Plants growing on the river margin (riparian plants) are species that thrive in sites subject to constant fluctuations in the level of circulating water currents. There is little information on this group of plants at the regional level. This work presents a floristic list of the riparian vegetation of the San José del Este river located south of the municipality of Carmen and connected to the Laguna de Términos. Data are provided on their geographic origin (native/introduced), growth habits and species at risk. Periodic and preferential walks were made along the river margin to collect specimens of fertile vascular plants. The origin, habit and species at risk were obtained from existing literature. The list includes 68 families, 160 genera and 185 species of vascular plants, flowering plants (Angiosperms) include the largest number of species (166), of which 111 are dicotyledonous (Magnoliophyta) and 65 monocotyledonous (Liliopsida), eight species of ferns and one gymnosperm are also recorded. Herbs constituted the dominant growth habit. Seventy-three point six percent of the species were native, the introduced species came mainly from the Old World, especially from Europe. The riparian flora has important ecological functions for the river ecosystem and provide environmental services for local inhabitants, however, they are currently threatened by factors such as irregular human settlements, deforestation, illegal logging, agricultural activities, forest fires and pollution. This contribution provides basic information for future conservation and restoration plans.

Keywords: flora of campeche, vascular plants, riparian vegetation, aquatic vegetation

The term riparian vegetation describes the plant communities that develop along riverbanks, including floating, submerged or emergent aquatic plants, making it a very variable community structurally and physiognomically.¹ Riparian vegetation is composed of a great diversity of species that often have denser cover and larger individuals compared to adjacent communities.²

The diversity and structure of riparian vegetation is conditioned by the hydrological and geomorphological characteristics of the river,¹ due to this, physiognomic differences can be observed along the riverbank, since the species must be adapted to develop in specific areas of the riverbank and withstand changes in the frequency, magnitude and duration of the flood-drought regime and its respective fluctuations of nutrients.³

This variation also occurs laterally, as plants present distribution patterns according to their life forms from the riverbed, towards dry land, in the water and river banks of develop those species adapted to high or permanent flooding periods, low nutrient environments and sunlight exposure, while as the soil moves away from the water current, species with longer life cycles, such as trees, shrubs and shade-tolerant plants, become established.³

Generally there is an alternation of species, although sometimes some plants form dominant communities of a single species, these differences can change in a short distance or else present in combinations of plant associations that can fit into different vegetation types,¹ in addition, vertically different species of epiphytic plants and woody climbers can develop.⁴

Riparian vegetation provides different ecological functions: it is the habitat for different aquatic and riverine species,⁵ constitutes corridors for wildlife dispersal,^5,6 and is the main food source for various species of fish, birds, mammals and invertebrates,^8,9 stabilizes river banks, preventing erosion,¹⁰ provides fallen branches, dead trunks and roots, which favor sediment retention and the creation of new habitats,¹¹ and contributes to regulating the microclimatic conditions of the environment such as temperature, sunlight, humidity and wind,^12,13 directly influencing the populations of bacteria, plants and animals.⁵

Riparian vegetation is exposed to various stressors, mainly anthropogenic, such as the extraction of timber and non-timber products, changes in land use for agricultural and livestock activities, and the growth of irregular human settlements that threaten the ecological functioning of the entire riparian ecosystem¹⁴ because when this vegetation is altered, there is an imbalance in the existing relationships with other biological groups.¹⁵

The importance of riparian vegetation is such that it is necessary to have information not only for its conservation but also for its restoration.

Conservation but also for its restoration. Although adjacent areas have been studied previously, research has been focused on aquatic plants,^16,17 or on specific families;¹⁸ however, there are areas, which due to the difficulties to access them, their floristic knowledge has not been explored. For this reason, the objective of this research is to present the floristic list and description of the vegetation of the San José del Este river to contribute to the floristic knowledge of the region, the municipality of Carmen and the state of Campeche and at the same time to have elements to create strategies to ensure the maintenance of these resources over time. (Figure 1)

Figure 1 San José del Este river and its riparian vegetation.

Study area. The San José del Este river is located southeast of the municipality of Carmen in the state of Campeche, its extension is approximately five kilometers, it is a tributary of the Palizada river that flows into the Laguna de Términos. The study area is located between the coordinates 18°20'27.92 "N, 91°41'2.97 "W and 18°18'5.75 "N 91°40'35.70 "W (Figure 2).

Figure 2 Location of the study area.

The prevailing climate is (Amw "ig) warm humid with a mean annual temperature of 27.2°C.¹⁹ Four climatic seasons are present throughout the year, dry from February to April, a transition month (May), rains from June to September and nortes from October to March.²⁰ The physiography is dominated by flat plains with elevations no higher than 2.5 m.²¹

The predominant soils are Gleysols, composed mainly of clays and silts, rich in organic matter that is carried by currents and deposited on the plains of Laguna de Términos.²¹ The prevailing vegetation is mainly lowland flooded forest, flooded palm grove, riparian forest, and aquatic and underwater vegetation.^22,23

Methods

Floristic composition. During the period from 2018 to 2020, the study area was explored by periodic river walks to make botanical collections using the technique proposed by Lot and Chiang,²⁴ covering from the river bank to approximately 20 meters inland, covering about 10 linear kilometers counting both margins of the river. Botanical collections were made in well-conserved areas, areas with different stages of recovery, as well as anthropized areas, mainly due to agricultural and livestock activities. Due to the conditions and accessibility of the area studied, some species were only documented by photographic record.

The determination of the specimens was carried out using different identification keys and regional floras,^3,16,25-27 collation of herbarium specimens and in some cases with the support of specialists. The herborized specimens were distributed in the UCAM and XAL herbaria.

The conservation status of the species was identified in the Norma Oficial Mexicana NOM-059-SEMARNAT-2010.²⁸ The origin (native/introduced) and geographic distribution of the species in the Yucatan Peninsula (including Tabasco due to its proximity) were determined through existing literature.^29-6

For the species listing (Annex 1), pteridophyte families and genera were assigned based on Mickel and Smith;³² gymnosperms based on Christenhusz et al.³³ and angiosperms were arranged according to the taxonomic classes Liliopsida (monocotyledons) and Magnoliopsida (dicotyledons) proposed by Cronquist,³⁴ families were named according to the Angiosperm Phylogeny Group IV classification system³⁵ and Species names were corroborated in the Missouri Botanical Garden TROPICOS database during 2019-2020.³⁶

Vegetation description

The vegetation communities identified along the river were described vertically and horizontally, considering the vertical structure taking into account the physiognomy, floristic composition, and height estimation. The name of the communities was assigned according to Miranda and Hernández-X⁴⁰ and Ocaña and Lot.⁴¹

Floristic composition. Sixty-eight families, 156 genera and 185 species were recorded (Annex 1). The dicotyledons were the most diverse group, with more than 60 % of the families, genera and species (Table 1). Fifty-six percent of the species belonged to the six most diverse families, of which Poaceae, Fabaceae and Malvaceae had the highest species richness (Table 2). The best represented genera in terms of species were Cyperus, Cenchrus and Panicum with four species each.

The predominant growth habit was terrestrial herbs, with a total of 85 species (85.6 %). Shrub and tree elements were represented by 15 species (12.0 %) and three species (2.4 %), respectively (Table 3).

Vegetation description

Perpendicularly along the river, the vegetation is heterogeneous, generally intermingled communities are observed, but also pure patches of species such as tasiste (Acelorrhaphe wrightii), and tifa (Thypha dominguensis); On the river bank, tree species adapted to flooding are established, such as the water sapote (Pachira acuatica) and red mangrove (Rhizophora mangle), as well as aquatic herbaceous species such as the lily (Hymenocaullis litoralis), and Sagittaria lancifolia; further inland, the species change,

In relation to the plants established in the water curtain, Floating water lettuce (Pistia stratiotes) and water hyacinth (Eichornia crassipes) are occasionally present and submerged we find foxtail...

Vertically there are abundant vines such as Dalbergia brownei and Pentalion andreuxy that grow above the shrub and tree vegetation, also present are various species of epiphytic plants, mainly orchids and bromeliads, but also cacti such as Strophocactus testudo and Hylocereus undatus that are established in trees at varying heights or on fallen trunks and branches.

The 185 species recorded in this research represent 56.75 % of the aquatic vascular and flood zone flora documented in the Laguna de Términos Flora and Fauna Protection Area, considering the 326 species reported by Bonilla-Barbosa,¹⁶ this number is also relatively similar to the 133 species previously recorded for the Palizada-del Este deltaic fluvial-lake system (mouth of the San José del Este river).¹⁷

The high number of widely distributed herbaceous species can be explained by the disturbed environments present in the river margin, where the most successful species tend to be herbaceous, because they have short life cycles and generally grow and reproduce rapidly.

The present contribution is relevant because the studied area is disturbed by factors such as fires, change of land use (mainly for rice crops), and the greater proximity to human settlements. Although it is not possible to offer environmental arguments to explain a greater number of species in the area, it is not possible to explain a greater number of species in the area.

The floristic lists related to natural protected areas provide basic and important information for the planning of conservation policies and the sustainable use of biological resources and can add value to the conservation of these areas.^42,43

The heterogeneity of microenvironments and their floristic composition may be due to changes in topography and channel morphology that directly influence the riparian flora.⁴⁴

Land use change is one of the main threats to natural resources in southeastern Mexico; the conversion of forests to pastures has a double negative impact; with the loss of natural vegetation, plant species are reduced or become extinct and wildlife species lose their habitat, and secondly, the forest that disappears ceases to be a sink and reserve for atmospheric CO2 and becomes a source of greenhouse gases (gHg).

During the development of this study, it was possible to confirm the strong influence of human activities on these communities, such as agricultural practices on nearby river terraces or within the riverbed, the contribution of organic waste to the water, as well as the destruction of trees for fuel and the presence of fences within the riverbed. The changes in use have caused the loss of continuity of the community, fragmenting the vegetation, which is why it is necessary to maintain a monitoring system that allows for efficient detection of the condition and stability of these communities.

Ecological changes occur along a river and different communities appear in each particular zone of the river with species combinations that are constantly repeated and, in general, without a clear dominant that imprints the physiognomy of the landscape.

None.

No conflicts of interest.

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