Extreme forestry: what does bungee jumping say about parasitic vines?

Natural History, July-August, 2003 by Adam Summers

It is May, time for naghol--a centuries-old fertility ritual practiced on Pentecost Island in Vanuatu, in the South Pacific. The participants--the island's young men--perform land dives to obtain a blessing for their people's crops. As a crowd of islanders watches, each young participant scales a rickety scaffold of branches to a platform some seventy feet above the tilled earth. The diver pauses for a moment, then leans forward and plunges headfirst off the platform, trailing vines tied to his ankles. If he has chosen the vines well, they will pull taut and stretch like a natural bungee cord, just enough to gently arrest his fall. If he has chosen poorly, he may slam into the ground or be yanked back against the platform.

The success of such derring-do evidently depends on the material properties of lianas, or woody vines. But how can woody vines stretch like oversize rubber bands? To answer that question, let's go to the other side of the world: to the forests of French Guiana on the northern coast of South America, where a German investigator, Thomas Speck, gingerly tests the strength of a kind of liana called monkey ladder (Bauhinia guianensis). Satisfied that the vine is sound, he hoists himself off the ground and swings back and forth on the vine like Tarzan of the apes. Speck and his colleague Benedikt Hoffmann, both biomechanicists at the University of Freiburg in Germany, analyze the peculiar material properties of woody vines. They are working to find out what makes the structure of lianas different from that of trees and shrubs--hence what makes lianas usable as elastic, weight-bearing rope.

Wood is a composite material made of two principal substances: cellulose, a complex carbohydrate that is the chief structural part of most plant-cell walls; and lignin, which binds the components of cell walls together. The same kind of pairing shows up in familiar man-made products: in the modern tennis racket, for instance, carbon fibers are mixed together with epoxy. The fibers provide tensile stiffness and strength, while the epoxy keeps the fibers properly oriented and binds them together.

Although composite materials can be made artificially, the properties of wood cannot be duplicated. Osage orange wood, not fiberglass, still gives the best power and feel for archers' bows; old-growth spruce adds vibrancy and color to the tones of the finest violins; and many major-league baseball players, long accustomed to bats made from ash, now swear that sugar maple is the finest for swatting one out to the center-field bleachers.

Lianas are woody vines that parasitize trees for structural support. A liana climbs its host tree, called a trellis, by laying down a network of tendrils, spikes, and hooks. Thus, it reaches the light of the upper canopy without having to invest in building up enough wood in its stem to support its weight. In American tropical forests, lianas may account for nearly half the leaf productivity, yet they amount to less than 5 percent of the biomass.

But a liana does not begin life as a parasite. It grows on its own until it finds a tree to cling to. Monkey ladder can reach a height of nearly six feet as a freestanding shrub. But when it finally finds a trellis, the vine begins to grow rapidly, the stem cross section becomes thicker and rectangular, and the material properties of the stem change radically.

The wood of a self-supporting monkey ladder shrub can be as stiff and dense as the hard, heavy wood of black locust trees that commonly occur in North American deciduous forests. The wood developed by the vine during the climbing, parasitic phase is less dense--the vessels in the wood that conduct water up the stem become much larger, and the wood itself absorbs more water. The increased water content makes the mature monkey ladder vine as much as three times more elastic than the shrub. So it's not surprising that the land-diving ceremony on Pentecost Island is held just after the wet season, when the vines are presumably filled with water and become most springy.

According to Speck's work on South American lianas, if a land diver were attached to a fifty-foot-long vine of shrub wood, it would stretch only five more feet. That would leave the diver far short of the ground (recall that the towers in the naghol ceremony are seventy feet high). But perhaps more important to the diver, the vine's arrest of his fall would be so abrupt that he would risk injury to both ankles (assuming the sudden loading didn't simply break the vine).

How do lianas make the transition from shrub to creeper? Speck and Hoffmann have shown that both monkey ladder and an unrelated South American liana, Condylocarpon guianense, undergo a marked drop in the cellulose, or fiber, content of the wood. (The amount of lignin--the epoxy analogue--remains the same in both kinds of liana.) But in all its stages of life Condylocarpon contains between 10 and 20 percent less cellulose than does the monkey ladder, and is about a third as stiff. Those data suggest that cellulose content is critical to stiffness.