Plant Species Diversity and the Interconnection of Ritual Beliefs and Local Horticulture in Heet Sip Song Ceremonies, Roi Et Province, Northeastern Thailand

Abstract

This study explores the ethnobotanical significance of plant species used in the Heet Sip Song (Twelve Monthly Merit-Making) ceremonies in Roi Et Province, Northeastern Thailand. A total of 80 plant species across 73 genera and 42 families were documented. The findings reveal that plants play multifaceted roles in ceremonial life, serving both symbolic and practical purposes rooted in spiritual belief systems and seasonal agricultural cycles. Quantitative analyses using Cultural Significance Index (CSI), Species Use Value (SUV), Genera Use Value (GUV), and Relative Frequency of Citation (RFC) highlighted the prominence of key species such as Oryza sativa, Musa acuminata, and Saccharum officinarum in ritual contexts. While staple crops dominate in frequency and cultural value, less commonly cited wild species fulfill specialized functions, reflecting deep local ecological knowledge. The integration of ritual and plant use promotes biodiversity conservation by maintaining plant populations and reinforcing sustainable harvesting practices. These results emphasize the vital role of traditional knowledge in conserving both biological and cultural diversity. As environmental pressures increase, this study underscores the importance of supporting community-led conservation efforts that honor indigenous practices and their contributions to ecological resilience.

1. Introduction

Plants, as both symbolic and functional entities, are integral to ceremonies meant to communicate with spirits [1]. Certain plants are believed to possess unique properties that make them suitable for specific rituals. In ceremonies related to life transitions (birth, marriage, and death), agricultural events (planting and harvesting), and religious observances (merit-making—the act of performing good deeds or religious activities, such as offering alms or participating in rituals, to accumulate spiritual merit for oneself or others in accordance with Buddhist beliefs), plants are used to invoke blessings, protection, or healing [2,3]. Buddhism has played a significant and enduring role in shaping the way of life in Thai society, influencing various aspects of culture both directly and indirectly [4]. This influence is particularly pronounced among the Isan people, whose customs and daily practices are deeply intertwined with Buddhist teachings. One of the most important expressions of this connection is the tradition of merit-making, which has long been a cornerstone of Isan culture. The Heet Sip Song ceremonies, a set of twelve annual Buddhist practices, stands out as a key ritual through which the Isan people engage in merit-making, reinforcing both their spiritual beliefs and their communal ties. These twelve practices include Bun Khao Kam, Bun Khun Lan, Bun Khao Jee, Bun Pha Wet, Bun Songkran, Bun Bang Fai, Bun Samha, Bun Khao Phansa, Bun Khao Pradap Din, Bun Khao Sak, Bun Ok Phansa, and Bun Kathin [5].

There have been several studies reporting on the ritual use of plants in many countries, where indigenous communities have integrated these plants into their spiritual, healing, and cultural practices. These plants are often used as a medium to connect with the sacred, ancestors, or spirits, and are used in rituals ranging from daily offerings to complex healing rituals [6,7,8,9,10,11].

A variety of plants have long been utilized in Thai cultural and traditional ceremonies. For example, Curcuma supraneeana (W.J.Kress & K.Larsen) Skornick., Etlingera elatior (Jack) R.M.Sm., Globba colpicola K.Schum., G. schomburgkii Hook.f., and G. williamsiana M.F.Newman & Sangvir. are used as offerings during the worship of the Buddha’s footprint in the Tak Bat Dok Mai festival and as floral bouquets for paying respects to Buddha [12]. Bambusa bambos (L.) Voss, × Thyrsocalamus liang Sungkaew & W.L.Goh, and Dendrocalamus asper (Schult. & Schult.f.) Backer play a significant role in the main pillar ceremony conducted before house construction. They are crafted into Chalaew, containers used for offerings dedicated to the Mae Phosop goddess during rituals. Additionally, they are fashioned into Khong, containers designed for holding fish and are employed in ceremonies to ward off malevolent spirits [13]. Calotropis gigantea (L.) W.T.Aiton is used to make garlands for religious offerings; Dracaena fragrans (L.) Ker Gawl. is cultivated to bring good fortune, and Oryza sativa L. is used in religious ceremonies [14]. Kaempferia galanga L. and K. rotunda L. are planted to promote auspiciousness, charm, popularity, and to attract affection [15].

In Roi Et Province, plants are vital to the daily lives of local communities, serving a wide range of purposes, from food, spices, and medicinal remedies to producing dyes, cosmetics, and perfumes. They are also essential in creating clothing, accessories, and construction materials, as well as playing a key role in religious rituals. Ethnobotanical studies have highlighted the diverse roles plants play in local culture and daily activities [14]. However, to our knowledge, there are currently no documented records of ritual plants used in the Heet Sip Song tradition of Roi Et Province.

This study aims to document and analyze the diversity of local plant species, cultivated and wild, used in the Heet Sip Song ceremonies in Roi Et Province, Northeastern Thailand. It focuses on the symbolic and functional roles of these plants within local belief systems and Buddhist-inspired horticultural traditions. Specifically, the research investigates how ritual practices are interlinked with local ethnobotanical knowledge, including local plant name, specific plant parts used, methods of ritual application, and their symbolic meanings. The study also examines the conservation implications of using non-cultivated (wild) plant species, particularly concerning potential overharvesting and sustainable use. By highlighting the cultural, spiritual, and ecological roles of these plants, this work contributes to the preservation of traditional ecological knowledge and offers practical insights into how local traditions can support, or potentially threaten, biodiversity conservation [16].

2. Materials and Methods

2.1. Study Area

Roi Et Province is located in Northeastern Thailand. This province is divided into 20 districts (Figure 1), characterized by its rural landscapes, agricultural practices, and rich cultural heritage in areas such as Tung-Kula-Rong-Hai, Wat Pha Nam Yoi, Chi River, and several near the Mueang District, etc. It has a tropical climate with distinct wet and dry seasons, influencing the biodiversity of plant species available for local use [17].

2.2. Data Collection and Plants Identification

Data collection was conducted from January to December 2024 across various locations in Roi Et Province, Northeastern Thailand. This study explored plant species diversity and their roles in the Heet Sip Song ceremonies, focusing on the interplay between ritual beliefs and local horticultural practices.

A mixed-methods approach was employed, integrating ethnobotanical surveys, field observations, and semi-structured interviews with 200 informants, including village elders, traditional healers, ritual leaders, monks, and horticulturists.

Surveys were conducted in home gardens, temple compounds, agricultural plots, and community forests, all commonly associated with the cultivation or collection of ritual plants. The data collected supported the following analytical categories, corresponding directly to the Results section:

2.2.1. Plant Diversity Used in Heet Sip Song Ceremonies

Species presence was documented through field surveys and interviews. Plants used in ceremonies were identified by local informants, and plant specimens were collected (where permitted) for taxonomic identification and classification at the family, genus, and species levels.

2.2.2. Cultural Significance Index (CSI)

Cultural importance was assessed based on informants’ perceptions of ritual function, symbolic meaning, frequency of use, and exclusivity of use in ceremonies. These data were used to calculate the Cultural Significance Index (CSI) for each species.

2.2.3. Species Use Value (SUV)

The frequency with which each species was mentioned by informants for any ceremonial use was recorded to calculate the Species Use Value (SUV), reflecting each plant’s ethnobotanical prominence.

2.2.4. Genera Use Value (GUV)

Use data were aggregated at the genus level across all informants to calculate the Genera Use Value (GUV), allowing insights into the broader ethnobotanical relevance of taxonomic groups.

2.2.5. Relative Frequency of Citation (RFC)

Informant citation frequencies for each species were used to calculate the Relative Frequency of Citation (RFC), representing the proportion of informants who recognized or used the species in rituals.

2.2.6. Plants Used in the Heet Sip Song Ceremonies (Monthly Breakdown)

Ethnobotanical information was organized according to the twelve-month Heet Sip Song cycle. Informants identified which plants were specifically used in each monthly ceremony (e.g., Bun Khao Kam, Bun Khun Lan, etc.). Detailed data on plant uses, cultural roles, preparation methods, and seasonal availability were recorded.

In community-managed areas, plant specimens were collected and preserved in 70% ethanol for deposition at the Vascular Plant Herbarium, Mahasarakham University (VMSU). In protected areas, only photographs and field notes were taken to minimize ecological disturbance.

Plant identification followed standard taxonomic procedures using herbarium references, regional floras, and botanical keys. Scientific names were verified against the Plants of the World Online (POWO) database [18] to ensure nomenclatural accuracy.

2.3. Conservation Status

The conservation status of the ritual plants in Roi Et Province was evaluated using the criteria outlined by the International Union for Conservation of Nature (IUCN) Red List, following the Guidelines for Using the IUCN Red List Categories and Criteria Version 16 (IUCN 2024) [19]. This assessment involved a detailed review of the species’ population sizes, distribution ranges, and potential threats to their survival, including habitat loss, over-exploitation, and environmental changes.

2.4. Data Analysis

2.4.1. Cultural Significance Index (CSI)

The data were analyzed by calculating the Cultural Significance Index (CSI) using the method developed by Turner (1988) [20]. The CSI was determined using the following formula:

$$\mathrm{CSI}=\sum _{\mathrm{i}=1}^{\mathrm{n}}{(\mathrm{q}}_{\mathrm{i}}{\times \mathrm{i}}_{\mathrm{i}} \times {\mathrm{e}}_{\mathrm{i}}{)\mathrm{n}}_{\mathrm{i}}$$

(1)

where CSI represents the Cultural Significance Index; q denotes the quality value (in this study, the focus is solely on ritual use, so q = 2 according to Turner’s method [20]); i stands for the intensity value (ranging from 1 to 5, where 5 indicates very high intensity—major attention focused on the taxon on a daily or seasonal basis with significant effects on daily or yearly living patterns; 4 indicates moderately high intensity; 3, medium intensity; 2, low intensity; and 1, minimal intensity—used or acknowledged only rarely, with negligible impact on daily or seasonal living patterns); e refers to the exclusivity value; and n is the number of months during which the plant was reported to be used.

2.4.2. Relative Frequency of Citation (RFC)

The Relative Frequency of Citation (RFC) is an index developed by Tardío and Pardo de Santayana [21], used to measure the local importance of each plant species in the study area based on how frequently informants mention them. This index ranges from 0 to 1 and is computed using the following formula:

RFC = FC/N (0 < RFC < 1)

(2)

where RFC indicates the relative frequency of citations, FC represents the number of informants who mentioned a particular species (citation frequency), and N is the total number of informants interviewed in the study [21].

2.4.3. Species Use Value (SUV)

The Species Use Value (SUV) reflects the significance of a ritual plant species as perceived by the people of Roi Et Province. The SUV is calculated using the method described by Hoffman and Gallaher [22] and is expressed as follows:

$$\mathrm{SUV}={\displaystyle \frac{\sum {\mathrm{UV}}_{\mathrm{is}}}{{\mathrm{n}}_{\mathrm{i}}}}$$

(3)

In this formula, UV_is represents the use values reported by individual informants for a specific ritual species, and Ni denotes the total number of informants. The SUV is determined by summing the use values given by all informants for a particular species and dividing the total by the number of respondents.

2.4.4. Genera Use Value (GUV)

The Genera Use Value (GUV) reflects how important a genus of ritual plants is to the people of Roi Et Province. The GUV is calculated using a method adapted from Hoffman and Gallaher [22] and is expressed as:

$$\mathrm{GUV}={\displaystyle \frac{\sum \mathrm{SUV}}{{\mathrm{N}}_{\mathrm{s}}}}$$

(4)

In the formula, the following variables are defined:

∑SUV = total of the species uses values in the same genus.

Ns = total number of species withing a given genus.

The GUV is calculated by summing the use values for all the species within a given genus and dividing by Ns.

The Genera Use Value (GUV) is calculated by averaging the Species Use Values (SUVs) of all species within a particular genus. The maximum value of GUV occurs when all species within the genus are highly cited and used for multiple purposes, leading to a high average value. It is possible for the GUV to exceed 1.0 if all species within the genus have an SUV greater than 1.0. On the other hand, the minimum value of GUV is 0.0, which means that none of the species in the genus were cited or used by informants. When the GUV value is near 0.0, it indicates that the genus holds little or no perceived significance or utility within the community.

3. Results

3.1. Plant Diversity Used in Heet Sip Song Ceremonies

A total of 80 plant species, representing 73 genera and 42 families, were documented as being used in the Heet Sip Song ceremonies across communities in Roi Et Province, Northeastern Thailand (

Table 1. Recorded plant species in Heet Sip Song ceremonies in Roi Et Province, Thailand, together with their vernacular names, conservation status, distribution status for Thailand, plant habits, resource, used parts, ceremony months, CSI, SUV, and RFC.

No.	Family	Scientific Name	Vernacular Name	Conservation Status	Distribution Status for Thailand	Plant Habits	Resource	Used Parts	Ceremony Months	CSI	SUV	RFC
1.	Amaranthaceae	Gomphrena globosa L.	Barn Mai Roo Roy	NE	Introduced	Shrub	Cultivated	Inflorescence	1, 2, 8, 11, 12	20	0.25	0.25
2.	Amaryllidaceae	Allium cepa L.	Hohm Daeng	NE	Introduced	Herb	Cultivated	Bulb	7, 9, 10	60	0.75	0.30
3.	Amaryllidaceae	Allium sativum L.	Gra Tiam	NE	Introduced	Herb	Cultivated	Bulb	7, 9, 10	60	0.78	0.28
4.	Anacardiaceae	Mangifera indica L.	Buk Muang	DD	Native	Tree	Cultivated	Fruit	10	8	0.50	0.38
5.	Anacardiaceae	Spondias mombin L.	Ma Gok	LC	Native	Tree	Cultivated	Fruit	9	8	0.35	0.33
6.	Annonaceae	Annona squamosa L.	Noi Na	LC	Introduced	Tree	Cultivated	Fruit	7, 9, 10	24	0.45	0.35
7.	Annonaceae	Uvaria siamensis (Scheff.) L.L.Zhou, Y.C.F.Su & R.M.K.Saunders	Lum Duan	LC	Native	Tree	Cultivated	Inflorescence	4	4	0.15	0.30
8.	Apocynaceae	Calotropis gigantea (L.) W.T.Aiton	Ruk	NE	Native	Shrub	Cultivated	Inflorescence	1, 4, 8, 11, 12	30	0.83	0.40
9.	Apocynaceae	Tabernaemontana divaricata (L.) R.Br. ex Roem. & Schult.	Put	LC	Native	Tree	Cultivated	Leave and inflorescence	1, 2, 4, 5, 7, 8, 10, 11, 12	72	0.88	0.33
10.	Araceae	Colocasia esculenta (L.) Schott	Pueak	LC	Native	Herb	Cultivated	Tuber	2, 10, 11	18	0.38	0.30
11.	Araceae	Lemna minor L.	Nae	LC	Native	Herb	Wild/AQ	Whole plant	4	1	0.13	0.25
12.	Araceae	Pistia stratiotes L.	Jok	LC	Native	Herb	Wild/AQ	Whole plant	4	1	0.13	0.28
13.	Arecaceae	Areca catechu L.	Mak	LC	Introduced	Tree	Cultivated	Fruit	1, 2, 4, 7, 9, 10	48	0.88	0.23
14.	Arecaceae	Borassus flabellifer L.	Tarn	LC	Introduced	Tree	Cultivated	Fruit	4	6	0.55	0.25
15.	Arecaceae	Calamus caesius Blume	Wary	NE	Native	Shrub	Wild/TF	Fruit	4, 6	8	0.48	0.38
16.	Arecaceae	Cocos nucifera L.	Ma Prow	NE	Introduced	Tree	Cultivated	Fruit	3, 4, 9, 10, 11, 12	96	0.88	0.33
17.	Asteraceae	Chromolaena odorata (L.) R.M.King & H.Rob.	Sarb Suea	NE	Introduced	Herb	Wild/DA	Stem	6	1	0.10	0.40
18.	Asteraceae	Tagetes erecta L.	Down Rueang	NE	Introduced	Herb	Cultivated	Inflorescence	1, 2, 5, 8, 9, 11, 12	64	0.50	0.23
19.	Bignoniaceae	Millingtonia hortensis L.f.	Peeb Khow	LC	Native	Tree	Cultivated	Inflorescence	4	4	0.48	0.25
20.	Bignoniaceae	Oroxylum indicum (L.) Kurz	Pe-ga	LC	Native	Tree	Cultivated	Fruit	4	6	0.73	0.35
21.	Bromeliaceae	Ananas comosus (L.) Merr.	Sapparod	NE	Introduced	Herb	Cultivated	Fruit	12	8	0.70	0.38
22.	Cactaceae	Selenicereus undatus (Haw.) D.R.Hunt	Gaew Mung Gorn	NE	Introduced	Climber	Cultivated	Fruit	9	8	0.70	0.43
23.	Caricaceae	Carica papaya L.	Malagor	DD	Introduced	Tree	Cultivated	Fruit	7, 9, 12	40	0.75	0.30
24.	Clusiaceae	Garcinia mangostana L.	Mung Kut	DD	Introduced	Tree	Cultivated	Fruit	7, 10	16	0.63	0.28
25.	Combretaceae	Getonia floribunda Roxb.	Nguang Sum	NE	Native	Climber	Wild/DDF, DEF	Inflorescence	4	1	0.05	0.20
26.	Convolvulaceae	Ipomoea batatas (L.) Lam.	Mun Tade	DD	Introduced	Herb	Cultivated	Tuber	2, 9, 10, 11	12	0.75	0.23
27.	Cucurbitaceae	Benincasa hispida (Thunb.) Cogn.	Fug	NE	Introduced	Climber	Cultivated	Fruit	12	8	0.73	0.38
28.	Cucurbitaceae	Cucumis sativus L.	Taeng Gwar	NE	Native	Climber	Cultivated	Fruit	7, 10	24	0.70	0.43
29.	Cucurbitaceae	Cucurbita maxima Duchesne	Fug Tong	NE	Introduced	Climber	Cultivated	Fruit	9, 10, 11	16	0.63	0.25
30.	Cucurbitaceae	Citrullus lanatus (Thunb.) Matsum. & Nakai	Taeng Mo	NE	Introduced	Climber	Cultivated	Fruit	10	24	0.50	0.28
31.	Dioscoreaceae	Dioscorea hispida Dennst.	Gloy	NE	Native	Climber	Wild/DF, MDF, MEF	Tuber	10	2	0.28	0.18
32.	Dioscoreaceae	Dioscorea pseudotomentosa Prain & Burkill	Mun Nok	NE	Native	Climber	Cultivated	Tuber	9, 10	4	0.28	0.20
33.	Dipterocarpaceae	Anthoshorea roxburghii (G.Don) P.S.Ashton & J.Heck.	Payom	NE	Native	Tree	Wild/MDF	Inflorescence	4	1	0.13	0.40
34.	Dipterocarpaceae	Dipterocarpus intricatus Dyer	Sa Bang	EN	Native	Tree	Wild/DDF, MDF	Inflorescence	4	1	0.13	0.30
35.	Dipterocarpaceae	Pentacme siamensis (Miq.) Kurz	Rung	NE	Native	Tree	Wild/DDF, DEF, MDF	Inflorescence	4	1	0.13	0.45
36.	Fabaceae	Arachis hypogaea L.	Tour Li Song	NE	Introduced	Herb	Cultivated	Fruit	9, 10, 11	24	0.70	0.48
37.	Fabaceae	Butea monosperma (Lam.) Kuntze	Tong Kwarw	LC	Introduced	Tree	Wild/DF, GL	Inflorescence	4	1	0.30	0.50
38.	Fabaceae	Cassia fistula L.	Khoon	LC	Doubtful	Tree	Cultivated	Stem and inflorescence	1, 2, 4, 5, 7, 9, 10	140	0.75	0.35
39.	Fabaceae	Erythrina variegata L.	Tong Lang	LC	Native	Tree	Cultivated	Inflorescence	6	1	0.23	0.38
40.	Fabaceae	Glycine max (L.) Merr.	Tour Lueang	NE	Native	Herb	Cultivated	Fruit	11	6	0.53	0.33
41.	Fabaceae	Pachyrhizus erosus (L.) Urb.	Mun Gaew	LC	Introduced	Herb	Cultivated	Tuber	9, 10	6	0.50	0.25
42.	Fabaceae	Samanea saman (Jacq.) Merr.	Chum Cha	LC	Introduced	Tree	Cultivated	Stem	6	1	0.13	0.48
43.	Fabaceae	Sesbania javanica Miq.	Sa No	LC	Native	Herb	Wild/AQ	Inflorescence	4	2	0.38	0.28
44.	Fabaceae	Vigna radiata (L.) R.Wilczek	Tour Keaw	LC	Native	Herb	Cultivated	Fruit	11	3	0.55	0.45
45.	Fabaceae	Vigna unguiculata subsp. sesquipedalis (L.) Verdc.	Tour Fug Yown	NE	Introduced	Climber	Cultivated	Fruit	10	8	0.85	0.25
46.	Irvingiaceae	Irvingia malayana Oliv. ex A.W.Benn.	Gra Bog	LC	Native	Tree	Wild/FE, DEF, DF, MDF, RF	Fruit	10	1	0.08	0.23
47.	Lamiaceae	Ocimum × africanum Lour.	Ho Ra Pha	NE	Native	Herb	Cultivated	Leave and inflorescence	10	16	0.88	0.20
48.	Lythraceae	Punica granatum L.	Tun Tim	LC	Introduced	Tree	Cultivated	Fruit	10	6	0.53	0.25
49.	Marantaceae	Maranta arundinacea L.	Mun Sa Khoo	NE	Native	Herb	Cultivated	Tuber	11	2	0.48	0.18
50.	Meliaceae	Lansium domesticum Corrêa	Long Gong	NE	Native	Tree	Cultivated	Fruit	7, 9, 10	42	0.60	0.15
51.	Musaceae	Musa acuminata Colla	Guay Hohm	LC	Native	Herb	Cultivated	Fruit, leave, and stem	1, 2, 4, 5, 7, 9, 10, 11, 12	180	0.88	0.30
52.	Myrtaceae	Psidium guajava L.	Fa Rung	LC	Introduced	Tree	Cultivated	Fruit	9, 10, 12	24	0.70	0.33
53.	Nelumbonaceae	Nelumbo nucifera Gaertn.	Bua Luang	NE	Native	Herb	Cultivated	Inflorescence	1, 2, 4, 5, 8, 11	42	0.73	0.15
54.	Nymphaeaceae	Nymphaea nouchali Burm.f.	Bua Fuean	LC	Native	Herb	Cultivated	Inflorescence	4	4	0.63	0.13
55.	Oleaceae	Jasminum officinale L.	Ma Li	NE	Introduced	Shrub	Cultivated	Inflorescence	1, 5, 8, 11	24	0.75	0.20
56.	Pandanaceae	Pandanus amaryllifolius Roxb. ex Lindl.	Toei Hom	DD	Introduced	Herb	Cultivated	Leave	1, 5, 8, 9, 10, 11	120	0.75	0.40
57.	Pedaliaceae	Sesamum indicum L.	Nga Dum	NE	Introduced	Herb	Cultivated	Seed	3, 9, 11	18	0.60	0.23
58.	Phyllanthaceae	Antidesma puncticulatum Miq.	Bak Mao	LC	Native	Tree	Cultivated	Fruit	10	4	0.48	0.50
59.	Piperaceae	Piper betle L.	Ploo	LC	Native	Climber	Cultivated	Leave	1, 2, 4, 7, 9, 10	120	0.75	0.38
60.	Piperaceae	Piper sarmentosum Roxb.	Cha Ploo	NE	Native	Herb	Cultivated	Leave	10	6	0.68	0.40
61.	Poaceae	× Thyrsocalamus liang Sungkaew & W.L.Goh	Phai Liang	NE	Native	Tree	Cultivated	Stem	2, 3, 4, 7, 9, 10	120	0.75	0.35
62.	Poaceae	Bambusa burmanica Gamble	Phai Wahn	NE	Native	Tree	Cultivated	Stem	6	12	0.80	0.30
63.	Poaceae	Oryza sativa L.	Khao	LC	Introduced	Herb	Cultivated	Fruit	1, 2, 3, 4, 6, 7, 9, 10, 11	180	0.98	0.53
64.	Poaceae	Saccharum officinarum L.	Oy	NE	Introduced	Herb	Cultivated	Stem	3, 4, 7, 9, 11, 12	96	0.93	0.45
65.	Poaceae	Zea mays L.	Khao Pod	LC	Introduced	Herb	Cultivated	Fruit	10, 11	12	0.90	0.38
66.	Pontederiaceae	Pontederia hastata L.	Pug Tob Cha Wa	NE	Native	Herb	Wild/AQ	Whole plant	4	1	0.05	0.45
67.	Rosaceae	Malus domestica (Suckow) Borkh.	Apple	NE	Introduced	Tree	Cultivated	Fruit	10	6	0.43	0.33
68.	Rubiaceae	Morinda citrifolia L.	Yor	LC	Native	Shrub	Cultivated	Leave	2	1	0.30	0.25
69.	Rubiaceae	Oxyceros horridus Lour.	Kud Khao	LC	Native	Climber	Cultivated	Inflorescence	4	1	0.23	0.43
70.	Rubiaceae	Paederia linearis Hook.f.	Tod Moo Tod Ma	NE	Native	Vine	Wild/DF, MDF	Leave	3	1	0.15	0.15
71.	Rutaceae	Citrus × aurantiifolia (Christm.) Swingle	Ma Now	NE	Introduced	Shrub	Cultivated	Fruit	7, 9, 10	60	0.88	0.38
72.	Rutaceae	Citrus × aurantium L.	Som	NE	Introduced	Tree	Cultivated	Fruit	10	6	0.65	0.18
73.	Rutaceae	Citrus maxima (Burm.) Merr.	Som-O	LC	Native	Tree	Cultivated	Fruit	9, 10, 12	18	0.70	0.10
74.	Sapindaceae	Nephelium lappaceum L.	Ngor	LC	Native	Tree	Cultivated	Fruit	7, 10	16	0.68	0.48
75.	Solanaceae	Capsicum frutescens L.	Prig	LC	Introduced	Shrub	Cultivated	Fruit	7, 9, 10	60	0.88	0.15
76.	Solanaceae	Nicotiana tabacum L.	Yaa Soob	NE	Introduced	Herb	Cultivated	Leave	1, 2, 4, 7, 9, 10	96	0.73	0.13
77.	Solanaceae	Solanum torvum Sw.	Ma Khuea Puang	NE	Introduced	Shrub	Cultivated	Fruit	7, 9	12	0.68	0.20
78.	Solanaceae	Solanum virginianum L.	Ma Khuea Proa	NE	Introduced	Shrub	Cultivated	Fruit	7	6	0.70	0.23
79.	Vitaceae	Vitis vinifera L.	A-ngun	LC	Introduced	Climber	Cultivated	Fruit	9, 10	12	0.63	0.38
80.	Zingiberaceae	Curcuma longa L.	Khamin Chan	DD	Introduced	Herb	Cultivated	Rhizome	7	6	0.73	0.25

Abbreviations: Conservation status; data deficient (DD), least concern (LC), not evaluated (NE). Ceremony months; Bun Khao Kam (1), Bun Khun Lan (2), Bun Khao Jee (3), Bun Pha Wet (4), Bun Songkran (5), Bun Bang Fai (6), Bun Samha (7), Bun Khao Phansa (8), Bun Khao Pradap Din (9), Bun Khao Sak (10), Bun Ok Phansa (11), Bun Kathin (12). Resource; aquatic area (AQ), dry evergreen forest (DEF), deciduous forest (DF), forest edge (FE), grassland (GL), mixed deciduous forest (MDF), and rainforest (RF).

, Figure 2). These species were identified through a combination of ethnobotanical surveys, interviews with local ritual specialists, and direct observations conducted during ceremonial events. For clarity, the twelve Heet Sip Song ceremonies are aligned with the Gregorian calendar months as follows: Month 1 = January (Bun Khao Kam), Month 2 = February (Bun Khun Lan), Month 3 = March (Bun Khao Jee), Month 4 = April (Bun Pha Wet), Month 5 = May (Bun Songkran), Month 6 = June (Bun Bang Fai), Month 7 = July (Bun Samha), Month 8 = August (Bun Khao Phansa), Month 9 = September (Bun Khao Pradap Din), Month 10 = October (Bun Khao Sak), Month 11 = November (Bun Ok Phansa), and Month 12 = December (Bun Kathin).

Among the plant families recorded, Fabaceae was the most dominant, comprising 10 species, reflecting its significance in both symbolic and practical contexts, including use as food, medicine, and in spiritual offerings. This was followed by Poaceae with 5 species, and Arecaceae, Cucurbitaceae, and Solanaceae, each represented by 4 species. These families include commonly cultivated or foraged plants in the region, often associated with agrarian rituals and seasonal transitions.

Additional families such as Araceae, Dipterocarpaceae, Rubiaceae, and Rutaceae were each represented by 3 species, while Amaryllidaceae, Anacardiaceae, Annonaceae, Apocynaceae, Asteraceae, Bignoniaceae, Dioscoreaceae, and Piperaceae included 2 species each. The remaining 25 families were represented by a single species. This wide taxonomic representation highlights the rich ethnobotanical knowledge within the local communities and the diverse roles plants play in the cultural and spiritual practices tied to the Heet Sip Song tradition.

Of the species identified, 66 species (82.50%) are cultivated, while the remaining 14 species (17.50%) consist of wild plants. Most of the plants were tree (30 species, 37.50%), followed by herbs (28 species, 35%) and climbers (12 species, 15%). Other habits included shrubs (9 species, 11.25%) and vines (1 species, 1.25%). In terms of distribution status, species were categorized into three groups: 48.75% (39 species) were classified as introduced, 50% (40 species) as native, and 1.25% (1 species, Cassia fistula L.) as doubtful, reflecting uncertainty regarding its origin or distribution.

In addition to the taxonomic classification, understanding the distinction between cultivated and wild plants provides valuable insight into the sociocultural dynamics underpinning the Heet Sip Song ceremonies. Cultivated species, which make up the majority of documented plants, are often integral to agrarian rituals and food-related offerings, reflecting the communities’ reliance on agricultural cycles and human stewardship. Conversely, wild plants tend to be gathered for their unique spiritual or medicinal properties, symbolizing a direct connection to the natural environment and ancestral traditions. Similarly, the categorization of species into native and introduced groups reveals patterns of cultural adaptation and ecological knowledge. Native species often carry deep-rooted symbolic significance, embedded in local myths and ritual practices, whereas introduced species demonstrate the dynamic nature of cultural exchange and the incorporation of new botanical resources into traditional frameworks. These distinctions enrich the interpretation of indices such as the Cultural Significance Index (CSI), Use Value (SUV), and Relative Frequency of Citation (RFC), highlighting how the cultural importance of plants is not solely dependent on taxonomic identity but also on their origins and roles within social and ritual contexts.

3.2. Cultural Significance Index (CSI) of Plant Species Used in Heet Sip Song Ceremonies

The Cultural Significance Index (CSI) was calculated to assess the relative importance of each plant species used in the Heet Sip Song ceremonies. To enhance clarity, we examined CSI values within the framework of plant origin (cultivated vs. wild, introduced vs. native) and their use throughout the twelve monthly ceremonies (Table 1).

Species with the highest CSI values, such as Musa acuminata and Oryza sativa (both scoring 180), are primarily cultivated and introduced species that play key symbolic and functional roles across many ceremonies, especially those related to fertility, sustenance, and merit-making. Cassia fistula (CSI = 140), also cultivated, is significant in purification and blessing rituals. Other high-CSI species like Pandanus amaryllifolius, Piper betle, and × Thyrsocalamus liang (each scoring 120) serve as important offerings, decorations, and components of ritual tools.

In contrast, many wild and native species such as Chromolaena odorata, Dipterocarpus intricatus, Getonia floribunda, and Pontederia hastata exhibit lower CSI values (1–4), indicating more specialized or occasional roles in rituals, often tied to particular months or localized ceremonies.

Moderately significant species, including Cocos nucifera, Saccharum officinarum, and Nicotiana tabacum (CSI = 96), represent a blend of cultivated and native plants commonly used in ancestral worship and agricultural rites throughout the year. Culinary and medicinal plants such as Allium cepa, A. sativum, Capsicum frutescens, and Citrus × aurantiifolia (CSI = 60) highlight species with dual roles in daily life and ritual practice.

This framework highlights how cultivated and introduced species generally hold higher cultural significance due to their widespread use across multiple ceremonies, while wild and native plants contribute more specialized and seasonal roles within the ritual calendar. Such distinctions reflect the complex and dynamic cultural relationships between plant use, origin, and ceremonial timing.

3.3. Species Use Value (SUV) of Plant Species Used in Heet Sip Song Ceremonies

A comprehensive list of plant species along with their corresponding Species Use Values (SUVs) is provided in Table 1. The analysis categorizes species into three groups based on their SUV scores: high (≥0.75), medium (0.50–0.74), and low (<0.50), reflecting the relative frequency and versatility of each plant’s use across food, medicinal, ritual, and commercial contexts.

This category is dominated by plants that play foundational roles in global food systems, traditional medicine, and local economies. Oryza sativa recorded the highest SUV (0.98), highlighting its vital importance as a staple crop and ceremonial offering. Other key species include Saccharum officinarum (SUV = 0.93) and Zea mays (SUV = 0.90), both central to subsistence agriculture and ritual contexts.

Species with an SUV of 0.88 included Tabernaemontana divaricata, Areca catechu, Cocos nucifera, Ocimum × africanum, Musa acuminata, Citrus × aurantiifolia, and Capsicum frutescens, commonly used in rituals for protection, purification, and food preparation. Additional high-use species include Vigna unguiculata subsp. sesquipedalis (SUV = 0.85), Calotropis gigantea (SUV = 0.83), Bambusa burmanica (SUV = 0.80), and Allium sativum (SUV = 0.78).

In contrast, species with an SUV of 0.75—such as Allium cepa, Carica papaya, Ipomoea batatas, Cassia fistula, Jasminum officinale, Pandanus amaryllifolius, Piper betle, and × Thyrsocalamus liang, also play essential roles in both ritual and culinary traditions, though they are used slightly less intensively.

This group includes plants with moderate utility in food, medicine, or ritual use. Though not as dominant as those in the high SUV category, they still contribute significantly to local traditions. Notable species include Oroxylum indicum, Benincasa hispida, Nelumbo nucifera, Nicotiana tabacum, and Curcuma longa (SUV = 0.73).

Other species in this category include Ananas comosus, Selenicereus undatus, Cucumis sativus, Arachis hypogaea, Psidium guajava, Citrus maxima, and Solanum virginianum (SUV = 0.70). Plants such as Piper sarmentosum, Nephelium lappaceum, and S. torvum (SUV = 0.68), C. × aurantium (SUV = 0.65), and Garcinia mangostana, Cucurbita maxima, Nymphaea nouchali, and Vitis vinifera (SUV = 0.63) also fall into this category.

Additional medium-use species include Lansium domesticum and Sesamum indicum (SUV = 0.60), Borassus flabellifer and Vigna radiata (SUV = 0.55), along with Glycine max, Punica granatum, Mangifera indica, Tagetes erecta, Citrullus lanatus, and Pachyrhizus erosus (SUV = 0.50).

Species in this category exhibited limited applications in food, medicinal, or commercial contexts. These plants are typically of local or symbolic significance, valued for their specific roles in particular rituals rather than broad utility. While they may not contribute significantly to economic systems, their presence in ceremonial contexts highlights the rich diversity of traditional knowledge and the cultural specificity of plant use in the Heet Sip Song traditions.

3.4. Genera Use Value (GUV) of Plant Species Used in Heet Sip Song Ceremonies

Table 2. Genera Use Value (GUV) of plant species used in Heet Sip Song ceremonies in Roi Et Province, Thailand, sorted from highest to lowest value.

Genera	GUV	Genera	GUV	Genera	GUV
Oryza	0.98	Ananas	0.70	Annona	0.45
Saccharum	0.93	Arachis	0.70	Malus	0.43
Zea	0.90	Cucumis	0.70	Colocasia	0.38
Areca	0.88	Psidium	0.70	Sesbania	0.38
Capsicum	0.88	Selenicereus	0.70	Spondias	0.35
Cocos	0.88	Vigna	0.70	Butea	0.30
Musa	0.88	Solanum	0.69	Morinda	0.30
Ocimum	0.88	Nephelium	0.68	Dioscorea	0.28
Tabernaemontana	0.88	Cucurbita	0.63	Gomphrena	0.25
Calotropis	0.83	Garcinia	0.63	Erythrina	0.23
Bambusa	0.80	Nymphaea	0.63	Paederia	0.23
Allium	0.76	Vitis	0.63	Oxyceros	0.15
Carica	0.75	Lansium	0.60	Uvaria	0.15
Cassia	0.75	Sesamum	0.60	Anthoshorea	0.13
Ipomoea	0.75	Borassus	0.55	Dipterocarpus	0.13
Jasminum	0.75	Glycine	0.53	Lemna	0.13
Pandanus	0.75	Punica	0.53	Pentacme	0.13
Thyrsocalamus	0.75	Citrullus	0.50	Pistia	0.13
Citrus	0.74	Mangifera	0.50	Samanea	0.13
Benincasa	0.73	Pachyrhizus	0.50	Chromolaena	0.10
Curcuma	0.73	Tagetes	0.50	Irvingia	0.08
Nelumbo	0.73	Antidesma	0.48	Getonia	0.05
Nicotiana	0.73	Calamus	0.48	Pontederia	0.05
Oroxylum	0.73	Maranta	0.48
Piper	0.71	Millingtonia	0.48

presents the Genera Use Value (GUV) of plant species used in the Heet Sip Song ceremonies in Roi Et Province, Thailand. The data reveal that a wide range of genera play relatively significant roles in these traditional practices. The genus Oryza recorded the highest GUV (0.98), reflecting its central importance in ceremonial rituals, likely due to its role as a staple food and symbol of prosperity. This was followed by Saccharum (GUV = 0.93) and Zea (GUV = 0.90), which are also essential food crops commonly integrated into religious and communal activities.

Several genera shared identical GUV values, including Areca, Capsicum, Cocos, Musa, Ocimum, and Tabernaemontana (all at GUV = 0.88), indicating their broad utility and symbolic relevance across various Heet Sip Song events. Genera such as Ananas, Arachis, Cucumis, Psidium, and Selenicereus also scored high (GUV = 0.70), suggesting a significant, though slightly lesser, level of cultural and functional integration.

As the GUV values declined, genera like Colocasia, Sesbania, Spondias, and Butea (ranging from 0.38 to 0.30) remained important, albeit with more specialized or context-dependent uses. The lower end of the spectrum included genera such as Chromolaena (0.10), Irvingia (0.08), Getonia (0.05), and Pontederia (0.05), indicating more limited or localized applications within the ceremonial context.

Overall, the results highlight the diverse ethnobotanical knowledge embedded in Heet Sip Song traditions, with food, medicinal, and symbolic plants all contributing to the spiritual and practical fabric of these cultural ceremonies. The GUV metric effectively captures the relative significance of each genus, reflecting both frequency and versatility of use across the community.

3.5. Relative Frequency of Citation (RFC) of Plant Species Used in Heet Sip Song Ceremonies

An overall analysis of the Species Use Value (SUV) and Relative Frequency of Citation (RFC) across all species in this study (Table 1) revealed a negligible linear correlation, with a Pearson correlation coefficient (r) of −0.01795. This finding suggests that, in a comprehensive view of the dataset, changes in SUV values do not exhibit a meaningful linear relationship with changes in RFC values, and vice versa.

While this comprehensive linear analysis indicates a lack of overall linear correlation, an initial observation focusing on the top ten plant species with the highest SUV scores (

Table 3. Recorded top 10 plant species with the highest Species Use Value (SUV) used in Heet Sip Song Ceremonies and their Relative Frequency of Citation (RFC) in Roi Et Province.

Scientific Name	SUV	RFC
Oryza sativa L.	0.98	0.53
Saccharum officinarum L.	0.93	0.45
Zea mays L.	0.90	0.38
Citrus × aurantiifolia (Christm.) Swingle	0.88	0.38
Tabernaemontana divaricata (L.) R.Br. ex Roem. & Schult.	0.88	0.33
Cocos nucifera L.	0.88	0.33
Musa acuminata Colla	0.88	0.30
Areca catechu L.	0.88	0.23
Ocimum × africanum Lour.	0.88	0.20
Capsicum frutescens L.	0.88	0.15

) appeared to show a consistent pattern. For these selected species, there was a visual tendency for those used more extensively (higher SUV) to also be mentioned more frequently by informants (higher RFC). This localized alignment might suggest a connection between a plant’s practical importance and its prominence in local knowledge for highly utilized species. SUV measures the breadth of a plant’s uses, while RFC reflects its common recognition and citation within the community. However, as the overall correlation demonstrates, this apparent parallel increase observed in a subset does not generalize across the entire spectrum of species studied.

For instance, Oryza sativa recorded the highest SUV (0.98) and a notably high RFC (0.53), highlighting its central role not only as a staple food crop but also as a culturally important species recognized by nearly all informants. Similarly, Saccharum officinarum (SUV = 0.93, RFC = 0.45) and Zea mays (SUV = 0.90, RFC = 0.38) also exhibited high values in both metrics, reinforcing their individual importance in practical day-to-day life and in collective traditional knowledge. These crops are indeed essential to the local agricultural system and dietary practices, and their prominence in both use and mention reflects their longstanding presence and integration into community life. Nevertheless, these specific instances of high co-occurrence do not represent a universal linear trend across all species when the entire dataset is considered.

Other species such as Citrus × aurantiifolia, Tabernaemontana divaricata, and Cocos nucifera also had high SUV values (each at 0.88) and relatively high RFC values (ranging from 0.33 to 0.38). While these values indicate a substantial level of community recognition and diverse usage, particularly in both nutritional and medicinal contexts, the overall low Pearson correlation coefficient confirms that a simple linear proportionality between SUV and RFC is not a consistent feature across all data points. Furthermore, even among species towards the lower end of the top ten in terms of RFC, such as Ocimum × africanum (RFC = 0.20) and Capsicum frutescens (RFC = 0.15), SUV values remained relatively high (0.88). This discrepancy suggests that while these plants may be used in diverse ways, their widespread recognition or citation by informants might be less universal, possibly due to more specialized or situational uses or varying levels of accessibility and preference across different demographic groups.

Therefore, based on the comprehensive analysis of the provided data, the present findings do not support a substantial linear association between SUV and RFC across all species, despite some individual highly prominent species exhibiting high values in both metrics.

3.6. Plants Used in the Heet Sip Song Ceremonies in Roi Et Province

The twelve-month traditions observed by the people of Roi Et are deeply rooted in Buddhist principles, local beliefs, and agricultural practices that have been passed down through generations. Each month is associated with specific rituals designed to bring good fortune and prosperity. A significant aspect of these traditions is the use of local plants, which play an essential role in various ceremonies. The plants utilized by the Roi Et people in these traditional rituals are listed in

Table 4. Diversity of plant species in Heet Sip Song ceremonies in Roi Et Province, Thailand.

Ceremonial	Plant Species
Bun Khao Kam (Month 1)	Areca catechu L., Calotropis gigantea (L.) W.T.Aiton, Cassia fistula L., Gomphrena globosa L., Jasminum officinale L., Musa acuminata Colla, Nelumbo nucifera Gaertn., Nicotiana tabacum L., Oryza sativa L., Pandanus amaryllifolius Roxb. ex Lindl., Piper betle L., Tabernaemontana divaricata (L.) R.Br. ex Roem. & Schult., Tagetes erecta L.
Bun Khun Lan (Month 2)	× Thyrsocalamus liang Sungkaew & W.L.Goh, Areca catechu L., Cassia fistula L., Colocasia esculenta (L.) Schott, Gomphrena globosa L., Ipomoea batatas (L.) Lam., Morinda citrifolia L., Musa acuminata Colla, Nelumbo nucifera Gaertn., Nicotiana tabacum L., Oryza sativa L., Piper betle L., Tabernaemontana divaricata (L.) R.Br. ex Roem. & Schult., Tagetes erecta L.
Bun Khao Jee (Month 3)	× Thyrsocalamus liang Sungkaew & W.L.Goh, Cocos nucifera L., Oryza sativa L., Paederia linearis Hook.f., Saccharum officinarum L., Sesamum indicum L.
Bun Pha Wet (Month 4)	× Thyrsocalamus liang Sungkaew & W.L.Goh, Anthoshorea roxburghii (G.Don) P.S.Ashton & J.Heck., Areca catechu L., Borassus flabellifer L., Butea monosperma (Lam.) Kuntze, Calamus caesius Blume, Calotropis gigantea (L.) W.T.Aiton, Cassia fistula L., Cocos nucifera L., Dipterocarpus intricatus Dyer, Getonia floribunda Roxb., Lemna minor L., Millingtonia hortensis L.f., Musa acuminata Colla, Nelumbo nucifera Gaertn., Nicotiana tabacum L., Nymphaea nouchali Burm.f., Oroxylum indicum (L.) Kurz, Oryza sativa L., Oxyceros horridus Lour., Pentacme siamensis (Miq.) Kurz, Piper betle L., Pistia stratiotes L., Pontederia hastata L., Saccharum officinarum L., Sesbania javanica Miq., Tabernaemontana divaricata (L.) R.Br. ex Roem. & Schult., Uvaria siamensis (Scheff.) L.L.Zhou, Y.C.F.Su & R.M.K.Saunders
Bun Songkran (Month 5)	Tabernaemontana divaricata (L.) R.Br. ex Roem. & Schult., Cassia fistula L., Jasminum officinale L., Musa acuminata Colla, Nelumbo nucifera Gaertn., Pandanus amaryllifolius Roxb. ex Lindl., Tagetes erecta L.
Bun Bang Fai (Month 6)	Bambusa burmanica Gamble, Calamus caesius Blume, Chromolaena odorata (L.) R.M.King & H.Rob., Erythrina variegata L., Oryza sativa L., Samanea saman (Jacq.) Merr.
Bun Samha (Month 7)	× Thyrsocalamus liang Sungkaew & W.L.Goh, Allium cepa L., Allium sativum L., Annona squamosa L., Areca catechu L., Capsicum frutescens L., Carica papaya L., Cassia fistula L., Citrus × aurantiifolia (Christm.) Swingle, Cucumis sativus L., Curcuma longa L., Garcinia mangostana L., Lansium domesticum Corrêa, Musa acuminata Colla, Nephelium lappaceum L., Nicotiana tabacum L., Oryza sativa L., Piper betle L., Saccharum officinarum L., Solanum torvum Sw., Solanum virginianum L., Tabernaemontana divaricata (L.) R.Br. ex Roem. & Schult.
Bun Khao Phansa (Month 8)	Calotropis gigantea (L.) W.T.Aiton, Gomphrena globosa L., Jasminum officinale L., Nelumbo nucifera Gaertn., Pandanus amaryllifolius Roxb. ex Lindl., Tabernaemontana divaricata (L.) R.Br. ex Roem. & Schult., Tagetes erecta L.
Bun Khao Pradap Din (Month 9)	× Thyrsocalamus liang Sungkaew & W.L.Goh, Allium cepa L., Allium sativum L., Annona squamosa L., Arachis hypogaea L., Areca catechu L., Capsicum frutescens L., Carica papaya L., Cassia fistula L., Citrus × aurantiifolia (Christm.) Swingle, Citrus maxima (Burm.) Merr., Cocos nucifera L., Cucurbita maxima Duchesne, Dioscorea pseudotomentosa Prain & Burkill, Ipomoea batatas (L.) Lam., Lansium domesticum Corrêa, Musa acuminata Colla, Nicotiana tabacum L., Oryza sativa L., Pachyrhizus erosus (L.) Urb., Pandanus amaryllifolius Roxb. ex Lindl., Piper betle L., Psidium guajava L., Saccharum officinarum L., Selenicereus undatus (Haw.) D.R.Hunt, Sesamum indicum L., Solanum torvum Sw., Spondias mombin L., Tagetes erecta L., Vitis vinifera L.
Bun Khao Sak (Month 10)	× Thyrsocalamus liang Sungkaew & W.L.Goh, Allium cepa L., Allium sativum L., Annona squamosa L., Antidesma puncticulatum Miq., Arachis hypogaea L., Areca catechu L., Capsicum frutescens L., Carica papaya L., Cassia fistula L., Citrullus lanatus (Thunb.) Matsum. & Nakai, Citrus × aurantiifolia (Christm.) Swingle, Citrus × aurantium L., Citrus maxima (Burm.) Merr., Cocos nucifera L., Colocasia esculenta (L.) Schott, Cucumis sativus L., Cucurbita maxima Duchesne, Dioscorea hispida Dennst., Dioscorea pseudotomentosa Prain & Burkill, Garcinia mangostana L., Ipomoea batatas (L.) Lam., Irvingia malayana Oliv. ex A.W.Benn., Lansium domesticum, Malus domestica (Suckow) Borkh., Mangifera indica L., Musa acuminata Colla, Nephelium lappaceum L., Nicotiana tabacum L., Ocimum × africanum Lour., Oryza sativa L., Pachyrhizus erosus (L.) Urb., Pandanus amaryllifolius Roxb. ex Lindl., Piper betle L., Piper sarmentosum Roxb., Psidium guajava L., Punica granatum L., Tabernaemontana divaricata (L.) R.Br. ex Roem. & Schult., Tagetes erecta L., Vigna unguiculata subsp. sesquipedalis (L.) Verdc., Vitis vinifera L., Zea mays L.
Bun Ok Phansa (Month 11)	Arachis hypogaea L., Calotropis gigantea (L.) W.T.Aiton, Cocos nucifera L., Colocasia esculenta (L.) Schott, Cucurbita maxima Duchesne, Glycine max (L.) Merr., Gomphrena globosa L., Ipomoea batatas (L.) Lam., Jasminum officinale L., Maranta arundinacea L., Musa acuminata Colla, Nelumbo nucifera Gaertn., Oryza sativa L., Pandanus amaryllifolius Roxb. ex Lindl., Saccharum officinarum L., Sesamum indicum L., Tabernaemontana divaricata (L.) R.Br. ex Roem. & Schult., Tagetes erecta L., Vigna radiata (L.) R.Wilczek, Zea mays L.
Bun Kathin (Month 12)	Ananas comosus (L.) Merr., Benincasa hispida (Thunb.) Cogn., Calotropis gigantea (L.) W.T.Aiton, Carica papaya L., Citrus maxima (Burm.) Merr., Cocos nucifera L., Gomphrena globosa L., Musa acuminata Colla, Nelumbo nucifera Gaertn., Psidium guajava L., Saccharum officinarum L., Tabernaemontana divaricata (L.) R.Br. ex Roem. & Schult., Tagetes erecta L.

.

The availability of plant species used in the Heet Sip Song ceremonies varies. Some plants are strictly seasonal and used during specific months based on their natural flowering or fruiting periods. Others are capable of producing flowers or usable parts year-round, especially those cultivated in home gardens. Additionally, certain species such as rhizomes, dried fruits, or seeds are harvested during peak seasons and preserved for later use across multiple months. No plants included in this study were grown under protected conditions such as greenhouses.

To provide a clear overview of the Heet Sip Song ceremonies central to this study,

Table 5. Heet Sip Song ceremonies in Roi Et Province, Thailand: lunar month and aims.

Month (Lunar Calendar)	Name of Ceremony	Aim of Ceremony
1	Bun Khao Kam	To foster community gatherings, rituals, and offerings, enhancing cultural identity and communal ties.
2	Bun Khun Lan	To honor Mother Earth and Mother Rice, engage in merit-making for good fortune, and raise the rice spirit among farmers.
3	Bun Khao Jee	To offer gratitude and respect during the post-harvest season by preparing and offering Khao Jee (grilled sticky rice balls) to the Buddha.
4	Bun Pha Wet	To commemorate the story of Phra Maha Vessantara Jataka, illustrating the Buddha’s ten perfections, through elaborate rituals and offerings.
5	Bun Songkran	To symbolize a request for forgiveness and blessings by pouring perfumed water over Buddha images, monks, and elders, and to express deep respect for cultural and spiritual practices.
6	Bun Bang Fai	To worship the city pillar’s guardian spirit, make merit, and request timely rain from Phaya Taen (the rain god) for the upcoming agricultural season.
7	Bun Samha	To drive out evil spirits, ghosts, demons, and other malevolent entities from the village, fostering community solidarity and cultural identity.
8	Bun Khao Phansa	To mark the beginning of the three-month Buddhist Lent, make merit by offering alms and essential items to monks, cultivate spiritual merit, and reinforce community bonds.
9	Bun Khao Pradap Din	To make merit for the deceased and extend compassion to spirits, ghosts, and stray animals during a sacred time when the gates of hell are believed to open.
10	Bun Khao Sak	To make merit for deceased relatives by preparing and offering food trays (including sticky rice, dried foods, and small rice bags) at the temple.
11	Bun Ok Phansa	To mark the conclusion of Buddhist Lent, engage in merit-making (giving alms, offering food, listening to sermons), and show devotion to the Triple Gem and ancestors.
12	Bun Kathin	To foster unity within the community by offering new robes to monks, symbolizing renewal and support, and emphasizing communal harmony and collective merit-making.

details each of the twelve monthly rituals. It outlines the lunar calendar month in which each ceremony takes place, its specific name, and its primary aim or purpose within the cultural and religious traditions of Roi Et Province. This table serves as a foundational guide to understanding the seasonal rhythm and profound significance of these plant-based ceremonies.

3.6.1. Plants Used in Bun Khao Kam (Month 1)

Bun Khao Kam, commonly referred to as the Bun Khao Kam ceremony, is a significant cultural and religious event celebrated in the northeastern provinces of Thailand. Occurring in the first month of the lunar calendar, this ceremony marks a crucial time for community gatherings, rituals, and offerings, fostering a strong sense of cultural identity and communal ties among the participants.

The plants utilized in the Bun Khao Kam ceremony predominantly serve worship purposes and include a variety of significant species (Table 1 and Table 4). Notable examples are Areca catechu, Calotropis gigantea, Cassia fistula, Gomphrena globosa, Jasminum officinale, Musa acuminata, Nelumbo nucifera, Nicotiana tabacum, Oryza sativa, Pandanus amaryllifolius, Piper betle, Tabernaemontana divaricata, and Tagetes erecta.

3.6.2. Plants Used in Bun Khun Lan (Month 2)

Bun Khun Lan, also known as the rice spirit-raising ceremony, is a vital tradition among farmers in the northeastern region of Thailand. Held in the second month of the Isan calendar, it is often referred to as the Bun Duan Yi event. During this ceremony, farmers gather threshed rice and engage in merit-making practices to ensure good fortune. Prior to threshing the rice, a ritual is performed to honor Mother Earth and to inform Mother Rice. Essential preparations for the ceremony include seven leaves of Cassia fistula and Morinda citrifolia, a pair of buffalo or cow horns, one egg, tubers of Ipomoea batatas and Colocasia esculenta, four rolls of Nicotiana tabacum wrapped in Piper betle leaves, four pieces of Areca catechu and glutinous rice (Oryza sativa) steamed in Musa acuminata leaves. Additionally, the ceremony requires a bowl of water, candles, and flowers such as Gomphrena globosa, Tabernaemontana divaricata, Tagetes erecta, and Nelumbo nucifera. All items, except for the water and buffalo or cow horns, are placed into a bamboo woven rice container made from × Thyrsocalamus liang.

3.6.3. Plants Used in Bun Khao Jee (Month 3)

After the villagers have completed the rice harvest and stored it in the granary, they prepare the rice to create offerings for the Buddha. Using the sticky rice they have gathered, they steam it until fully cooked, then shape it into fist-sized balls. These rice balls are skewered on sticks made from the stems of × Thyrsocalamus liang and grilled over an open fire until charred, resulting in a dish known as Khao Jee. The ingredients for Khao Jee include Cocos nucifera, Oryza sativa, Paederia linearis, Saccharum officinarum, and Sesamum indicum. This traditional preparation not only serves as a culinary offering but also reflects the community’s gratitude and respect during the post-harvest season.

3.6.4. Plants Used in Bun Pha Wet (Month 4)

The Bun Pha Wet ceremony is intricately linked to the story of Phra Maha Vessantara Jataka, which illustrates the ten perfections practiced by the Buddha in his final life before attaining enlightenment. The ceremonial site is prepared with a Thung Chai or long-tailed Thung, tied to the end of a × Thyrsocalamus liang stick, planted at all eight cardinal directions. Bowls of rice balls for worship are placed on these Thung poles, arranged around the pulpit. Khan Mak Beng, crafted from Musa acuminata leaves, Calotropis gigantea flowers, and Tabernaemontana divaricata flowers, is used in worship rituals and to honor the Rattana Tri in Buddhism. Additional offerings include Areca catechu, Piper betle, and Nicotiana tabacum, which are also attached to the Thung poles surrounding the area. Strips of Sesbania javanica trees are dyed to create colorful decorations for the pulpit, where monks and novices sit to preach. Garlands are fashioned from the flowers of Dipterocarpus intricatus, fruits of Calamus caesius and Oroxylum indicum, alongside cut-paper garlands, popped rice (Oryza sativa), and clusters of Musa acuminata, Saccharum officinarum, Cocos nucifera, and Borassus flabellifer. Wildflowers, including Butea monosperma, Getonia floribunda, Uvaria siamensis, Oxyceros horridus, Anthoshorea roxburghii, Pentacme siamensis, Millingtonia hortensis, and Cassia fistula are hung in the pavilion area. Additionally, four water jars, symbolizing four ponds, are placed around the pulpit, each containing Pistia stratiotes, Lemna minor, Nelumbo nucifera, Nymphaea nouchali, and Pontederia hastata. The most prominent plants identified in this ritual belong to the Dipterocarpaceae family, which are abundant in the region and are currently in their flowering and fruiting season.

3.6.5. Plants Used in Bun Songkran (Month 5)

Songkran, also known as Bun Song Nam, is a traditional festival that involves pouring water over Buddha images, monks, and elders using perfumed water infused with Jasminum officinale flowers. This act symbolizes a request for forgiveness and blessings. During the ritual of bathing the Buddha statue, participants adorn the statue with various flowers, including Tabernaemontana divaricata, Tagetes erecta, Cassia fistula, and Nelumbo nucifera, along with leaves of Pandanus amaryllifolius. These elements not only enhance the beauty of the ritual but also reflect the community’s deep respect for their cultural and spiritual practices.

3.6.6. Plants Used in Bun Bang Fai (Month 6)

The Bun Bang Fai ceremony is a vibrant tradition celebrated in the sixth lunar month, dedicated to worshipping the guardian spirit of the city pillar. This ceremony serves as an occasion for making merit and requesting rain from Phaya Taen, the rain god, to ensure timely rainfall for the upcoming season. The rockets used in the festival are constructed from Bambusa burmanica trunks, which are wrapped in Calamus caesius stems and bound together with glue made from Oryza sativa. These rockets are filled with gunpowder, composed of a mixture of bat guano, charcoal derived from Chromolaena odorata, and wood from Erythrina variegata and Samanea saman. This unique blend of materials underscores the festival’s cultural significance and its deep-rooted connections to agricultural practices and local beliefs.

3.6.7. Plants Used in Bun Samha (Month 7)

Bun Sam Ha, also known as Bun Beik Ban, is a ceremonial event aimed at driving out evil spirits, ghosts, demons, and other malevolent entities from the village. Villagers construct a ceremonial pavilion at the center of the community, adorned with Tabernaemontana divaricata flowers, incense, candles, and water bowls. Additional offerings include bowls filled with gravel and sand, along with cotton tied around participants’ arms. For the sacrificial offerings, a basket made from Musa acuminata leaves is divided into four sections, containing black rice, red rice (a mix of Oryza sativa and Curcuma longa), cooked rice, and various foods and fruits. These offerings typically include Allium cepa, Allium sativum, Annona squamosa, Areca catechu, Carica papaya, Garcinia mangostana, Cucumis sativus, Cassia fistula, Lansium domesticum, × Thyrsocalamus liang, Saccharum officinarum, Citrus × aurantiifolia, Nephelium lappaceum, Piper betle, and Capsicum frutescens. This ritual serves not only to purify the village but also to foster community solidarity and cultural identity.

3.6.8. Plants Used in Bun Khao Phansa (Month 8)

Bun Khao Phansa is an important Buddhist tradition in Thailand that marks the beginning of the three-month Buddhist Lent. During this period, laypeople make merit by offering alms and essential items to monks, who usually stay within their monasteries to focus on study and meditation. These offerings typically include food and daily necessities such as soap, toothpaste, candles, and bathing robes, since monks have no personal income and depend entirely on the generosity of the community.

Various species of flowers are also presented during the ritual. These flowers, including Calotropis gigantea, Gomphrena globosa, Tabernaemontana divaricata, Tagetes erecta, Nelumbo nucifera, Jasminum officinale, and Pandanus amaryllifolius, are often arranged into bouquets or garlands as symbols of respect and devotion. Offering flowers serves both as a visual tribute and a spiritual gesture, symbolizing the beauty and impermanence of life. These floral offerings are considered Buddha-puja—acts of homage and reverence to the Buddha—and are believed to bring merit to those who make them [12].

In addition to flowers, offerings commonly include incense, candles, and practical items such as soap and toothpaste, all presented to monks and novices as acts of devotion and respect. This ritual not only helps participants cultivate spiritual merit but also reinforces community bonds and promotes the values of generosity and compassion central to Buddhist teachings.

3.6.9. Plants Used in Bun Khao Pradap Din (Month 9)

Bun Khao Pradap Din is a meaningful tradition focused on making merit for the deceased. According to local beliefs, on the 14th waning day of the 9th lunar month, the gates of hell open, allowing the spirits of the deceased, and ghosts and other spirits from hell, to enter the human world. In response, the Bun Khao Pradap Din ceremony was established as a way for the living to offer food and make merit on behalf of these spirits. This practice not only honors the departed but also extends compassion to poor and hungry stray animals, ensuring that all beings are nourished during this sacred time.

The offerings mainly consist of food and fresh fruits, carefully arranged on banana leaves. The plants and foods included are diverse and plentiful, × Thyrsocalamus liang, Allium cepa, Allium sativum, Annona squamosa, Arachis hypogaea, Areca catechu, Capsicum frutescens, Carica papaya, Cassia fistula, Citrus × aurantiifolia, Citrus maxima, Cocos nucifera, Cucurbita maxima, Dioscorea pseudotomentosa, Ipomoea batatas, Lansium domesticum, Musa acuminata, Nicotiana tabacum, Oryza sativa, Pachyrhizus erosus, Pandanus amaryllifolius, Piper betle, Psidium guajava, Saccharum officinarum, Selenicereus undatus, Sesamum indicum, Solanum torvum, Spondias mombin, Tagetes erecta, and Vitis vinifera. After the ceremony, some of the food may be left unused and eventually spoil, attracting animals or insects in the surrounding area.

3.6.10. Plants Used in Bun Khao Sak (Month 10)

The Khao Sak ceremony, celebrated on the 15th day of the waxing moon, involves villagers preparing a food tray that includes sticky rice, dried foods such as grilled fish and meat, fermented fish chili paste, and small rice bags dedicated to deceased relatives for merit-making at the temple. In this study, we identified the following plants packed within small, handcrafted containers made from banana leaves: × Thyrsocalamus liang, Allium cepa, A. sativum, Annona squamosa, Antidesma puncticulatum, Arachis hypogaea, Areca catechu, Capsicum frutescens, Carica papaya, Cassia fistula, Citrullus lanatus, Citrus × aurantiifolia, C. × aurantium, C. maxima, Cocos nucifera, Colocasia esculenta, Cucumis sativus, Cucurbita maxima, Dioscorea hispida, D. pseudotomentosa, Garcinia mangostana, Ipomoea batatas, Irvingia malayana, Lansium domesticum, Malus domestica, Mangifera indica, Musa acuminata, Nephelium lappaceum, Nicotiana tabacum, Ocimum × africanum, Oryza sativa, Pachyrhizus erosus, Pandanus amaryllifolius, Piper betle, P. sarmentosum, Psidium guajava, Punica granatum, Tabernaemontana divaricata, Tagetes erecta, Vigna unguiculata subsp. sesquipedalis, Vitis vinifera, and Zea mays.

3.6.11. Plants Used in Bun Ok Phansa (Month 11)

The Ok Phansa ceremony marks the conclusion of the Buddhist Lent, a significant period during which monks retreat to temples or designated places to fulfill their vows. On this day, Buddhists engage in various acts of merit-making, including giving alms in the morning, offering food to monks, and visiting temples to listen to sermons. These practices often continue into the following day. Common offerings to the Buddha include flowers of Gomphrena globosa, Calotropis gigantea, Tabernaemontana divaricata, Tagetes erecta, Nelumbo nucifera, and Jasminum officinale, along with leaves of Pandanus amaryllifolius, incense, and candles—symbols of devotion and gratitude within the community. This study found that the lotus (Nelumbo nucifera) is prominently used in various rituals, as it is regarded by Buddhists as a symbol of purity and sacredness.

A cherished tradition involves preparing Khao Thip to offer to monks, to honor the Triple Gem, and to dedicate merit to the deceased. Khao Thip, a special rice dish, is made with a variety of ingredients, including Colocasia esculenta, Cocos nucifera, Ipomoea batatas, Cucurbita maxima, Arachis hypogaea, Glycine max, Vigna radiata, Maranta arundinacea, Musa acuminata, Sesamum indicum, Oryza sativa, Saccharum officinarum, and Zea mays, which create a sweet and aromatic flavor. This practice not only nourishes the monks but also serves as a meaningful expression of respect and gratitude, connecting the living with their ancestors and the spiritual community.

3.6.12. Plants Used in Bun Kathin (Month 12)

After the conclusion of Buddhist Lent, Buddhists come together to participate in a significant merit-making event known as the Kathin ceremony, which fosters unity within the community. Kathin refers to the new robes presented to monks, symbolizing renewal and support. The term originates from Pali, meaning “a stretching frame” or “a wooden frame,” historically used to stretch fabric for sewing robes.

The ceremony, known as “offering Kathin robes,” provides an opportunity for the Sangha to receive new garments, allowing monks to replace old or worn-out robes. This annual event, held within a specific period after the end of Lent, emphasizes communal harmony and collective merit-making. During the Kathin ceremony, offerings are made alongside a beautifully decorated display of Kathin robes, often accompanied by a variety of fruits such as Ananas comosus, Benincasa hispida, Carica papaya, Citrus maxima, Cocos nucifera, Musa acuminata, Psidium guajava, and Saccharum officinarum.

In addition to fruits, various flowers are offered in reverence to the Buddha, symbolizing purity, beauty, and the transient nature of life. Common floral offerings include Nelumbo nucifera, which holds particular spiritual significance in Buddhism as a symbol of enlightenment and purity. Other frequently used flowers include Calotropis gigantea, Gomphrena globosa, Tabernaemontana divaricata, and Tagetes erecta, each contributing its own symbolism and fragrance. These floral offerings enrich the spiritual ambiance and reflect a heartfelt expression of devotion and gratitude within the Buddhist tradition.

4. Discussion

The findings from this study provide significant insights into the intricate relationships between plant species diversity, cultural practices, and ritual beliefs in Roi Et Province, Northeastern Thailand. The documentation of 80 plant species across 73 genera and 42 families emphasizes the rich ethnobotanical knowledge embedded within local communities. It highlights not only the wide variety of plants used in the Heet Sip Song ceremonies but also their profound cultural, symbolic, and functional roles in the rituals that are an integral part of the local belief system.

The diversity of plant species identified in this study demonstrates the deep connection between local horticulture and cultural practices. Many of the species used in the Heet Sip Song ceremonies are intentionally cultivated in home gardens, temple grounds, and communal spaces, indicating an active engagement in horticultural management. For example, plants such as Oryza sativa, Musa acuminata, Saccharum officinarum, and Curcuma longa are not only essential to the ceremonies but are also grown through traditional agricultural techniques passed down through generations. These practices include rotational cropping, intercropping with vegetables or herbs, and organic fertilization using compost derived from plant and animal waste. In contrast, several plant species are harvested from nearby forests, rice paddies, and natural habitats, reflecting a continued reliance on wild plant resources. Species like Chromolaena odorata, Dipterocarpus intricatus, and Butea monosperma are typically collected seasonally and only when specific rituals call for their use. Local communities exercise ecological knowledge in their wild harvesting practices—avoiding overharvesting, selecting mature plants, and harvesting at optimal times to ensure plant regeneration. For example, wildflowers and leaves used for decoration during Bun Bang Fai are gathered shortly before the ceremonies to preserve freshness while minimizing environmental disruption. The distinction between cultivated and wild plant sources also reflects a dual strategy in resource use: cultivation ensures the availability of key ritual plants throughout the year, while wild harvesting offers flexibility and access to less frequently used or context-specific species. This integration of horticultural and foraging practices illustrates a sustainable approach to plant use that supports both ritual needs and biodiversity conservation [23,24,25]. The presence of species from different ecological zones—ranging from cultivated plants to wild species—also underscores the adaptability of local communities in utilizing available plant resources for ritual purposes [26,27].

Importantly, the most commonly used plant families, such as Fabaceae, Poaceae, and Arecaceae, highlight the role of agriculturally significant crops in the Heet Sip Song ceremonies. Oryza sativa and Saccharum officinarum exemplify the centrality of staple crops in both daily life and spiritual life, reinforcing the idea that agricultural products are symbolic of life cycles, fertility, and prosperity [28,29]. This aligns with the broader ethnobotanical literature, which often links agricultural plants to concepts of fertility and life-sustaining forces in indigenous cultures [30,31].

The Cultural Significance Index (CSI) and the Species Use Value (SUV) analyses further illustrate the varying levels of importance assigned to different plants in the Heet Sip Song ceremonies. It is evident that a small number of plant species, such as Musa acuminata and Oryza sativa, hold high cultural value, serving both functional and symbolic roles across a wide range of ritual activities [6,32,33]. These species’ high CSI and SUV values underscore their centrality in the ritual context, where their presence is crucial for conveying meaning related to fertility, abundance, and spiritual connection [34,35].

In contrast, plants with lower CSI and SUV values, such as wild species like Chromolaena odorata and Dipterocarpus intricatus, serve more specialized or occasional functions. These plants may be used in specific rituals or for symbolic gestures, highlighting the context-specific nature of plant usage in the Heet Sip Song ceremonies. The range of plant species utilized reflects the local communities’ selectivity in incorporating plants that align with the ritualistic demands of each event, as well as their ecological knowledge in sourcing plants from different environments [36,37].

While quantitative metrics such as the Cultural Significance Index (CSI) and Species Use Value (SUV) offer useful insights into the relative importance and prevalence of plant species, they can oversimplify the complex cultural relationships and symbolic meanings that certain plants embody. For example, high CSI values for staple crops like Oryza sativa primarily reflect their widespread use rather than deeper ritualistic or symbolic significance. To gain a more comprehensive understanding, future research should. integrate qualitative ethnographic approaches that explore the specific cultural contexts, rituals, and beliefs associated with key species. Such mixed-method frameworks would provide a richer, more nuanced interpretation of plant cultural significance beyond numerical indices.

The Genera Use Value (GUV) analysis, which is derived from the aggregated Species Use Value (SUV) of plants within each genus, reveals that certain plant genera, notably Oryza, Saccharum, and Zea, dominate both in cultural importance and frequency of use. For these specific highly utilized genera, there was an observed alignment between their high GUV and high Relative Frequency of Citation (RFC), suggesting that such commonly used plants are also the most widely recognized and culturally significant within local communities [38,39,40]. This observed alignment underscores the concept of cultural transmission, where frequently used plants are more likely to be remembered and passed down through generations. As seen with species like Oryza sativa and Saccharum officinarum, such plants become deeply ingrained in the cultural identity of the community [14,28,41]. However, it is crucial to note that while these highly prominent genera demonstrate this strong co-occurrence, a comprehensive linear correlation analysis at the species level (SUV vs. RFC across all studied species) revealed a negligible relationship (r = −0.01795), indicating that this strong alignment is not a generalized linear pattern across the entire ethnobotanical landscape.

Furthermore, the study also demonstrates the presence of species with lower RFC values, such as Ocimum × africanum and Capsicum frutescens, despite their diverse uses as indicated by their Species Use Value contributions. This variation in plant use and citation frequency, where high use value does not always translate to universally high recognition, highlights the localization of cultural practices. Certain species might be more significant in particular villages or among specific groups within the community, or their uses might be more specialized, which can contribute to a lower overall linear correlation between use value and citation frequency [42,43].

The significant role of 200 local interviewees in contributing knowledge underscores the wealth of traditional botanical knowledge maintained by local communities [44]. The use of semi-structured interviews and direct observations allowed for the collection of both qualitative and quantitative data, providing a holistic understanding of plant use in these ceremonies [45]. These findings align with the growing recognition of the importance of local knowledge in preserving plant biodiversity and managing agricultural systems sustainably [46,47]. Local communities’ understanding of plants and their relationship with the spiritual world is not only an essential aspect of maintaining cultural identity but also a vital tool for the conservation of plant species [48].

In addition, the study reveals how cultural practices such as the Heet Sip Song ceremonies contribute to maintaining plant species diversity by encouraging the continued use, cultivation, and transmission of knowledge about various plant species—many of which are locally grown or managed in home gardens. While some species are harvested from the wild, communities often engage in sustainable practices or cultivate key species to ensure their availability for future use, thereby reducing pressure on natural populations [24,49]. Rituals often involve specific planting and harvesting practices, which help preserve plant populations and ensure their availability for future generations [50]. This highlights the reciprocal relationship between culture and biodiversity, where plant knowledge contributes to the survival of both species and cultural heritage [51].

The ceremonies themselves reflect seasonal rhythms and agricultural cycles. In the early months, such as Bun Khun Lan (Month 2) and Bun Khao Jee (Month 3), rituals focus on rice harvesting and preparation, using rice-related plants like Colocasia esculenta, Ipomoea batatas, Saccharum officinarum, and Sesamum indicum. These plants embody themes of fertility, nourishment, and gratitude [52]. In contrast, mid-year ceremonies like Bun Bang Fai (Month 6), intended to invoke rainfall, incorporate bamboo (Bambusa burmanica), rattan (Calamus caesius), and plant species that are burned to produce charcoal, which is then used as a component of gunpowder in traditional rockets for the Bun Bang Fai festival such as Chromolaena odorata and Erythrina variegata to construct the traditional rockets (Bung Fai) are large, handmade projectiles constructed using bamboo or PVC tubes filled with gunpowder, launched during cultural festivals—particularly the Bun Bang Fai (Rocket Festival)—as a way to celebrate local traditions and invoke rain—indicating a close tie between ecological knowledge and ceremonial practice [53].

Several ceremonies also reflect beliefs related to spirits and the afterlife. Bun Samha (Month 7) and Bun Khao Pradap Din (Month 9) feature elaborate plant offerings, including herbs, vegetables, and fruits such as Allium cepa, Citrus spp., Annona squamosa, and Carica papaya, symbolizing abundance and the transference of merit to the spiritual realm. These offerings are carefully gathered and arranged into banana leaves container— to show respect and devotion. The plants used are chosen for their symbolic meanings, cultural importance, and local availability. These practices illustrate a profound respect for the unseen world and ancestral spirits, reinforcing the social and moral fabric of the community. After the rituals, the offerings are generally left at the ritual site as symbolic gifts to the spirits or allowed to naturally decompose [54].

Another critical insight is the prominent use of local plant resources for crafting ceremonial items and decorations. For example, × Thyrsocalamus liang is commonly used to create bamboo containers, skewers, and poles. Its recurring use across months suggests both its material utility and cultural symbolism [13,55]. Similarly, the use of wildflowers and foliage—such as Butea monosperma, Getonia floribunda, and Dipterocarpaceae members—in decorating altars and pavilions reflects seasonal floral availability and deep aesthetic sensibilities rooted in the local landscape [56].

Moreover, certain ceremonies—particularly Bun Kathin (Month 12) and Bun Ok Phansa (Month 11)—emphasize collective merit-making (the act of performing good deeds or religious activities, such as offering alms or participating in rituals, to accumulate spiritual merit for oneself or others in accordance with Buddhist beliefs) and generosity, as demonstrated by the rich assortment of fruits and flowers used in offerings. These rituals underscore the symbolic role of plants in fostering communal bonds and embodying Buddhist values of giving, mindfulness, and impermanence [28].

Taken together, the findings reveal that the use of plants in the Heet Sip Song traditions is not arbitrary but rather highly intentional, informed by local ecology, spiritual belief systems, and seasonal availability. This dynamic plant–ceremony relationship provides a window into the cultural biocentrism of Roi Et communities, where plants are seen not merely as resources but as sacred partners in maintaining social and spiritual harmony.

The heat map (Figure 3) presents the seasonal intensity of ceremonial plant use for 80 species, using a color scale from blue (no use) to red (frequent use, value = 4). The data reveal that the rainy season is the most active period, with a notable concentration of orange and red shades. This suggests that many species are commonly incorporated into ceremonies during the rainy season, likely due to increased availability of flowers, fruits, and other plant parts, as well as the alignment with key cultural festivals.

In contrast, the summer and winter seasons show predominantly blue to light blue hues, indicating lower overall usage. However, certain species—such as Curcuma longa, Musa acuminata, and Tagetes erecta—maintain moderate to high use across multiple seasons, reflecting their year-round availability or significant ritual value. The clear seasonal variation in plant use underscores the interdependence between cultural practices and ecological cycles, with the rainy season playing a central role in shaping ceremonial plant selection.

The results from this study emphasize the need to preserve both the plant species that are used in the Heet Sip Song ceremonies and the traditional knowledge that sustains their use. As global challenges such as climate change and habitat loss continue to threaten biodiversity, the role of traditional knowledge in sustainable resource management becomes even more critical. Local rituals and practices not only foster a deeper connection with nature but also provide effective means of conserving endangered plant species that may otherwise be overlooked by formal conservation strategies [16,57].

Future research could further explore the specific ecological relationships between these plants and the environments in which they thrive, and the influence of modern agricultural practices on the availability of ritual plants. Comparative studies between regions or communities that perform similar rituals could provide valuable insights into the universality and variation in plant uses in ritual contexts.

It is important to acknowledge the specific scope and inherent limitations of this ethnobotanical study. Our primary objective was to document the traditional uses and community recognition (Species Use Value and Relative Frequency of Citation) of plant species within the local community context, relying on informant interviews rather than ecological field surveys. Consequently, this methodology did not encompass a quantitative assessment of environmental pressures, such as deforestation or climate change, affecting the documented plant species in their natural habitats. Furthermore, a formal evaluation of species’ conservation status according to criteria like the IUCN Red List was beyond the scope of this community-centric research.

Despite these methodological boundaries, this study makes a significant contribution by comprehensively identifying culturally important and highly utilized plant species, which is a crucial first step in any conservation initiative. The findings provide valuable baseline data on species that are deeply integrated into local life and knowledge. We strongly recommend that species identified with high cultural use values be prioritized for subsequent ecological and conservation-focused research. Such future studies could involve detailed field surveys to assess population dynamics, quantify specific threats to these species in their wild populations, and ultimately inform more targeted conservation strategies and Red List assessments.

Additionally, given the community’s reliance on these plant resources, we suggest that nearby forest areas that are not currently under formal protection be considered for designation as Other Effective Area-Based Conservation Measures (OECMs) to ensure the long-term sustainability of biodiversity and traditional resource availability.

5. Conclusions

This study explores the intricate relationships between plant biodiversity, cultural heritage, and ritual practice within the Heet Sip Song traditions of Roi Et Province, Northeastern Thailand. By documenting 80 plant species across a range of genera and families, the research underscores that plant use within these ceremonies encompasses not only utilitarian purposes but also symbolic, spiritual, and culturally embedded meanings. While the findings suggest that traditional ecological knowledge plays a role in shaping the selection and use of plants in ritual contexts, a more in-depth analysis would be necessary to fully understand the complexities and transmission of this knowledge system.

The application of quantitative indices—such as the Cultural Significance Index, Species Use Value, Genera Use Value, and Relative Frequency of Citation—provides insight into the relative importance of various species. However, these tools primarily capture utilitarian and observable dimensions of plant use, and they may not fully reflect the broader symbolic, emotional, or existence-based values associated with traditional ecological knowledge. For example, the prominence of cultivated or commercially valuable species such as Oryza sativa and Saccharum officinarum, while significant to both agriculture and ritual, may also indicate ongoing shifts in local practices, including processes of species substitution or gradual knowledge erosion.

Nevertheless, the integration of plants into ceremonial life appears to support culturally meaningful relationships with the local environment. The cyclical and community-based nature of these ceremonies may foster forms of environmental awareness and stewardship that contribute—albeit indirectly—to the maintenance of local plant diversity. Rather than asserting a direct conservation effect, this study highlights the potential of ritual practice to sustain both culturally significant species and the ecological contexts in which they are embedded.

As global environmental and cultural transformations continue, preserving and understanding traditional knowledge systems such as those found in the Heet Sip Song rituals remains a vital task. These systems offer nuanced, context-specific insights into human–environment relationships that deserve further investigation—especially in connection with ecological sustainability and biocultural resilience. Future research should aim to deepen the understanding of traditional ecological knowledge, examine how it evolves in response to external pressures, and explore how it might inform conservation strategies in ways that are inclusive of local perspectives and cultural values.