When a woody vine, or liana, wraps around a tree it stands to reason that there will be a cost to the tree. Does that hurt the tree's ability to produce fruit? What does that cost add up to in an entire forest? If climate scientists need to calculate how much carbon can be stored in a neo-tropical forest, are the lianas throwing off their results?
Lianas are a common sight in Central American neo-tropical forests. By growing up along the trunks of existing trees, the plants can reach sunlight faster and more reliably than if they had to grow a thick, woody trunk strong enough to support their full height. Compared with the trees they twist around, lianas are able to put more energy and carbon into the production of leaves and seeds and less towards growing a trunk.
The study, led by Panamanian researcher Maria Garcia León, who began the research while still an undergraduate student and intern, compared 16 different plots of land with one another on Barro Colorado Island in Panama. In eight of the forested plots, every liana was severed and killed. In the other eight plots of land that made up a control group, no lianas were severed. After five years of study, the scientists were able to see what differences the lianas made.
Trees in the canopy bearing fruits were 150 percent greater in the liana-free zones and there were 109 more species of fruiting trees. A tree covered with lianas faced double the chance of dying as compared with a tree without lianas.
Traditionally, ecologists have tended to think of carbon as a zero-sum game. That the amount of carbon in any ecosystem tends to be constant, while competition between species moves the same amount of carbon around in different directions and forms. But that model “fails when we think of tree-liana competition because lianas themselves store less carbon,” says Stefan Schnitzer, a coauthor of the paper, research associate at STRI and Mellon distinguished professor of biology at Marquette University.
“It can be 75 percent per year less carbon uptake when you have lianas versus no lianas,” Schnitzer says. “When lianas produce more leaves, they fall off and they are turning over carbon faster and you are getting more flux in those ecosystems.”
These findings are potentially significant as scientists around the world race to understand the likely impact of manmade global climate change. Having accurate models of how different types of forest can absorb carbon can help to predict, or possibly reduce, the exact degree of warming and sea level rise that will take place over coming decades and centuries.
According to the paper, lianas seem to hurt tree growth by several different means. First, their efforts to choke and shade the leaves of trees directly reduce the energy those tree have to put into growth and fruit production. Next, the lowered fruit production means that fewer seeds are dispersed to grow new trees. And then even when gaps in the canopy open up and allow new seedlings a chance at becoming trees, lianas tend to completely smother those seedlings and kill them. Lianas are driving the structure of forests to a major degree that was not previously understood.
This is not to say that lianas are a bad influence on forests overall. They can provide ecological benefits. Lianas form pathways from tree to tree that are used by monkeys, squirrels, sloths and thousands of species of insects and other small arthropods. The same group of scientists published a previous paper in 2016, which found that neo-tropical forests without lianas can become death traps for tiny arboreal creature.
“When you have a tree with no lianas on it, it is basically an island for non-flying invertebrates,” Schnitzer says. “If there is a really aggressive species like an azteca ant it will go up there and kill everything. But when there are lianas, the invertebrates can go in and feed and then escape.”
Availability of fruit and seeds in liana-heavy neo-tropical forests may also be reduced.
“Without lianas there would be no grapes,” Schnitzer observed. “Without grapes there would be no wine. However, in the neo-tropics most lianas are wind-dispersed. Small seeds and big wings on the seeds. They aren't good food sources for animals. . . The trees are producing fruits that animals prefer.”