Trees are full of song. Wind clatters and hisses through leaves and needles, insects stridulate, ice rends weakened wood, people chatter on the street below, and mechanical noises reverberate within trunks. Some tree sounds are too high for our ears, but can be heard with the right microphones. Under the acoustic surface are the hidden songs, the stories heard when we follow sounds to their sources. To attend to a tree’s song is therefore to touch a stethoscope to the skin of a landscape, hearing what stirs below. Here are ten ways to listen:
1. Fingers. I hold two slabs of maple in my hands, each the weight of a heavy book. The blocks are hewn into wedges with surfaces rough enough to graze my skin. I hold seemingly soundless lumber, inert blocks. I pick up each one in turn and, as I’ve been taught, I hold the wood pinched between fingers, its weight dangling from my left hand. I tap the wood. My skin feels the answering reflections. One block has more clarity, it feels more bright, open, and lean muscled. The second is similar, yet tinged with granularity and turbidity. From the first, my teacher, the luthier, will craft the back of a violin. Its curved skin will give this wood its second life, a new song.
2. Soles of feet. As the hickory log hits the blade, the saw’s judder and scream enter me through the sawmill’s floor, a blast of vibratory energy. The circular saw is five feet high, mounted vertically so that the log is cut lengthwise as it slides down the conveyor. Spinning metal shears the wood, turning it to boards and studs. This lumber holds up my house, an architecture supported with carbon dioxide annealed by sunlight. Two-by-fours are welded air. I burn the wood scraps and around the fire, talk among friends turns to the realm of the imagination, of connections seen and unseen. Wood fire fuses the human community, joining strands.
3. Electric caliper. A metal clamp gently grasps a twig and measures diameter every 15 minutes. Over hours, days, weeks, the twig’s girth is scribed into electronic memory. On a 24 hour cycle, the twig pulses with a sun-powered heartbeat, contracting in the day time as water flows through it from stem to leaf. At night the twig fattens, rehydrating with water drawn from the roots. Twigs on moribund branches—too shaded and low in the canopy—have weak pulses, daily cycles that barely stir under the metal fingertip. On sun-happy branches, systole and diastole surge and draw back, the forest’s subsonic hum. In late spring, this daily oscillation is overlain with an upward surge in diameter as the twig fattens with new layers of wood.
4. Ultrasound. A thumb-sized sensor presses against a pine twig’s thin bark. On a computer screen, a line scribes the number of high-pitched clicks and fizzles hitting the sensor, the sounds of water columns breaking inside the twig’s narrow-bored vessels. For the tree, these are sounds of impending hunger. If too many water columns break – as happens during drought – the twig’s needles must close their breathing pores to save water. No air moving through pores means no photosynthesis. The sounds follow a daily rhythm: In the morning, the tree’s slaked roots provide ample moisture and few sounds emerge from the twig. By afternoon the soil is depleted. The silk-thin strands of water inside the plant’s vessels break with a snap. After sundown, tree roots and soil fungi conspire to defy gravity and draw water from the deeper layers of soil. The lacework of roots connected to threads of fungi acts like a vast piece of blotting paper, sliced and loosely woven into the soil’s depths. Water flows through this meshwork as the laws of physics dictate, from wet to dry, and water moves upward through the night. Evening’s dust-dry soil surface is thus dampened by morning, and the twig’s acoustic distress is eased.
5. Conversation. Under branches, people gather. The tree is a nexus of human connection. In Manhattan, street vendors and pedestrians rest and chatter in the shade, out of the bruising jostle of the sidewalk. The cooling leaf canopy above them saves the city 11 million dollars in air conditioning costs each year. On a dry hillside in the West Bank, farmers and their families cluster on ladders propped in trees, human fingers working through twigs, bringing in the olive harvest. By the time the grove has been picked, tens of thousands of words have flown from mouth to ear as people work side-by-side in the tree branches. In a park in downtown Denver, kids gather under a cottonwood, the pattering sound of its wind-blown leaves soon engulfed by their shrieks as they leap into the river’s mountain-chilled waters. Decades ago this was a waste dump. Now, children run laughing from tree to water and back again. Parents sit chatting in the shade.
6. The sigh of indrawn breath and the flick of a switch. The oxygen we inhale came from trees and the oceans. We exhale and leaves snatch our unwanted carbon dioxide, then lodge it in wood, a timbered memory of breath. Click the light switch: Coal burns, ancient memories are released.
7. At a library desk. Swish as I smooth my palm over the page of the scientific journal. I’m gazing at cellulosic sheets of tree, the wood’s stiffening lignin molecules purged by the paper-making process. Inked on their pages, data about the fate of forests: Summed over the globe, the area of land covered by forests is plunging. The first dozen years of the millennium saw 2.3 million square kilometers of forest lost but only 800,000 regrown. I turn the page, a crackle of fibers loose their complaint as the page buckles.
8. Snoozing under a tree. I wake from my nap to the smell of sun-warmed ponderosa resin. In the forest, relationships between trees and other species is a chemical process, so I listen with my nose. The golden sap between dark plates of ponderosa bark has the vigorous odor of rosin and turpentine: oily, acidic, and bright. The scent is a deterrent against attacking insects. Sticky resin gums wood-boring insects, and resinous chemicals are poisonous. These defenses are adequate against most insects and in most years, but lately ponderosa and other pines have been dying by the millions, killed by beetles. Paradoxically, the odors that protect the tree are the same ones that lead these beetles to their target. These attacks have become so widespread in the Rocky Mountains that it is common to see whole valleys turn from living green to dead-needle brown and finally to bleached-wood gray. Pine beetles have always lived in these mountains. But now their populations are surging, pushed upward by a landscape full of drought- and heat-weakened trees. I smell fire on the wind.
9. Through insect sounds. Clicks and rasps under the under the bark: Beetle larvae auger the tree with jut-bladed mouths. The young insects swallow the fine sawdust, passing it to guts populated by symbiotic microbes. Woodpeckers listen, hoping to catch a sound that betrays an insect’s presence in the tree. Pressing my ear to the tree, I hear the wood-carried sound of beetle mouth and bird beak. In the canopy, caterpillars chew on leaves. The tree, too, listens. When the sounds of munching caterpillars are played to a tree, the plant releases defensive chemicals.
10. Stop, listen. Gusts of wind sonify plant diversity. Oak’s voice is coarse-grained, throaty; maple’s is sandy and light. These differences have their origins in plant evolution and adaption. Drought-resistant oak leaves are thicker, tougher than the water-hungry maple. The different sounds of trees on a dry mountain ridge and in a moist forested hollow speak to the particularities of the ecology of each place. Ponderosa pine sings sweetly in the winds of California, its long needles were, John Muir wrote, “finest music” and a “free, wing-like hum”. But in Colorado, pines have evolved shorter, stiffer needles to cope with heavy loads of snow and ice. There, the trees wail as their wiry needles harrow the wind.
Step outside, attend to your ears. In your home, what do the sounds of trees reveal?
David George Haskell (@dghaskell), is the author of the newly released “The Songs of Trees”, an exploration of science and ethics through the lives of a dozen trees around the world. He is professor of biology at The University of the South is the author of “The Forest Unseen”, winner of the 2013 National Academies’ Best Book Award and Pulitzer finalist.