Trees that kill their attackers. It sounds like science fiction, but it’s actually scientific fact.
Researchers investigating the mysterious death of hundreds of kudu in South Africa in the 1990s made a startling discovery: the antelope were killed by the acacia trees that form part of their regular diet.
It turned out that, to protect themselves from the voracious animals, the acacias flooded their leaves with lethal quantities of poisonous tannins; they also spewed ethylene gas into the air to warn nearby trees of the impending danger.
The antelopes’ experiences hold important lessons for humanity. If we continue to mismanage and mistreat the world’s forests, we could suffer the same fate as the kudu.
According to the UN Food and Agriculture Organization, industrialised farming and urbanisation have shrunk the world’s forests by 129 million hectares in the past 25 years, an area equivalent to the size of South Africa.
The amount of carbon stored by the world’s forests has consequently fallen by almost 11 gigatonnes, equivalent to about a third of the amount generated by human activities, causing a sharp rise in the concentration of planet-warming carbon dioxide (CO2) in the atmosphere.
As of May 2019, the concentration of CO2 in the air reached 415 parts per million, the highest in human history.
So, with the world struggling to limit global warming to just 1.5°C from pre-industrial levels and slash CO2 emissions by 45 per cent by 2030, our failure to use timber wisely becomes ever more absurd.
“We’re destroying trees and forests, but they can be our greatest ally in combating climate change," says Dr Ernst Zürcher, Emeritus Professor of Wood Science at the Bern University of Applied Sciences.
400 million years of experience
In recent years, trees’ carbon-capturing qualities have been overshadowed by newer, more technologically complex solutions.
Among the most popular is Carbon Capture and Storage (CCS), which involves building a large-scale engineering facility to trap waste CO2 from industrial power plants and store it underground.
However, this method requires big set-up costs and can lose up to 75 per cent of carbon to leakage.
There’s also an emerging field of geoengineering, a radical plan to artificially cool the earth by spraying aerosols into the stratosphere or sending a giant mirror into orbit to reflect sunlight away – but experts are divided over its social, ethical and political implications.
Trees, on the other hand, are relatively cheaper to plant and have 400 million years of experience in capturing carbon. Trees also cool the air by releasing water into the atmosphere in a process known as evapotranspiration. What is more, carbon does not leak out of them, unless they are burnt.
Dr Zürcher says a young willow tree building up a dry biomass of 75kg in the first five years of growth capture 140kg of CO2, equivalent to the emissions of one car over 1,000 km.
“Trees are an efficient carbon absorber, they are like a pump. It’s more efficient than geological CO2 capture, which... would have an incalculable negative impact on landscapes,” he says.
According to various academic studies, reforestation is among the cheapest carbon-capture methods, costing around USD25-50 to capture one tonne of CO2. Other estimates put the cost at a far lower USD3.5.
Seeing the forest for trees
But there is more to Dr Zürcher’s thinking than reforestation.
Trees are an efficient carbon-storer, he explains, whether they are standing or not. So even when a tree is cut down for construction or other uses, for example furniture, it continues to store large amounts of CO2.
Which is why, he says, that using wood more extensively as a construction material will play a key role in putting the world on a more sustainable footing.
Wood also helps save energy over the life of a building thanks to its superior thermal insulation properties. Every cubic metre of wood used as a substitute for steel or aluminium reduces CO2 emissions to the atmosphere by an average of 0.9 tonnes.
Even when the cost of processing and transporting wood is taken into account, its CO2 footprint is negative over its entire lifecycle.
Dr Zürcher cites a study undertaken in Germany which found that the fossil fuel energy required to process and transport wood amounts to just 15 per cent of the total amount of energy that’s locked within it.Herzog, Natterer, Schweitzer, Volz, Winter: Timber Construction Manual
“It’s an interesting calculation: if you use 1 calorie of fossil fuel, you get 7 calories blocked or sequestered – it gives you a new perspective,” he says.
“What shall we do with our last potential of fossil energy? Do we burn it until the end like we’ve done, or shall we use it to ensure the ecological and energy transition? It’s an investment choice.”
