As trees age, their climate benefit grows

Trees play an important role in the fight against climate change. They capture and store carbon in their biomass — their roots, stumps and branches.

According to new European research, when it comes to a tree's climate benefit, as a tree get older it also stores more carbon.

What does this new study suggest about older trees and climate change?

The University of Hamburg study suggests that old trees know best. Researchers studied unmanaged tropical forests in Suriname, on the northeastern Atlantic coast of South America, and looked at three different species of trees that ranged in age from 84 to 255 years old. They aren't the oldest trees on the planet, but they make up a complete wilderness of unmanaged forests.

The study found that the older a tree is, the better it absorbs carbon from the atmosphere. In fact, the research suggests that almost 70 per cent of all the carbon stored in trees is accumulated in the last half of their lives.

Why do old trees store more carbon than younger trees?

The researchers aren't sure yet, but the best hypothesis is that old trees store more carbon because they are taller and form the upper crown canopy of these rain forests. Old trees aren't storing more carbon because they are bigger. They store more carbon in proportion to their size.

The key seems to be that older trees can reach the top of the canopy and have consistent access to the sun. Because trees store carbon by photosynthesis, they take in the energy from the sun as well as carbon dioxide from the atmosphere and convert it into carbon-based sugars to fuel their tissues.

Younger trees are more sensitive to the changing conditions of rainfall and sunlight than older trees. And the tree ring data that the scientists studied reflects that. The group found that in the three species studied, the trees accumulated more than 40 per cent of their lifetime's worth of carbon in just the last quarter of their lives.  This points to a real benefit of old age trees in our battle against climate change.

How much carbon are we talking about?

There are some impressive numbers. Ninety per cent of the biomass on the Earth's surface is stored in the forests (that doesn't include the oceans: another important carbon storage area). Tropical forests in particular remove 0.5 gigatonnes of carbon globally every year.

Plants generally take up about half of the annual carbon release, so our forests are really crucial in the battle to curb global release of carbon dioxide into the atmosphere. Plants do an incredible job sucking out carbon dioxide from the atmosphere and converting it into a form that is hard to release back. But here's the big problem: deforestation and forest damage removes about a gigatonne of capacity each and every year.  

What do these findings suggest about forest management and climate change?

Unfortunately, this study shows it's not just a matter of planting many more trees to make up for deforestation. Previous studies done on managed forests — places where trees have been planted at about the same time and are all similar species — show that managed forests take up a lot less carbon than unmanaged forests of the same age. This poses a problem. It could be that it's part of natural selection in unmanaged forests with competition, that creates the best carbon sinks. We can't replace century old trees in a biosphere and expect the same results that nature has evolved.

The best option, as this paper and its authors suggest, is recognizing the importance of old growth forests in their carbon storage capacity as efficient and important parts of our battle against climate change.

Of course the solution could always be limiting carbon release, but we're past the point of solving climate change being as simple as ceasing to drive cars and eat beef. It also requires methods to remove the pre-existing carbon from our atmosphere and nothing is better at it than trees.