An epiphyte is a plant that grows harmlessly upon another plant, utilizing the host for physical support but not for nourishment. The term itself is derived from Greek, meaning “upon a plant.” Because they have no direct connection to the soil, these organisms are commonly referred to as “air plants.” Epiphytes derive their moisture and mineral nutrients entirely from the surrounding atmosphere, including rain, fog, dew, and accumulated organic debris. They are most diverse and abundant in humid, tropical environments like rainforest canopies, where competition for sunlight is high.
Defining the Epiphytic Lifestyle
The relationship between an epiphyte and its host plant is a form of commensalism, where the epiphyte benefits while the host remains unaffected. The epiphyte gains height, providing access to intense sunlight filtering through the dense forest canopy.
Epiphytes are autotrophs that produce their own food through photosynthesis. They differ from parasitic plants, such as mistletoe, which actively extract water and nutrients from the host’s vascular system. Epiphytes simply use the host plant as an anchor and never penetrate the host’s tissues to steal resources.
This aerial existence provides access to the light-rich zone above the forest floor, but the trade-off is a lack of constant access to soil-based water and nutrients. Epiphytes must possess specialized mechanisms to manage the intermittent availability of these resources in the canopy.
The biomass of epiphytes can be substantial, sometimes weighing several tonnes on a single rainforest tree. These plants play a role in nutrient cycling and water retention, catching rainfall and debris that would otherwise wash directly to the forest floor.
Specialized Adaptations for Resource Acquisition
To survive without soil, epiphytes have evolved specific anatomical and physiological features focused on water and nutrient capture. A primary adaptation is the development of aerial roots, common in orchids. These roots are often covered by a thick, white or silvery, spongy layer of dead cells called the velamen.
The velamen functions like a sponge, rapidly absorbing atmospheric moisture, dew, and water running down the host tree’s bark. Once saturated, the velamen becomes transparent, allowing light to reach internal photosynthetic tissues in some species. This outer layer also protects the root from rapid water loss during dry periods.
Many Bromeliaceae species, such as certain Tillandsia, form a rosette of tightly overlapping leaves. This structure creates a watertight “tank” or “cistern” that collects and holds water, along with falling debris and animal waste. Specialized cells at the base of the leaves absorb the water and nutrients released from the decaying organic matter within the tank.
In some tank bromeliads, this reservoir supports various arthropods and amphibians, whose waste further enriches the plant’s nutrient supply. Other epiphytes, like Spanish moss (Tillandsia usneoides), lack traditional roots entirely and rely on tiny, silvery leaf scales called trichomes. These trichomes absorb water and mineral ions directly from the humid air and rain.
Many epiphytes, including a significant proportion of orchids, also employ Crassulacean Acid Metabolism (CAM) photosynthesis. This metabolic pathway allows the plant to open its stomata to take in carbon dioxide only at night, drastically reducing water loss from transpiration during the day.
Classification and Common Examples
Epiphytic plants are broadly categorized based on their life cycle and dependence on the host plant. The first group is the holoepiphyte, or true epiphyte, which spends its entire life cycle—from germination to reproduction—exclusively on the supporting plant or structure. These plants never establish root contact with the ground.
Many species of orchids, the largest family of flowering plants, and the well-known air plants (Tillandsia) exemplify the holoepiphytic lifestyle. They rely entirely on the aerial environment, leading to the specialized adaptations necessary for life above the soil.
The second major category is the hemiepiphyte, a plant that is epiphytic for only a portion of its life. Primary hemiepiphytes germinate high in the canopy and initially grow as true epiphytes. They later send down long aerial roots that eventually reach and penetrate the soil, allowing them to access ground-based water and nutrients.
The strangler fig is a classic example of a primary hemiepiphyte, beginning its life high in a host tree’s crown before its roots descend and thicken, sometimes encircling and outcompeting the host. Mosses, ferns, and lichens are also common epiphytes, often forming dense mats that contribute significantly to the biodiversity and water retention of the forest canopy.