Their carbon-storing potential is being blunted Brooke Whatnall/Getty

Our planet is getting greener thanks in part to the growth-boosting effects of extra carbon dioxide in the air. But this greening won’t soak up quite as much carbon dioxide from the atmosphere as climate scientists have been projecting.

That’s the conclusion of the first experiment to test the effect of raised CO 2 levels on trees growing in soil low in phosphorus, which is common in the tropics and subtropics.

The finding suggests forests will store around a tenth less carbon than expected, meaning CO 2 levels will rise even faster than computer models are projecting. “It can make a huge difference in the projections,” says David Ellsworth of Western Sydney University in Australia, whose team carried out the experiment.


Satellite studies show that while plant growth has declined in some regions, overall the planet is getting greener as it warms. Higher temperatures are usually the main factor in colder regions, while in warmer regions rising CO 2 levels can be the main factor boosting growth.

Higher CO 2 levels can boost the growth of some plants providing they have enough of all the other nutrients they require. Projections of future CO 2 factor in this fertilising effect. However, they may be overestimating it because they don’t take into account the fact that many soils in tropical and subtropical regions are low in phosphorus – an essential nutrient.

To find out if this matters, Ellsworth’s team raised CO 2 levels in six small plots in a mature eucalyptus forest near Sydney, growing in phosphorus-poor soil. The trees studied were a mix of species and ages.

Similar experiments in temperate forests with phosphorus-rich soil have boosted growth by as much as 20 per cent. But Ellsworth’s team found no evidence of any growth boost at all.

A lack of phosphorus is just one of many factors that will limit the CO 2 fertilisation effect. And even when plant growth increases, carbon storage does not necessarily go up.

A study led by Martin Brandt of the University of Copenhagen in Denmark shows the effect of another factor limiting carbon storage – us. Their satellite study suggests that there has been an overall increase in woody vegetation in Africa.

However, in humid areas with high population levels, woody vegetation levels have fallen despite the growth boost, they conclude – presumably because of the felling of forests.

The biggest increases in woody vegetation were in dry areas with a low population, but it is not yet clear if this is making up for the release of carbon from forests elsewhere.

Estimating amounts of carbon locked away in forests will require very high-resolution satellite data that reveal individual tree canopies and their height. “It is challenging, but we are working on that,” Brandt says.

Journal reference: Nature Climate Change, DOI: 10.1038/NCLIMATE3235

Journal reference: Nature Ecology & Evolution, DOI: 10.1038/s41559-017-0081