A Tree Hugger, With a Twist
Monday, May 23 2011 @ 08:34 PM UTC
Contributed by: Don Winner
“This is really typical of lots of tropical forest,” said Dr. 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.” Dr. Schnitzer knows as much about liana-tree interactions as anyone, and what he knows is troubling. In a recent paper in Ecology Letters that looked at all the research on the topic, he confirmed what was first documented nearly a decade ago: that throughout tropical forests in Central and South America, vines are slowly taking over. (more)
“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, Dr. 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,” Dr. 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. They are a diverse group, with common names like monkey ladder and water vine, and are not invasive species like kudzu, an Asian native that grows out of control in the southeastern United States.
Thickets like the one that caught Dr. Schnitzer’s eye on a brief hike around the island are the most obvious examples of the power of lianas to affect tropical forests. But even in less disturbed areas, studies have shown increases in their abundance.
Oliver Phillips, a researcher at the University of Leeds in England, published the first paper documenting the phenomenon — in the western Amazon in parts of Bolivia, Ecuador and Peru — in the journal Nature in 2002. The study was met with some skepticism, with critics saying his sample was not representative because he looked only at large-diameter lianas.
Since then, though, the basic thesis has been borne out by more research in the Amazon, northern South America and Central America. On Barro Colorado, for example, where some areas have been intensively studied for decades, a 2007 survey by Dr. Schnitzer and colleagues found that in some plots, the crowns of about 75 percent of trees with trunks larger than eight inches in diameter were infested with lianas, a 57 percent increase since 1980.
“Almost everywhere he’s looked, or other people have looked, they’ve found essentially the same pattern,” Dr. Phillips said.
Lianas do provide some benefits, most notably to animals. The vines are often a source of food during the dry season, because they tend to flower and fruit in that part of the year, when many trees do not. Because lianas tend to snake their way through the treetops, they also provide a highway of sorts for animals that travel through the canopy. (Animals don’t use the vines as swings, Tarzan-like, because the vines are rooted firmly in the ground. Swinging vines are a Hollywood creation.)
But lianas outcompete trees for soil nutrients, water and light. That can stunt trees’ growth and, over time, kill them.
Trees can also suffer what Dr. Phillips described as “dynamic death,” becoming so infested that the sheer weight of the lianas’ stems and leaves — particularly in the rain and wind — produces so much mechanical stress that the trees come down. Any lianas attached to the tree come down, too, but they are flexible enough that they are not damaged, and quickly resprout. That may be what happened in the clearing on Barro Colorado.
While lianas store some carbon in their stems and leaves, they store a lot less of it than trees, with their woody trunks. So displacing a lot of trees with lianas means a net reduction in a forest’s carbon-storage capacity. Even if a tree survives an infestation, its stunted growth means it stores less carbon than a fully grown one.
Lianas can also reduce carbon storage by affecting forest diversity, Dr. Phillips said. Tree species that can grow quickly are better able to “escape” infestations, so forests with heavy liana growth tend to have more of these species. But faster-growing trees have wood of lighter density, and thus store less carbon. In Peru, Dr. 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 outcompeting trees, but researchers have some ideas. Carbon dioxide itself 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,” Dr. 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. “They’re growing, when trees are starting to shut down,” Dr. Schnitzer said. “It’s a water-based competitive advantage.”
That advantage may increase if, as many scientists say is likely, climate change results in longer dry seasons.
Testing this and other hypotheses is difficult, but Dr. Schnitzer is starting to do so.
“I knew how to work a machete, so I said, Hey, let’s go in and start cutting things down,” he said.
In one experiment, involving 16 260-by-260-foot forest plots on a nearby peninsula, Dr. Schnitzer’s team is cutting all the lianas from half of them and will study how the plots change, hopefully over several decades. In another, the researchers are using instruments to study the immediate effects of liana removal on trees, including their ability to take up water, to test the hypothesis that lianas do not affect tree species uniformly.
Dr. Schnitzer’s experiments “are going to be very insightful,” said Dr. Phillips, who is testing many of the same hypotheses. Rather than studying small plots, however, Dr. Phillips is looking for patterns across a whole forest. If lianas are better able to use water during dry seasons, for example, then he expects to find a change in liana dominance across a forest that becomes progressively drier or wetter from one side to another. “It’s a completely different approach to Stefan’s, but complementary,” Dr. Phillips said.
Most of these experiments are long-term ones, so it may be a while before researchers fully understand why lianas are becoming dominant. In the meantime, though, it is difficult to escape the evidence.
Later on during the hike, Dr. Schnitzer came across another break in the forest. This one appeared to be a much more recent tree fall, and while lianas had come down with the tree, they had yet to resprout and take over.
“If you come back in a year, it will have changed,” he said. “There will be a whole lot of vines in there, all rooted, all growing. And all the trees will be very unhappy.”