And that means we may have overestimated the amount of carbon that trees will pluck from the air in the future.

What does the future of forests look like in a warming world? Will Increased Atmospheric Carbon Dioxide Help Trees Grow? Or will extreme temperatures and precipitation hold back growth? The answer depends on whether tree growth is more limited by photosynthesis or by cell growth – a fundamental question in tree biology that scientists have been unable to answer until now. But a new study lifts a corner of the veil.

Horse and carriage

“A growing tree is like a horse and carriage moving along the road,” said study researcher William Anderegg. “What we just don’t know is whether the horse represents photosynthesis or cell expansion and division. This is a long standing and difficult question in the field. However, it is very important to know the answer to better understand how trees will respond to climate change.”

Photosynthesis and cell growth

In primary school we learned that trees produce their own food through photosynthesis. Trees absorb sunlight, carbon dioxide and water and turn this into leaves and wood. However, there is more. In order to convert carbon obtained by photosynthesis into wood, the cells must expand and divide. Basically, trees extract carbon from the atmosphere through photosynthesis. This is the carbon source for trees. They then use that carbon to build new wood cells – the carbon sink from the tree.

Carbon source or carbon sink?

Now the question is which of these two has a greater influence on the growth of trees. If tree growth is limited by the carbon source, this means that growth is limited only by the degree of photosynthesis. In that case, tree growth would be easy to predict using a mathematical model. And a rising concentration of carbon dioxide in the atmosphere could boost tree growth in this case.

Instead, if growth is limited by the carbon sink, the tree can only grow as fast as its cells can divide. Many factors can directly affect both photosynthesis and cell growth rate, including temperature and water or nutrient availability.

Study

To get to the bottom of things, researchers delved into the carbon sources and sinks of trees in North America, Europe, Japan and Australia in a new study. However, the researchers found no evidence confirming that tree growth and photosynthesis are linked. In other words, when photosynthesis increased or decreased, there was no parallel increase or decrease in tree growth. “This is our main argument for concluding that tree growth is not tied to the carbon source,” said researcher Antoine Cabon.

cell growth

The researchers conclude that tree growth is thus generally not limited by photosynthesis, but rather by cell growth. And that means that trees grow slightly differently than expected. “Almost all vegetation models assume that tree growth is ‘resource limited’,” says Cabon. That this has now been debunked suggests that we need to rethink the way we forecast forest growth in a changing climate. In addition, it means that we may have overestimated the amount of carbon that trees will pluck from the air in the future; they probably won’t absorb as much carbon from the atmosphere as previously thought.

This has far-reaching consequences. Forests currently absorb and store about a quarter of our current CO2 emissions. However, as tree growth slows, so does the ability of forests to absorb carbon and slow down climate change. And that in turn can influence how we try to tackle the climate problem.