Not all trees are equal in the fight against carbon

Replanting trees in cleared regions helps mitigate global warming, right? Well, not always. It turns out if you plant the wrong species of tree, not only can you increase carbon in the atmosphere, temperatures can rise too.

In the journal Science, University of Paris-Saclay environmental scientist Kim Naudts and colleagues built a model that analysed European forest management data over the past 260 years – which, for the first time, incorporated tree species.

They found even though the continent added nearly 200,000 square kilometres to their forest stocks – or the area of Hungary, Slovakia and Slovenia combined – between 1750 and 2010, the atmospheric temperature in summertime increased by 0.12 °C.

What’s more, they calculated around 3.1 million tonnes of carbon was released to the atmosphere during the same period.

Why? Much of it came down to a question of foliage.

In 1750, European forests comprised 70% broad-leafed trees, such as oaks. Sunlight tended to reflect off the wide shiny leaves, cooling the local area.

But many native forests were cleared. When it came to replanting, instead of restoring the same species, more commercially profitable – but darker – conifer species were favoured, such as beech and pine.

Forests in Australia managed for timber production fall well short of their carbon-storage capacity.

By 2010, conifers made up more than half of European forests. The less-reflective needles absorbed light and heat.

Broad leaves also lost more water than conifers, acting as a kind of evaporative cooling.

So what about carbon? Studies in Australia suggest that removing old growth forest increases atmospheric carbon. 

While all trees suck carbon dioxide from the air to make sugar, forests in Australia managed for timber production fall well short of their carbon-storage capacity, says Philip Gibbons, an environmental scientist at the Australian National University in Canberra.

“By logging our forests we’re actually denying them 40% of their carbon-carrying capacity.”

What are missing from logged forests, he adds, are the big trees. A 2014 Nature paper showed big, old trees grow faster and absorb carbon dioxide more rapidly than saplings.

But logging only allows trees to grow a few decades before being mown down. And tearing out trees also releases carbon that would usually be stored in soil and litter.

Naudts and colleagues acknowledge that their analysis is restricted to Europe. They suspect, though, that countries undergoing large-scale tree planting – such as China, Russia and the US – will also see similar effects.

Planting trees on cleared agricultural land is better than nothing, Gibbons says. “But it’s better to not clear at all!”