BARRO COLORADO ISLAND, Panama – Stefan Schnitzer paused along one of the trails that crisscross this forested island in the Panama Canal waterway. Around him were trees, their high canopies muting the light from the tropical sun, the occasional woody vine, or liana, climbing up their thick trunks.
But Schnitzer’s attention was turned to a break in the forest just a few meters off the trail.
There, in harsher sunlight, a tree stump was all but obscured by a riot of lianas, their tangled stems forming a heavy thicket. Clearly the tree had come down at some point, which created an opening in the forest canopy that allowed the vines to run amok.
“This is really typical of lots of tropical forest,” said Schnitzer, a biologist at the University of Wisconsin-Milwaukee and an associate of the Smithsonian Tropical Research Institute, which is based in Panama City and operates a field station here, about halfway across the isthmus. ‘`Where you get some disturbance, you get this massive influx of vines. They come down in the disturbance, but they don’t die. They just start putting out these stems everywhere.
“This is the liana-tree interaction at its most horrible.”
In a recent paper in Ecology Letters that looked at all the research on the topic, Schnitzer confirmed what was first documented nearly a decade ago: throughout tropical forests in Central and South America, vines are slowly taking over.
“Lianas are increasing in tropical forests, no doubt about it,” he said. “But what’s most important is that they are increasing relative to trees.”
Now, through a series of experiments here, Schnitzer is trying to determine why these changes are taking place.
Understanding why vines are increasing in dominance is important in part because of their potential to reduce tropical forests’ capacity to act as a carbon sink, packing carbon away in trunks and other woody tissues through photosynthesis. That has implications for climate change, since storage helps to regulate the amount of heat-trapping carbon dioxide in the atmosphere.
“Tropical forests store around one-third of the terrestrial carbon on the planet, so big changes in tropical forests will mean a huge change to the global carbon cycle,” Schnitzer said.
Lianas are structural parasites, using trees to support their thin stems as they climb to the forest canopy, where they produce a profusion of leaves.
On Barro Colorado, for example, where some areas have been intensively studied for decades, a 2007 survey by Schnitzer and colleagues found that in some plots, the crowns of about 75 percent of trees with trunks larger than 20 centimetres in diameter were infested with lianas, a 57 percent increase since 1980.
Lianas out-compete trees for soil nutrients, water and light. That can stunt trees’ growth and, over time, kill them.
Lianas can also reduce carbon storage by affecting forest diversity, Phillips said. In Peru, Phillips said, he and his colleagues figure that infestations are reducing the carbon-storage capacity by about 10 percent.
No one knows precisely why lianas are out-competing trees. Carbon dioxide may play a role – vines may be better able to make use of it, which would give them an advantage with carbon dioxide levels increasing because of human activity.
Water may also be an important factor. “We think that lianas are really good at pulling up water from the soil,” Schnitzer said. The vascular systems in their small stems have to be super-efficient, to move an enormous amount of water up to the leaves in the canopy. “They’re like straws,” he said.
Trees are not as efficient in drawing water, he said, so that means that during the dry season – in Panama, roughly December through April – vines tend to thrive.
That advantage may increase if, as many scientists say is likely, climate change results in longer dry seasons.
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