When the trunk or limb of a tree provides the substrate on which an epiphytic orchid grows, the arrangement benefits the orchid, which gets nutrients from the air and moisture from aerial roots, while the tree is unaffected. The relationship is referred to as

The symbiotic relationship that occurs between an epiphytic orchid and a tree would be referred to as commensalism.

Explanation:

Commensalism, in biology, is a relationship between two individual species in which one of the species obtains food or other benefits from the other (orchid) without either harming or benefiting the latter (the tree).