Old trees 'star players' in carbon uptake

The researchers found that old-growth forests shouldn't be left out of the carbon trading equation (Source: Markku Saarinen)

Anna Salleh
ABC

The largest trees in the world grow more quickly, and thus take up more carbon dioxide, than younger, smaller trees, say researchers.

The findings, published today in the journal Nature add to our understanding of how tree growth rates change over time.

"We found that tree growth rates increased continuously with tree size so, on average, the biggest trees in the forest are the fastest growing trees in the forest in terms of mass", says lead author, Dr Nate Stephenson, a research ecologist with the US Geological Survey.

Stephenson says the majority view to date has been that trees have a similar growth pattern to humans - starting off slowly, getting a growth spurt in adolescence, and then slowing down again as they get older.

But, he says, a number of scientists have argued trees continue to grow faster and faster the larger they get.

"It turns out that that's the case for the large majority of tree species."

Stephenson says the latest evidence comes from the largest study to date on the growth rate of trees across the globe.

He and colleagues measured the change in diameter of 650,000 individual trees from 403 species in both temperate and tropical areas.

"On average, every year trees put on more mass than they put on the year before," says Stephenson.
"Old trees are the ones putting on the most bulk in old forests. They are the star players."

"We think it strongly supports the minority view about tree growth rate."

Paradox

Stephenson says the majority view has been supported by previous research that, among other things, has shown as a tree gets older its growth rate per leaf decreases.

"A given leaf on a big tall eucalyptus tree photosynthesises less than one on a smaller tree," says Stephenson.

Studies have also found that, overall, older forest stands grow at a slower rate compared to younger stands.

But, says Stephenson, such measurements have been made at the scale of individual leaves and at the scale of forest stands.

"Most people have just inferred that the same has to be happening at the scale of the individual tree but it's not," he says, adding the latest findings are not incompatible with earlier observations at the leaf and forest stand scales.

"Our observation is compatible with those other observations but it just may not be intuitive," says Stephenson.

He says even though productivity per leaf in older trees decreases, this is counteracted by the fact that these trees have a lot more leaves that collectively result in the tree's faster growth rate.

"A tree that is a metre in diameter has, on average, 100 times as much leave mass as one that is only 10 centimetres in diameter."

At the other end of the scale, more young trees than old trees are packed into the same area of forest, and so this explains why collectively forest stands consisting of young trees put on more mass than those consisting of fewer old trees.

Implications

Stephenson says the findings will have implications for our understanding the role of trees in the carbon cycle and greenhouse gas concentrations in the atmosphere.

"We already know that the biggest trees are the most important ones to preserve for carbon sequestration because they already hold so much carbon in them. It think the finding adds a little more support to the idea we need to protect these big trees," he says.

"Not only do they hold the most carbon but they are putting on a huge amount every single year."