Most modern bridges are constructed from lots of concrete and steel with heavy machinery and infrastructure. But did you know there exist bridges solely made from living tree roots that regularly transport people and can survive for centuries?
I am in India for a few weeks and I recently learned about a fascinating type of bridge in the eastern Indian state of Meghalaya. These bridges are not like modern bridges such as the Golden Gate Bridge – they are not made of concrete and steel beams or cables. Instead they consist solely of interconnected, living tree roots.
“Double-decker” living root bridge in the Indian state of Meghalaya. The top deck was built for use when the bottom deck is flooded.
These bridges are vital in the rainforest climate of Meghalaya, where they connect isolated villages by providing transportation over frequently-flooded rivers. Other types of bridges are difficult to build and maintain in this wet climate. Steel bridges are expensive to construct and begin to rust quickly, and lightweight bamboo bridges may be swept away by heavy winds. The living root bridges instead use natural resources and actually become stronger over time as the roots thicken. Hence these bridges are a clever, sustainable solution for the unique, tropical climate.
Let’s start with how these bridges are made. They are typically made of the aerial roots of the Indian rubber tree called Ficus elastica. These roots grow vertically downward from the trunk, remain above ground (hence “aerial roots”), and are initially very flexible. Once they are long enough, they can be tied to some support structure to guide their growth. To build a bridge like the one shown above, various strategies are used to horizontally orient and strengthen the roots over a period of 10-15 years. These include manually manipulating the roots by anchoring them to the sides of the riverbanks, tying them to one another, and using a wood or bamboo scaffold for the roots to grow around. Once tied around each other, some roots slowly fuse together through a process similar to grafting known as "inosculation". The roots of F. elastica are particularly good at this and overtime yield strong structures which can support the weight of a person. Here are some pictures of root inosculation (copied from [1]).
Man-made inosculations in living root bridges [1]. (a) young nodal connection, (b) a similar connection after tightening and the beginning of inosculation, (c) a fully established inosculated node, (d) a young root used to tie together two older ones, (e) a typical example of a twisted linear connection with first signs of inosculation, (f) a structure with thickened roots showing no inosculation.
In fact, a living root bridge never finishes construction since the roots generally grow and strengthen over time. Surveys and measurements of root geometries have elucidated this evolving load-carrying capability of the bridges [1]. For instance, while vertically grown roots of F. elastica have a circular cross section, the horizontally-oriented roots are subject to a bending moment from self-weight and begin to develop an elliptical or T-shaped cross section that is more structurally efficient. As the roots become strong enough to support the weight of a person, their cross section continues to evolve, with more material growing on the side of the root under tension, yielding tear-dropped or T-shaped cross sections. Some studies have even suggested that I-shaped roots could form in living root bridges (although rare). This is basically nature’s topological optimization!
Compared to a steel bridge, tree root bridges are truly “living” bridges and require maintenance to ensure they stay strong. Maintenance includes properly orienting stray roots and tying newer roots to the existing structure, and this is often a social endeavor.
Maintenance of a root bridge in the East Khasi Hills [2].
With proper maintenance, these bridges can grow to be over tens of meters long and last for centuries. In Meghalaya, two tribes – the Khasi and Jainita – have built and maintained over 70 living root bridges, with many estimated to be centuries old. One bridge, linking two sides of the Nongbareh village, is estimated to be as old as the village itself, around 700 years. Another bridge, in the Rangthylliang village, is considered to be the longest living root bridge, with a length of over 50 meters.
Section of the Rangthylliang-1 root bridge, with a total length of over 50 m [2].
Wah Lar Ung bridge in Nongbareh, estimated to be over 700 years old [1].
Today many of these root bridges have become tourist destinations (one day I shall visit!). At the same time, some bridges have started to degrade without proper maintenance and conservation efforts, which is a shame because there is a lot to learn from these bridges: They show us a pathway to sustainable architecture, and that sometimes nature is truly the best engineer.
References/further reading:
[1] “How are living root bridges made?” 2015.
[2] “Living root bridge,” Wikipedia, accessed Jan 2024.
[3] “The living root bridge project,” accessed Jan 2024.
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