Plants growing on other trees represent a diverse ecological phenomenon where one species uses another for structural support or nutritional resources. This strategy, known as corticolous or epiphyllous growth, allows plants to gain access to the sun-drenched upper canopy of a forest. The relationships between the host tree and the plant growing upon it are not uniform, ranging from a benign association to an outright parasitic attack. These plants use the host tree to gain height and capture sunlight, but their methods for obtaining water and nutrients vary significantly.
Epiphytes: Plants That Live on Air
Epiphytes are commonly known as “air plants” because they anchor themselves to a host tree purely for physical support, drawing no nutrients or water directly from its tissues. These organisms, which include the majority of plants found growing on tree trunks and branches, are particularly abundant in tropical and humid forests. They survive by capturing all their water and mineral requirements from the atmosphere.
Water is absorbed from rainfall, fog, mist, and water vapor, while nutrients are derived from dust and decaying organic debris that collects on the bark. Vascular epiphytes, such as orchids and bromeliads, have developed specialized structures to cope with the lack of soil. Many orchids possess thick, spongy aerial roots covered in velamen, a white tissue that quickly absorbs moisture and minerals.
Bromeliads, including “tank bromeliads,” often form tight, overlapping leaf bases that create a reservoir capable of holding significant amounts of water. This mechanism allows them to survive dry periods and serves as a miniature ecosystem where decaying matter provides nutrients. Non-vascular epiphytes, such as mosses and lichens, lack specialized internal transport systems. Instead, they absorb water and nutrients directly over their entire surface area, flourishing in damp environments.
Vascular Parasites and Hemiparasites
In contrast to epiphytes, vascular parasites actively invade the host tree’s tissues to steal resources. They develop a specialized root-like structure called a haustorium, which penetrates the host to establish a direct connection with its vascular system. This bridge allows the parasite to tap into the host’s internal plumbing, drawing out water, minerals, and sugars.
Parasitic plants are categorized based on their reliance on the host for nutrition. Holoparasites, such as dodder, lack the ability to photosynthesize. They steal all their fixed carbon, water, and minerals from the host through haustorial connections to both the xylem and the phloem. This total reliance makes them detrimental to the host tree’s health.
Hemiparasites, like mistletoe, retain green leaves and perform photosynthesis to produce their own sugars. They use their haustoria to tap into the host’s xylem, the tissue responsible for water transport, to steal water and dissolved mineral nutrients. By taking a constant supply of water, hemiparasites reduce the host’s growth and vigor, and large infestations can lead to increased mortality.
Lianas and Strangler Figs: Ground-Rooted Climbers
Climbers initiate their life cycle on the forest floor but rely entirely on the host for vertical support. Lianas are woody vines that spend little energy building a self-supporting trunk. Instead, they use mechanisms like twining stems, clasping tendrils, or adhesive adventitious roots to ascend the host tree and reach the canopy sunlight.
As lianas mature, their thick, cable-like stems spread across multiple trees, creating a dense network that competes with the host for light and nutrients. This competition, combined with the weight of the vines, can smother the host tree’s canopy or make it susceptible to being toppled in high winds. The liana’s growth strategy prioritizes height over structural strength, making the host’s trunk an indispensable scaffold.
Strangler figs are unique climbers that often begin life as an epiphyte, germinating from a seed deposited in a high branch by an animal. The seedling sends down aerial roots that grow along the host’s trunk toward the forest floor, rooting into the soil. Once grounded, the fig’s roots thicken and fuse, forming a lattice structure that completely encircles the host tree.
The strangler fig competes vigorously for light while its woody sheath compresses the host’s trunk, restricting the flow of nutrients and water in the vascular tissues. Over time, the host tree often dies and decays, leaving behind a hollow, self-supporting cylinder of fig wood. This aggressive growth strategy allows the strangler fig to claim the host’s vertical space and light access.