Trees are alive in every sense biologists use. They grow, use energy, respond to the world around them, and eventually die. But a tree is alive in an unusual way, because most of a mature trunk is made of wood that is no longer living.1Treehugger. How Much of a Tree Is Alive? Understanding Living and Non-Living Tree Cells A tree is less like a single living lump and more like a living skin built around a structure it made itself.
Which Parts of a Tree Are Living
Just under the bark lies the cambium, a growth layer only one to several cells thick.2PubMed. Vascular cambium stem cells: past, present and future Every year it adds new wood on its inside and new inner bark on its outside.3PubMed. Vascular cambium stem cells: past, present and future
Wood cells, oddly, die to do their job. Most of them die once they mature, leaving hollow tubes that carry water and hold the tree up. So in a dormant mature tree, only a very small share of the trunk is living tissue.4Treehugger. How Much of a Tree Is Alive? Understanding Living and Non-Living Tree Cells The outer wood, called sapwood, still carries water and contains living cells that store food. Deeper in, those last living cells make protective chemicals and then die, and the core becomes heartwood.5Journal of Wood Science. Cell death of long-lived ray parenchyma cells during heartwood formation in trees Heartwood is dead, yet it helps the trunk resist rot.6Journal of Wood Science. Cell death of long-lived ray parenchyma cells during heartwood formation in trees
A cut log or stump works as a map of all this. The outer bark is a dead protective coat, much like the outer layer of your skin. Just inside it sits the thin, living inner bark, which carries sugar down from the leaves. Next comes the pale band of sapwood, moving water up and still holding living storage cells. The darker center is the heartwood, dead tissue that still holds up the trunk.
This is why a hollow tree can be alive and healthy. The core is already dead, so rot inside it doesn’t kill the tree directly. What matters is that the outer ring of bark, growth layer and sapwood stays unbroken. A large hollow can still weaken the trunk, but that is a safety question rather than a question of life.
The reverse is also true: stripping the bark all the way around a trunk kills a tree. A ring cut through the bark severs the living inner bark and the growth layer. The roots then stop receiving sugar from the leaves and slowly starve. That’s why lawn mower and string-trimmer damage at the base of a trunk matters so much.
Trees can’t heal the way skin does. When a branch is cut or bark is gouged, the tree doesn’t regrow what is missing. It seals the wound off and grows new wood and bark around the edges. Over several years, the growth layer at the wound’s rim lays down a rolling lip of new tissue that slowly closes inward, and a small wound may close completely. The old wood beneath stays damaged, so the tree is walled off around the injury rather than repaired, and a wound can hide decay inside a tree that looks healthy.
How Trees Breathe, Drink and Store Energy
Living cells in the trunk and branches use oxygen and release carbon dioxide, just as ours do. So bark has small raised pores whose main job is letting those gases in and out.7IAWA Journal. Breathing life into trees: the physiological and biomechanical functions of lenticels They work for the trunk much as leaf pores work for leaves.8IAWA Journal. Breathing life into trees: the physiological and biomechanical functions of lenticels
Water moves through a tree by being pulled, not pumped. Trees have no heart. Water evaporating from the leaves pulls a continuous column of water up through the wood. Water molecules cling tightly together, so the pull at the top travels all the way down to the roots.9Annals of Botany. Sap Ascent in Vascular Plants: Challengers to the Cohesion Theory Ignore the Significance of Immature Xylem and the Recycling of Münch Water
Leaves make food by day, but the whole tree breathes around the clock. In daylight, leaves use sunlight to turn carbon dioxide and water into sugar and release oxygen, a process called photosynthesis. Breathing, meaning cells burning sugar for energy, goes on day and night in every living part, including roots that never see light. Over a year a healthy tree makes more sugar than it burns, and the surplus becomes wood, new leaves and stored reserves.
Those reserves matter. Trees store sugars and starch to cover the gap between when food is made and when it is needed, from day to night and from season to season.10PubMed. Understanding the roles of nonstructural carbohydrates in forest trees – from what we can measure to what we want to know In leaf-dropping trees, stored reserves usually peak around October, before winter.11PubMed Central. Whole‐tree nonstructural carbohydrate storage and seasonal dynamics in five temperate species Over the winter, starch is converted to sugars, and sugar levels peak in the cold months.12PubMed Central. Whole‐tree nonstructural carbohydrate storage and seasonal dynamics in five temperate species Some of the stored carbon used today was made years or even decades earlier.13PubMed. Understanding the roles of nonstructural carbohydrates in forest trees – from what we can measure to what we want to know
How Trees Sense and Signal
Trees produce small electrical impulses that travel through their tissues when conditions around them change.14PubMed Central. Electrical signals in avocado trees When insects chew leaves, plants also release scented chemicals that switch on their own defenses.15PubMed Central. Plant Secondary Metabolites as Defense Tools against Herbivores for Sustainable Crop Protection A damaged tree can therefore prepare unhurt parts of itself before the attack spreads.
Those defenses are chemical. Trees can respond to chewing insects by producing compounds that make leaves harder to digest or less appealing. Defenses cost energy, so a tree switches them on in response to danger rather than running them constantly.
Neighbors can pick up those airborne warnings. Plants release airborne chemicals that warn others of danger, and nearby plants ready their defenses in response.16PubMed Central. Plant communication In natural beech forests, trees close downwind of a damaged tree built up more defense hormones and suffered less damage from insects and disease.17Journal of Plant Interactions. Volatile-mediated plant–plant communication in natural beech forests Trees upwind showed no such effect, so it depends on the breeze carrying the scent.18Journal of Plant Interactions. Volatile-mediated plant–plant communication in natural beech forests The warning worked whether or not the trees were related.19Journal of Plant Interactions. Volatile-mediated plant–plant communication in natural beech forests
Underground, most trees partner with root fungi that trade soil nutrients for the tree’s sugars. Carbon has been shown moving between trees through these fungal networks, first in the lab and later in forests.20PubMed Central. Belowground carbon transfer across mycorrhizal networks among trees: Facts, not fantasy One field study found networks moving carbon, nitrogen and phosphorus and helping connected seedlings, with a single fungus spanning about 2 km.21Plant Science Archives. Exploring the Complex underground social networks between Plants and Mycorrhizal Fungi known as the Wood Wide Web But in aspen, which share root connections, damaged trees did not trigger defenses in connected neighbors.22PubMed. Herbivore-simulated induction of defenses in clonal networks of trembling aspen (Populus tremuloides) “Communication” here means chemical cues that other trees respond to, not intent or conversation.
The popular “wood wide web” stories deserve some care too. They often describe mother trees feeding their young on purpose. What has been shown is that resources can move through fungi, not that trees choose to send them.
Is a Bare Tree in Winter Alive
A leafless tree is usually alive and dormant. Leaf-dropping trees slow their activity in winter and rely on the food stored in autumn. Cold is the danger, because ice forming inside a living cell kills it.23bioRxiv. Freeze dehydration vs. supercooling in tree stems: physical and physiological modelling Tree cells avoid this either by drying out slightly or by staying liquid below freezing without turning to ice.24bioRxiv. Freeze dehydration vs. supercooling in tree stems: physical and physiological modelling
To check a twig, scrape a little bark off a thin one with a thumbnail or knife. Green, moist tissue underneath usually means the twig is alive. Brown and dry means that twig is dead, so test several, moving toward the trunk.
Buds offer other clues. Look for plump, firm buds rather than shriveled or hollow ones. Evergreens keep their leaves but still slow down for winter. And wait for spring leaf-out before judging a tree dead, since some species leaf late.
Dropping leaves is a trade. Broad, thin leaves lose water quickly and are easily damaged by freezing, and in winter frozen soil makes it hard for roots to supply water. Shedding them avoids that loss, at the cost of growing new ones in spring. Before the leaves fall, the tree pulls back some of their useful nutrients, which is part of why leaves change color in autumn.
How Trees Die
In a fast drought, a tree can die when its water column breaks and stops flowing, even with food reserves left.25PubMed Central. How do trees die? A test of the hydraulic failure and carbon starvation hypotheses In deep shade, a tree can instead starve slowly as it burns through stored food.26PubMed Central. How do trees die? A test of the hydraulic failure and carbon starvation hypotheses In slower droughts both happen together, and low food reserves help decide when water flow finally fails.27PubMed Central. How do trees die? A test of the hydraulic failure and carbon starvation hypotheses In one study, half of trees died once they had lost about 80% of their ability to move water.28PubMed Central. Dead or dying? Quantifying the point of no return from hydraulic failure in drought‐induced tree mortality
“Breaking the water column” has a plain meaning. When soil is very dry, the pull on the water in the wood becomes so strong that air bubbles form in the tubes and block them, like a bubble in a drinking straw. Blocked tubes can’t carry water, so the leaves above them dry out, and enough blocked tubes cut off the whole crown.
Death can also show up late. A stressed tree can look fine for a long time while its reserves or its water-carrying capacity quietly drop. It may die a year or more after the drought or injury that caused the damage, which makes the cause hard to link to the result. Stress also weakens defenses, so insects and decay fungi often finish off a tree that was already failing.
On your own tree, watch for leaves that wilt, brown or drop out of season. Bark peeling away to reveal dry, dead wood beneath is another sign, as are mushrooms growing at the base or on the trunk. So is branch dieback starting at the top of the crown.
Tree inspectors can look inside a trunk without cutting it down. They use visual checks, sound-based devices, fine drills, pulling tests and scans.29Forests. Methods for Nondestructive Testing of Urban Trees Some scans measure how electricity passes through the trunk, which may reveal water stress before visible symptoms.30PubMed Central. Noninvasive Analysis of Tree Stems by Electrical Resistivity Tomography: Unraveling the Effects of Temperature, Water Status, and Electrode Installation
Call an arborist when a large tree near a house, path or road shows several warning signs. An arborist can test soundness and say whether a tree is dying or just stressed. Deep watering during a drought can help a stressed tree, though it may not save one that has already lost too much of its ability to move water.
Can a Tree Live Forever
Some plants keep making new cells for thousands of years.31PubMed Central. Single-Cell Telomere-Length Quantification Couples Telomere Length to Meristem Activity and Stem Cell Development in Arabidopsis Plant growth cells don’t appear to wear out with repeated division the way animal cells do.32PubMed Central. Growing old while staying young They repair DNA well and divide rarely, keeping a reserve of little-used cells to limit copying errors.33PubMed Central. Growing old while staying young The protective caps on chromosomes also stay longest in these growth cells.34PubMed Central. Single-Cell Telomere-Length Quantification Couples Telomere Length to Meristem Activity and Stem Cell Development in Arabidopsis
A very old tree is also mostly recent living tissue wrapped around old, dead wood. A tree can drop leaves, branches and even large limbs and replace them with new growth.
Still, trees die from drought, disease, storms and decay. Not aging internally is different from being impossible to kill. Large old trees face growing risks from wind, rot and drought stress, and the oldest living trees survive in harsh, sparse places with few competitors and pests.
Death isn’t the end of a tree’s role. A fallen tree stores nutrients and releases them slowly as fungi, insects and worms break it down.35Italian Journal of Agronomy. Deadwood as a driver of forest functions A dead log can feed the forest for more than 100 years.36Italian Journal of Agronomy. Deadwood as a driver of forest functions
In your yard, a dead tree far from buildings can be left as habitat for birds and insects. One near people should be checked, since it can fall unpredictably. Leaving logs or stumps in a garden corner returns nutrients to the soil.
Trees Compared With Animals
Scientists define life as matter carrying out biological processes, which trees clearly do.37PubMed Central. Life as we don’t know it: plant life in the history and philosophy of science an introduction Whether plants feel or think is a separate, open debate.38PubMed Central. Life as we don’t know it: plant life in the history and philosophy of science an introduction Trees also keep growing their whole lives rather than stopping at an adult size.39PubMed. Evolution of arborescence at hydraulic, structural, and developmental limits40PubMed. Evolution of arborescence at hydraulic, structural, and developmental limits
Growing for life changes how a tree is built. An animal builds its body once, but a tree keeps adding new branches, roots and leaves for as long as it lives. So a tree can lose a limb or a set of leaves and replace them, which an animal usually can’t do. The cost is that a tree can’t flee a threat, so it relies on defenses, repair and sheer redundancy.
Trees respond to damage, but there is no evidence they feel pain the way we do. Reacting needs only chemistry, while feeling, as science defines it, is linked to a nervous system.