When growing down isn't good enough.

When growing down isn't good enough

There are "renegade' tree roots in the Amazonian rain forestthat apparently listen to the beat of their own drummer. Rather than grow downward into the soil as good roots should, they grow vertically up the trunks of neighboring trees--most likely in search of a better meal.

A study of these so-called apogeotropic roots by Robert L.Sanford Jr. of North Carolina State University in Raleigh is reported in the Feb. 27 SCIENCE. During field experiments in Venezuela, Sanford found that the roots of at least 12 species of tropical trees send out climbing roots that originate in the soil and then decide to take the high road--often creeping more than 13 meters up the trunks of other trees, as fast as 5.6 centimeters in three days.

Sanford proposes that this"nutrient cycling pathway' develops in response to the nutrient-poor soil found in tropical forests. Previous studies have found that the canopy, or thick-foliage top, of such forests scavenges nutrients from rainfall. A certain amount of that precipitation flows down tree trunks.

To test his hypothesis that this "stem flow' is related toupward-growing roots, Sanford used artificial tree stems of plastic pipes, and near their tops attached cylinders containing manure, forest soil or nothing. He found that those with nutrient-rich manure being leached down by rain supported climbing roots from nearby trees sooner and more abundantly.