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Trees make scientific waves
27 OCTOBER 2005

About 276,000 people died during the titanic Indian Ocean tsunami of Dec. 26, 2004. Like the human toll, the physical damage was beyond counting. But here's a glimmer of hope amid the devastation: Nature can reduce the damage of a horrific natural disaster. Thick stands of trees along India's shore slowed the massive waves and reduced the damage to villages further inland.

A study in this week's (Oct. 28, 2005) Science magazine shows that trees soaked up energy from the huge waves. When an international group of researchers examined land along the coast, they found that trees offered serious protection from the surge of water.

Village full of grass huts on beach is in partial ruins
This part of a village, in the study area, was partly damaged. Other parts of this village were obliterated. Photo: ©Science

The protection was particular strong from dense stands of mangrove trees: Only 0.5 percent of the area behind them was damaged, compared to 15 percent of land behind more open stands of trees, and 35 percent of land without any shelter from trees. Put another way, 96 percent of land sheltered by dense trees was undamaged, compared to 38 percent of land without trees.

There is nothing new to the idea that vegetation cuts coastal erosion, but "this is, to our knowledge, the first strong evidence that trees could dampen the destruction from a tsunami," says Finn Danielsen, an ecologist with the Nordic Agency for Development and Ecology in Copenhagen, Denmark, who was the study's primary author.

After the tsunami, he says, ecologists heard "a lot of stories" about protection from forests, especially from villages in Thailand that had made a point of protecting mangroves. "They said, 'We have been protecting our trees for a long time, and our village survived with very few casualties thanks to the mangroves." The neighbors that did not invest time and effort in protecting the forest lost everything."

satellite image of India (at night) with Cuddalore highlightedThe coast in the Cuddalore District of Tamil Nadu, India, was straight and regular; ideal for studying how trees affect tsunami waves. Original photo of India, at night from NGDC.

Natural experiment
The tsunami, in other words, added up to a natural test of whether nature could protect against a natural calamity. "We had to try to find out, is there something to this?" Danielsen says. "Can this be documented, or is it only hearsay?" The researchers chose to study a 21-kilometer stretch of South India because the coastline, both above and below water, was straight and regular. According to the best estimates, the coast was flooded by a fairly uniform, 4.5 meters wall of water. (Since the study site was more than 1,500 kilometers from the gigantic earthquake that caused the tsunami, the flood was far less intense than the one that destroyed Sumatra's coast. The scientists are not claiming that trees would have protected against the 10- to 20-meter wall of water closer to the quake.)

Data from satellites and on-the-ground inspections showed that at the study site, "trees had this ability to break the waves or absorb their energy," Danielsen says. "It's very clear. There were five villages around the area with mangroves, two in front, and three behind. The two in front were completely destroyed, and the three behind were untouched, even though they were the same distance from the sea as other [unprotected] areas that were damaged."

Thick tree roots jut out in jungle-like setting. The mangrove's aerial roots enable them to tolerate extreme variations in water depth and salinity in their home in the tidal zone. Photo: Finn Danielsen

Although it's uncertain if the trees reduced the overall height of the flood, Danielsen says they certainly slowed the floodwaters. The trees also blocked floating logs, buildings and cars, which, in many places, destroyed houses and killed victims trapped in the floodwaters.

Trees = Life-savers?
The forests had a third benefit. In many places, the receding water pulled people to their death. But behind the trees, Danielsen says," When the waves went out, many people were able to cling to the trees and not get taken out to sea. We have no doubt that a lot of people survived because they were able to grab the trees and not get taken out to sea."

Mangroves are dense stands of trees that are adapted to life in tropical, coastal wetlands. An estimated 26 percent of mangroves have been destroyed around the Indian Ocean through conversion to farm fields, aquaculture ponds, or from other causes, exposing the coast to accelerated erosion.

The study also found that beach forest trees that had been planted to protect against typhoons (as hurricanes are called in the region) helped protect land.

Blue ocean is visible through heavily deforested area Casuarina trees planted years ago to protect against typhoons also helped temper the tsunami waves. This plantation is seen after the tsunami. Photo ©Science

You might respond, duh, George, of course trees protect shorelines. But Danielsen notes that tsunamis are quite different from regular sea waves, in their origin, timing, and of course size, and it's important to know if they can protect against the rare, but horrific, tsunamis.

By trying to extract some benefit from a disaster, the study echoes the 60-year effort to explore the health effects of radiation in Japan, Danielsen said. "We felt the tsunami was such a huge human tragedy, the least we could do was to try and find out what can we learn from this."

-- David Tenenbaum


Bibliography
Asian Tsunami: A Protective Role for Coastal Vegetation, Finn Danielsen et al, Science, Oct. 28, 2005.

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