Lianas are competing with forest trees and, in some cases, reducing the amount of carbon they can store
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In 2020, American billionaire and internet entrepreneur Marc Benioff announced a plan to plant one trillion trees worldwide by the end of the decade, an amount, he claimed, that would absorb 200 gigatonnes of carbon dioxide from the atmosphere. Benioff’s initiative is one of three ‘trillion trees’ campaigns launched in recent years to tackle the climate crisis, alongside more than 100 government tree-planting pledges and many more vegetation-restoration projects supporting everything from salt marshes to hedgerows. When it comes to increasing carbon sequestration, however, scientists now know that not all plants are helping.
In the canopies of forests worldwide, lianas – fast-growing, woody climbing vines – are competing for space. Lianas are most easily recognisable in tropical rainforests, where they can grow to 60 centimetres in diameter and more than 100 metres in length, but Ethan Belair, a research forester at US-based conservation organisation the Nature Conservancy, says that they’re found across temperate and boreal forests too. Sometimes called ‘structural parasites’, lianas use their host tree (or trees) to reach up into the canopy, which can be more than 40 metres above the ground, where they form up to 40 per cent of the foliage. They are essential components of the natural forest ecosystem, providing habitat and food for birds and primates and forming arboreal corridors for a range of species. But in many forests, the proliferation
of lianas is becoming a problem.
‘We tend to see a higher density of lianas in places with historically high human land use,’ says Belair. ‘Areas that have been cleared for temporary agriculture or where there is selective timber logging, for example. In those scenarios, we end up with a lot more sunlight hitting the ground than in a closed-canopy forest and that helps plants like lianas and herbaceous grasses to flourish – but the lianas are the ones we’re worried about.’
If left to their own devices, lianas can quickly take over a forest and crowd out emerging tree seedlings while impeding the growth of their host tree, and consequently its ability to sequester carbon dioxide. According to Belair, a number of recent field studies have indicated that liana removal has the potential to improve carbon sequestration in forests, an idea supported by decades of field observations. ‘My colleague, ecologist Francis Putz, has seen first-hand the increases in growth and quality from the forest when you remove lianas,’ he says. Together, they calculated that selectively cutting back lianas could remove as much as 800 million tonnes of carbon dioxide from the atmosphere.
Belair explains that while lianas also store carbon – ‘any woody plant is going to have a carbon-storage component’ – they store much less than a liana-free tree. That said, he isn’t calling for a ‘blanket removal of lianas’. ‘We don’t want to have such a negative impact that the benefits they provide for wildlife and Indigenous people are lost.’ But in areas where there is an ‘artificial surplus’ due to activities such as selective logging, cutting them back wouldn’t just restore forests to a more natural condition, it could be a cost-effective – even profitable – climate solution.
Liana cutting is easily incorporated into existing forest-management practices, says Belair, and the cost of the treatment is estimated at less than US$1 per tonne of carbon dioxide. At the same time, it’s expected to boost timber yields and potential timber revenue, which helps to protect the land from more damaging extractive industries. ‘In a forestry setting,’ explains Belair, ‘the environmentally beneficial thing and the profitable thing are not usually the same thing; in fact, oftentimes they are at odds.’
Belair hopes to take advantage of this by working with forest-concession managers in the tropics, large owners such as states and governments – people he says have the opportunity to influence the way that tens or hundreds of thousands of hectares are managed. ‘I want them to realise the alignment of incentives here – that selective liana removal scratches our back, but it also scratches theirs.’
‘Liana removal in managed forests is a climate no-brainer with a double win for climate at a cost of pennies per tonne of carbon,’ agrees Peter Ellis, global director of natural climate solutions science at the Nature Conservancy. ‘If you’re looking for a ready-made “low-hanging” natural climate solution, I can’t think of a better option.’
