Tropical rainforests hold more plant species than any other land habitat on Earth.1Plant Systematics and Evolution. Odd man out: why are there fewer plant species in African rain forests? The plants living there are less a single list than a set of lifestyles stacked from the treetops to the forest floor. In such dense layering, a plant’s biggest problem is often not soil but where it sits relative to light and water. Some of the solutions look more like animal behaviour than plant life, from trapping prey to slowly killing a host tree.
Giant Canopy Trees and Their Buttress Roots
The canopy is the continuous roof of leaves formed by the tallest trees, often 30 to 45 metres up. In some long-lived canopy trees, a dense outer layer of leaves blocks about 61% of the light within half a metre of the crown’s surface.2PubMed Central. Variation in crown light utilization characteristics among tropical canopy trees The forest floor below gets only a sliver of the sun the treetops receive. As rainforest trees grow taller, their crowns get deeper and pack in more layers of leaves.3Journal of Ecology. Crown development in tropical rain forest trees: patterns with tree height and light availability
Not every tree plays the long game. Fast-growing pioneer trees such as Cecropia have thin, open crowns with short-lived leaves.4PubMed Central. Variation in crown light utilization characteristics among tropical canopy trees They shoot up quickly in gaps left by fallen trees and spend less on each leaf, replacing leaves fast. Slow giants instead build dense, long-lasting leaf layers.
Holding up such a tall tree is its own challenge. Many rainforest trees grow buttresses, tall flaring fins of root at the base of the trunk. Buttressed trees can resist being pulled over with almost twice the strength of similar trees without them.5Journal of Experimental Botany. The function of buttress roots: a comparative study of the anchorage systems of buttressed (Aglaia and Nephelium ramboutan species) and non-buttressed (Mallotus wrayi) tropical trees Buttresses supply around 60% of such a tree’s anchorage, working both by pulling and pushing as the tree sways.6Journal of Experimental Botany. The function of buttress roots: a comparative study of the anchorage systems of buttressed (Aglaia and Nephelium ramboutan species) and non-buttressed (Mallotus wrayi) tropical trees They are more common where the soil is less stable and as trees grow taller.7Biotropica. When are buttresses and stilt roots necessary for a tree in terra‐firme Amazonian forests? Stilt roots do the job differently, propping the trunk up on separate arching legs.
Shade Plants of the Forest Floor
Below the roof, the understory holds shrubs, ferns, palms, and saplings waiting for a gap to open above. Shade-adapted understory plants have thinner leaves that need less light than sun-loving plants to start making food.8PubMed. Trade-offs between light interception and leaf water shedding: a comparison of shade- and sun-adapted species in a subtropical rainforest Their leaves also tend to sit flatter rather than tilting steeply.9PubMed. Trade-offs between light interception and leaf water shedding: a comparison of shade- and sun-adapted species in a subtropical rainforest
How dim it gets depends on the forest. Wetter rainforests are darker at ground level, with about half the yearly understory light of drier ones.10PubMed Central. Do differences in understory light contribute to species distributions along a tropical rainfall gradient? Shade plants in wet forests rely on tough, long-lasting leaves, while those in drier forests focus on catching light and saving water.11PubMed. Leaf traits show different relationships with shade tolerance in moist versus dry tropical forests
Rain is the other problem. Many rainforest leaves end in a long pointed tip called a drip tip.12Functional Ecology. Influence of tree height and age on leaf drip‐tip morphology in lowland tropical rainforest trees The tip helps rainwater run off so the leaf dries and can get back to making food.13Functional Ecology. Influence of tree height and age on leaf drip‐tip morphology in lowland tropical rainforest trees A drier leaf surface is also harder for moss, lichens and algae to colonise.14Biotropica. Drip‐tips are Associated with Intensity of Precipitation in the Amazon Rain Forest Shade plants have especially long drip tips,15PubMed. Trade-offs between light interception and leaf water shedding: a comparison of shade- and sun-adapted species in a subtropical rainforest and drip tips are more common where rain falls harder.16Biotropica. Drip‐tips are Associated with Intensity of Precipitation in the Amazon Rain Forest
Woody Vines That Climb to the Light
Lianas are woody vines that root in the soil and climb trees to reach the sunlit canopy. Because they use other plants as ladders, they skip building a thick trunk of their own. Even so, liana branches are better than tree branches at bending without breaking under outside forces,17PubMed. Fully exposed canopy tree and liana branches in a tropical forest differ in mechanical traits but are similar in hydraulic traits and they move water through their stems about as well as trees do.18PubMed. Fully exposed canopy tree and liana branches in a tropical forest differ in mechanical traits but are similar in hydraulic traits
Their growth habits vary by region. In American rainforests, many lianas spread several small leafy crowns across the leafy top layer that drift apart over time.19Botanical Journal of the Linnean Society. Liana structure, function and selection: a comparative study of xylem cylinders of tropical rainforest species in Africa and America Nearly 80% of American liana species studied had stems built from multiple bundles, compared with under 50% in Africa.20Botanical Journal of the Linnean Society. Liana structure, function and selection: a comparative study of xylem cylinders of tropical rainforest species in Africa and America
Rattan furniture comes from climbing palms, and the vine behind curare is also a liana. In the forest, lianas link neighbouring tree crowns into a tangled web.
For the trees that carry them, vines are a mixed burden. They compete with their hosts for light at the top, and heavy vines can weigh down branches. Foresters sometimes cut them to help timber trees.
Air Plants Living on Branches
Epiphytes are plants that grow on other plants without taking food from them. They make up about 9% of all plant species and are abundant in rainforest canopies.21University of Wisconsin-Madison. Examining Foliar Water Uptake in Neotropical Epiphytes With no roots in soil, they live in surprisingly dry conditions up on the branches.22University of Wisconsin-Madison. Examining Foliar Water Uptake in Neotropical Epiphytes Many can absorb water directly through their leaves.23University of Wisconsin-Madison. Examining Foliar Water Uptake in Neotropical Epiphytes
Orchids and bromeliads solve the problem differently. Some epiphytic orchids save water with a thick waxy skin on their leaves.24PubMed Central. Two strategies by epiphytic orchids for maintaining water balance: thick cuticles in leaves and water storage in pseudobulbs Other orchids have thin leaves and instead store water in swollen stems.25PubMed Central. Two strategies by epiphytic orchids for maintaining water balance: thick cuticles in leaves and water storage in pseudobulbs Tank bromeliads hold a pool of water in a cup of overlapping leaves.26PubMed. High but not dry: diverse epiphytic bromeliad adaptations to exposure within a seasonally dry tropical forest community Some thin-leaved air plants in the treetops act as fog catchers, soaking up mist.27PubMed. High but not dry: diverse epiphytic bromeliad adaptations to exposure within a seasonally dry tropical forest community
A bromeliad’s cup works like a tiny pond. The pooled water collects fallen leaves and insects, and frogs and insects can live and breed in it, which makes the plant a habitat in the canopy. As the debris breaks down, it also feeds the plant.
Strangler Figs That Take Over Host Trees
Strangler figs begin life perched on a host tree, then become free-standing trees that kill their hosts.28PubMed. Adaptations of strangler figs to life in the rainforest canopy Ordinary figs, by contrast, start on the forest floor.29PubMed. Adaptations of strangler figs to life in the rainforest canopy Seeds usually sprout on large, older trees, likely because those have humus-filled, rotting pockets that feed the seedling.30Journal of Tropical Ecology. Light or presence of host trees: which is more important for the strangler fig? Once the fig’s aerial roots reach the ground, the host’s leaves lose nitrogen and phosphorus.31PLANTS, PEOPLE, PLANET. Strangler fig–host tree associations: Insights into the ecology and management of tropical urban green spaces The fig competes for soil nutrients as well as squeezing the trunk.
The whole process unfolds in stages. A bird or bat drops a seed in a branch fork. Dangling roots grow down and reach the soil, then thicken and fuse into a lattice round the trunk. Finally the fig’s crown shades the host until it dies and rots, leaving a hollow fig.
Carnivorous Pitcher Plants
Tropical pitcher plants of the genus Nepenthes grow where nitrogen is scarce and catch insects in fluid-filled leaf traps.32PubMed Central. Carnivorous Nepenthes Pitchers with Less Acidic Fluid House Nitrogen-Fixing Bacteria The plants digest their prey to take up nitrogen and minerals.33PubMed. Enzyme activities in two sister-species of carnivorous pitcher plants (Nepenthes) with contrasting nutrient sequestration strategies In acidic pitcher fluid, the plant’s own digestive juices break down the insects.34PubMed Central. Carnivorous Nepenthes Pitchers with Less Acidic Fluid House Nitrogen-Fixing Bacteria In less acidic pitchers, bacteria in the fluid capture nitrogen from the air that the plant can use.35PubMed Central. Carnivorous Nepenthes Pitchers with Less Acidic Fluid House Nitrogen-Fixing Bacteria
The trap is a modified leaf with a lid. Colour and nectar lure prey, a slippery rim makes insects fall in, and the fluid at the bottom drowns and dissolves them.
A few Nepenthes species attract mammals and collect their droppings and urine instead of insects.36PubMed. Enzyme activities in two sister-species of carnivorous pitcher plants (Nepenthes) with contrasting nutrient sequestration strategies Insects are scarce on tropical peaks above 2,200 m, so tree-shrew droppings are a richer source.37Sci.News: Breaking Science News. Study: Some Tropical Pitcher Plants Capture Mammal Droppings These species capture more than twice as much nitrogen as other pitcher plants.38Sci.News: Breaking Science News. Study: Some Tropical Pitcher Plants Capture Mammal Droppings
Rafflesia and Other Hidden Parasites
Rafflesia produces the world’s largest flowers.39Journal of Systematics and Evolution. The big, the bad, and the beautiful: Biology of the world’s largest flowers The rest of the plant is just strands of cells inside its host vine, with no leaves, stems or roots.40Journal of Systematics and Evolution. The big, the bad, and the beautiful: Biology of the world’s largest flowers It takes all its food from that one specific host,41PubMed. Dissecting the Biology of Rafflesia Species: Current Progress and Future Directions Made Possible with High-Throughput Sequencing Data so it is invisible until it flowers.42Journal of Systematics and Evolution. The big, the bad, and the beautiful: Biology of the world’s largest flowers
The flower can be around a metre across, red and blotched, and it smells of rotting meat. The smell draws carrion flies as pollinators.
That dependence makes Rafflesia hard to protect. It cannot be grown without its host vine, and forest clearing removes the host.
Plants That Partner With Animals
Cecropia trees house Azteca ant colonies inside their hollow stems. Colony and tree grow at matching rates, which keeps the partnership stable.43PubMed Central. Ant-plant sociometry in the Azteca-Cecropia mutualism The size of the worker ants tracks the size of the tree rather than the colony’s age.44PubMed Central. Ant-plant sociometry in the Azteca-Cecropia mutualism In return for shelter and food from the tree, the ants attack leaf-eating insects and clear climbing vines.
Pollination partnerships are just as common. In one Brazilian understory, about 62% of plant species had small white dish-shaped flowers suited to bees.45Austral Ecology. Plant–animal interactions of understory species in an area of tropical rainforest, north‐eastern Brazil Other flowers are built for hummingbirds, butterflies, flies or hawk moths.46Austral Ecology. Plant–animal interactions of understory species in an area of tropical rainforest, north‐eastern Brazil Red tubular flowers often suit hummingbirds, and pale flowers opening at night often suit moths, though flower shape and colour are only rough guides to who pollinates them.
Medicines From Rainforest Plants
Quinine, the first effective malaria medicine, comes from the bark of the cinchona tree, first used by the Quechua people.47Global Citizen. 4 Known Medicines With Roots In The Amazon Rainforest Curare, an arrow poison from a South American vine, led to the muscle relaxant tubocurarine.48Global Citizen. 4 Known Medicines With Roots In The Amazon Rainforest The cancer drugs vincristine and vinblastine come from the Madagascar periwinkle and are on the WHO list of essential medicines.49Global Citizen. 4 Known Medicines With Roots In The Amazon Rainforest About 75% of plant-derived medicines in use were found from plants with the same or related traditional use.50University of Delaware. Medicines from the Rainforest: Biodiversity and Biologically Active Natural Products
Everyday products come from the forest too. Chocolate comes from cacao, a rainforest understory tree, and rubber came from rainforest latex. Vanilla is a climbing rainforest orchid, and many spices and fruits also originate there.
How Rainforest Plants Differ Around the World
The three main rainforest regions are the Americas, Southeast Asia and Africa, and they are not equally rich.51Plant Systematics and Evolution. Odd man out: why are there fewer plant species in African rain forests? African rainforests average about half as many tree species in a patch as Amazon forests,52Journal of Ecology. The odd man out? Might climate explain the lower tree α‐diversity of African rain forests relative to Amazonian rain forests? perhaps because Africa has a smaller pool of tree species suited to warm, wet conditions.53Journal of Ecology. The odd man out? Might climate explain the lower tree α‐diversity of African rain forests relative to Amazonian rain forests? Within one forest, the plants also change along wet ground, with fewer species in long-flooded river forests.54Europe PMC. Response of black-water floodplain (igapó) forests to flood pulse regulation in a dammed Amazonian river
Each region has signature plants. Bromeliads are almost entirely American, as is Cecropia. Nepenthes and Rafflesia are Southeast Asian, and Africa has fewer epiphytes.
How Drought Threatens Rainforest Plants
Intact rainforests on all three continents are vulnerable to severe drought.55PubMed Central. Increasing and widespread vulnerability of intact tropical rainforests to repeated droughts American rainforests often fail to recover after severe droughts, while African and Asian forests bounce back better.56PubMed Central. Increasing and widespread vulnerability of intact tropical rainforests to repeated droughts Under drought, understory saplings and shrubs grow less and suffer more disease and insect attack,57Forest Ecology and Management. Drought reduces the growth and health of tropical rainforest understory plants so shade plants of the forest floor are likely the first to suffer. Some tree groups resist drought much better than others.58PubMed Central. Plasticity in leaf‐level water relations of tropical rainforest trees in response to experimental drought