VOA标准英语2011--Vines Overwhelming Trees in Rain Forests

Vines Overwhelming Trees in Rain Forests

Tropical rain forests in the American Hemisphere and other regions of the world are changing rapidly. Studies conducted over recent decades indicate that increasingly, most of the trees in South and Central American rain forests must compete for light and nutrients with "lianas" or woody vines. Studies also show that lianas are growing so abundant as a result of climate change and higher levels of carbon dioxide in the atmosphere that they are overwhelming their host trees, and in some cases, killing them.
Deep in the tropical forest of Panama, on the island known as Barro Colorado, scientists from the Smithsonian Tropical Research Institute have been studying the flora and fauna for nearly 100 years.
Stefan Schnitzer is one of those scientists. He studies "lianas" or woody vines, structural parasites that require trees for support. He found out that vines growing on tree crowns have more than doubled over the last four decades.
"What we found is that probably about 80 percent of the trees that are around us now have some competition with vines," Schnitzer noted. "Vines are affecting nearly all of the trees. So most of their growth rate is being reduced because of vines."
Not only growth. Lianas compete for nutrients in the soil, deprive trees of light, and can choke and kill them.
"You can call them natural born killers," Schnitzer added. "What's very interesting about these lianas is they climb up a tree, that tree falls, the liana falls with it, but the tree dies and the liana stays alive."
Schnitzer estimates that some lianas are hundreds of years old, cover dozens of trees at one time and stretch for nearly a kilometer over tree tops.
Although some lianas produce fruits and habitat for animals, they are devastating for most trees. One liana we found had more than 10 different rooting points within a few square meters.
"This is an enormous liana and it's rooted right there next to the tree; it's competing for all sorts of resources," Schnitzer explained. "And then, to add insult to injury, it's sending this giant stem up into the canopy where is competing for light as well."
Schnitzer says he believes lianas are growing faster because of drought and warmer temperatures. He says these woody vines prevent trees from growing and capturing carbon dioxide.
In another clearing we saw, there were no trees, the vines had completely taken over.
"What you get is tree gaps that never recover or take them 20-30 years to recover back to full canopy, because there are so many lianas preventing trees from growing," Schnitzer said.
Schnitzer and his team are also working on the nearby peninsula of Gigante in another protected area. Here, hundreds of tree seedlings from 14 native species are ready to be planted.
Schnitzer's team will plant these seedlings in 16 plots, half with lianas and half without them.
"We will measure mortality or survival every 2-3 months, and then we measure growth every 6 to 12 months. And after two or three years we'll know which tree species are going to regenerate better when lianas are present versus when lianas are absent," Schnitzer explained.
At the end of 20 years, Schnitzer says they will know how the tropical forest is changing and how to restore and conserve it.
"The forests are changing and moving in a direction that may result in more liana- dominated forest and forests dominated by trees that can tolerate lianas," Schnitzer said.
Why should we care? Because, Schnitzer says, tropical forests contain about one third of all global terrestrial carbon. Without trees to capture that carbon dioxide, global climate change would be an even greater threat.