The stranglers that save lives when cyclones strike

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They are demonised as brutal killers but, as two studies published this month show, strangler figs can be lifesavers — of both plants and people.

According to research in the Australian Journal of Management, they contributed to the low death toll of Cyclone Pam, which hit the Pacific island nation of Vanuatu in 2015. When the cyclone destroyed homes there, people survived by taking shelter among the aerial roots of giant strangler figs.

This is more than just an anecdote. People in Vanuatu have been using giant fig trees to escape from cyclones for thousands of years, say Lachlan Forsyth and Rebecca Olul of UNICEF.

“For as long as people have lived on these islands, the trees have been a traditional cyclone shelter,” they write in an article published this week. “During cyclones, entire villages would wriggle down into the tree’s extensive root system, tucking children into the cavities, and waiting out the storm. Even if the tree were uprooted, the entire root structure would come up, leaving those within safely cocooned and protected from flying debris.”

These refuges exist because strangler figs start out in life as seeds that germinate high on other trees. They then send down roots that merge and split and merge again, thickening into a mesh that envelopes the host tree and — in time — erases all trace of it.

While most strangler figs grow on a single host tree, some — known collectively as banyans — are much bigger specimens. Their branches produce aerial roots that thicken into stout pillars that resemble tree trunks and enable the banyan to continue to expand as it ages.

The mass of roots that descends from a banyan’s body and branches creates a matrix of hollows and thick woody walls. In Vanuatu, these chambers have saved people from cyclones for generations.

Strangulation might sound like a brutal fate for the host tree, but before a strangler fig overcomes its host it can also protect it, say biologists Leora Richard and Sylvia Halkin in the new issue of Symbiosis. When they studied the aftermath of Cyclone Oswald, which hit Lamington National Park in Australia in 2013, they found that large trees were less likely to be uprooted if they hosted strangler figs.

By building a scaffold around their host and by anchoring their aerial roots in both the ground and around other trees, the strangler figs appear to protect the trees on which they grow.

So the stranglers can not only protect people directly, as living shelters, but can also help ensure fewer trees fall when a cyclone strikes. Elsewhere fig trees protect people from drought and from extreme rainfall. They sustain more wildlife than any other trees and can accelerate rainforest regeneration.

Collectively, they have much to offer as we come to terms with our fast changing climate.

References:

Handmer, J. & Iveson, H. 2017. Cyclone Pam in Vanuatu: learning from the low death toll. Australian Journal of Management 32 https://ajem.infoservices.com.au/items/AJEM-32-02-22

Richard, L.S. & Halkin, S.L. 2017. Strangler figs may support their host trees during severe storms. Symbiosis 72: 153–157. https://link.springer.com/article/10.1007/s13199-017-0484-5

Forsyth, L. & Olul, R. 2017. Unicef NZ: Climate change and preparing for the coming storm. stuff.co.nz (28 June 2017). https://www.stuff.co.nz/world/south-pacific/94193406/unicef-nz-climate-change-and-preparing-for-the-coming-storm

Photo credits:

Children play among the roots of a giant strangler fig in Vanuatu (UNICEF New Zealand)

 

Can living fig-tree bridges save lives in a changing climate?

By AditiVerma2193 (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons

In 1841, a young Scotsman called Henry Yule was exploring the Khasi Hills of north-east India when he came upon something no other European had ever reported. There, in that challenging landscape of thick rainforest and perilous gorges, was a most extraordinary structure — a living bridge formed from the roots of a gnarly old fig tree.

The tree’s roots had somehow reached more than 20 metres across a river and taken hold on the far side. Over time, they had thickened and interwoven to form a walkway, onto which Yule now had to step. One of the roots, which in places was thicker than his thigh, provided a handrail. Side roots had descended from it and merged into the walkway, making the whole structure strong and secure. Yule could cross with confidence.

The bridge was no miracle. Long before, human hands had guided the tree’s roots across the river, training them into a shape that could promise safe passage. Yule had assumed the bridge to be “unique, perhaps half accidental”, but he soon saw several more. Their architects were local Khasi people, whose attitudes towards time, the environment and their unborn descendants we are sorely lacking in our fast-warming world.

Fig tree bridge. Credit: Laurence Mitchell (https://eastofelveden.wordpress.com/2011/03/06/crossing-the-bridge/)

The Khasi Hills are in Meghalaya, an Indian state whose name means ‘land of the clouds’. The clouds cry often on this land, for nowhere else on Earth must people endure such heavy rainfall. Twelve metres of it falls in a typical year. After monsoon rains, the region’s rivers rise. The rush of water racing downstream renders steep gorges impassable, isolating villages and endangering lives. Yet people have lived in the Khasi Hills for at least 3200 years. For generations, they have overcome this extreme environment by harnessing the strong yet pliant roots of Ficus elastica, a fig species best known as the Indian rubber tree.

Fig roots are exceptional. They grow fast, long and strong. They can even rip apart bare lava and concrete. In many fig species, including Ficus elastica, the roots aren’t all underground. These figs produce aerial roots that flow down their trunks and drop from their branches. Their roots can merge and split and merge again, forming strong, mesh-like structures. Long ago, the Khasi people worked out how to get such fig roots to do their bidding.

It’s a practice still alive today. The Khasi and neighbouring Jaintia peoples have shaped fig roots into living ladders, whose rungs ease journeys up steep slopes. They have woven the roots into nets that hold banks of earth in place, preventing landslides and soil erosion. They have even forced fig roots to form a platform, from which to watch football games. But the masterworks of Khasi architecture are their bridges. Most — like one Yule sketched, below — form from the roots of two fig trees, one on either side of a river. The longest spans more than 50 metres, the oldest an estimated 500 years.

ficus-bridge-henry-yule-1844-khasi-hills

Building these bridges involves first guiding slender fig roots through hollow trunks of betel palms, which support, nourish and protect the roots as they lengthen. Once the fig roots are long and strong, the bridge builders bind them to those of the opposing tree or embed them in the ground on the far side of the river. They shape secondary roots that grow from these mainstays into a net that will form a walkway. They use stones and soil to plug any gaps then wait for the roots to thicken and hold everything in place.

It can take 15-30 years before the bridge is strong enough to use. But in Khasi time this is an eye-blink. The Khasi people are investors in the future. People living there today benefit from the ingenuity and foresight of their ancestors and today’s bridge builders bequeath security to future generations. The fig tree bridges don’t only make commerce and romance possible between otherwise isolated villages. They also save lives. As the climate changes, this ancient approach to bioengineering has never been more relevant.

Meghalaya was already the wettest place in the world when Henry Yule explored its forested hills in 1841. Back then, the atmospheric concentration of carbon dioxide was 283 parts per million. Today it is more than 400. More carbon means warmer air, and as warmer air carries more moisture, this means more rain. The state government’s climate change action plan says rainfall has increased in most districts of Meghalaya in the past century, with the highest increase in the West Khasi Hills. It warns that as temperatures continue to rise, so will rainfall and the risk of floods and landslides.

To architect Sanjeev Shankar, these threats call for renewed attention to living bridges. In a research paper he presented in 2015, he warned that they are being replaced by “inappropriate solutions”. Quick-fix bamboo bridges buckle and break – they can’t withstand the monsoon rains. People have died as a result. Modern steel bridges corrode, their cables weaken and snap. And because repairs are rare, these bridges last just 40-50 years compared to hundreds for living bridges.

Shankar says the living fig bridges cost next to nothing and become stronger, more robust and resilient with time and use – unlike expensive, short-lived steel suspension bridges. Indeed, some of the bridges Henry Yule saw in the 1840s are still saving lives today. Shankar urges a revival of fig-tree bridge-building, and even foresees bridge that are strong enough for vehicles to cross.

The only downside is the time it takes the bridges to grow. But Shankar sees potential to blend the old with the new. Having seen how Khasi people have used fig roots to mend steel bridges, he envisages planned hybrid structures — steel bridges that fig trees envelop with their roots and make stronger. Shankar wonders if other fig species could perform this role in other countries, helping people adapt to the changing climate.

The question is, will there be time? It’s a question for us all, as climate change doesn’t respect the short-term thinking that tends to rule our lives. The Khasi people’s approach is instructive. To build living bridges, they invest time and effort knowing they might not personally benefit but that their children surely will. Such foresight, patience and selflessness is rare.

This post was first published by the Thomson Reuters Foundation and is reproduced here with permission.

IMAGE CREDITS: Top (Aditi Verma, via Wikimedia Commons); middle (Laurence Mitchell); bottom (Henry Yule, from Yule, H. (1844). “Notes on the Khasia Hills, and people”. Journal of the Asiatic Society of Bengal. 14 Part 2, Jul-Dec (152): 612–631).

My new book —published in the UK as Ladders to Heaven and in North America as Gods, Wasps and Stranglers tells how fig trees have shaped our world, influenced diverse cultures and can help us restore life to degraded rainforests.

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