Freeing trees of their liana load can boost carbon sequestration in tropical forests

The liana is an opportunistic plant, a vine that, in the right conditions, can smother a tree as it hitches a ride up to the top of the forest canopy. It’s estimated that 250 million hectares of managed forest are affected by rampant liana growth, as the plant thrives where the forest floor has been disturbed by clearing activities like logging, as well as natural events such as wildfires and hurricanes.

“Liana” is a catch-all name for long, woody vining plants, many species of which are native to tropical forests around the world. While the negative effects of lianas on tree growth have been known for some time, a study published in 2023 in the journal Forest Ecology and Management has revealed the carbon benefits of stripping back these woody vines and allowing the trees to grow unencumbered.

The study found that in selectively logged forests, where certain tree species are commercially cut and the rest left standing as an alternative to clear-cut felling, freeing just five trees per hectare of their liana load across the 250 million hectares of degraded managed land could remove 800 million tons of CO2 from the atmosphere over 30 years — averaging 3.2 tons of CO2 removed per hectare — as well as boosting sustainable timber production. And all at a cost of just $1.50 per hectare.

The research is based on managed forests that have previously been disturbed, explains Ethan Belair, a natural climate solutions forester at The Nature Conservancy (TNC), and one of the report’s authors.

“Lianas in those areas are at higher density than they would have existed normally,” he says.

The process of cutting the lianas is kept as simple as possible, Belair continues, with the vines cut with a machete at ground level and simply left to rot on the tree. Trying to pull them down, he says, is both dangerous and can damage the tree.

“As the liana dies, its impact on the tree’s growth dissipates, so there is no real need to remove the liana,” he says.

To be effective, Belair believes liana cutting needs to becomes a new norm in forest management and incorporated into national carbon policy frameworks. It can also give forest mangers the opportunity to access voluntary carbon markets and diversify their incomes, he adds.

But he also stresses that this is not about the blanket removal of lianas.

“We have identified what we think are safe liana cutting densities of around five trees per hectare in managed forests,” Belair says. “This is a small overall impact on the density of lianas in a whole forest, but brings a large impact in increased growth of targeted trees for timber and carbon benefits.”

A trial in a former logging concession in the Belize Maya Forest, which was under threat of complete deforestation until it was purchased by TNC, is being conducted by TNC to determine the most effective way of removing lianas. It is also assessing the effects that removing lianas has on different trees and connectivity through the forest, as well as potential impacts on biodiversity.

Denver Cayetano is a forest biologist who is helping run the trial. He explains that many species of liana are native to Belize, and as such they have a value in the ecosystem both for people and animals.  They provide food and traditional medicines, while arboreal animals use them to move through the canopy, helping them to avoid predators that hunt on the ground. It makes liana cutting: “a delicate balancing act,” he says. “What is a reasonable amount of liana to remove?”

As well as stifling individual tree growth, a proliferation of lianas can also lead to arrested succession, he continues, and prevent a forest that has been damaged by logging or natural events from regrowing into a mature forest.

“Removing lianas opens up the canopy,” he says, “(and) could speed up or restore that process of succession,” making liana cutting a treatment that can help bring back forests to their natural state.

The Belize trial is focusing on six timber species including Drypetes brownii (bullhoof), which has medicinal uses, and Swietenia macrophylla (mahogany), in order to assess how each tree species reacts to losing its liana load. Initial results from the trial have been delayed by a hurricane that hit the forest last year.

The trial is also quantifying the potential of the liana itself to store carbon, comparing this with the extra growth put on by the tree, to produce a net benefit. However, because lianas focus their efforts on leaf growth and reproduction rather than the woody growth that stores carbon, the carbon benefits from extra tree growth is likely to far outweigh the liana loss, Cayetano says.

Another important aspect of liana cutting is that it provides local employment, he adds, and an opportunity to get local people more involved with the environment and forestry.

While the concept of liana cutting is generally supported, some academics have echoed the report’s concerns around the possible impact on forest biodiversity.

“I think it’s always good to be cautious in dealing with something as complicated as a tropical rainforest, it’s a challenge,” says Bill Laurance, director of the Centre for Tropical Environmental & Sustainability Science (TESS) at James Cook University in Queensland, Australia.

“Vines are an important source of fruit and flowers and leaves and other resources,” he continues, with animals such as orangutans and gorillas also using the vines to make nests and dens.

Laurance also believes that liana cutting should only be carried out in managed forests, where access already exists. “If you start talking about old growth forests, somebody is going to start putting roads or trails or something in which are then going to be increasing access to the forest for poachers, and land grabbers … and all the different people that will invade the forest.

“There is no way this should ever be an excuse for anybody accessing forests that are not used by people [already].”

Liana cutting is not limited to the Western Hemisphere. There are also trials in Gabon and Indonesia, at the Wana Bakti timber concession, in East Kalimantan province, also owned by TNC, where the work is part of on-going efforts to reduce the impact of commercial forestry.

According to Delon Marthinus, a climate and forestry specialist for Yayasan Konservasi Alam Nusantara (YKAN), which operates TNC’s Indonesia program, liana cutting is being introduced alongside other technologies such as remote sensing of trees using LiDAR, and a new reduced impact logging system called Logfisher. It is also linked to the concept of Reduced Impact Logging Practices (RIL-C), which includes practices such as improved road narrowing and skid trail planning, which creates routes that allow logs to be dragged out of the forest without damaging trees that are still standing.

As well as carbon sequestration, liana cutting can also boost timber production while reducing collateral damage to the surrounding forest, Marthinus says, as lianas often connect trees together so that when a targeted tree is being cut, it may also bring down others that are tangled up in shared vines.

The vines can also deform trees, Laurance adds, as they force them to grow round the thick stems, making them difficult to mill and leading to excessive waste.

“The timber that is being extracted from these forests is often very valuable,”and is often used to make furniture and veneers, Laurance says. “Anything to optimize the production (of) marketable wood … would be a big advantage.”

Belair agrees that the idea of improving the quality and quantity of timber that comes from selectively logged forests is important. And while he concedes that: “one of the easiest ways to increase carbon in forests is to not manage it … we also know that people use wood products … better a paper plate than a plastic one.”

Timber also supports local economies, he says, and: “trade-offs are common when doing conservation work in forests that are logged.”

But while it’s usually a case of carbon or timber, he continues, liana cutting is an opportunity for conservationists and timber companies to work together, in theory boosting both carbon sequestration and the economic value of the timber at the same time.

“Timber companies manage a lot of land; they can have more on-the-ground impact than I can,” Belair says. “By showing that I’m helping to support their business model, I can then start asking things of them.

“I’ll sort out the lianas, and in return you set aside certain areas of the forest; they’re more willing to compromise if we’re working with them instead of against them.”

Banner image by Dinesh Valke via Wikimedia Commons (CC BY-SA 2.0).

See related: 

Tropical forests share similar mix of common and rare tree species, study shows

Citation: Putz, F. E., et al. (2023) Liana cutting in selectively logged forests increases both carbon sequestration and timber yields. Forest Ecology and Management, 539 doi:10.1016/j.foreco.2023.121038.

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