Wynne Parry livescience.com Yahoo News 15 Feb 11;
Trees are the backbone of a forest, but in tropical forests throughout the Americas, trees appear to be losing ground to the woody vines that climb them in a race to reach the sunlight above. This shift could have important implications for tropical ecosystems and for the globe, according to researchers.
"This is the first major structural change in tropical ecosystems that we have witnessed. That is key," said Stefan Schnitzer, an associate professor at the University of Wisconsin in Milwaukee. Schnitzer is one of the two researchers who pulled together evidence from eight studies that, collective, show a pattern of woody vine growth in American tropical and subtropical forests.
"That is going to have cascading effects on things like species diversity, tropical forest functioning in carbon storage and whole forest water use — really important and practical things that will change the way these forests work," said Schnitzer, who is also a research associate at the Smithsonian Tropical Research Institute in Panama.
Evidence climbs
The first evidence of this pattern emerged in 2002, when a 23-year study showed that woody vines were becoming more abundant relative to trees in the Amazon rain forest, northwest South America and Central America. Since then, other studies have shown an increase in woody vines in Panama, French Guiana and the Bolivian Amazon. For instance, on Barro Colorado Island in Panama, the relative abundance of vines in tree crowns has more than doubled over the past 40 years.
In places like these, the vines are native species, but farther north in subtropical places such as Florida and South Carolina, invasive species, like the infamous kudzu, contribute to the problem, Schnitzer said.
The colder weather of higher latitudes keeps the vines in check, write Schnitzer and his fellow researcher, Frans Bongers from Wageningen University in The Netherlands, in an article published online today (Feb. 14) in the journal Ecology Letters.
The researchers speculate about possible causes: Drier weather in the tropics may aid vines, which, unlike trees, can continue to grow during the dry season. Woody vines are also adept at taking advantage of disturbances in the forest, such as openings created when a tree falls. Once they find a gap, their growth rate far exceeds those of trees. Logging and other human alterations to forests may give woody vines an advantage, and there is also evidence that higher levels of carbon dioxide in the atmosphere — the most prominent greenhouse gas — may benefit woody vines more than trees, they write.
And not only do woody vines possess all these advantages; the presence of more woody vines appears to slow tree growth and increases tree death.
An altered ecosystem?
As the world faces global warming linked to increased greenhouse gas emissions, tropical forests provide an important "carbon sink," by tying up the carbon from the dominant greenhouse gas, carbon dioxide in their wood as they grow, according to Schnitzer.
By interfering with the growth of trees, and increasing tree deaths, the vines reduce the amount of carbon the trees can sequester, but the vines themselves have less wood and store less carbon than the trees they are replacing.
"Vines use tree architecture to ascend to the light. They are more like structural parasites. They use trees to get their leaves to the sun, they don't store very much carbon," he said.
It's possible an increase in woody vines could change the nutrient dynamics of forests, in part because of differences between their leaves and the leaves of tropical trees, all of which ultimately fertilize the forest floor. Water dynamics may also be affected because woody vines appear to exhale more water vapor through their leaves during dry times, researchers said.
The growth of woody vines does not appear to be a worldwide phenomenon, however. Two studies in Africa found evidence of decreases or stable growth.
Tree Stranglers Overtaking Tropical Forests
Andrea Mustain LiveScience.com 7 Apr 11;
A creeping menace is taking over the planet's tropical forests, a brand of tree-hugging vine whose embrace — if you're a tree — is of the sinister kind.
Lianas, a type of woody vine, are increasingly abundant across forests in Central and South America, according to new research on the plants.
"They seem to be increasing while trees are not," said Stefan Schnitzer, a professor at the University of Wisconsin-Milwaukee and co-author on a recent study, the first of its kind, which combined data from eight previous investigations of lianas across Central and South America.
In fact, some parts of the Amazon are now called liana forests, Schnitzer said, because the climbing vines have taken over. "And they're not particularly pretty forests, I may add," Schnitzer said.
Schnitzer said it's a scene that would be familiar to anyone who has spent time in the southeastern United States, where another type of prolific liana — kudzu — is taking over the trees.
However, the creeping abundance of lianas has implications beyond mere aesthetics.
"Lianas prevent trees from growing," Schintzer told OurAmazingPlanet. "They're really good competitors."
Tree backstabbers
Lianas use the sturdy trunks of trees as a kind of living trellis, allowing the vines to slither up the length of a tree and reach the canopy, beating out the trees for resources such as water and sunlight.
And a shift in the makeup of forests — a higher percentage of lianas, a lower percentage of trees — can have big ramifications for everyone on the planet, because trees are an excellent carbon sink (meaning they suck up carbon dioxide from the atmosphere and store it for many years), whereas lianas are not.
Trees put a lot of their energy into growing sturdy trunks. "That's where all the carbon is stored, and it can be stored for hundreds and hundreds of years," Schnitzer said.
Lianas don't have to bother — because they hitch a ride on tree trunks. "They have a lot less wood volume, so they don't store nearly as much carbon," Schnitzer said.
What's to blame
Schnitzer said the study data suggest there are several main culprits behind the increase in lianas.
First, if trees are removed from forests, whether by natural or human means, more sunlight penetrates the darkness of the forest floor, which allows lianas to flourish. In addition, it appears that lianas can thrive in drier conditions, and when greater amounts of carbon dioxide are found in the atmosphere, two conditions now seen in some places in the Amazon.
Schnitzer said more study is required to see how big a part all these factors play, and how they may be intertwined.
However, the invading vines aren't bad news for all the residents of the tropics.
Sloth highway
"Lianas are important for animals in tropical forests, because they connect from one tree to the next tree, and provide a lot of resources," Schnitzer said.
Because the hardy vines can thrive in the dry season, they can flower and produce fruit when other plants don't, providing a source of food.
In addition, lianas' vast network of entangled tendrils provide an inter-tree expressway, allowing creatures to climb from tree to tree with relative ease, and providing more real estate for nesting, sleeping and eating.
"These things are really important structural components of the forest for animals," Schnitzer said. "Monkeys, sloths, they're always climbing on lianas. So you can't go in and say, 'Let's just cut down all the lianas, and the forest will be OK.''
Schnitzer said the increase of the vines is a more nuanced issue. "We're hoping that this study will get people to realize this is an important phenomenon," Schnitzer said, "and in another five or 10 years, we'll have more data."
Schnitzer spoke with OurAmazingPlanet from Panama, where he works with the Smithsonian Tropical Research Institute. The study he co-authored is published in the journal Ecology Letters.
