The case of the Itata estuary (Bio-Bio Region-Chile) plant formations: anthro- pogenic interference or natural disturbance-induced diversity enrichment?

The current study examined the relationship between native and introduced plant species, as indicators of the state of anthropogenic influence on plant formations in the area of the Itata estuary (Bío-Bío Region, Chile). A total of 186 vegetation samples were collected in different plant communities in the wetlands and adjacent areas of the Itata River during 2011 and 2012. The communities of four terrestrial plant formations were sampled within dunes, prairies, shrub lands and food crops, and within two aquatic habitats (freshwater and salt marsh). The total flora comprised of 222 species, these were dominated by introduced taxa with thirty-three species considered invasive for Chile. The high percentage contribution of these alien weeds to the total community could be interpreted as signs of a strong degree of anthropogenic interference in the natural plant formations. However, some habitats such as salt marshes are subjected to periodic natural disturbances (e.g. tsunamis), lacking human interference. Consequently, in some habitats, alien species, which are more resilient, represent the primary plant formations.


Introduction
One of the great utopian desires of human beings is to return to the past. The common notion is that in the past everything was better because the environment was in a pristine state, undisturbed, and landscape destruction and contamination were not an issue. This heavenly-like perception was already well displayed in Don Quixote's speech thanking the goatherds (Cervantes, 2004). However, parallel with the evolution of hominids, a continuous modification and alteration of the natural, pristine environment took place in order to facilitate human life, and with these changes began the process of anthropogenic landscape transformation (Lovelock, 2000;Frey & Loesch, 2010). This process, speeded up by the technical revolution, has led to large-scale environmental destructions, causing a general imbalance in the biogeochemical cycles that led to the extinction of species, often harming dominant species the most (Kauffman & Harries, 1996;Barnoski & al., 2012;Pott, 2013). One could also speculate that before Homo sapiens, the extinction of dinosaurs was self-inflicted, as this mass extinction might have been caused by their massive impact on the environment. As Margalef (1996) stated, perhaps so many complacent meteorites are not necessary to produce extinction.
Nevertheless, assuming a pristine state of the earth, or at least a state without human interference, is central to assess levels of ecological landscape degradation. This assumption has important consequences for the conservation of species, their habitats and their ecological interactions in order to understand the magnitude of damage environmental alterations may cause.Thus, particularly a dominant species' ecology such as its activity affects biodiversity, which in turn is central to maintain ecosystem stability (Streit, 2007;Schulze & al., 2002;MacDougall & al. 2013).
Rather pristine landscapes in sparsely populated locations enable the study of plant communities, which represent the zero point of an anthropogenic induced degradation process and others, which represent different degrees of alteration. This facilitates the assessment of degradation processes, its stages of secondary succession and, consequently, the understanding what might have caused these state changes Alarcón, 2014).
The original, pristine communities without human intervention are considered primary and their stands represent the reference point in respect to landscape degradation. In Chile, these primary communities were mainly forests between the Aconcagua River (Valparaíso Region) and the Magellan Strait (Reiche, 2013). The anthropogenic processes, which are the 'slash and burn' strategies, give rise to the development of secondary communities. In the case of forests, these secondary communities correspond to shrub communities (secondary thickets), from which the original forest could be regenerated, in the absence of other disturbances such as cattle grazing. Generally, it is known that cattle grazing induce the development of a secondary prairie community (Alarcón, 2014). If the latter is overgrazed and further degraded, it will be populated by invasive shrub species, forming a tertiary shrub community. This tertiary community does not allow a potential regeneration of the original forests. The main changes of the flora and vegetation in this degradation process are outlined in Figure 1. However, ironically stated, human interference can also diversify the landscape by giving rise to secondary and tertiary communities that in turn increase floristic biodiversity, because of the introduction of alien weeds (Murphy & Romanuk, 2004;Richardson & al., 2000;Sandoval & al., 2016).
The present study is a floristic survey of the plant formations of a coastal wetland in South Central Chile affected by frequent natural disturbances. Our study was based on the assumption that in such disturbed habitats, the presence of alien species would not necessarily be a consequence of human interference (Figure 1) changing the perception that alien species reflect secondary and tertiary conditions. Our aim was to identify, characterise the flora, including alien species, inhabiting six plant formations within different habitats in the area of the Itata estuary (Bío-Bío Region, Chile). We also evaluated the life forms within those plant formations and assessed the scale of human interference using different indexes of landscape degradation. Figure 1. Changes in species richness and vegetation structure (increase or decrease) due to landscape degradation in South Central Chile (after Ramírez et al., 1992a).

Study sites
The study sites comprised the plant formations inhabiting dunes, a salt marsh, prairies, shrub lands, freshwater and food crops within the estuary of the Itata River (Ñuble province, Bío-Bío Region, Chile) ( Figure 2). A sand bar at the estuary´s mouth reduces the surface flow of the river (except during periods of high water), where the estuary expands into a large lagoon which gives rise to a salt marsh. The estuary consists of a narrow water channel between dunes, flanked by the sector of Boca Itata (36°22'28''S-72°50'50'' W) to the North and Las Vegas del Itata (36°24'0''S-72°50'57''W) to the South ( Figure 2). The climate of this region is oceanic temperate with Mediterranean influence. The climatic diagram of Punta Tumbes (36°37'59''S-73°06'57''W), 12 km north of the city of Talcahuano, shows a marked seasonality with long winters characterised by high precipitation and short but dry summers (Hajek & Di Castri, 1975) (Figure 2). The annual average monthly temperature reaches 12.3°C with an average maximum and minimum of 15.6°C and 9°C, respectively. The climate in this region can be considered humid Meso-Mediterranean with an annual precipitation reaching 829 mm (Amigo & Ramírez, 1998).Depending on the topography, the original forest vegetation along the coastal zone of the Bío-Bío Region was formed in depressions characterised by water-logged soil of swamp forests of the Temo-Pitra association (Temo-Myrceugenietum exsuccae) and at elevated locations by partially sclerophyllous forests dominated by roble (Nothofagus obliqua) of the Nothofago-Perseetum boldetosum association (Oberdorfer, 1960). However, both communities were destroyed and have been replaced by "junquillo" wet grassland association (Juncetum procerii) biotopes and sclerophyllous shrubs, such as "boldo" (Peumus boldus) and "yelmo" (Griselinia racemosa), respectively (Ramírez & al., 2014).  Hajek & Di Castri, 1975); x-axis displays months from July to June; left y-axis represents temperature (°C) and right y-axis precipitation (mm). Continuous and broken lines shows monthly precipitation (mm) and temperature (°C). Dotted and lined areas shows dry (arid period, P<2T) and wet months. Black areas indicate months with precipitation over 100 mm.
Moreover, the boundary hedges of the roads in that zone are presently covered by invasive blackberry bushes (Rubo-cestretum parquii) (Amigo & al., 2007). The surrounding land of the estuary hosts large plantations of Pinus radiata and Eucaliptus globulus.
The sand dunes were covered with vegetation on both sides of the estuary. The salt marsh was characterised by brackish swamps exhibiting extreme conditions under the tidal influence. On the other hand, the prairies, which were located inland, were anthropogenic formations used for cattle grazing. The shrub lands consisted of secondary plant communities, which originated from anthropogenic destruction of the primitive sclerophyllous forest vegetation and the introduction of blackberry shrubs as hedges on roadsides. The aquatic vegetation in freshwater habitats comprised the free water zone of saltwaters and the boggy zones near riverbanks. The food crops were mainly cereals and potatoes.

Sampling
The present study represents an assessment of the terrestrial and aquatic vegetation of the northern and southern reaches of the Itata river mouth, applying the phytosociological methodology of Southern Europe (Braun-Blanquet, 1979;Dengler & al., 2008).
A total of 186 sample units of vegetation were surveyed from all distinguishable plant communities at the southern estuary reach (Las Vegas del Itata) as well as from the dunes present on both sides of the estuary.The study comprises six plant formations with 30 communities (plant associations) ( Table 1).The latter will be analysed in a forthcoming publication. The differentiation of plant formations was based on the biological spectra and environmental conditions depicted in Table 1 following Schmithüsen (1968 In the spring and summer, 2011 and 2012, surveys were carried out on 25 m 2 (5x5m) plots arranged within the six plant formations (see Table 1). For each sample unit of vegetation, the list of plant species present in the plot was first noted, followed by a subsequent abundance estimation of each species in the plots, represented as total percentage cover (Dierschke, 1994). For plants with less than 1% cover, we used a "+" symbol to denote the presence of several individuals of a species, and the "r" symbol to indicate the presence of a single individual of a species (Knapp, 1984). These symbols were later changed to a unit of one to facilitate further calculations.
Unknown plant species were collected, dried and preserved in a herbarium to aid later identification, using the taxonomic literature (Ramírez & al., 1989;Matthei, 1995;Ramírez & Álvarez, 2017;Ramírez & San Martín, 2006).These preserved specimens were deposited in the VALD Herbarium of the Universidad Austral de Chile (Valdivia-Chile).The nomenclature was updated following the Plant List website (http:// www.theplantlist.org) while their native or introduced status followed the assessment of Zuloaga & al. (2008). The classification into large groups considered the dicotyledons, including the basal Angiosperms and the Magnoliidae. Life forms were assigned using the key of Mueller-Dombois & Ellenberg (1974). The proportion of life forms including phanerophytes (woody plants), chamaephytes (subshrubs), hemicryptophytes (perennial plants) and therophytes (annual and biannual plants) were used to depict biological spectra following the approach of Raunkiaer (1937). The classification into invasive species was based on Fuentes & al. (2014). The stages of anthropogenic modifications followed those defined in González (2000), Schroeder (1998), Frey & Loesch (2010) and Steinhardt & al. (1999).

Classification of the introduced and native flora
A total of 222 plant species were identified from the northern and southern reaches of the Itata estuary (Appendix 1). Among those species, only 44.15% were native plants while the remainder (55.85%) was composed of alien species (Table 2, Figure 3). The angiosperms widely dominated with 219 species (98.65%) the plant formations, while the dicotyledonous (in a wider sense) were more abundant (148 species, 66.66%) than monocotyledonous species (71 species, 31.98%). The gymnosperms were represented by a single, introduced species used for forest plantations, Pinus radiata D. Don ('Pino insigne') and two ferns: Adiantum chilense Kaulf. ('Culantrillo') and Azolla filiculoides Lam. ('Flor del pato') ( Table 2, Appendix 1). A. chilense inhabits the understory of forests and shrub lands, while A. filiculoides is a floating aquatic plant. The 222 plant species are distributed in 165 genera and 65 families. The species-richest genus is Juncus with seven species, followed by the genera Ranunculus and Trifolium, each with four species, and ten genera (Acacia, Carex, Cyperus, Hordeum, Lolium, Lythrum, Plantago, Polygonum, Rumex and Salix) each with three species. However, most genera (153) were represented with either two or one species. When considering species richness among families, the Poaceae (grasses), Fabaceae (legumes), Asteraceae (composites) and Cyperaceae dominated with 38, 19, 18 and 13 species, respectively. Nevertheless, if the number of species of the Cichorioideae (5 species), a subfamily of the composites is added to the Asteraceae, the latter becomes the second species-richest family with 23 species. Furthermore, the families of Scrophulariaceae were represented with eight, both the Juncaceae, and the Polygonaceae with seven, the Apiaceae (Umbelliferae) with six, and both the Chenopodiaceae and Cichoriaceae with five species. More than three-quarters (84.61%) of all families were represented with fewer than five species (Appendix 1).

Biological spectrum
The biological spectrum was dominated by hemicryptophytes (perennial plants) with 86 species (38.74%) and therophytes (annual and biannual plants) with 57 (25.68%) species (Table 2). Thus, both herbaceous life forms include 143 species (64.42%), while all other plant life forms including the phanerophytes (woody plants), chamaephytes (sub-bushes and erect grasses) and cryptophytes have fewer species (see Table 2). The biological spectrum found at the estuary of the Itata River does not represent any typical phyto-climate but demonstrated altered vegetation at the study sites. In particular, the high percentages of annual plants (therophytes) and perennial (hemicryptophytes) groups that incorporated only neophytes indicate a high degree of anthropogenic modification of the flora.
Comparing the commonness of plant species among the different habitats, revealed that 115 species occurred in one, 55 in two, 35 in three and 12 in four plant formations (Table 3). Four non-native weed species (Lupinus arboreus, Plantago lanceolata, Anthemis cotula and Calystegia sepium) occurred in all but one plant formation, indicating a strong anthropogenic influence. In contrast, only one species, Galega officinalis, occurred in all of them (Table 3).

Introduced and native species within plant formations
All plant formations were dominated (> 50%) by introduced species (Figure 4). This high percentage implies a perturbed plant community altering plant formations. Only within the studied shrubs and freshwaters, native species reached 50% of the whole plant communities. Only food crops, as expected, were dominated by introduced species (Figure 4). In all other plant formations, the woody plants played a minor role while being absent in the food crops. In the dunes, the contribution in species number (15) by the chamaephytes (sub-shrubs) to the plant formation was higher than in all other studied habitats (Table 4, Figure 5). The hemicryptophytes (perennial weeds) dominated in the plant formations of four habitats (dunes, salt marsh, prairies and freshwater) with the highest number of species (46) in the prairies. The highest number of cryptophytes species (17) was found in freshwater habitats including swamps. Geophytes, a subgroup of the cryptophytes, reached the highest number of species (6) in the prairies. In contrast, this group was only represented by one species (Alium vineale) in the food crops and was missing in the salt marsh.  The annual and biannual life forms (therophytes) were common in all studied habitats, reaching their highest species number in dunes and prairies (29 species each; Table 4). In addition, they were only represented with one species in the crops and were absent in the salt marsh. Finally, therophytes (annual and biannual plants) are abundant in all formations, but especially in dunes and prairies each with 29 species (Table 4, Figure 5).
Comparing the percentage contribution of each life form within each plant formation demonstrated that the chamaephytes were evenly represented across the dune, salt marsh and food crop habitats (Table 5). Similarly well represented were the hemicryptophytes in the plant formations of the salt marsh, prairies and freshwater (Table 5). In contrast, phanerophytes and therophytes dominated the plant formations in the shrub lands and food crops, respectively ( Table 5). The cryptophytes, on the other hand, were only well represented in the freshwater habitats, while the therophytes reached a high percentage contribution among all plant life forms in the dunes and dominated in the food crops.

Invasive plants
Thirty-three species of the 124 introduced ones could be considered invasive (Tables 6 and 7). These constitute species that invade natural and anthropogenic altered ecosystems, changing the floristic structure of the existing plant communities.
The biological spectrum of this group is dominated by the therophytes with 48% (16 species) followed by the phanerophytes and hemicryptophytes (8 species each), while cryptophytes are only represented by one invasive aquatic species, Veronica anagallisaquatica.

Discussion
The high species richness, in total 222 plant species, within the studied sites of the Itata estuary was expected, particularly when considering the vegetation diversity of six plant formations and 30 plant associations (see Table 1). However, this high floristic diversity was mostly due to the presence of angiosperms, which in turn comprised a high number of neophytes. The latter evidences a high degree of anthropogenic alterations of the environment. This result contrasts with a rather balanced biological spectrum (among life forms between 10.36 and 38.74%). In the Itata estuary, both the scare remnants of the original forest flora and numerous invasive plants are characterised by phanerophytes. Among the species of the biological spectrum, sub-shrubs (chamaephytes) thrive in extreme environmental conditions (Raunkiaer, 1937: Cain, 1950. Among the habitats they inhabit in the area of the estuary are the dunes that are a nutrient poor, wind-swept, and, therefore, an unstable environment with little water retention. The other habitat colonised by sub-shrubs is the salt marsh, with a substrate of silt and organic matter, exposed to tidal water level changes. The weed representatives of therophytes and hemicryptophytes reflect a high anthropogenic intervention into ecosystems, among which alien species changed the source spectrum of the native flora. In particular, therophytes show a higher prevalence for compacted soils, while hemicryptophytes prefer 'loose' soils with higher porosity . More generally, the presence of invasive species in the study area could allow their future dispersal into other plant communities and food crops, thus, altering further their present species composition. In particular, according to the proposed scheme of González (2000) shown in Table 8, a strong anthropogenic influence on the Itata's estuary plant formations can be depicted, since more than 30% of the plant assemblages ( Figure 4) are constituted by neophyte species. Consequently, all studied plant formations are impacted and were in a stage of secondary or tertiary plant formation. Similarly, a strong to very strong anthropogenic impact could be deduced from the degree of hemeroby following Sukopp (1976), Dierschke (1994) and Schroeder (1998) (see Table 9). This index considers three parameters, which include the percentage composition of neophytes, therophytes and the loss of native species, to allow an impact assessment. Thus, the studied plant formations can be depicted as eu-and polyhemerobe with an absence of the original plant formation (Peña-Cortés & al., 2006), although a lower number of therophyte species (annual plants) may be the consequence that part of the studied area covers wetlands.  However, some neophyte species that occurred in the salt marsh and in dunes might have reached Chilean shores by dispersing over long distances such as via seacurrents (Ramírez & Romero, 1978 (Ramírez & Romero, 1978) although Zuloaga & al. (2008) and Bortolus (2006) considered it as native to Chile.
As suggested by Ramírez et al. (1989), and defined by Walter (1997), the Chilean coastal wetlands and in particular the plant formations in dunes and salt marshes could be considered 'native azonal primary communities', despite the dominance of non-native plant species. The native plants that would able to colonize such extreme environments (i.e. sandy soils) are absent (Ramírez & al., 1992b;. The lack of original plant formations in these zones is puzzling as no succession in degradation processes could be observed when compared to other environments (Ramírez & al., 2014;. As a consequence of this lack of native plant populations, wind-and water-dispersed seeds of alien species could colonise these coastal habitats, forming 'autochthonous' assemblages. Remarkable is the high resilience of these neophytes (and some native species) as they are able to withstand high levels of natural disturbances including surface gravity waves, mud intrusions, earthquake induced permanent water level changes and saline intrusions due to tsunamis (Valdovinos & al., 2012). Furthermore, the lack of Chilean native species may also be attributed to the expansion of coastal dunes since the early 20th century. This dune development extending to the southern parts of Chile might have in part its origins in bad soil management linked to agricultural practices (Ramírez & al., 1992b). In Chile, salt marshes are older in origin but are also affected by natural disturbances such the flooding after tsunamis (González & al., 2012), but not by human interference.

Conclusions
Although human interference plays a big role in the alteration of original plant formations, the number of introduced plant species is not necessarily a good indicator of direct anthropogenic landscape manipulations. In particular, the azonal communities of the Itata salt marsh and its dunes, were dominated by alien, resilient species withstanding frequent natural disturbances. This may suggest that these assemblages constitute the primary communities of these habitats.

Appendix 1.
Nomenclature and taxonomic status of the plant species found in the estuary of the Itata River (Bío-Bío Region, Chile). The data includes phytogeographic origin and life forms.