Vines may be bad news for forest carbon storage

A recent study finds that vines can reduce the carbon storage capacity of tropical forests by crowding out and sometimes even killing trees.

Vines may be bad news for forest carbon storage
  • Researchers determined that lianas could potentially reduce long-term carbon storage in tropical forests by one-third or more.

A recent study published in the Proceedings of the National Academy of Sciences finds that vines called lianas reduce the carbon storage capacity of tropical forests by crowding out and sometimes even killing trees. With tropical forests acting as one of the world’s greatest defenses against global warming, the study’s authors say that understanding the impact of lianas on carbon sequestration is important to figuring out just how much carbon forests will be able to store in the coming years.

Lianas are woody vines that use other plants – mainly trees – as support by climbing on them to reach higher, sunnier localles. These vines are native to tropical forests, which exhibit the greatest abundance and diversity of these climber species. Research suggests lianas have been increasing in number and bulk in tropical forests recently. They can also inhibit tree growth by up to 84 percent and significantly increase the chances of a tree’s death.

Tropical forests are major carbon storehouses, sequestering nearly 30 percent of the world’s terrestrial carbon by taking it out of the air through photosynthesis. The carbon storage capacity of the tropics is already being undermined in many areas as humans fell forests to produce timber, clear land for crops, and build roads. Global Forest Watch Climate shows that tropical deforestation led to the release of more than 2,200 million metric tons of carbon every year between 2001 and 2013.

The world’s tropical forests store around 30 percent of the world’s terrestrial carbon. Brazil’s forests alone held more than 64 million metric tons of carbon in 2000, according to data from Global Forest Watch Climate. However, land use changes such as conversion of forest to cropland and pasture are releasing that stored carbon into the atmosphere, further exacerbating global warming. Even the state of Amazonas, which holds a large share of Brazil’s Amazon rainforest and is relatively unscathed compared to more southerly states, is trending upwards in terms of carbon emissions. Between 2001 and 2014 the state released 660 metric tons of carbon due to deforestation activities.
The world’s tropical forests store around 30 percent of the world’s terrestrial carbon. Brazil’s forests alone held more than 64 million metric tons of carbon in 2000, according to data from Global Forest Watch Climate. However, land use changes such as conversion of forest to cropland and pasture are releasing that stored carbon into the atmosphere, further exacerbating global warming. Even the state of Amazonas, which contains a large share of Brazil’s Amazon rainforest and is relatively unscathed compared to more southerly states, is trending upwards in terms of carbon emissions. Between 2001 and 2014 the state released 660 metric tons of carbon due to deforestation activities.

As many countries prepare to reduce deforestation-caused emissions to minimize global warming and keep its worst consequences at bay, could other factors like a surge in lianas hinder the capacity of existing forests to sequester carbon?

Researchers from Marquette University, University of Nottingham and the University of Minnesota undertook a study to figure this out by comparing forest plots with lianas to those where lianas had been removed. They conducted their study in Panama’s Barro Colorado Nature Monument.

After their three-year liana removal experiment, the researchers found that the study plots that had lianas accumulated 76 percent less biomass than in the plots without lianas. The researchers attribute this to reduced tree growth and increased tree death due to liana presence.

“The study shows that forests, not individual trees, where lianas are present take up 76% less carbon than forests where lianas were removed,” said study coauthor Geertje M.F. van der Heijden in an email to Mongabay. “So this means that in forests with lianas present, 76% less carbon is being captured in plant tissue – and therefore is left in the atmosphere – than in the forests where lianas were removed.”

Extrapolating these results and applying them to tropical forests as a whole, the team determined that lianas could potentially reduce long-term carbon storage in tropical forests by one-third or more.

“This is the first time the effect of lianas on carbon cycling has been shown in such detail and on a large scale,” Jennifer Powers, a faculty member of the University of Minnesota’s College of Biological Sciences and one of the authors of the paper, said in a press release. “It suggests that over the decades to come, lianas could increasingly suppress the ability of tropical forests to sequester carbon.”

The authors explain that the increase in liana abundance and biomass in these forests could be driven by a number of factors, several human-caused.

Lianas are woody vines that use other plants for structural support. Photo by Geertje M.F. van der Heijden.
Lianas are woody vines that use other plants for structural support. Photo by Geertje M.F. van der Heijden.

“There are several hypotheses proposed, but none of them are yet proven to be the main driver and they could all play a role simultaneously as well,” said van der Heijden, who is affiliated with Marquette University and University of Nottingham. “A few of the proposed mechanisms are an increase in atmospheric carbon dioxide concentrations, increase in (anthropogenic and natural) disturbance, reduction in rainfall and land use change.”

The amount of carbon stored wasn’t the only difference between the liana-present and liana-free forest plots. The researchers also found that lianas may change how the above-ground carbon was stored. Specifically, liana presence was correlated to a shift in carbon storage to above-ground vegetation with a shorter lifespan, which will release its carbon into the atmosphere more quickly than if that carbon were stored in the trunk of a long-lived tree. Furthermore, because lianas hinder tree growth, many trees that are present in a liana-filled environment may not be able to reach prime carbon-storage size.

Taken together, the researchers found that these liana-influenced changes limited forests to only 24 percent of their carbon sink potential.

The authors caution that lianas by themselves aren’t bad, and this study should not be seen as advocating their complete removal. They are important and native components of lowland tropical forests that naturally contribute up to 25 percent of an area’s plant biodiversity.

“The last thing we want to do is therefore claim that we should go out and cut them down!” Lianas certainly have their advantages,” Van der Heijden said. “They are a very important food source for animals, birds and insects. They also provide alternative pathways into the canopy for animals, birds and insects. So even though they have a negative impact on the amount of carbon being stored and sequestered in tropical forests, they provide numerous advantages and are generally very important in tropical forests.”

Instead, the researchers hope that their work will add to the paucity of research on lianas, and help guide scientists who are attempting to predict the trajectory of carbon storage as atmospheric concentrations increase and the world heats up. Van der Heijden also said that this type of research is important for effective forest management.

“We should not underestimate the impact of [lianas],” van der Heijden said. “Compared to trees, lianas have been chronically understudied until recently and were thought to not be important for processes like carbon storage and sequestration. Our research demonstrates that the presence of lianas can have a huge impact on the capacity of tropical forests to store and sequester carbon, which also has implications for climate change if lianas keep increasing…”

 

Citation:

  • van der Heijden, G. M., Powers, J. S., & Schnitzer, S. A. (2015). Lianas reduce carbon accumulation and storage in tropical forests. Proceedings of the National Academy of Sciences, 112(43), 13267-13271.

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