Two ways of using a tree
Trees are not just plants; they are also real estate. In tropical forests, two groups of plants take advantage of trees without being trees themselves. Lianas are woody climbing vines that start rooted in the ground and use other plants to reach the light. Epiphytes are plants that live on trees, high above the soil. They sit on branches and trunks for support, not as parasites, and take in water and nutrients from rain, mist and debris instead.
The two groups are often confused because both cover tree crowns, but their lifestyles are quite different.
Lianas: climbing to the light
A liana germinates on the forest floor, where light is scarce. Rather than invest in a thick self-supporting trunk, it grows a flexible stem that climbs up a host tree using tendrils, hooks, twining stems or adhesive roots. Once in the canopy, it spreads leaves across the crowns of many trees and can link them together.
Lianas can be very long, with stems that wander and loop between trees, sometimes reaching considerable length. They are common in tropical forests worldwide, and are generally more abundant in seasonally dry tropical forest than in the wettest forests, although patterns vary. Their stems are often unusual inside, with wide water-conducting vessels that carry water efficiently. This lets a thin stem supply a big mass of leaves.
Effects on host trees
Lianas are not harmless passengers. A heavy load of vines competes with the host for light, since liana leaves grow in the upper canopy above and among the tree’s own foliage. They also compete for water and nutrients, as their roots draw from the same soil. Heavy lianas may add weight and wind resistance, which can increase the risk of branches breaking or trees falling. Some studies suggest that trees carrying many lianas grow more slowly and may be more likely to die, but the strength of this effect varies between forests and species.
Lianas also provide benefits to the forest. They create pathways through the canopy for animals such as monkeys, sloths and tree-dwelling mammals, and they produce flowers and fruit that feed wildlife, including forest birds and bats. Cutting lianas is sometimes done in forestry to help valuable timber trees grow, a practice used in some selective harvesting and restoration systems, and it can help reduce damage when a tree is felled by pulling down its neighbors.
There is also ongoing research on whether lianas are becoming more common in some tropical forests. Some studies report increases, and possible causes are debated; the evidence is not uniform across regions, so the question remains open.
Epiphytes: gardens in the air
Epiphytes include orchids, bromeliads, ferns, aroids, mosses, liverworts and lichens. A single large tree may carry hundreds of individuals from many species. They are especially abundant in humid and cloud forests, where constant moisture allows them to flourish; the article on cloud forests discusses this in more detail.
Because they have no access to soil, epiphytes have developed ways to get by:
- Water storage. Some, such as bromeliads, form a rosette of leaves that holds a pool of water. Others have thick, succulent leaves or swollen stems.
- Absorbent roots. Many orchids have roots covered in a spongy layer that quickly soaks up rain and dew.
- Catching debris. Some ferns grow in a basket shape that traps falling leaves and other matter, which rots into a pocket of soil.
- Leaf scales. Certain bromeliads absorb water and nutrients through scales on their leaves.
Effects of epiphytes and their role in the forest
In most cases epiphytes do little direct harm to the host, though very heavy loads can weigh down branches, especially when soaked with water. More often they contribute to the forest. They store large amounts of water, release it slowly and can affect the humidity of the canopy. The water pools in bromeliads support tiny ecosystems with insect larvae, frogs and other small animals. The plants also provide nectar, fruit and nesting material, and attract forest pollinators.
Dead plant material collects among the epiphytes and decays into a canopy soil, which in some forests even grows roots from the host tree itself, taking nutrients straight from the mat. This is one more way the forest recycles nutrients and keeps them out of the poor ground below.
Vulnerability
Epiphytes are sensitive to changes in humidity and temperature. When forest is cut, the exposed remaining trees often lose their epiphytes, and recovery in regrown forest can be slow because the plants need mature branches and steady moisture. Hotter or drier conditions could stress them further. Since many animals depend on them, a loss of epiphytes can have knock-on effects on the canopy community.
In short, lianas and epiphytes are not just decoration. They shape how the canopy works, and they illustrate how tightly the layers and organisms in a rainforest are connected.