Field surveys were conducted on the floodplain of Songkhla Lake Basin (7°00′00″N to 7°50′00″ N and 100°15′00″ E to 100°45′00″ E), Songkhla and Phatthalung Provinces. This extensive lake basin is located on the east coast of the peninsular Thailand.The area is fed by numerous tributaries originating from the surrounding mountain ranges, including the Banthat Range to the west and other highland areas that form the watershed divide.

The Songkhla Lake Basin experiences a tropical monsoon climate (Am), classified according to the Köppen climate classification (Kottek et al., 2006). The mean annual temperature is approximately 28.2 °C . The average annual rainfall 2,089.5 mm. The region is characterized by distinct wet and dry seasons, with the heaviest rainfall occurring from July to December. Runoff is highest during this period and lowest from January to April (Department, 2024), significantly influencing the floodplain›s hydrology.

The Songkhla Lake Basin, Thailand’s largest natural lake system, covers approximately 8,729 km², including 1,017 km² of open water (Chesoh et al., 2009). Despite its size, much of the floodplain vegetation has been degraded due to agriculture, aquaculture, urbanization, and deforestation (Doungsuwan et al., 2013).

Only scattered patches of remnant vegetation remain along the lake’s edge and tributaries. Study plots were selected within these patches Figure 1. Ten study plots were selected within the remnants of the floodplain vegetation. Flood duration time refers to the period during which each study plot is inundated in the study year, based on observations made in the field.

This variable reflects the hydrological regime influencing vegetation structure and species composition, especially in flood-prone ecosystems. The characteristics of each study plot were shown in Table 1.

Cluster analysis was performed using Python (Version 3.10; Python Software Foundation, 2024). Specifically, K-Means clustering was applied to the dataset via the scikit-learn library (Scikit-learn: Machine learning in Python, 2011). Before clustering, data preprocessing and manipulation were performed using the Pandas library (McKinney, 2010), and visualizations were generated with Matplotlib (Hunter, 2007). Each group was subsequently identified as a distinct vegetation unit. These units were labelled approximately based on dominant species, structural appearance, and environmental setting.

To explain differences in structure among community types identified by vegetation classification, all vascular plant species found in the study plots were grouped according to various growth strategies and life forms. This classification assumes that variations in plant form and strategy reflect adaptations to different environmental conditions. Often, vegetation stands with very different species composition share similar structure and life forms due to the influence of similar environmental factors. Therefore, analyzing the dominant growth strategies and life forms of each community type, together with environmental factors such as land use history, flood duration, and water depth, can help us better understand spatial variation among these communities (Pérez-Harguindeguy et al., 2013).

The relative abundance of each life form within a community type was calculated by summing the percentage cover of all species sharing the same growth strategy or life form. The results were then presented as percentages to reflect the dominance of each life form category within the respective vegetation community. Plant species were broadly classified as either annual or perennial.

Annuals encompass all vascular plants that complete their life cycle within a single year and endure unfavorable conditions as seeds. In certain instances, species generally regarded as perennial in different environments were classified as annuals here if they exhibited significant seasonal variations in above-ground presence within the floodplain.

Perennials are vascular plants that live for more than 1 year. Some may temporarily disappear during dry or unfavorable periods but persist through underground structures such as rhizomes or corms. Perennials are further classified as either terrestrial or aquatic based on their primary habitat.

This study exclusively targeted terrestrial areas subject to seasonal flooding. Within our defined study boundaries, all plant species encountered were required to experience inundation for at least 1 month during the study year; no arid habitats were included in the study. This specific water regime, characterized by seasonal flooding that occurs at least once a month, defines the vegetation as swamp vegetation, indicative of a seasonal wetland ecosystem. Furthermore, these criteria for seasonal and occasional flooding may be applied to categorize plant groups within the study.

In this context, aquatic plants are strictly defined as species that grow exclusively in fully submerged or floating aquatic environments. Conversely, helophytes are classified as plants that can thrive in continuously waterlogged conditions or saturated soils, characterized by their root systems and basal stem portions being submerged beneath the water surface or within the muddy substrate. In contrast, their central stems, leaves, and inflorescences are elevated above the water level. This distinction is crucial for understanding the various growth strategies and adaptations of the flora within these unique, periodically inundated terrestrial habitats.

This community type is primarily characterized

by the presence of Perennial Seasonal Flooded Tree (PsfT) species. Dominant plant species within the PsfT life form included Barringtonia acutangula (L.) Gaertn., Diospyros rhodocalyx Kurz, and Lagerstroemia speciosa (L.) Pers. Perennial helophyte (Phl) species were also prevalent, particularly in areas subjected to at least 2 months of inundation, with a strong representation from the Cyperaceae, Poaceae, and Pontederiaceae families. Commonly observed species in this category included Cayratia trifolia (L.) Mabb. & J.Wen and Combretum trifoliatum Vent. Notably, 19 species were exclusively recorded in this vegetation type, absent from all other vegetation forms (Table 2), while 21 species were consistently observed across all study plots.

The most frequently observed life form was the Perennial seasonal flooded tree (PsfT), representing 16 species. Prominent species within this group included Antidesma ghaesembilla Gaertn. and Phyllanthus subscandens (Zoll. & Moritzi) Müll. Arg., as well as Shirakiopsis indica (Willd.) Esser. Perennial seasonal flooded woody climbers (PsfWc) comprised 12 species, with 1/3 belonging to the Fabaceae family. Perennial helophytes (Phl) accounted for 10 species, predominantly from the Poaceae family. Additionally, seven species were uniquely found in this vegetation type, not observed in any other vegetation form. These unique species included Colocasia esculenta (L.) Schott, Crinum amoenum Ker Gawl. ex Roxb., Nymphoides parviflora (Wall. ex G.Don) Tippery, Phyllanthus lanceolarius (Roxb.) Müll.Arg., Potamogeton nodosus Poir., Tinospora sp., and Wrightia religiosa (Teijsm. & Binn.) Benth. ex Kurz.

In the relatively disturbed dry areas, only 2 species of perennial helophytes (Phl) were found: Chrysopogon festucoides (J. Presl) Veldkamp and Cynodon dactylon (L.) Pers. The most abundant life forms were the perennial, seasonal flooded trees (PsfT) and the perennial, seasonal flooded woody climbers (PsfWc), each comprising eight species. During the short-duration flooded season (with water levels not exceeding 30 cm), 2 aquatic plant species were observed: Nymphoides indica (L.) Kuntze and Nymphaea nouchali Burm.f. Additionally, 2 relatively rare species, Corypha utan Lam. and Dolichandrone columnaris Santisuk, were found in this group.

This is a mixed community of helophytes and small shrubs, seemingly dominated by perennial helophytes (Phl) species. It was observed in areas subjected to relatively long-term inundation (up to 50 cm deep for 3 months). During the wet season, the vegetation was dominated by numerous Phl species, specifically Cyperus compactus Retz. and Chrysopogon aciculatus (Retz.) Trin., Eleocharis acutangula (Roxb.), and other members of the genus Cyperus. This group also included one species of perennial helophyte fern (PhlF), Acrostichum aureum

L. The aquatic plants present were Nymphaea nouchali Burm.f. and Nymphoides indica (L.). Kuntze, Ottelia alismoides (L.) Pers., and Utricularia aurea Lour. Conversely, during the dry period, some tree and shrub species persisted, such as Shirakiopsis indica (Willd.) Essere and Combretum trifoliatum Vent.

This area, located along the stream banks, is mainly inhabited by plants adapted to flooding. The dominant vegetation included perennial helophytes (Phl) from the Cyperaceae and Poaceae families. Further from the edge, Lagerstroemia speciosa (L.) Pers. and Barringtonia acutangula (L.) Gaertn. were seen. During periods of heavy flooding, when water levels rise by about 1 meter, Phl species became submerged. At these times, free-floating aquatic plants like Nymphaea nouchali Burm.f. and Nymphoides indica (L.) Kuntze became common. However, once water levels return to normal, the Phl community reappeared.

At a 20% similarity threshold, the cluster analysis allowed for a broad categorization of plant communities into 2 major groups, with inundation depth serving as the primary distinguishing factor. One group was characterized by high water levels (up to 1.5 m), while the other comprised communities adapted to low water levels (up to 0.5 m) Figure. 3, Line B). The two identified vegetation groups display not only unique habitat distributions but also distinct structural variations.

This study examined the spatial patterns of floodplain vegetation within the context of these two groups. Vegetation analysis results indicated that broad-scale differences in species composition and structure are influenced by successional stages, which are linked to past land uses. However, at a finer scale, the water regime appears to be a more significant determining factor.

The Barringtonia-DiospyrosLagerstroemia woodland in the Songkhla Lake Basin exhibits a relatively complex vegetation structure compared to other local vegetation types, suggesting it represents a more or less «primary vegetation» that historically occurred in the study area. This aligns with the key features of primary floodplain vegetation, which thrives in dynamic river environments by adapting to floods through the development of specialized roots or physiological changes. These adaptations encompass a diverse array of life forms, ranging from fast-growing annual herbs to resilient perennial woody species, and hold significant ecological importance in flood control, erosion prevention, water quality, and habitat provision (Wellstein et al., 2003).

This study identified several flood-adapted species, including Sonneratia caseolaris (L.). Engl., which possesses pneumatophores. Lenticels were also observed in Brachypterum scandens (Roxb.) Wight & Arn. ex Miq. Furthermore, the floodplain species of the Songkhla Lake Basin displayed various adaptive traits for germination and establishment in inundated conditions. Many exhibited hydrochory (water dispersal) via their diaspore characteristics. Notable examples included the cork-like pericarp of Barringtonia acutangula (L.) Gaertn., winged fruits in Combretum trifoliatum Vent. and Hymenocardia punctata Wall., and spongy syncarps in Nauclea orientalis L. These features not only facilitate flotation and water dispersal but also maintain seed viability by protecting tissues from oxygen-depleted conditions. This seed buoyancy is a crucial adaptation, consistent with observations in other inundated habitats like Central American floodplains (Lopez, 2001), the Drenthse and Vechtplassen area in the Netherlands, the Peene River valley in Germany (Broek T et al., 2005), and northern Australian floodplain plant communities (Finlayson, 2005). Nevertheless, further information on the vegetation history is needed to fully confirm these observations.

The study area is characterized by a distinct embankment that creates a trough, separating the open floodplain from the lake. This structure prevents direct lake flooding during periods of elevated water levels, effectively shielding the adjacent landscape from inundation. Consequently, increased water levels within the study area primarily result from accumulated surface runoff generated by local precipitation, which eventually drains into the lake. The proximity to agricultural land contributes to ongoing anthropogenic disturbances, classifying the area as undergoing secondary succession.

In this successional context, the vegetation is dominated by pioneer wetland species, particularly members of the genera Cyperus and Fimbristylis, which exhibit key adaptive traits such as rapid growth, short life cycles, and high tolerance to hydrological fluctuations (flooding and drying). In areas where disturbance has stabilized, mid-successional species, such as Nymphaea sp. and Ottelia sp., may establish. As succession progresses toward a more stable state, climax communities tend to be composed of helophytic species, such as Lepironia articulata (Retz.) Domin, and riparian woody species, such as Barringtonia acutangula (L.) (Valk AG, 1981). This pattern aligns with findings by (Parolin et al., 2004) and (Keddy, 2010), who reported that late-successional tree species in floodplain systems often develop specific adaptations enabling them to withstand prolonged inundation. This finding is consistent with the present study, in which helophytic species were more prevalent than woody species within the low-water-level group Table 2. This species composition further supports the interpretation that the area is currently undergoing secondary succession.