Tropical forests, sometimes referred to as the Earth’s lungs, are having trouble breathing.
Woody vines, known as lianas, are slowly outcompeting trees in tropical forests, making it harder for those forests to inhale the vast quantities of carbon dioxide that humans pump into the atmosphere every year. In a new study published last week in the Proceedings of the National Academy of Sciences of the United States of America, scientists estimate that without the lianas’ strangling vines, tropical forests could soak up 4 times as much carbon dioxide as they do now.
The vines have grown more widespread in recent decades—increasing in some forests by as much as 75%, said Geertje van der Heijden, the study’s lead author and a tropical ecologist at the University of Nottingham, in the United Kingdom. No one knows for certain why lianas are proliferating, she noted, but one explanation could be that they thrive under higher concentrations of carbon dioxide or drier conditions. Another theory is that because trees are dying faster, lianas can quickly move into the resulting gaps in the canopy, van der Heijden said.
The Amazon forest—which is the largest tropical forest in the world—absorbs more carbon than it releases, making it a significant global carbon sink. It sequesters just under 1 billion metric tons of carbon every year through photosynthesis and stores it in the woody tissues of trees.
However, climate change and other human influences are taking their toll on the Earth’s forests, especially the tropical ones. Since 1990, the world has lost 129 million hectares to deforestation, predominately in tropical South America and Africa, according to a recent report from the Food and Agriculture Organization of the United Nations. A study in Nature published in March reported that Amazon trees have been dying at higher rates in the past few decades—possibly because of the atmosphere’s increasing carbon dioxide concentrations.
The increasingly abundant lianas are also outcompeting tropical forests’ trees, van der Heijden said. Their extensive root systems more efficiently take up water, whereas their leaves blanket the canopy, blocking out sunlight. The more lianas there are, the faster trees die, allowing the lianas to spread further.
Most importantly, compared to trees, lianas produce less woody tissue—where carbon is stored in the long term—and more leaves. In other words, what lianas make up for in total biomass, they lack in carbon storage.
“Lianas, on the forest level, make sure that the carbon taken up from the atmosphere is less and that the carbon released back to the atmosphere is higher,” said van der Heijden.
Previous studies have shown that lianas diminish tropical forests’ carbon sequestration, but van der Heijden said this new study quantifies that impact for the first time on a whole-forest level. Researchers cordoned off 16 plots in a 60-year-old forest on the Gigante Peninsula in Panama in 2008 and measured total biomass and the amount of carbon taken up by each plot. Three years later, the researchers removed lianas from eight of the plots and again measured biomass and carbon uptake over another 3 years, comparing the result to plots where lianas remained. The researchers found that the lianas reduced carbon uptake by about 76% in the third year, which translates to 2.43 metric tons of carbon per hectare per year.
Given that the Amazon rain forest, for example, sprawls across 550 million hectares, the loss of carbon-storage capability “is quite a lot,” said van der Heijden.
“The cost to the planet of liana-laden trees is decreased rates of carbon uptake and storage by forests. This research shows that liana control makes sense as a global climate change mitigation strategy,” said Francis Putz, a professor of biology at the University of Florida, Gainesville, who was not involved in the research.
However, despite their negative impact on carbon storage, lianas provide food and shelter to many organisms and biodiversity to the forest as a whole and shouldn’t be blindly chopped down, van der Heijden said. Rather, the liana’s impact should be incorporated into climate models to better understand how tropical forests’ carbon intake abilities may change over time, she said.
“The importance of this [study] is that increasing lianas and lianas themselves are really not taken into account in any of the models that try to predict what the climate and tropical forests will look like,” she continued. “Lianas are increasing in tropical forests, [and] that needs to be taken into account if we want to make more accurate predictions of what’s going to happen with these forests and with climate change in the future.”
—JoAnna Wendel, Staff Writer
Citation: Wendel, J. (2015), Woody vines limit how much carbon tropical forests sequester, Eos, 96, doi:10.1029/2015EO037881. Published on 21 October 2015.