The great climate change challenge should consider the world’s trees. New wooden cities and suburbs − that is, new homes fashioned from wood rather than bricks and mortar − could consume 55 million tonnes of carbon dioxide (CO2) a year: that adds up to almost half of the annual greenhouse gas emissions from Europe’s cement industry.
And the bigger and more substantial the tree, the more value in the arboreal effort to limit global warming and contain climate change. A US study has found that large trees − those with trunks of 53 cms at breast height − might make up only 3% of a measured plot, but contain 42% of all the above-ground carbon.
And trees could enhance human health as well as capture carbon: an international team believes that tree-sourced food − think mangoes, avocados, Brazil nuts and so on − could deliver much more nourishment for tomorrow’s supper tables.
The planet is home to at least 7,000 edible plants. Half the world’s calories come from just four crops, all high in calories but low in nutrients − wheat, rice, sugar cane and maize − that simultaneously fuel both malnutrition and obesity. There are 50,000 tree species in the tropics alone, a number of them potentially new sources of high quality food.
The conclusions of all three studies are tentative. But they are also familiar: that is, other research teams have for years been investigating trees as fabric, trees as absorbers of atmospheric carbon, and trees as enhanced forms of farming.
“This is the first time that the carbon storage potential of wooden building construction has been evaluated on the European level”
But all three offer a new and more detailed look, and confirm the big picture: when it comes to climate, the world’s trees are among the most important things on the planet.
Finnish scientists report in the journal Environmental Research Letters that they looked again at 50 case studies of timber as a way of growing cities: Europe builds about 190 million square metres of housing each year, largely in cities, and this demand for new homes is growing at 1% a year. Buildings worldwide − concrete, steel, glass, bricks, tiles, paving and so on − account for one third of global greenhouse gas emissions.
If however 80% of new residential buildings in Europe were built of, clad with and furnished from timber from sustainable forests, then this could represent a carbon sink of 55 million tonnes of CO2 a year, represent a 47% cut in greenhouse gas emissions from Europe’s cement-makers, and deliver energy-efficient homes.
“This is the first time that the carbon storage potential of wooden building construction has been evaluated on the European level, in different scenarios,” said Ali Amiri, of Aalto University, who led the study. “We hope that our model could be used as a roadmap to increase wooden construction in Europe.”
US scientists report in the journal Frontiers in Forests and Global Change that they took a close look at large diameter trees on National Forest lands in the states of Oregon and Washington.
Size matters
Trees with diameters greater than 21 inches, or 53.3 cms, accounted for only 3% of the total number of trees in the plots they chose to study. But when it came to absorbing atmospheric carbon, these were the real heavyweights. They contained 42% of all the above-ground carbon in the entire measured ecosystem.
Trees bigger than 30 inches, or 76 cms in diameter, made up only 0.6% of the total number, but accounted for 16% of the total above-ground carbon. The message was, the bigger the better.
The forest giants are themselves natural habitat: they support birds, mammals, insects, microbes and other plants; they serve as soaring water towers, tapping groundwater and cooling the environment through evotranspiration. And their value as a store of atmospheric carbon has been confirmed again and again.
“If you think of adding a ring of new growth to the circumference of a large tree and its branches every year, that ring adds up to a lot more carbon than the ring of a small tree,” said David Mildrexler, of Eastern Oregon Legacy Lands, who led the research. “This is why specifically letting large trees grow larger is important for climate change.”
And trees, researchers from five nations argue in the journal People and Nature, could be the healthy solution both to the climate crisis and to poor diet.
Better fed
Of the world’s 100 most nourishing foods, 14 come from trees. The planet is home to 60,000 species of tree, and many − especially in the tropics − provide nutritious fruits, nuts, leaves and seeds. Many are exploited only by small rural communities.
In the Amazon basin, for instance, a shrub called Myrciaria dubia was found to have a vitamin C content 54 times that of an orange. The scientists looked at seven tropical nations to identify foods from 90 tree species: these provided local families with 11% of diet by mass but 31% of the daily intake of vitamins A and C.
Never mind the giant commercial palm oil plantations and cacao harvests: the researchers see tree crops as something that could sustainably help hundreds of millions of the world’s smallholder farmers, by diversifying income and providing more and healthier food with a very low investment.
“The right type of trees in the right place can provide nutritious foods to improve diets sustainably while providing other valuable ecosystem services such as carbon sequestration,” said Merel Jansen, of the Swiss Federal Institute of Technology known as ETH Zurich, who led the investigation.
“It can also contribute to development issues related to poverty reduction, biodiversity conservation, and food security.”
by Tim Radford
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