By Agnès Schermann Legionnet, Lecturer, University of Rennes
While we are increasingly understanding the many cooperative relationships that can exist between plants, in the jungle, one lifestyle remains quite mysterious: that of strangler trees that kill their cooperators.
Example of a strangler fig in Kerala, India. L. Shyamal/Wikimedia, CC BY
If you have a friendly Ficus benjamina at home, you may not know that in the jungle, this species can be a formidable "strangler fig." Should you therefore fear that your little companion tree might wrap a small branch around your neck and squeeze until death ensues?
Rest assured, no such accident has ever been reported. Even the Buddha, who is said to have meditated for a very long time under a fig tree and attained Nirvana there, was not a victim of strangulation. His tree, which later became the sacred tree of Buddhism, is also a strangler fig.
Figs that strangle other trees
The only victims of these strangler figs are actually other trees, after a process that can take several decades.
It all starts with a seemingly innocuous event: a seed germinating. The seeds of strangler trees germinate in places that may seem unusual: on other trees. From this position, the young plants send stems upward and roots downward. These aerial roots encircle their host tree to the point of sometimes causing its death.
While there is much talk these days about cooperation among trees, what could be the origin of this very uncooperative behavior: killing the one who allowed you to rise? Let's review what current research tells us about these stranglers found in tropical and equatorial forests worldwide.
Seeds that germinate on other trees
So, let's start with a seed germinating on a tree. The young tree that results then develops as an epiphyte: this is what we call it when a plant grows on another plant. In temperate zones, you may have already seen mosses, ferns, or other plants growing in a hollow of a tree, on a fork, or a horizontal branch. However, this habitat is not very popular in these latitudes, as the water supply is often insufficient.
But in rainforests, the atmosphere is humid year-round, water runs down trunks and stagnates in hollows, with debris accumulating and forming a semblance of humus. Under these conditions, epiphytic life is much more common. It is estimated that there are about 25,000 species of plants that grow on other plants.
Tillandsia. Muzina_Shanghai/Flickr, CC BY
For example, Tillandsia, those strange plants that grow without water or soil, are epiphytic plants. Some can survive without roots, simply collecting moisture from the air through their leaves.
Aerial roots
Some other strict epiphytes (meaning they spend their entire lives without contact with the soil) produce roots, such as orchids of the genus Phalaenopsis, well-known as ornamental plants.
These roots are capable of photosynthesis and also of collecting atmospheric water through a fine veil that surrounds them. They never bury themselves in a substrate. Cultivated Phalaenopsis are even sold in transparent, very aerated plastic pots, which are well-suited to their cultivation. Indeed, their survival depends on the functioning of the aerial roots, which die if they are buried away from air and light.
Some other plants that begin their lives as epiphytes produce downward-oriented roots that do bury themselves in the soil. They can measure several meters or even tens of meters long! They draw water and mineral salts from the soil, like the roots of a "classic" plant. These plants that grow on other plants but are rooted in the soil are called hemiepiphytes (etymologically, they live on other plants, but only halfway), and there are about 800 species of them.
A host tree that may eventually disappear
Detail of a host trunk encircled by aerial roots. Natasha De Vere/Flickr, CC BY
It happens that the host tree is entirely surrounded by these aerial roots, which are capable of fusing together and forming a very dense grid. This is when the situation becomes dangerous: for most trees, survival depends on the ability to grow in thickness, as only the young tissues produced on the periphery of the trunk transport the sap. If the trunk cannot thicken, the tree can no longer feed itself. The host tree dies when the roots of its strangler no longer allow it to grow.
There are about 500 species of strangler trees, and the majority belong to the genus Ficus. So, they are figs. But this very large genus includes 850 species. Thus, while stranglers are mostly figs, not all figs are hemiepiphytes, and even fewer are stranglers!
These figs can also use other supports than a tree to grow, such as a rocky escarpment; there is even mention of a factory chimney in Brazil. These realities prove that, as is the case for ivy climbing a pole, the host tree of the strangler fig is not a source of food.
After the death and decomposition of the host tree, the strangler tree becomes autonomous, resembling a "normal" tree except that its trunk is a strange hollow cylinder, formed by a dense network of roots. It often displays impressive buttress roots. Some stranglers become very large trees, reaching the top of the canopy.
But what is the point of developing on a host tree, then killing it?
All this, then, to become a "normal" tree with roots in the ground, a trunk, and branches reaching for the sky...
What could be the reasons that push certain trees to become stranglers?
As always in biology, trying to explain natural phenomena by a "reason," in the sense of reasoning produced by a brain with cognitive abilities, is futile.
But biological evolution as described by Darwin and developed by his successors explains the immense diversity of life by the random appearance of variations and the sorting of these variations by natural selection.
The famous evolutionary biologist Theodosius Dobjansky, author of the aphorism "nothing in biology makes sense except in the light of evolution," has addressed this mystery of strangler trees. He relied on the existence of many intermediate forms between autonomous trees and strangler trees to propose hypothetical evolutionary steps that led to their existence. Since his 1954 article on this type of tree, knowledge about life in tropical forests has expanded and allows us to paint the following picture.
Pressure on dispersal
First, life in tropical forests exerts particularly strong pressure on seed dispersal: it has been shown that seedling survival is much better if they are located far from their mother than if they are planted nearby. This is explained by the fact that the young plant growing at the foot of a tree of the same species finds itself in an area infested with specific diseases and herbivores.
Now, figs have a very particular mode of reproduction: they produce figs, that is, botanically speaking, clusters of flowers, which then turn into fruits, well protected in an almost entirely closed envelope. When the seeds are mature, the figs become edible and are sought after by different species of animals capable of traveling long distances (birds, monkeys, bats). The seeds thus ingested then travel through the forest along with these frugivorous animals.
After some time, the seeds exit the animal in its droppings. As a result, they end up under the places where the animals perch, often the branches of trees. If these places accumulate droppings and other organic debris and a bit of water, they are favorable for germination. The selection of effective dispersal methods (and probably also pollination methods, as the fig is involved in both stages of reproduction) may have resulted in germination often occurring as an epiphyte.
Small perched plants
Germination is a vulnerable stage in the life of plants. Those that germinate on the ground can succumb to burial, trampling, fires, floods, unlike those that grow perched. But the latter risk falling and lacking water. It is observed that young epiphytic shoots often produce particular roots, adhesive, which prevent them from falling, as well as stems or roots filled with water, allowing them to survive periods of drought: these are adaptations to this particular mode of germination.
Plants that germinate on the high branches of trees are theoretically more exposed to sunlight than those that germinate on the ground. One might thus think that one of the main selection pressures in favor of epiphytism or hemiepiphytism would be competition for light, but biologist Gerhard Zotz and his colleagues have shown that young shoots are generally located in very shaded areas, whether inside the foliage of host trees or on the ground. Competition for light therefore does not seem to be decisive in the evolution towards the strangler tree lifestyle, at least at the young stage.
Pressure on rooting
Unlike strict epiphytes that do not root in the ground, hemiepiphytes gain a certain advantage in terms of water and mineral supply. This allows them to become true large trees.
But what could be the selection pressures in favor of killing the host tree?
Let's first consider strength: hemiepiphytes depend on the survival of their host. If it dies, or is simply knocked down or broken, the hemiepiphytes are thrown to the ground and lose much of their chance of survival.
While they were specifically looking to find out what favored tree survival during a hurricane, researchers Richard and Halkin found that trees carrying a strangler fig had much better resisted uprooting than others, because they had benefited from the ground anchors of their hemiepiphyte, as well as its many roots implanted all around, solidifying the whole like the ropes planted around tents in strong winds. Thus, in some cases, carrying a hemiepiphyte is even beneficial for host trees, which are thereby strengthened.
It therefore seems paradoxical that strangler trees end up killing their host tree and invest in an essential and costly supporting infrastructure (trunk, roots, buttresses) when their host provided it before they killed it. One can imagine that the selection pressure first acted on access to soil resources (production of aerial roots), then on the strength of the whole against storms (production of many strong aerial roots) and that finally the death of the host tree is more due to excessive consolidation than to an advantage linked to its killing.
Moreover, if the host tree is one of those rare trees that do not need to grow in thickness, namely a palm tree or a tree fern, it is rarely killed by a so-called strangler tree.
However, the strangler tree still benefits from the death of its host, as it is best placed to literally take its place in the sun, and also benefits from its decomposition, which provides nutrients.
In conclusion, we still do not know what evolutionary scenario led to this strange lifestyle. If being a strangler had only advantages, one would expect them to be much more widespread than they currently are.
Stranglers make bridges!
Strangler trees have indeed not yet revealed all their secrets, but human populations have managed to make clever uses of them, as inventoried by Gerhard Zotz and his colleagues. For example, planting a strangler fig right on the edge of a cliff and waiting for the roots to grow to use them as a ladder.
The most spectacular use is perhaps the living bridge: having observed the ability of figs to make many strong roots, some rural Indian communities traditionally have them build bridges by directing the growth of roots in the desired direction through entanglements of branches, then braiding them.
The roots of a tree planted on the bank root on the other bank, then emit other roots that are in turn directed, braided... the construction of such a bridge takes a long time, a few decades, but its strength increases over time and it requires neither engineering nor materials to extract and transport, just a Ficus elastica, like the one that may be at your home next to its cousin Ficus benjamina!