28 THE TROPICAL Garden 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 epiphytes (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 littertrappers. 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 waterfilled 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. previous page The upward-growing roots on this Cow’s Horn Orchid (Cyrtopodium punctatum) trap litter, while other roots secure the orchid to the palm. ABOVE This Anthurium jenmanii has a private compost pile in the center of its crown.
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