How Does a Plant Live in a Tree Without Roots in Soil? It Never Needed Any
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Plants growing on other plants rather than in the ground make up a substantial share of rainforest diversity. They take nothing from their host and everything from the air and the rain.
What the strategy is
The plant germinates on a branch, a trunk or another plant and spends its whole life there without ever reaching the soil. It is not a parasite, since it takes nothing from the plant it sits on and does not penetrate its tissues, and the host is simply a perch. What the arrangement buys is light, which is the scarcest resource in a forest, since the floor of a dense canopy receives a very small fraction of what falls on the top. What it costs is access to water and nutrients, which a plant in soil takes for granted and which must be obtained some other way entirely.
How they get what they need
The solutions vary between groups and are all recognisable:
- •Roots that grip the bark and absorb water directly from rain and mist
- •A spongy layer on those roots acting as a wick and a store
- •Leaves arranged as a tank holding several litres of rainwater
- •Scales on the leaf surface absorbing water and dust from the air
- •Thick leaves and water storage, since supply is intermittent
- •Debris trapping, building a small pocket of soil in place
Why they matter to the forest
The habit accounts for an unexpectedly large part of what lives in a rainforest, which is why canopy access changed tropical ecology. Something approaching a quarter of vascular plant species in some wet forests grow this way, with orchids and bromeliads dominating, along with ferns, mosses and cacti. The water held in tank-forming plants is an entire aquatic habitat in the canopy, supporting insect larvae, frogs that breed nowhere else and small invertebrates, so a single tree may hold hundreds of separate ponds. They also intercept a substantial quantity of rain, nutrients and dust before it reaches the ground, which changes the forest's whole nutrient economy.
The ones that turn on the host
A small number of plants begin this way and do not stay harmless, and the transition is worth separating out. Strangler figs germinate in a branch high in the canopy, grow as an ordinary epiphyte for years, and then send roots down the trunk to the ground. Once those roots reach soil the plant has access to water and nutrients, grows rapidly, and its descending roots fuse into a lattice enclosing the host trunk, competing above for light and below for soil. The host is eventually killed and rots away, leaving a hollow cylinder of fig roots standing on its own. That is parasitism arrived at from a harmless starting point.
The load problem
Carrying a large population of them is not costless to the host, and the effects are measurable. A mature tree in a wet forest may carry several tonnes of these plants and the water, debris and soil they accumulate, which is a real structural load and increases the chance of a branch failing in a storm. Dense growth on a trunk can shade the host's own leaves. Some hosts respond with bark that sheds in flakes, which discards anything attached to it and is a recognised strategy. Against that, the trapped debris eventually becomes nutrients that the host tree accesses by growing its own roots upward into the mass, which several species do.
The takeaway
Growing on another plant buys light, which is the scarce resource in a forest, at the cost of losing access to soil, so these plants take water from rain and mist through gripping roots, spongy root coverings, leaf tanks and absorbent scales. They can be a quarter of the plant species in a wet forest, and their tanks are whole aquatic habitats. A heavily loaded tree carries tonnes of them.