A symbiotic relationship is a relationship between two organisms that can be helpful, harmful, or have no effect. A mutualistic relationship benefits both species involved in the relationship. Commensalism is a type of a symbiotic relationship in which one species benefits while the other species is neither harmed nor helped. A parasitic relationship helps one of the species involved in the relationship but harms the other organism.
The symbiotic relationship that occurs between an orchid and a tree would be classified as commensalism. Most orchids are epiphytes, which mean that that they grow on other plants. This benefits the orchids because they can grow on top of the canopy, which prevents the orchids from being walked on or eaten by ground-dwelling organisms. Being in the canopy also allows the orchids to receive more sunlight for photosynthesis. However, the orchids receive nutrients from the air and compost on the trees. Thus, they do not harm the plants on which they grow by drawing nutrients from them.