First Author

Forests both store and release carbon, and are a crucial piece of the global climate puzzle. Until now, it was impossible to discriminate between the effects of carbon dioxide levels, forest age, management practices, temperature and pollution on the carbon storing abilities of boreal and temperate forests. Established forests were not thought to offer the valuable storage potential of younger forests. But the work of an international collaboration, reported on page 849, finds that pollution, in the form of increased nitrogen, allows established forests to soak up carbon. Federico Magnani, a forest ecologist at the University of Bologna in Italy, spoke to Nature about the work.

What allowed your team to finally separate the age and nitrogen deposition effects?

First we removed the effects of age by creating a massive database with carbon fluctuation data from publications and five European forests of different ages. We then used new gridded maps of nitrogen deposition in the Northern Hemisphere, which allowed us to show that carbon storage is substantially enhanced by nitrogen pollution. This work shows that it is possible to transform ecology into an exact science.

How do natural nitrogen levels compare with human-induced nitrogen deposition?

At present, nitrogen levels in some forests are more than an order of magnitude greater than would be expected under natural conditions. Natural levels result primarily from lightning strikes and microbial nitrogen fixation. Nitrogen was a limiting nutrient until combustion and intensive agriculture greatly increased the amount pouring into some ecosystems.

So nitrogen pollution is 'valuable' in the context of encouraging carbon storage?

To a point. In the 1980s, when we began to consider whether acid rain and nitrogen deposition were threats, long-term ecological studies were established. In the 1990s, we realized that the trees in those plots were growing faster, and wondered whether that would translate into increased carbon storage. We've found that it does, but you can still have too much of a good thing.

What is the biggest misconception about the forest carbon cycle?

That forest management translates automatically into a loss of carbon. We should ask where the carbon is going. If it ends up in a wooden building rather than as carbon dioxide in the atmosphere, the tree loss may be justified. But if we remove too much carbon or other nutrients, we will, in the long run, undermine the forest's abilities.