., 2016); 21. Posada (Melis et al., 2018); 22. Aleria Del Sale (Currás et al., 2017); a. Portus Pisanus (Bertacchi et al., 2008); b. Pyrgi - Santa Severa (Coccolini and Follieri, 1980); c. Le Cerquete-Fianello (Celant, 2002); d. Lingua d'Oca-Interporto (Di Rita et al., 2010); e. Tor Pagnotta (unpublished); f. Casale di Valleranello (unpublished); g. Longola di Poggiomarino (Heussner, 2012). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2. Modern distribution map of Alnus glutinosa from Euforgen (http://www.euforgen.org/) and location of the Sites of Community Importance (SCI) belonging to Natura 2000 habitats 91E0 and 91F0 (triangles) within the Tyrrhenian plains. A tentative reconstruction of the distribution of the lost alder forests is represented. Dots indicate the pollen and macrofossil sites listed in Fig. 1.

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3. Materials and methods

along the Tyrrhenian coast, - identifying the factors driving the alder vegetation dynamics at a regional scale, - characterizing the past alder vegetation as biodiversity reservoir for plant species of economic value, - discussing conservation strategies, in a long-term perspective.

Following a careful screening of pollen records from Italy, Sardinia, and Corsica, a total of 22 pollen records, considered in this work, were found that are located facing the Tyrrhenian Sea (Fig. 1). In addition, the available macrofossil data from the same coastal areas were considered. The chronological control of the pollen and macrofossil records is based on radiocarbon dates and tephra layers, although in some cases (Lago d'Averno, Neapolis harbour, Portus Pisanus, Longola di Poggiomarino, and Pyrgi-Santa Severa) archaeological evidence was used to establish the age of the remains. In pollen records, a value of 2.5% was considered indicative of the local presence of alder (Lisitsyna et al., 2011), while values > 10% were considered to reflect Alnus-dominated vegetation within the site catchment (Huntley and Birks, 1983). We considered that values > 35% in pollen records may represent an alder carr, based on surface pollen analysis from NE-Germany (Prager et al., 2012). We are aware that water/river transport may affect the pollen representation in plain landscapes. Pollen records for which pollen counts are not available to us, have been digitized from published pollen diagrams. The pollen diagrams in Fig. 3 were plotted using the program Psimpoll (Bennett, 2008). All ages are expressed as calendar years. Wood remains recovered from archaeological sites were possibly collected by humans, but in all cases we suppose they were of local origin, as they were used for activities of ordinary life. We did not consider wood remains of alder from boats and ships. In one case, also leaves and fruits of Alnus were recovered (Bertacchi et al., 2008).

2. Floodplain forests and alder carrs Alnus glutinosa (L.) Gaertn. has a wide distribution in Europe, being diffused from Scandinavia to the Mediterranean countries (Durrant et al., 2016). It is the only species of Alnus living in marshy waterlogged sites and floodplain forests in the coastal areas along the Tyrrhenian Sea (Cutini et al., 2010). Its natural distribution (Fig. 2), generally below 1000 m in elevation, indicates that the species is adapted to a wide range of temperatures but is limited by aridity (where annual rainfall is below 500 mm), so that at the drier limits of its range, it finds refuge in the humid microclimates of valleys (Claessens et al., 2010). In fact, it requires high water availability to thrive and can survive flooding better than most other forest tree species. It is typically a component of mixed broadleaved forest, and it contributes particularly to riparian ecosystems where it assists with water filtration and purification in waterlogged soils. The timber of alder is very durable under water, and the root system is adapted to penetrate deeply into wet and anaerobic soils and helps to control floods and stabilize riverbanks (Claessens et al., 2010). In addition to riverside sites where A. glutinosa is often mixed with Fraxinus, Ulmus, Acer, Salix and other tree species adapted to alluvial sites, there are two main habitats where A. glutinosa usually lives (Claessens et al., 2010):

4. Results

• in sites with a temporary water-table, it may be found in forests with

•

The history of Alnus populations along the Tyrrhenian coasts has been reconstructed following the main coastal plains from Tuscany to Calabria and then from Corsica to Sardinia (Fig. 1). In the Versilia Plain, the pollen record from Lake Massaciuccoli (Fig. 3), which originated as a retrodunal basin after the last sea transgression (Versilian transgression), shows values of Alnus > 10% between 7000 and 4800 BP, with a peak of 30% at 5200 BP (Colombaroli et al., 2007). These values indicate a significant presence of alder in the coastal floodplain forest. According to Colombaroli et al. (2007), the development of Alnus was in response to fire activity, favouring the re-sprout of fire-resistant species like A. glutinosa. After 4800 BP, especially between 4000 and 2800 BP, a possible shift to drier climate conditions may have disadvantaged drought-sensitive Alnus, as reflected in the decline of pollen values (5–10%), which however indicate continuous local presence. Between 2800 and 700 BP, a new remarkable expansion of local Alnus populations is testified by frequencies up to 35%, suggesting the temporary existence of an alder carr. At the margin of Lake Massaciuccoli, the low-resolution records ENEA1 and ENEA2 show continuous presence of Alnus between 10,000 BP and the Roman times (Bellini et al., 2009). A considerable development of the local riparian vegetation, represented by Alnus > 40%, occurred especially from ca. 5600 to 2700 BP, coupled with an expansion of Vitis and Salix after 4200 BP. In the Arno Plain (Fig. 1), Amorosi et al. (2009) found significant values of riparian trees (> 40%), including Alnus, during transgressive phases of the early Holocene before 8000 BP. After 8000 BP, a long pollen record from the harbour area of Pisa (Portus Pisanus) (Fig. 3) shows a discontinuous curve of Alnus, never exceeding 5% (Kaniewski et al., 2018). It points to the absence of local stands of alder-dominated forest, but it does not exclude scattered local occurrences of alder trees. In fact, the local presence of alder is testified by macrofossil remains of fruits, leaves and cortex of Alnus in the Roman harbour of Pisa (Bertacchi et al., 2008). In the Ombrone Plain, a discontinuous presence of floodplain forest

other mesophilous species (floodplain alder forests). These forests are characterized by the dominance of hardwood tree species with alder (Alnus glutinosa), ash (Fraxinus angustifolia, F. excelsior), elm (Ulmus laevis, U. minor), poplar (Populus alba, P. nigra), oak (Quercus spp.) and willow (Salix spp.) as the most frequent elements (Brullo and Spampinato, 1999; Douda et al., 2016). in marshy areas with soils waterlogged throughout the year, such as lakes, swamps, and waterlogged depressions in floodplains, it forms almost monospecific stands (alder carr) owing to its physiological adaptations to anaerobic soils (Döring-Mederake, 1990; Ellenberg and Leuschner, 2010; Douda et al., 2012).;

These two types of alder forests are currently classified into the separate vegetation classes of the Salici purpureae-Populetea nigrae and the Alnetea glutinosae, respectively (Biondi et al., 2014). In large river plains, both floodplain forests and alder carrs may occur next to each other, forming temporally variable habitat mosaics (Douda et al., 2016). These forests are included in the Annex I of the Habitats Directive 92/43/EEC (priority site 91E0: Alluvial forests with Alnus glutinosa and Fraxinus excelsior). Along the Tyrrhenian coasts, only a limited number of Sites of Community Importance (SCI) fall in this category (Fig. 2). The largest areas are found in the Macchia Lucchese (40.6 ha) and Selva Pisana (482.85 ha). Altogether, the remaining sites in Tuscany, Sardinia and Corsica reach 17 ha. In addition, it should be considered that protected areas of habitat 91F0 (Riparian mixed forests of Quercus robur, Ulmus laevis and Ulmus minor, Fraxinus excelsior or Fraxinus angustifolia, along the great rivers) may also include Alnus as a secondary component in coastal floodplain forests. Along the Tyrrhenian coasts, this second habitat is found in the Foresta Demaniale and Parco Nazionale del Circeo in southern Latium, in addition to the Macchia Lucchese and Selva Pisana in Tuscany (http://natura2000.eea.europa. eu). 3

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Fig. 3. Pollen percentage records of Alnus and occurrences of Vitis from selected sites located along the Tyrrhenian coasts (Fig. 1). Different colours indicate extralocal presence of Alnus (< 2.5%), local presence (2.5–10%), Alnus-dominated floodplain forest (10–35%), and Alnus carr (> 35%).

et al., 2010). After this interval, Alnus dramatically decreased and even temporarily disappeared between 2900 and 2000 BP, possibly in relation to a change of fluvial inflow into the pond, determined by a migration of the river mouth and coastline progradation (Milli et al., 2013). In this phase, the Tiber River played a major influence on the Ostia pond located in the southern side of the deltaic region, as testified by an increase in Alnus (ca. 10%) in the pollen record of the Ostia C5 core (Bellotti et al., 2011) between 2500 and 2000 BP, and confirmed by high values of Alnus (15%) in the Fiume Morto pollen record at the same time (Pepe et al., 2016). Comparable frequencies of Alnus in Imperial Times were also found in the pollen record of the harbour of Portus (Pepe et al., 2013). During the last 2000 years, on the northern side of the Tiber delta, in the Lingua d'Oca-Interporto record, the frequencies of Alnus slightly increased up to ca. 10%, suggesting a new development of floodplain forest with alder populations (Di Rita et al., 2010). The area was completely reclaimed and traversed by drainage canals starting in the 19th century and is currently occupied by agricultural fields and by the Fiumicino airport. The Pontine plain (Fig. 1), in southern Latium, was occupied by a large marshland until massive reclamation works, carried out by the fascist regime in the 1920's, turned it into agricultural land. A pollen record from the inland margin of the ancient marsh (Mezzaluna) shows two considerable expansions of Alnus in the mid- and late Holocene (Eisner et al., 1986) whose age is uncertain because of the “hard water” effect on radiocarbon dates (Sevink et al., 2013). In the Fondi Plain, the pollen record from Femmina Morta provides just a snapshot of the vegetation conditions between ca. 3900 and 3500 BP, when the frequencies of Alnus generally ranged from 10 to 20% with a remarkable peak of ca. 70% at 3700 BP, which suggests the presence of a temporary alder carr, although no alder macroremains were found in the rich macrofossil assemblage that was recovered (Doorenbosch and Field, 2019).

with Alnus is suggested by a pollen record during the last 7200 years (Biserni and van Geel, 2005). In the southern margin of the Latium Maremma (Fig. 1), macrofossil remains from the Etruscan site of Pyrgi-Santa Severa (200–250 a.C.) document the presence of Alnus in the area (Coccolini and Follieri, 1980). In the Tiber Plain (Fig. 1), the pollen record from Pesce Luna (PLU) (ca. 12,900–8400 BP), analyzed between 45 and 33.5 m depth of a 100 m-long sediment core drilled approximately 5 km north of the Tiber River mouth, is among the few records in the central Mediterranean providing vegetation information from coastal areas at the Pleistocene to Holocene transition (Milli et al., 2013; Di Rita et al., 2015). Alnus is recorded in almost all the samples between 12,900 and 11,400 BP with frequencies never exceeding 3%, then it significantly increased with a peak of 15% around 10,600 BP, in relation to the development of a marshy environment hosting a riparian woodland. From 10,600 to 8400 BP, Alnus decreased to mean values of ca. 5%. Between 8400 and 5500 BP, the nearby record from Lingua d'Oca-Interporto (Fig. 3) indicates that woodlands with a significant presence of Alnus populations were found in the wet soils of the northern Tiber Plain, as testified by alder pollen ranging from 8 to 12% (Di Rita et al., 2010). From 5500 to 5000 BP, a remarkable decrease in the water level induced the development of an extensive reed swamp and a rapid decline of riparian trees. However, Alnus was still present in the plain, as many wood fragments were found in the Eneolithic settlement of Le Cerquete-Fianello, at the margin of the wetland, radiocarbon dated between 5300 and 5000 BP (Celant, 2002). Between 5000 and 2900 BP there was a new remarkable expansion of riparian trees, mostly represented by Alnus (40–80%), accompanied by Salix and Fraxinus, with an understory dominated by Vitis. This situation suggests a permanently flooded plain, with the consequent formation of an alder carr, testified also by the deposition of a peat layer including several wood macroremains of alder (Di Rita 4

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(< 1%). The site of Posada (Fig. 1) is located in the alluvial-coastal plain of the 50-kilometer-long Posada River. The cores collected in this site, including fine sediments alternating with sand and coarse sediments, resulted in a fragmentary pollen record with poor pollen preservation, spanning the time-interval from 7500 to 5500 BP (Melis et al., 2018). Alnus shows scattered occurrences before 5800 BP, followed by a remarkable increase (38%) between 5800 and 5500 BP, suggesting the development of alder carr formations in the local coastal floodplain (Fig. 3). This is consistent with the pollen record of Torregrande in the Tirso coastal plain (central-western Sardinia), where alder forests were found between 6000 and 5400 BP (Melis et al., 2017). In the Corsican coastal plains, most pollen sites are supported by weak chronologies (Reille, 1984, 1992) or are located in the western side of the island, where Alnus glutinosa floodplain formations were well developed during the Holocene, most frequently during the subboreal phase (5000–2500 BP) (Reille, 1992). On the Tyrrhenian side of Corsica, the lagoon of Aleria del Sale, located in the middle part of the Eastern Plain (the so-called Aleria Plain), allows tracing back the history of Alnus (Currás et al., 2017). In the pollen record spanning the last 5400 years, Alnus frequencies, corroborated by high pollen concentrations, range between 10% and 20% until 1800 CE, pointing to a continuous and significant presence of alder communities in the plain (Fig. 3). Between 1800 and 1950 CE the pollen record is characterized by a 30-cm-thick barren interval of sediments consequent to drainage works carried out in the lagoon. When the pollen record starts again, around 1950, it shows a significant decrease in Alnus percentages below 5%, consistent with a rapid decline of alder woodland formations related to the changes of limnological conditions following the drainage activities. These determined the disappearance of hygrophilous vegetation and favoured the development of pioneer species of halophile chenopods.

In the Minturno Plain, two pollen records spanning the last 8200 years from the mouth of the Minturno River show the presence of wet habitats rich in riparian trees such as Alnus (up to 80%) and Salix (up to 10%) which alternately expanded and contracted (Bellotti et al., 2016), suggesting discontinuous presence of an alder carr. The pollen record dated at around 8400 BP from the Vendicio Plain, between the Fondi and Minturno Plains, also shows high frequencies of alder (> 40%) pointing to a possible regional development of the Alnusdominated forest during the early Holocene (Aiello et al., 2007). Between 7500 and 2800 BP the floodplain riparian forest with Alnus at the mouth of the Minturno River was much reduced (5–10%), except for a temporary new increase between 6500 and 6000 BP (20%). In the Campanian Plain, insights into the vegetation history of coastal areas come from lagoon, volcanic, and harbour sedimentary archives. In the pollen record from Lago Patria (Fig. 3), a lagoon in the coastal floodplain region of the Volturno and Regi Lagni rivers, Alnus shows frequencies exceeding 20% in the time intervals 4800–4000 BP and 2600–2200 BP, coupled with pollen of Vitis from natural grapevine populations. Comparable frequencies are recorded in a level dated ca. 180 BP, which suggests that alder populations were abundant in the local floodplain landscape also in modern times before reclamation works, which started by the end of the 16th century under the Spanish domination (Di Rita et al., 2018c). In the time interval between 2200 and 200 BP, information on the history of Alnus can be retrieved from the nearby pollen record of the volcanic Lago d'Averno in the Phlegraean Fields, spanning the time interval from ca. 800 BCE to 800 CE (Grüger and Thulin, 1998). In this sequence, low frequencies of Alnus are found in Roman times (ca. 2%), while in the post-Roman phase an alder increase (> 5%) is consistent with the presence of a small alder population within the lake in a developing freshwater habitat. In the pollen record from the Neapolis harbour, spanning from the 1st century BCE to the 5th century CE, Alnus shows even lower frequencies, with only scattered occurrences (Russo Ermolli et al., 2014). In the Sarno Plain, south of Naples, the presence of Alnus is documented by the macrofossil assemblage from the Longola di Poggiomarino archaeological site, occupied between approx. 3500 and 2600 BP, where Alnus was a minor component of an oak dominated vegetation (Heussner, 2012). Evidence for the presence of Alnus along the Campanian coast is provided by the marine pollen records from the Gulf of Gaeta in front of the Volturno Plain (Fig. 3; Di Rita et al., 2018b), spanning the last 5500 years, and from the Gulf of Salerno in front of the Sele Plain (Russo Ermolli and di Pasquale, 2002; Di Donato et al., 2008), spanning the last 34 kyr. They show values of Alnus generally ranging from 0 to 3% with only few peaks around 5%, reflecting a regional pollen representation of mixed floodplain forests. In the Sant'Eufemia Plain, a pollen record documents the coastal vegetation history in Calabria (Russo Ermolli et al., 2018). Alnus occurs in almost all the sequence, spanning from 8300 to 1240 BP, with two gaps at 4600–4000 and 3800–2800 BP (Fig. 3). Between 8300 and 6700 BP, Alnus shows frequencies generally around 20%, then it progressively declines to values around 2% around 5400 BP. In the few analyzed samples between 5400 and 2700 BP, Alnus presents a new remarkable increase, often with values around 40%, pointing to a development of an alder carr. The upper part of the pollen diagram, after 2200 BP, documents a new dramatic decline of alder populations. In the E-Sardinian coastal plains, two pollen records were studied. The wetlands that host Sa Curcurica (Fig. 1) are located within a slightly lowered area behind the seashore (Beffa et al., 2016). The pollen record, spanning the time interval 8150–0 BP, displays a continuous presence of Alnus with frequencies generally below 1% between 8150 and 5130 BP, suggesting scattered alder trees in the landscape. From 5300 BP Alnus shows a moderate increase with frequencies never exceeding 5%, testifying for a slight development of coastal alder populations, but not for the establishment of Alnus-dominated floodplain forests. Around 1000 BP Alnus almost disappears from the record

5. Discussion The postglacial history of the Alnus-dominated forests in the Tyrrhenian floodplains is characterized by overall different trends from one site to another. In many studied areas, even pollen records close to each other show contrasting frequencies of Alnus, suggesting that local edaphic conditions may have been a major factor influencing the development of alder communities. For example, a forest with sparse presence of Alnus (ca. 10%) at Lake Massaciuccoli between 4000 and 2800 BP (Colombaroli et al., 2007) is matched by the development of an alder carr (> 35%) at the margin of the same wide lake (Bellini et al., 2009). A dramatic drop in Alnus between 2900 and 2000 BP at Lingua d'Oca-Interporto, just north of the mouth of the Tiber River (Di Rita et al., 2010), corresponds to an increase in Alnus in the Ostia C5 (ca. 10%) and in the Fiume Morto (15%) pollen records (Bellotti et al., 2011; Pepe et al., 2016), south of the Tiber River. Pollen values indicating an Alnus-dominated floodplain forest at Lago Patria (Di Rita et al., 2018c) are not reflected in the record from Lago d'Averno, 12 km apart (Grüger and Thulin, 1998), and in the marine sediment core from the Gulf of Gaeta, representing the regional vegetation of the area (Di Rita et al., 2018b). Similarly, the two Sardinian records of Sa Curcurica and Posada (Beffa et al., 2016; Melis et al., 2018), located at ca. 50 km distance from each other, depict substantially different abundances of Alnus. Discordant results are sometimes obtained also when macrofossils and pollen of alder are recovered from the same plain. In Portus Pisanus, near Leghorn, a discontinuous pollen diagram of Alnus, never exceeding 5% (Kaniewski et al., 2018), contrasts with the finds of fruits, leaves and cortex of alder found in the Roman harbour near Pisa (Bertacchi et al., 2008), where pollen records from ships dated from the Etruscan to the late Roman Empire periods show variable values ranging from < 5% to > 25% (Mariotti Lippi et al., 2007). Conversely, at Fondi no alder macroremains were found in a rich macrofossil assemblage where pollen frequencies reached 70% (Doorenbosch and Field, 2019). These discordant results indicate that pollen and macroremains 5

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can be found regarding the duration of this phase, which ended at different times in different sites. The late Holocene is characterized by a general decline of the alderdominated floodplain forests, which was particularly marked in historical times, although it is difficult to recognize a trend common to all the pollen records. In some cases, such as at Lake Massaciuccoli (Colombaroli et al., 2007), a decline of the floodplain forests occurred only in the last 800 years, but remnants of the ancient populations are still found in a nearby SCI. A progressive disappearance of this vegetation type in the last two thousand years is recorded at Minturno (Bellotti et al., 2016). Conversely, in several Tyrrhenian floodplains there was a sudden demise of alder-dominated formations, mostly related to reclamation works for agrarian scopes in the last centuries. For example, the topmost samples from Lago Patria show an almost complete disappearance of Alnus in the interval interested by the reclamation works carried out by the Bourbons in the 19th century (Di Rita et al., 2018c). At Aleria del Sale in Corsica, alder-dominated forests were extirpated by drainage operations executed in the 19th and 20th centuries, after which time the wetland turned into its current brackish lagoon status (Currás et al., 2017). In other cases, the decline of the Alnus forest occurred in multiple phases. For example, in the sedimentary record from Lingua d'OcaInterporto (Di Rita et al., 2010) around 2900 BP a sudden change in sedimentation from peat to clay, possibly reflecting reduced fluvial inflow into the pond, corresponds to a dramatic drop of the alder carr. During Imperial times, a development of halophytes, due to intense saltworks activities in the wetland, limited the recovery of alder communities, despite a slight increase in Alnus is recorded after Medieval times. In the 19th and 20th centuries, land reclamation works produced the drying up of the marshland, topped by reworked sediments and agrarian soils, and definitely extirpated the local remnants of alder vegetation. Unfortunately, many pollen sites along the Tyrrhenian coast do not record the final stages of the Alnus-dominated forests, due to reclamation works in recent times, which interrupted the sediment accumulation in the basins and/or obliterated the pollen records. However, from historical documents and from the modern distribution of A. glutinosa (Fig. 2), we know the fate of these forest formations, that underwent a strong fragmentation since two thousand years ago, followed by a human-induced disruption in modern times. The disappearance of floodplain forests also entails the loss of alder companions, such as Fraxinus, Salix and Vitis. Among these, it is worth mentioning the history of wild grapevine, which is common and sometimes very abundant in the Mediterranean natural riparian vegetation in the form of a climber. The evidence of co-occurrence of Alnus and Vitis in the Italian Peninsula dates back to at least the Middle Pleistocene (Di Rita and Sottili, 2019 and references therein). In several pollen records from the Tyrrhenian coasts, Vitis parallels Alnus (Fig. 3), reaching even frequencies of 40% in the records of Lake Massaciuccoli (Mariotti Lippi et al., 2007) and Fango (Reille, 1992) during the midHolocene. In many sites, Alnus and Vitis are contemporarily recorded with no evidence of human impact (Mariotti Lippi et al., 2007; Di Rita et al., 2010; Currás et al., 2017; Di Rita et al., 2018c; Melis et al., 2018; Russo Ermolli et al., 2018), until Vitis was managed, probably since the Bronze Age (Aiello et al., 2007; Russo Ermolli et al., 2018). Cremaschi et al. (2016) show the role of Vitis in inland riparian associations even when vines grew close to prehistoric archaeological sites in the Middle Bronze Age. An extensive cultivation of Vitis is probably to be related to the Greeks and/or the Etruscans, who likely introduced and planted in Italy varieties originated from other regions of the Mediterranean and the Near East (Zecca et al., 2012; Wales et al., 2016; Terral et al., 2010). At the same time, wild grapevine could persist in the alder floodplain forests, which can be considered an important biodiversity reservoir for this climber species of considerable economic value. While the origin and development of alder forests appear markedly influenced by edaphic and ecological processes following the Holocene

give complementary evidence, as macroremains record a very local presence while pollen can be of regional origin. Despite this general heterogeneous situation, some features of regional dynamics can be traced by reviewing the origin, development and decline of the Alnusdominated formations. In the early Holocene, coastal forests were located tens of meters below the current coastline (Milli et al., 2013; Di Rita et al., 2015). Due to the former sea level position, the sedimentary archives recording these ancient floodplain forests are difficult to be retrieved without hiatuses, due to the dynamic nature of floodplain environments. They are rarely studied by palynologists, as they can be recovered only by deep near-shore drilling of lacustrine/lagoonal sediments rapidly submerged by fast sea-level rise. The few available long pollen records from the Tyrrhenian coast, in Tuscany and Latium, depict an early Holocene origin of the alder-dominated floodplain forests, as testified by Alnus frequencies exceeding 10% already in this phase (Amorosi et al., 2009; Bellini et al., 2009; Di Rita et al., 2015). The middle Holocene coincided with the establishment, development and stabilization of most of the Tyrrhenian coastal wetlands, which were formed following complex sedimentological, geomorphological, and ecological processes in response to the sea-level rise deceleration since around 8000 BP. The same pattern is also found in many coastal areas of Atlantic and Mediterranean regions of Europe (Pons, 1992; Behre, 2004; Deforce, 2011). In the Tyrrhenian coasts, pollen assemblages and/or sedimentological features often reveal that the vegetational development involving aquatic and hygrophilous plants followed a typical hydroseral succession, characterized by an initial phase of open water bodies with phytoplankton and aquatic plant communities (e.g., Rivulariaceae and Myriophyllum), followed by peaty ponds featured by reed-swamp and sedge marsh environments, and then by swamps with Alnus woodland/carr and peat deposition. In particular, the sites of Lingua d'Oca-Interporto (Di Rita et al., 2010), Posada (Melis et al., 2018), and Lake Massaciuccoli (Colombaroli et al., 2007) present clear examples of these ecological dynamics from limnic to terrestrial communities, which mirror the dynamics in many inland peatlands in Europe (Muller et al., 2012). In other Tyrrhenian records, this mid-Holocene pattern is hardly detectable or totally absent, either because of unfavourable physiographical conditions of the sites, or because in such dynamical environments as the floodplains, hydroseral successions may have occurred in different areas of the basins at different times, so that mature plants communities, established in other parts of the basin in previous phases, overshadow the pollen representation of early-stage communities. This may be the case of Sant'Eufemia Plain (Russo Ermolli et al., 2018), which outlines possible hydroseral successions in both the time intervals 8300–7800 BP and 6400–5400 BP, resulting in a mixed pollen representation of hygrophilous communities at different stage of development. Besides, in the estuarine sites from Corsica, Campania and Calabria, the Alnus record may include inputs of Alnus cordata pollen from inland populations, normally located at elevations higher than 300 m (Caudullo and Mauri, 2016). This pollen is not counted separately from A. glutinosa, except for a few records (Barra et al., 1999). A common feature shared by several sites is a remarkable increase in Alnus, occurred diachronically from 5800 to 5000 BP, overall tracing the phase of maximum spread of alder-dominated woodlands in the Tyrrhenian plains during the Holocene (Di Rita et al., 2010; Melis et al., 2018; Russo Ermolli et al., 2018). It coincides with the mature forest stage of the hydrosere succession and at some sites it matches the formation of alder carrs (Lingua d'Oca-Interporto, Sant'Eufemia Plain and Posada Plain in Fig. 3). Interestingly, in this time interval an increase in Alnus is also recorded in the marine sediment cores from the Gulfs of Gaeta (Fig. 3) and Salerno (Di Rita et al., 2018b; Russo Ermolli and di Pasquale, 2002), reflecting the development of alder forests at a regional scale. While the onset of the alder expansion is chronologically well constrained, with only moderate time differences probably depending on local geomorphic and ecological factors, no general patterns 6

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ago, are now almost completely lost. In Fig. 2, the modern range of A. glutinosa is integrated with the palaeobotanical evidence to reconstruct the former distribution before the recent disruption. While A. glutinosa cannot be considered a threatened species, as it is very common in Europe, the question of its disappearance from large Tyrrhenian areas cannot be neglected for several reasons. Firstly, A. glutinosa is one additional species to the list of woody taxa that have undergone a severe contraction in southern Europe since the last glacial period, including Buxus sempervirens (Di Domenico et al., 2012), Carpinus betulus (Muñoz Sobrino et al., 2017), Abies alba (Magri et al., 2015), and Picea abies (Magri et al., 2017). Secondly, the Mediterranean wetlands host a long list of plant and animal species, some of which are rare or even endemic, facing the risk of extinction together with the Alnus swamps, thus producing an irreparable impoverishment of the biodiversity levels (Perennou et al., 2012; Zacharias and Zamparas, 2010). A recent review of the plant taxa that disappeared from the Latium region, based on scientific literature and herbarium records, indicates that, apart from the city area of Rome, most losses occurred in the coastal floodplains at the time of the great reclamation works between 1870 and 1920 (Lucchese, 2018). Only a few restricted remnants of the lost forests are preserved within the Natura 2000 network (Fig. 2). The extension of Sites of Community Importance of habitat 91E0 (Alluvial forests with Alnus glutinosa and Fraxinus excelsior) is negligible compared to the extension of the vanished forests. Besides, a major problem is that restoration will be hardly implemented, as it is a low priority compared with other more pressing economic activities. In fact, the wide reclaimed areas along the Tyrrhenian coasts currently host highly productive agricultural land, industrial and urban centres, airports and primary communication routes, which cannot be easily dismissed. To counterbalance this unfortunate perspective, the Holocene history of vegetation in Tyrrhenian Italy indicates that alder forests can rapidly recover when flooded environments and suitable edaphic conditions are restored. In a sense, the instability of coastal environments on the one hand reveals a strong vulnerability, but on the other hand bears a strong potential for recovery.

sea-level rise, and their decline may be ascribed to human activities, what was the role of climate in the dynamics of floodplains? To what extent have climate changes affected the history of alder formations? The available literature on Alnus-dominated forests in Europe suggests that climate changes had a marginal influence on the evolution of coastal environments. However, climate influence cannot be totally ruled out, since A. glutinosa is favoured by humid conditions (Ellenberg et al., 1992). Alder requires high availability of water to thrive and the atmospheric humidity must keep high during all phases of its reproductive cycle (Durrant et al., 2016). Accordingly, alder had its maximum expansion along the Tyrrhenian coast in the mid-Holocene, which corresponds to an overall phase of high precipitations that may have favoured the development and persistence of alder-dominated forests in many sites. Climate influence is often called into question to explain the history of alder-dominated forests in European inland sites. For example, in the central French Alps, the progressive spread of alder populations would result from the interaction between climate forcing and sedimentation processes, which appear responsible for the past dynamics of the regional wetlands (Muller et al., 2012). The modern distribution of Alnus, currently absent from large areas of the Hungarian, Romanian and Ukrainian lowlands, and of the Iberian Peninsula, indicates that it is significantly affected by arid climate. In particular, Douda et al. (2014) suggest that the aridity of the Mediterranean regions may limit the establishment of new populations and locally induce population decreases. However, in the literature on pollen records from the central Mediterranean, the expansion/contraction of Alnus-dominated forests is never explicitly related to climate changes. A simple correlation with climate is partly prevented by an unclear correspondence of the increases in Alnus frequencies with shifts towards wetter climate, as described by the independent palaeoclimate proxy records available for the region (Lirer et al., 2013; Di Rita et al., 2018a; Bini et al., 2018). All the same, a relationship between shifts towards aridity and declines of Alnus-dominated forests appears insecure, although during the last few thousand years Alnus populations may have responded to a general aridification trend largely recognized in the Mediterranean regions (Reale and Dirmeyer, 2000; Di Rita et al., 2018a, 2018b, 2018c; Bini et al., 2018). A. glutinosa populations, characterized by fast growth and decline, may respond rapidly to dry climate events, thus leaving clear evidence of climate change in pollen records. Besides, climaticallydriven changes in the water table level of floodplains may indirectly affect the density and extent of alder populations on waterlogged-tomoist soils and the resulting pollen production. In Tyrrhenian Italy, despite no clear correlation between human demography (Stoddart et al., 2019) and the oscillations in the pollen record of Alnus may be found, human activity is considered the prominent factor determining the decline of alder-dominated forests, overshadowing the effects of climate changes. We have shown that the demise of these environments in the Tyrrhenian floodplains mostly occurred during the last two thousand years, when many coastal pollen records document a general opening of the forest, accompanied by an increase in cultivated and synanthropic plants, which testify for a remarkable human impact (Di Rita and Magri, 2012). This is consistent with coastal and inland sites of Europe, where the general decline of Alnus-dominated forests during the late Holocene is ascribed to an increase in human pressure on natural environments rather than to climate change (Hughes et al., 2012). Especially in the Mediterranean regions, extensive unhealthy areas, occupied by marshlands and floodplain forests, have been replaced by arable land and grazing pastures, because they have the most productive and humid soils within the arid catchments (Hughes et al., 2012).

7. Conclusions The present review offers new insights into the extent of Alnusdominated forests along the Tyrrhenian coasts during the Holocene. We have found that Alnus was present in all coastal floodplains before the reclamation works of the last centuries, which resulted in the almost complete disruption of the alder coastal habitats. Pollen and macrofossil records show that alder was present in the floodplain forests with different roles: in some cases, it formed pure stands of alder carrs, in other cases it was the dominant element of mixed forests, or just a companion in ash- and elm-dominated woodlands. In all the studied records, Alnus shows temporally fluctuating trends, which reflect rapid responses to local edaphic and hydrological variations, partly masking the influence of climate change on population dynamics. The few pollen records spanning the last glacial to interglacial transition document the local presence of alder already during the lateglacial period. During the middle Holocene, most of the Tyrrhenian coastal wetlands were established, following complex sedimentological, geomorphological, and ecological processes in response to the sea-level rise deceleration since around 8000 BP. Between 5800 and 5000 BP, most Alnus populations reached their maximum extent. The subsequent decline of alder forests occurred at different times and with different intensities along the Tyrrhenian coasts, depending on local geomorphic and hydrologic conditions, as well as on the impact of human activities. However, it cannot be excluded that a general aridification trend in the Mediterranean regions during the last few thousands of years may have catalysed the effects of human activity, accelerating the process of disruption of alder forests.

6. The vanished forests The former habitats with Alnus-dominated floodplains and carrs, which was widespread along the Tyrrhenian coast until a few centuries 7

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The final result of this process is a very restricted extent of the remnants of the former widespread coastal floodplain forests, preserved in a few SCI located in Tuscany, Sardinia and Corsica. Moreover, in most of the coastal plains of the Tyrrhenian side of the Italian Peninsula, a large number of species that occupied the former wetlands have disappeared together with the alder populations, producing a severe loss of biodiversity level (Lucchese, 2018). A major concern connected with the extirpation of alder communities along the Tyrrhenian coast is the formulation of adequate conservation programs for at least some of the endangered species. For example, wild populations of Vitis, which is a common element in alder forests, should be protected by taking into account both the natural postglacial refuge areas and their cultural refuge areas, so to preserve a genetic heritage of great natural and economic importance. While it appears impossible to restore the extensive alder forests that vanished from the Tyrrhenian coasts, now heavily occupied by urban, agricultural and industrial centres and related infrastructures, the history of A. glutinosa may still leave some glimmer of optimism. In many cases, fossil data show that rapid alder declines are followed by equally rapid expansions, which suggests a strong resilience of the species and a considerable recovery potential in those coastal areas that might be destined to be returned to their natural state.

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