Look closely at the rim of your coffee cup and you’ll notice the surface of the liquid stands a few fractions of an inch higher than the edge. That little bulge is capillary action at work — a surface effect that makes liquids climb up narrow spaces. The narrower the vessel, the higher the liquid can rise. Trees exploit the same physics: the vessels that carry water in deciduous trees measure barely 0.02 inches across, while conifers use even narrower tubes, as thin as 0.0008 inches.
Through the warmer seasons leaves and needles continually exhale water vapour. A mature beech, for example, can release hundreds of gallons of water in a single day. This steady transpiration creates suction, which draws water upward through the tree’s microscopic waterways. Suction depends on uninterrupted columns of water; bonding forces make water molecules stick to one another so they behave like links in a chain. When a leaf breathes out water, it creates space that the next bonded molecule fills, and the chain pulls itself a little higher up the trunk.
Yet measuring water pressure in trees reveals a surprising rhythm. Pressure is highest shortly before the leaves open in spring, when water surges up the trunk with enough force that a stethoscope pressed against the bark can register it. Scientists from the University of Bern, the Swiss Federal Institute for Forest, Snow, and Landscape Research, and the Swiss Federal Institute of Technology in Zurich listened even more carefully and recorded a soft nocturnal murmur in trees. At night, when photosynthesis and transpiration slow, most water is stored in the trunk and scarcely moves. The trunks sometimes swell as they fill, but with the inner tubes holding nearly immobile water.
So where do the murmurs come from if nothing is flowing? The researchers suspect tiny bubbles of carbon dioxide trapped in the narrow water-filled tubes. If bubbles interrupt the supposed continuous column of water thousands of times, then transpiration, cohesion and capillary action may contribute far less to water transport than once thought — a reminder that even familiar processes hold mysteries.
A tree’s outer layer—the bark—acts like our skin. It keeps inner fluids inside, blocks pathogens, absorbs and releases moisture and gas, and senses touch. Without bark a tree would quickly dry out and become vulnerable to fungi that cannot survive in healthy, moist wood. Insects, which require lower moisture levels, are likewise thwarted when the bark remains intact; a tree’s abundant internal water makes it an inhospitable environment for many pests.
Each year a healthy tree adds between 0.5 and 1 inch to its girth. To avoid splitting, the tree continually renews its outer skin, shedding vast flakes of bark up to 8 inches across. After windy or rainy weather you can often find these remnants beneath trunks; the red bark of pines is especially conspicuous. But not all surface marks heal cleanly. Attacks by bark flies can leave a tree scattered with tiny pits and pustules that never disappear, and sick trees may develop moist, festering wounds darkened by invading bacteria. Like human skin, a tree’s bark reflects its condition.
Growth itself is finite. At some point every tree stops getting taller: its roots and vascular system cannot push water and nutrients any higher without overexertion. Instead, growth continues outward. Over time the tree loses energy and can no longer sustain its highest twigs; these die back and are shed by storms. This gradual pruning reduces the crown little by little until only thicker lower branches remain — and eventually those too decline. The slow narrowing of crown and the loss of topmost branches reveal the tree’s advancing age, much as the body shows the marks of a long life.
Trees, then, are both mechanical and fragile: they pump and store enormous quantities of water, their inner plumbing operates at surprising pressures and with unexpected interruptions, and their bark protects, heals, and displays the effects of time and injury. In these quiet processes we see how physics, physiology and the passage of years shape the living architecture of forests.
Source : The Hidden Life of Trees: What They Feel, How They Communicate: Discoveries from a Secret World by Peter Wohlleben, Tim Flannery (Foreword), Jane Billinghurst (Translator)
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