This Ficus virens strangler growing next to the Observation Tower at the Maliau Basin, is an excellent example of how a strangling fig slowly kills the host tree.
Ficus roots have a peculiar ability for anastomosis where adjacent roots fuse together into a rigid network. The network of roots then girdles the host tree like a rigid cage or corset.
Woody trees grow by adding new fluid transport vessels in the cambium under the bark which deliver water and minerals from the roots to the leaves (xylem) , and sugars from the leaves to the roots (phloem). This means that diameter of a growing woody tree trunk is continually expanding. The phloem channels are softer and closer to the bark than the xylem. As the woody host trunk expands, the flow of water and nutrients gets restricted by the rigid corset of fig roots. The starving host tree slowly starts to die as the transport channels cease to function.
Also the roots of the fig compete for water and minerals and the leaves of the fig steal sunlight from the leaves of the host. This very effective 3 pronged attack on the host means that once a strangler roots have encircled a host tree, the host is usually doomed.
Note that because the trunks of palms are a fixed girth (diameter) from sapling stage palms cannot be strangled by figs.
There is no benefit to the fig tree if the death of the host tree happens too quickly, because then both the tree and the host will fall to the ground, so the “strangling” process has evolved to take place very slowly over a few years until the roots of the fig tree are strong enough to support the fig tree by themselves.
The Ficus virens fig illustrated in the article above is growing near the Observation Tower at the Maliau Basin in Sabah. The tree in the photo above growing behind the Observation Tower is Octomeles sumatrana (Local name Binuang) which is a fast growing tree normally found along river banks in Borneo.