While I was on the island of Kaua’i earlier this year, I discovered a small grove of Golden Trumpet Trees (Tabebuia chrysotricha) growing along a roadside near Poipu. The showy yellow flowers were what initially attracted this photographer, but when I hiked over to the grove of trees, I discovered something that REALLY peaked my interest: a Candelabra Tree cactus (Euphorbia sp.) perched high on the trunk of one of the Golden Trumpet Trees. The cactus was growing happily in the crotch of the tree’s two largest branches (see photo).
Epiphytic cactus (Euphorbia sp.) ensconced in a tree, Poipu Road, south Kaua'i.
Where I come from in the Desert Southwest of North America, self-respecting Euphorbia don’t behave like this! They stay firmly (and properly) rooted in soil, on the ground. I assumed that some joker had purposely uprooted this “sapling” and replanted it (along with a small handful of soil) in the crevice between the two branches of the tree. I began to notice that several of the other trees had similar Euphorbian “passengers” firmly attached to them. More mature cacti rooted in the ground amongst the trees were, in some cases, growing as tall as their non-cacti grove-mates.
Cacti rooted in soil in grove of trees, Poipu Road, Kaua'i.
“Could it be true?”, I thought: “Are some cacti ‘parasitic’?” Then a long-forgotten bit of terminology from my college biology classes popped into my head: epiphytes! After taking numerous photographs of the spectacular trees and their odd little succulent companions, I returned to Maui and began to fill in the gaps in my knowledge of epiphytic plants with some much-needed research. I soon discovered that I had photographed many different species of epiphytes in the islands without even knowing it.
What is an Epiphyte?
An epiphyte is any plant that grows on another plant; occasionally, epiphytes may also take root on a non-soil substrate, like a building’s rain gutters or a wooden fence post. A true epiphyte uses photosynthesis for energy and derives water and nutrients from the air, rain water, or cloud moisture on the host’s surface. It is believed that more than 15,000 species of epiphytes inhabit the tropical regions of the world; this figure doesn’t account for hundreds of epiphytic species that are, as yet, uncatalogued.
Maile-Scented Fern attached to a tree branch, Nahiku, east Maui.
Some Defining Characteristics of Epiphytes
Some species of epiphytes have developed specialized structures such as cups and scales to collect or hold moisture. However, many of Hawaii’s epiphytes seem to thrive in the numerous cracks, grooves, and pockets of a host plant’s surface (the bark of a tree, for example) where organic debris such as decaying leaves, rotting twigs, and bird droppings readily accumulates, providing an excellent medium to stimulate initial growth. True epiphytes (called “autotrophic epiphytes” or “holoepiphytes”) only employ roots for attaching themselves to a surface, NOT for extracting nutrients. These species never root in soil and remain attached to their hosts for their entire life cycle. They only use their hosts as a means of physical support and do not derive nutrition from them, as do parasitic plant species. However, many species of epiphytes grow so thickly upon the trunks, branches, and leaves of host plants that they restrict their growth to the point that the host plant is severely damaged or dies.
Harts Tongue Fern growing on a tree trunk, Kula, Maui.
Botanists do love their jargon, and so to add a bit of confusion to the meaning of the word “epiphyte”, more subdivisions were defined. A hemiepiphyte is a plant species that is exclusively epiphytic for one stage of its life cycle, but then becomes rooted in the soil during another stage. A primary hemiepiphyte is epiphytic only during the first half of its life cycle, while a secondary hemiepiphyte assumes an epiphytic lifestyle in the latter half of its life cycle. There’s more of this sort of lumping and splitting of terminology, but I think I’ve muddied the waters sufficiently for now.
Epiphytes exist in habitats where they are subjected to extended periods of environmental stress; these stresses are the driving forces behind their exceptional evolutionary diversification and adaptability. Epiphytes such as many species of orchids live in the upper canopy of the forest where water, minerals, and other nutrients are in short supply.
Orchid growing in the branches of a tree, Kula, Maui.
Epiphytic orchids have evolved specialized structures such as thick stems to store water and leaf hairs whose function is to close the plant’s stomata when dry conditions set in. Many species of bromeliads (known as “tank” bromeliads) are able to catch and hold water in their broad, stiff, upturned leaves.
Orchid growing on tree trunk, Poipu, Kaua'i.
Some species of secondary hemiepiphytes have become adapted to the low-light environment of the forest understory. Shaded by gigantic trees of the forest canopy, these species (such as philodendrons) spend their early lives rooted in the soil. As they mature, they grow by sending out long tendrils (internodes) along the ground, seeking the shade of the trunk of a large tree where the plant continues to grow. This behavior is known as skototropism.
Many epiphyte species occupy microhabitats that dry rapidly. Such an environment promotes strong selection among plant species for resisting drought conditions. One of the most common survival traits developed by epiphytes is Crassulacean Acid Metabolism (CAM), a system of enhanced photosynthesis that enables a substantial increase in the efficient use of water. CAM epiphytes characteristically have thick leaves or succulent green stems.
Epiphyte bromeliads (Tillandsia gardneri) growing on a dead tree trunk, Kula, Maui.
Bird’s Nest ferns (common in Hawaii) form complex tangles of roots and leaves that trap leaf-litter and other dead-and-decaying plant material that accumulates and forms humus. Species displaying this characteristic are called trash-basket epiphytes. The genus Asplenium sp. is an example of this type of epiphyte common in Hawaii’s rain forests.
Birds-Nest Ferns growing on a Coconut Palm trunk, Keanae Arboretum, east Maui.
Some Advantages of the Epiphyte Lifestyle
Epiphytes have a great many competitive advantages over other rain forest plants.
- The epiphytic lifestyle affords them access to more direct sunlight, a greater number of animal pollinators (birds and flying insects that reside in the forest canopy), and the capability for wider seed dispersal via the wind.
- Epiphytic plants attached to host trees and other plants that reside high in the forest canopy have an advantage over plant species restricted to the ground where there is less light and herbivorous animals (feral pigs, for example) may be more active.
- Epiphytes tend to produce far more seeds than species that root in the soil because large numbers of their seeds fail to reach a suitable substrate in which to germinate. For this reason, many of the seeds of epiphyte species have evolved specialized structures such as wings, glider-like air foils, and parachutes to enhance wind-borne dispersal. Many of the epiphyte species that bear fleshy fruits have developed the ability to crowd thousands of seeds into a single berry.
What About My “Cactus-In-A-Tree”?
It turns out that many species of cacti are able to adopt the epiphyte life style, and it is not unusual to find them growing on trees of tropical rain forests. The spines of tropical epiphytic cacti are much reduced or totally absent, presumably because over-sized spines would prevent sunlight from reaching the photosynthetic stems. Most species of epiphyte cacti have fewer ribs than their desert-dwelling counterparts. Their leaves and stems are more elongated, which enhances light absorption.
Tropical epiphyte cactus (Euphorbia sp.) living happily in the trees on Kaua'i.
So, unlike money, cacti DO grow on trees. Amazing…
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Tags: autotrophic epiphytes, bromeliads, cacti, cactus, CAM, Crassulacean Acid Metabolism, epiphitic, epiphitic cacti, epiphyte, epiphytes, Euphorbia, hemiepiphyte, holoepiphytes, Kauai, linkedin, Maui, orchid, orchids, parasitic plants, plant behavior, plants of Hawaii, plants of Kauai, plants of maui, skototropism
