Buddhism in Southeast Asia
Thailand, Cambodia, Laos & Burma

According to the layer of the Pali Canon most concerned with making merit, acts of merit are most fruitful when planted in the appropriate “field”: either the Buddha-field surrounding a relic (or, by extension, a Buddha image); or legitimate members of the monastic Sangha. The importance of the Buddha-field explains the cults of stupas, Buddha images, and Buddha amulets that thrive in the Theravadin world. The importance of the monastic Sangha as a field of merit explains the devotional cults surrounding individual monks or nuns who are believed to be noble disciples. [Buddhist Religions, 144]
General familiarity with rituals, both animistic and Brahmanical, has ... influenced the typical villager’s view of karma in that merit is often seen as residing, not in the quality of the intention behind an action, but in the proper performances of acts that are defined as meritorious. This view differs radically from the view of kamma as expressed in the earliest layer of Suttas. ... A prime example of this sort of accommodation is the typical merit-making ritual on the morning of the Uposatha, which occurs on the days of the full, new, and half moons. The primary function of the ritual is to make merit by presenting food to the monks and listening to a sermon. Monks and villagers gather in separate zones of the main meeting hall. The general mood of the gathering is cheerful. Before the food is presented, the villagers pay respect to the Buddha-image and request the Triple Refuge and Five Precepts from the chief monk.
For those who have had recourse to animist or Brahmanical [i.e. Hindu] practices since the last Uposatha service, and who believe that their allegiance to the Buddha, Dhamma, and Sangha has thus been tarnished, the request for the Triple Refuge offers the chance to renew allegiance with no questions asked. For others, it is an opportunity to renew their sense of prasada [serene confidence that one has found the way] in the Triple Gem. The request for the Five Precepts serves a similar dual function. On one level it is ritualistic: a donation is said to bear the greatest fruit if both the donor and the recipient live by the precepts. For those who have little intention of trying to observe the precepts after leaving the monastery, the act of taking the precepts puts them “in possession of” the precepts at least while the donation is being given. For those who do intend to keep the precepts, the request is an opportunity to renew their dedication and make a fresh start if they have broken any precepts since the last Uposatha.
Once the preliminaries are completed, the villagers as a group make a formal declaration of donating their offerings to the Sangha. This is because the act of making a donation to the Sangha is more meritorious than that of donating it to individual monks. ... Only then is the food formally handed to the monks, who chant blessings rejoicing in the merit the villagers have made. The monks then eat while the villagers look on, chatting informally about the events of the past week. [Buddhist Religions, 162-3]
Thailand is the only country in the world where the Buddhist monastic tradition is maintained by temporary ordination of young laymen coming from all classes of society. They can spend a few months of their life in monastic robes to deepen their spiritual life, strengthen their moral commitment, and broaden their knowledge of the Buddhist religion. They also believe that the merit of the ordination can be passed to their mothers. So each year, from the end of the summer to the beginning of the monsoon, hundreds of thousands of young Thai men are ordained in monasteries all across the country to spend a period of three months practicing Buddhism before they finally disrobe and return to lay life as religiously educated and morally formed men. [BIBE, 109]

Buddhism & Local Traditions
Wherever Theravada Buddhism has become the dominant religious ideology, it has also tended to coexist with beliefs in indigenous spirits and deities. In part this is due to the Theravada understanding of the Buddha himself: he is a “converter,” he is a “refuge,” but he is not an active “protector.” Though devotees may well turn to him in their soteriological concerns, he has basically transcended this world and does not interfere directly in the mundane concerns of people. Thus, in Sri Lanka and Southeast Asia, the Buddha occupies a position at the top or, perhaps better, “above the top” of a large pantheon of spirits who are concerned with mundane affairs. This pantheon is not always well organized, but it includes regional and local divinities, guardians of towns and villages, spirits of the dead, and demonic and autochtonous forces concerned with illness, fertility, protection, success, failure, and the like.
       The relationship of the worship of these spirits (called nat in Myanmar [Burma], and phi in Thailand and Laos) and the practice of Buddhism has long retained the interest of scholars. Although the situation can be quite complex, Buddhists in these countries do not generally see any contradiction between their veneration of the Buddha and their veneration of these deities. And though rituals toward them may vary (for example, certain indigenous spirits may be offered animal sacrifices or whiskey, while the Buddha and the more buddhaized spirits will receive only cooked food and flowers), often, in festivals, there is a mixture of elements that may baffle Westerners brought up in more exclusively monotheistic traditions. [The Experience of Buddhism, 219]

Spirits (Phi)

Everywhere in Thailand, some trees are wrapped with a cloth. Especially in temples but also in forests. It means that a spirit inhabits the tree. Of course the tree shall not be cut without warning the spirit in order to let him find another tree. [thaiworldview.com]

Spirit Houses
A spirit house or san phra phum in Thai (Thai ศาลพระภูมิ) is a shrine to animist spirits found in the Southeast Asian countries of Cambodia, Laos, and Thailand. Most houses and businesses have a spirit house placed in an auspicious spot, most often in a corner of the property. The location is often chosen after consultation with a Brahmin priest. The spirit house is normally in the form of a miniature temple and is mounted on a pillar or on a dais. The house is intended to provide a shelter for spirits which could cause problems for the people if not appeased. Offerings are left at the house to propitiate the spirits. [Spirit_house]

Bai Sii/Su Khwan
Baci (บายศรี) (pronounce Buy-see) (known by the commonly used words su kwan (สู่ขวัญ) in Thai language meaning “calling of the soul”) is an important ceremony practiced in Lao culture[1][2][3] and Northern and Isan Thai culture. Baci is an animist ritual used to celebrate important events and occasions, like births and marriages and also entering the monkhood, departing, returning, beginning a new year, and welcoming or bidding etc.[1] The ritual of the baci involves tying strings around a person’s wrist to preserve good luck, and has become a national custom. ... 
The crux of the ceremony is to invoke the kwan, which in specific terms is explained as:
An ancient belief in Laos that the human being is a union of 32 organs and that the kwan watch over and protect each one of them. It is of the utmost consequence that as many kwan as possible are kept together in the body at any one time. Since all kwan is often the attributed cause of an illness, the baci ceremony calls the kwan or souls from wherever they may be roaming, back to the body, secures them in place, and thus re-establishes equilibrium.[2]
The ceremony is performed by a senior person of the community who has been a Buddhist monk at some stage, and special arrangements are made for the occasion. The practice involves preparing the pah kwan or the flower trays and placing at a central location for people to gather around it in reverential prayers. ...
During the Baci ceremony, a white (symbolizes purity) thread of silk or cotton is tied on the right hand wrist of the individual who is being wished for his well being and good luck and also around the wrists of all guests who assemble to wish a person. The thread is first knotted before tying on the wrist of the person to be blessed and other guests. Before the thread is tied, the hand is held chest high as a mark of respect. The white thread is symbolic of “peace, harmony, good fortune, good health and human warmth and community.” The thread is worn by an individual normally for a minimum of three days and is untied thereafter (thread is not be cut). Recommended practice is to allow the thread to fall off on its own. In recent times the thread in yellow, red and black colours are also used representing particular occasion; red symbolizing bravery, yellow representing faith and black sharing a person’s loss or grief. [Baci]