Some species of the pantropical orchid genus Vanilla are widely used in a variety of industries, including food, pharmaceuticals, cosmetics, tobacco and traditional crafts. Vanilla siamensis Rolfe ex Downie is a leafy vine endemic to Thailand. Ecological factors affecting the distribution of V. siamensis are still unknown. This study aims to examine the distribution of V. siamensis in natural conditions as well as to investigate the impact of environmental factors on its distribution in forest ecosystems. In order to achieve the proposed objectives, a study was conducted in a tropical moist forest at Khao Soi Dao Wildlife Sanctuary. The plant community and environmental factors where V. siamensis occurs were assessed in two study sites. At each site, two plots (10 m×50 m) were set up near waterfalls (NWA) and in adjacent forest areas (AFA).
A total of 66 species of woody plants were recorded in the study plots. Higher values for importance value index (IV) were found for Pterygota alata (Roxb.) R.BR., Diospyros defectrix H.R. Fletcher, Strombosia ceylanica Gardner, Alchornea rugosa (Lour.) Mull. Arg., and Diospyros transitoria Bakh. The highest number of tree species, number of individuals, Shannon diversity index and evenness were found at the NWA compared to the AFA. A total of 19 woody plant species act as support trees of V. siamensis in the plant families Malvaceae, Annonaceae and Achariaceae. The results showed that V. siamensis might possible well adapted in the NWA study zone which contained a higher level of soil moisture content compared to the AFA. All results revealed that the distribution of V. siamensis in the study areas could be impacted by tree hosts. All tree host species have a larger diameter at breast height and thicker and softer bark when compared with non-tree hosts.