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Epicurean Epiphytes—How Canopy-Dwelling Plants Acquire Their Water and Nutrients
Plants & Gardens News | Volume 22, Number 1 | Spring 2007
by Niall Dunne
An epiphytic tillandsia in BBG's Robert W. Wilson Aquatic House.
While strolling through the Tropical Pavilion of BBG's Steinhardt Conservatory recently, I stopped to admire our epiphyte "tree"—a tall, thick-branched stump, wildly festooned with beautiful bromeliads, orchids, ferns, jungle cacti, and aroids, and representing a somewhat idealized plant community high up in the rainforest canopy. Many of the epiphytes (plants that grow on other living plants) up there seemed to be cultivars of wild species, but it was exciting to see them jostle and compete with each other for every scrap of space on the tree.
I was taking a lunch break but had some time to kill while the staff at the Garden café toasted me a ham-mozzarella-and-arugula panini. Food was on the brain. Staring at the arboreal splendor above me, I wondered: How in the name of fancy Italian sandwiches do these plants acquire nutriment? (True epiphytes are never in contact with the soil at any stage in their life cycle, and—unlike the mistletoes—they don't parasitize the vascular systems of their host trees.)
"Epiphytism has evolved many times in different lineages of vascular plants," says David Benzing, professor emeritus of biology at Oberlin College, in Oberlin, Ohio, and one of the world's foremost authorities on epiphytes (nonvascular groups such as mosses and lichens excluded). "Consequently, the mechanisms evolved to compensate for lack of root access to soil are diverse."
Among these mechanisms are many of the features that make epiphytes such ideal plants for the indoor gardener: slow growth, compact habits, and a whole suite of adaptations—such as waxy leaves and succulent stems—that allow them to store water and persist through periodic droughts. Also included are many fascinating strategies for obtaining water and mineral ions, such as the use of specialized and often bizarre vegetative structures to capture nutrient-enriched rainfall, symbiosis with insects and other animals, and—at least in the case of one species—a primitive form of carnivory.
Dr. Benzing kindly advised me, for the sake of organizing this article, to narrow my focus to four representative species/types: a trash-basket orchid, a tank bromeliad, an atmospheric bromeliad, and an ant-fed member of the family Rubiaceae. It must be added, though, that the feeding strategies described are not exhaustive, and no one group of epiphytes has a monopoly on any of them. For instance, some ferns (such as those in the staghorn genus, Platycerium) impound water and litter in "tanks," while some orchids (such as Schomburgkia tibicinis) are nourished by the activities of ants.
Orchid Adaptations
What better place to begin than with the tropical orchids, which form the largest contingent of epiphytic plants. As Dr. Benzing writes in his book Vascular Epiphytes: "About two out of three epiphytes are orchids; at least 70 percent of the family [Orchidaceae] are canopy adapted." Two out of three ain't bad—it adds up to roughly 14,000 epiphytic species.
Climate varies in the tropics, and epiphytic orchids have developed methods of water and nutrient storage accordingly. However, all epiphytic orchids have to deal with the cycles and vagaries of day-to-day weather and the frequent, drying breezes in the canopy habitat. To this end, they possess specially adapted aerial roots composed of a wiry central core surrounded by a thick, spongy, nonliving tissue called a velamen.
The velamen is highly absorptive but also has an insulating function, protecting the roots from desiccation. Velamentous roots allow epiphytic orchids to clasp the bark of trees and soak up rainwater or condensed fog as it runs down the tree branches and trunks.
Where do the nutrients come from? Well, sources are numerous, but the major one is probably slowly decaying organic matter (leaves, berries, carrion, bird feces, and other treats) that accumulates in tree crotches and bark, as well as among the roots, rhizomes, and leaves of the orchid plants themselves.
Some orchid species, such as those in the neotropical genus Catasetum, produce specialized short, upright roots that form a "trash basket" above the main root mass. These roots trap organic debris more efficiently, creating a miniature compost heap and a more reliable food source for the plants.
Bountiful Bromeliads
The bromeliad family (Bromeliaceae) is a diverse group of around 3,000 New World flowering plants, roughly half of which are epiphytic. Growth forms vary widely according to environmental conditions, but the typical bromeliad habit consists of a collection of wide, linear, straplike leaves with inflated bases that overlap to construct a tight rosette.
In humid forest environments, these rosettes often form single or multiple watertight phytotelmata—or tanks—capable of holding considerable amounts of fluid and litter. (The mature shoots of Glomeropitcairnia erectiflora, one of the larger species, can impound up to 20 liters of rainwater.) Tank bromeliads, such as the Brazilian Wittrockia superba, are especially popular among indoor landscapers.
Like the trash-basket orchids, tank bromeliads are essentially humus-based epiphytes; however, unlike many orchids, they are not limited to absorbing captured rainwater and nutrients released from decomposed litter solely through their roots: They also employ specialized absorptive trichomes—foliar hairs or scales—on the surfaces of their leaves. (In many bromeliads, the primary function of roots is to anchor the plants in the tree canopy.)
But the fun doesn't stop there. Tank bromeliads are famed for their ability to support complex communities of aquatic animals—including insects, frogs, and salamanders—in their water-collecting chambers. The decomposing waste products and debris of these organisms contribute to the nutrient load absorbed by the plants.
What percentage of the epiphytic bromeliad diet is vegetable- versus animal-derived? That hasn't been determined; however, a recent greenhouse experiment conducted by Brazilian researchers found that jumping spiders living and preying on other insects in the foliage of the tankless terrestrial bromeliad Bromelia balansae contribute a whopping 18 percent of the nitrogen within the bodies of their host plant. The researchers concluded that inputs from animals to epiphytic tank bromeliads may be even higher.
Does this make tank bromeliads carnivores (like pitcher plants and venus flytraps), at least in part? "Technically, no," says Dr. Benzing. "The plants are neither adapted for direct capture or killing of the animals, nor do they produce secretions in their tanks that digest prey. That said, bona fide carnivorous bladderworts—aquatic Utricularia species—are known to live in the tank water of some epiphytic South American bromeliads, and Catopsis berteroniana does seem to have a passive mechanism for causing flying insects to crash-land into its tank fluids."
An epiphytic bromeliad native from southern Florida to Brazil, Catopsis berteroniana lives high in the canopy and produces yellow-green leaves covered with a loose, whitish cuticular powder. The powder enhances the UV reflectance of the foliage, rendering it invisible to passing bugs. The insects hit the plant, tumble into the tank, and are prevented from escaping by slippery wax particles concentrated at the base of the leaves. The prey subsequently drowns and decomposes naturally. "The plant is considered to be a protocarnivore," says Dr. Benzing.
"Living on Air"
A significant number of epiphytic bromeliads—including several hundred Tillandsia species—are adapted to very dry and/or windy canopy habitats and are commonly referred to as "atmospherics" or "air plants." They are generally quite small and succulent, and their foliage (compared with that of tank bromeliads) is densely covered with scales, sometimes giving the plants a silvery appearance. Many atmospherics form compact rosettes of short, stout leaves, whereas others—such as Spanish moss (Tillandsia usneoides)—exhibit highly reduced stems and very slender leaves. Tanks are absent, and so the plants don't have recourse to humus reservoirs. Roots, when present—(mature Spanish moss plants are famously unattached; they just hang out!)—have little absorptive capacity.
So what do they live on? It's not really air (or nutrients drawn directly from the air), as some would have it. Basically, these plants rely on periodic pulses of rainfall. The rainwater washes dust particles out of the atmosphere and nutritious leachate from the bark and leaf canopy of host trees over the shoots of the plants, and the elaborate foliar scales quickly absorb what they can. Rainfall can hold significant amounts of dissolved nutrients; however, Dr. Benzing has shown in elaborate experiments with Tillandsia paucifolia in Florida that the runoff from the living canopy may be the key source of mineral ions for atmospherics.
Ant Gardens and Ant-Fed Plants
As with ants and certain plants in terrestrial habitats, symbiosis between these industrious insects and epiphytic flora is amazingly complex and varied. There are examples of ants guarding epiphytes from hungry herbivores in exchange for nectar and other rewards. More related to plant nutrition, a number of humus-based ephiphytes in the Araceae (aroid family) and Gesneriaceae are known to associate with arboreal ants in warm, humid neotropical forests by rooting in their tree-lodged "carton" nests. Put simply, the ants plant the seeds of these species in their earthen nests to structurally reinforce them, and in turn, the plants gain access to a rich source of nutrients and moisture.
Another complex ant-epiphyte mutualism occurs when plants provide domatia—essentially affordable housing—to ants within their tissues. A highly specialized example is found in the Southeast Asian genus Myrmecodia, a member of the Rubiaceae. These plants house entire ant colonies in ready-made cavities within their tuberlike stems. The ants are offered two types of cavity: a nonabsorptive smooth-walled one in which to raise their young, and a highly absorptive rough-walled one in which the ants place the remains of prey and other nutrient-rich debris. This "feeding of the host" is their rent! Over time, the animal tissues break down, and released nutrients are assimilated by the plants.
Now, if only I could pay my landlord with ham-and-cheese sandwiches.
Niall Dunne is the former editor of Plants & Gardens News.