CHAPTER ONE
DISASTER!
HAVE YOU EVER CLIMBED A TREE? Have you ever breathed in the spicy scent of pine trees or looked closely at a leaf?
Hermann Merkel was a boy who loved being around trees. He loved them so much that when he grew up, caring for trees became his job. Even before the New York Zoological Park hired Merkel in 1898 to be its chief forester, he could identify every tree in the park—and there were more than a thousand of them—by the shape of its leaves and the look of its bark. The trees were as familiar to him as the people in his neighborhood. Merkel spent hundreds of hours walking the park’s many paths, doing what he liked to do most: looking at trees. The park’s 1,500 American chestnut trees were among his favorites. Some of them had trunks so big that Merkel’s outstretched arms couldn’t completely encircle them. It took two—if not three—people holding hands to do it. Every year on hot summer days, Merkel cooled off in the shade of these trees. Every year after winter frosts arrived, he ate the trees’ roasted chestnuts as a sweet, crunchy snack. Every year, that is, until 1904, when disaster struck.
One summer day that year, Merkel found ugly wounds on the trunks of some of his splendid chestnut trees. Their bark had split open and, as with a serious cut in a human’s skin, he could see the tissue beneath. When he looked toward the treetops, he saw that the leaves on certain branches had wilted and turned brown. Concerned, Merkel checked other trees: the oaks, the locusts, the birches. All of them looked fine. Only the chestnut trees were hurt. Merkel was unsure what was happening, but it looked like a blight, a disease. He kept a close eye on the chestnut trees and waited to see what would happen next.
The situation got worse.
Within days, the injured bark encircled the branches and trunks. Then pinhead-sized orange bumps appeared in the areas near the damaged bark. Within weeks of their appearance, all the affected branches died. Merkel didn’t know what was attacking his chestnut trees, but he hoped that the approaching winter’s cold temperatures would stop it.
Unfortunately, the mysterious blight dashed his hopes the following spring. The disease remained. Even worse, it spread. It killed American chestnut seedlings sprouting in the park’s greenhouse. It infected chestnut saplings in the tree nursery outside. It attacked the enormous trunks of hundred-year-old chestnut trees. Determined to save them, Merkel cut off the infected branches. He sprayed the trunks with liquids formulated to kill tree diseases. Nothing halted the blight. One after another, the park’s magnificent chestnut trees began to die.
Desperate for help, Merkel contacted William Murrill, a scientist at the nearby New York Botanical Garden. Perhaps Murrill would know what was killing Merkel’s trees. Maybe he could save them.
THE CULPRIT
Murrill couldn’t help until he identified the culprit. The orange bumps that surrounded the ugly wounds gave him an important clue. He thought they looked like the reproductive parts of a fungus. Fungi are organisms that live and grow on organic matter such as plants and animals. This worried Murrill because he knew that some fungi are harmful, even deadly, to other forms of life.
To confirm his suspicions, Murrill collected material from the trees’ wounds and put it on a nutritious gel inside a shallow glass container called a petri dish. He covered the dish and waited. Within days, tiny white threads—the sign of a growing fungus—reached across the gel. As days passed, the fungus spread; its color changed from white to yellow. When the fungus was completely grown, it started reproducing. The reproductive parts of the fungus turned a deep orange—the same color as the tiny bumps that Merkel had found on the park’s chestnut trees. Murrill had solved the first mystery: A fungus was causing the blight.
Meanwhile, more of the park’s chestnut trees were getting sick. But until Murrill figured out how the fungus got inside the trees, he couldn’t give Merkel advice on how to stop it. Searching for a way to fight the fungus, Murrill purposely gave the blight to some healthy young chestnut trees in his greenhouse. He smeared a small amount of the blight fungus on their bark. Several weeks passed. Nothing happened.
Puzzled, Murrill tried again. This time he scraped a hole in the trees’ bark and exposed the tissue beneath. Then he put the fungus directly into the small wounds. Within four to six weeks, all the branches above the wound sites were dead. That solved the second mystery: Any wound that exposed the tissue layer beneath the bark—a scratch from a squirrel’s claw, a hole gouged by a woodpecker, a break caused by the wind snapping off a branch—provided a doorway for the fungus to reach the tree’s inner tissue.
BENEATH THE BARK
How could a tiny fungus kill a gigantic tree? The answer lies beneath the bark. In a sense, a tree resembles the human body. Its trunk encases the tree’s internal tissues in much the same way that the trunk of your body surrounds your heart, liver, and other internal organs. Like your arms, a chestnut tree’s branches are extensions off its trunk. Bark, the trunk’s outermost covering, is the tree’s skin. It protects the tree from pests and diseases and insulates it from extreme weather conditions.
Three layers of tissue lie just under the bark. They control how healthy the tree is and how well it grows. Two of the layers are made of tubular vessels—like super-thin drinking straws. One layer carries nutrients and water from the leaves down to the roots. The other transports nutrients that are absorbed by the tree’s roots up to the branches and leaves.
Between the two nutrient transport layers is the third layer, a paper-thin sheet of cells called the cambium. The cambium is very important because it produces the cells that a tree needs to grow. When a large area of the cambium is damaged, it is a severe injury, and death can occur.
Spotting the ugly wounds on a chestnut tree’s branches and trunk was easy. But Murrill was unable to solve the third mystery—how the fungus killed the tree—until he looked at the tissue beneath the bark. There he saw that the blight fungus’s microscopic threads had fanned out beneath the bark as they sought water and food. The thready network spread throughout the cambium until it encircled the trunk or a branch. Encircling, or girdling, a trunk or branch in this way cuts off the tree’s food-supply lines. The parts of the chestnut tree beyond the girdled area starved to death. Appalled, Murrill realized that the fungus was the most destructive kind of parasite—one that killed its host, the plant or animal it grew on. Although he had identified the culprit and discovered how it killed chestnut trees, Murrill was disappointed that he could not find a way to stop it. The only advice he could offer Merkel was to cut down and burn the sickened trees to keep the fungus from spreading.
In 1906, Murrill wrote, “My observations … have led me to take a gloomy view regarding the immediate future of the chestnut [tree]. The disease seems destined to run its course … and it will hardly be safe to plant young trees while the danger of infection is so great.” Throughout the summer he watched as Hermann Merkel lost heart. “I believe he considers the condition quite hopeless.… Practically all the chestnut trees within [Merkel’s] jurisdiction appear to be dying rapidly. Even the young trees in the nursery [at the zoo] have been either entirely killed or rendered worthless by the fungus.”
By 1911, only two of the 1,500 chestnut trees that had graced the grounds of the New York Zoological Park remained. And foresters, botanists, and chestnut growers in neighboring states reported more alarming news: The blight had spread and was killing the American chestnut trees in their areas, too. Some of them followed Murrill’s advice. They cut and burned infected trees. Some experimented with special sprays created to kill fungi. Others resorted to home remedies, such as pouring poison on chestnut tree roots or boring holes into the trunks and filling them with iron nails or sulfur. Nothing worked.
Were American chestnut trees doomed? And why were people so concerned?
Text copyright © 2018 by Sally M. Walker
