Climate change could be making forests shorter – this is how

Global warming is causing forests to lose many older trees, leaving the planet’s forests younger - and shorter, according to new research.

Using data from 160 previous studies and satellite imagery captured over decades, a team of more than 20 scientists analyzed the state of the world’s forests. The findings, published in Science, show that between 1900 and 2015 more than a third of the planet’s old-growth forests disappeared.

In some areas of Canada, the western US and Europe, tree loss rates have doubled over the past 40 years, affecting a disproportionately high number of mature trees. Alongside decades of logging activity and forest clearances, increasing temperatures from human-made climate change are seen as a significant factor.

As well as Brazil, where both human activity and wildfires have cleared large tracts of Amazon rainforest, Russia and North America have lost vast areas of tree canopy. More than 55,000 square kilometres of Russia’s tree cover was cleared in 2018 – roughly equivalent to the size of Croatia.

Stunted growth
The loss of mature trees is harming global biodiversity levels, as animal habitats disappear and ecosystems are disrupted. What’s left are predominantly younger trees, less able to reach for the skies due to warming temperatures, which limits photosynthesis and stunts tree growth.

“Unfortunately, mortality drivers like rising temperature and disturbances such as wildfire and insect outbreaks are on the rise and are expected to continue increasing in frequency and severity over the next century,” says the lead author of the study, Nate McDowell of the US Department of Energy’s Pacific Northwest National Laboratory.

“So reductions in average forest age and height are already happening, and they’re likely to continue to happen.”

Carbon sponges

Trees act as natural sponges to absorb carbon dioxide from the planet’s atmosphere. When mature forests – which are generally bigger and can hold more carbon than younger areas of canopy – die, the planet’s capacity to store CO2 is diminished, exacerbating climate change.

Another study, led by Martin Sullivan from the University of Leeds and Manchester Metropolitan University, suggests that tropical forests are resistant to small temperature changes until they reach a threshold of about 32℃. Beyond this, trees stop growing and begin to die, releasing carbon back into the atmosphere.

"The 32-degree threshold highlights the critical importance of urgently cutting our emissions to avoid pushing too many forests beyond the safety zone,” Sullivan says.

"For example, if we limit global average temperatures to a 2°C increase above pre-industrial levels this pushes nearly three-quarters of tropical forests above the heat threshold we identified. Any further increases in temperature will lead to rapid losses of forest carbon."

A trillion ways to tackle climate change

Aiming to support forest conservation efforts, the World Economic Forum has launched a global initiative to grow, restore and conserve 1 trillion trees. The 1t.org programme aims to connect, empower and mobilize tree planting and reforestation efforts to restore biodiversity and fight climate change.

The programme is designed to support the UN Decade of Ecosystem Restoration 2021-2030, led by UNEP and FAO, by bringing together governments, business leaders, entrepreneurs and other stakeholders.

It builds on existing reforestation programmes, providing a platform to help coordinate funds and political support, while connecting tens of thousands of small and large conservation groups.