Litter-Trapping Plants: Nature’s Compost Makers
The Tropical Garden, Fall 2016
By Scott Zona, Ph.D.
The nutrient-poor soils of tropical rainforests have pushed plants to evolve new ways to get the carbon, nitrogen, potassium and other nutrients that they need. Many tap into the rainforest’s constant drizzle of falling leaves, flowers, fruits and more to create their own private compost piles.
A great irony of tropical rainforests—and the cause of failure for many grand plantation schemes—is that the underlying soils are notoriously poor in nutrients. Heavily leeched by relentless rain and almost devoid of organic matter beneath the surface, the soils under most rainforests should not be able to support much plant growth. Yet, they support the most biologically rich forests on the planet. How is that possible? The answer lies in the forest itself, not the soils. The nutrients—carbon, nitrogen, phosphorus, potassium, etc.—are held by the living organisms of the forest, not by the soil. When a leaf falls, its nutrients are quickly released by decomposing organisms (bacteria and fungi) and immediately taken up again by the roots of plants. Those not quickly absorbed by organisms are washed away by the daily rains.
In the mid-20th century, ecologists illuminated the nutrient cycle, demonstrating the endless cycling and recycling of nutrients within this closed system. The only way nutrients leave the system is through leaching and erosion, oxidation (fire), or as timber on the back of a flatbed truck. In undisturbed forest, nutrients cycle through the system in a sustainable and predictable way. Some nutrients are shunted into subcycles, such as when an insect eats a leaf and a bird eats the insect. Ultimately, though, all nutrients are returned to the forest floor in a constant drizzle of litterfall, which includes not just fallen leaves, but also flowers, fruits, twigs and branches, animal droppings, effluvia and even fallen epiphyte (plants that grow harmlessly on other plants).
In tropical forests, some plants have evolved a way to shortcut the nutrient cycle by tapping into the litterfall, intercepting litter before it hits the ground. Plants that can capture litterfall gain access to its nutrients, which confers upon them a competitive advantage. These plants are called litter-trappers. They are plants that make their own compost, thereby giving themselves access to a private supply of nutrients and a leg… or, rather, a leaf, up on the competition. A casual stroll through the Garden reveals a number of litter-trapping plants, some familiar and others quite surprising.
Probably the most familiar litter-trappers are the staghorn ferns (Platycerium spp., Polypodiaceae) and the tank bromeliads (Bromeliaceae). Staghorn ferns are epiphytes that produce two distinct kinds of leaves. The shield, or sterile, leaves are rounded and erect, cupped against the trunk of their host tree, while the fertile leaves are lobed and pendulous, performing photosynthesis and reproduction. The shield leaves trap litter behind them, where it decomposes, and the fern sends its roots into that composted litter to extract its nutrients.
Tank bromeliads, those that form water- filled rosettes of leaves, may be epiphytic or terrestrial. The tank impounds a rich broth of decaying leaf litter, as well as aquatic and semi-aquatic organisms such as frogs, snails and mosquito larvae (and their waste products). The ability of bromeliad leaves to absorb nutrients directly from their private cesspools has been well-studied.
The bird’s nest fern (Asplenium nidus, Aspleniaceae) and bird’s nest anthuriums (Anthurium spp., Araceae) are another group of familiar litter-trappers. The bird’s nests are epiphytic plants and form a funnel-shaped rosette of leaves that captures and retains litter. Anthurium salvinii and A. schlechtendalii can be seen in the Garden’s Richard H. Simons Rainforest, their crowns of leaves capturing litter from the oak trees overhead. A fine specimen of Asplenium nidus is growing between the Corbin Education Center and the tram path. Look into the crowns of these plants and you’ll see their private compost piles, slowly releasing nutrients to the root zone below. It is likely that some nutrients might be taken up directly by the leaves, as they are in bromeliads.
Many palms are known to be litter-trappers, and several in-depth studies of litter-trapping have focused on litter-trapping palms. Some palms trap and retain litter in their leaf crowns; a few even hold water, like bromeliads. Salacca magnifica, Sommieria leucophylla, Calyptrogyne ghiesbreghtiana and Johannesteijsmannia altifrons (all in the Tropical Plant Conservatory) are all consummate litter-trappers, although the roof of the Conservatory prevents them from naturally catching litter in their crowns.
Other palms use another litter-trapping strategy, funneling litterfall toward the center of their crowns, where the litter drops to the ground below, forming a compost pile directly around the base of the trunk. An example of this kind of litter- trapping palm can be found among the many forest species of Licuala. I have seen Licuala species in Malaysia rainforests with the telltale mound of leaf litter heaped around their short trunks, augmenting the leaf litter that carpets the forest floor.
Just outside the Conservatory, you can see some young plants of Clavija domingensis and Theophrasta jussieui (both Primulaceae) grown from seeds collected by the Garden’s staff in the Dominican Republic. The genus Clavija comprises some 55 species from the American tropics, and all of them are litter-trappers. Theophrasta has just two species from Hispaniola, and both are litter-trappers. These are small trees with a palm-like growth habit, trapping litter in the crowns of their strap-shaped leaves. Walk over to Plot 39 to see Erythrochiton brasiliensis (Rutaceae), a Neotropical understory shrub that traps litter in its crown. A short distance away, in Plot 45, a young sapling of Gustavia monocaulis (Lecythidaceae) is doing the same thing.
An entirely different mode of litter-trapping is found in several genera of epiphytic orchids, including some species of Catasetum, Cymbidium, Grammatophyllum and Cyrtopodium. These orchids produce two kinds of roots: thick roots that hold the orchid to the tree on which it grows, and thin, upward-growing roots that trap litter. Cyrtopodium punctatum, an orchid native to Florida growing on several palms in the Montgomery Palmetum, readily exhibits these litter-trapping roots. The thin, wiry roots trap stray leaves and other debris and are also, presumably, able to absorb nutrients as they leech out of the trapped litter.
In the wild, the private compost piles accumulated by litter-trapping plants are home to an amazing array of organisms. Ants, collembolans and mites are some of the most common occupants of litter masses, but biologists have recorded hundreds of different kinds of animals living in litter- trappers. One ecologist estimated that the biomass of invertebrate species living in a forest canopy in Asia is doubled if one also counts the biomass of species living in bird’s nest ferns. Inventories of insects living in litter-trappers routinely turn up new, undescribed species, and probably many more insect species await discovery by intrepid biologists willing to dig into the private compost piles of litter-trapping plants.
Litter-trapping plants are quintessentially tropical. They didn’t evolve in the temperate zone, where soils are richer and the decay process is slower. Hundreds of litter-trapping species from more than 30 families of plants are found in tropical regions of Asia, Africa and the Americas. Australia appears to be poor in litter-trappers, but this may be an artifact of under-reporting. One of the big remaining questions is: Which plants are litter-trappers? In the past, field botanists cleared away the litter before taking photographs and making specimens (no one wants to press a mound of decaying leaves infested with ants!), so litter-trapping has been under-reported in the literature. As awareness of litter-trapping plants grows—through living plant displays at Fairchild, among other means—so will our understanding of this fascinating tropical phenomenon.