We all know what a tree is, but do we, really? As you will see, nailing down a single definition for such an extraordinarily broad and diverse collection of species within the entire Plant Kingdom is impractical if not impossible. Still, why are trees called trees?
Trees are called trees because they are tall, perennial, woody plants with a single unbranched self-supporting stem holding an elevated and distinct crown of branches. The word “tree” derives from the Old English word “treow” which primarily defined large woody plants.
However, there’s still MUCH more to it! This article will review 8 features of common trees that distinguish them from all other species of plants and get into the nitty gritty of just why trees are called trees. Let’s get to it!
All Trees Have A Trunk
The trunk is likely the most prominent visual feature most people associate with a tree. No other plant species on earth stands upon such a single, unbranched woody stem. While several species grow from a clump-like base, it is the uniqueness of the trunk that defines a tree’s visual identity.
While shared as a common feature, the trunks of various tree species show remarkable diversity. Consider shape and form. At one end of the spectrum, most pines, birches, and redwoods feature trunks that are nearly perfectly vertical. At the other end are species such as junipers, contorted beeches, and Japanese maples which feature twisted irregular trunks.
The girth of the world’s largest trees, as measured by circumference and diameter, is simply incredible. El Arbol del Tule in Oaxaca state, Mexico, stands alone atop the list. While only a modest 116 feet in height, the tree has a circumference of 137.8 feet and a diameter of just over 46.1 feet. It would take over 60 adults with outstretched hands to make it around its massive trunk. Wow!
The volume and weight of El Arbol del Tule are equally bewildering. Checking in at just under 25,000 cubic feet, it would fill every square inch of a 3,000 square-foot home with 8 feet high ceilings! If you put its 1.1 million pounds on one side of a balance scale, it would take 111 African Bush Elephants, the largest land mammal on earth, on the other side to match!
Not even the colossal blue whale comes close to rivaling the staggering scale of some trees, earth’s true largest living organisms.
All Trees Have Indeterminate Growth
Indeterminate growth refers to a tree’s ability to grow outward in all directions. Tree growth occurs in two ways: primary and secondary.
Primary growth involves increases in height and length via nodes, buds, and branches. A node is any place on a branch where leaves are produced. After one or more leaves emerge at a node, the stem extends to form a series of additional nodes followed by a terminal bud at the end of the stem.
In younger trees, one terminal bud typically grows straight up to form what is known as a leader. Other terminal buds grow more horizontally to form additional branches. This repeated pattern of growth over time results in the unique canopy of each tree species.
Secondary growth is the process by which tree stems, branches, and roots grow larger or thicker in diameter. If you’ve ever peeled back the bark on a live tree branch, you’ve probably observed the soft and typically moist inner layer just underneath. This is called the vascular cambium. Every year it produces new wood on the inside known as xylem, and new inner bark on the outside known as phloem.
Nowhere is the repeated series of secondary growth more recognizable than in the annual rings found within each trunk.
Every Tree Features Annual Rings
Trees are the only plants that methodically record their lifespan. This living record is found in the annual rings within the trunk.
In temperate climates, the vascular cambium begins growing when temperatures warm in the spring and stops growing when temperatures drop in fall or winter. When cambial growth stops, it leaves behind a dark line until new growth begins again.
As trees perpetually grow outward due to indeterminate growth, each dark line becomes an annual ring, just like the concentric circles that appear when a rock is thrown into still water.
Trees growing in tropical climates don’t typically have clearly visible annual rings. However, dendrologists, tree scientists, can still use chemical analysis to determine their age and the width of their rings. In years with above-average precipitation, the annual ring will be wider. In years with lower precipitation or even drought, the annual ring will be narrower.
Who knew trees also provide historical weather data?!
All Trees Have Bark
As with the wide variations found in trunks, similar diversity can be found in the bark that surrounds them. The bark is generally defined as the two outermost layers of the trunk. Inner bark consists of both living and dead phloem just outside of the vascular cambium described earlier. Outer bark consists of dead phloem and cork.
The external bark of a tree is its primary protection. Similar to human skin, bark provides a barrier to keep moisture in and disease and infection out. Trees have a highly developed injury defense system!
Through both chemical and physical processes, damage or wounds to the bark that threaten living tissues can be sealed off or callused. New bark eventually covers this callus growth to fully restore the tree to its pre-injury health.
Beyond the function of bark, humans have long been intrigued by its texture and appearance. How many thousands of couples have carved their initials in the smooth thin bark of an American beech?
Many aspens and birches feature similarly soft, smooth bark. Most oaks, pines, and hickory trees, however, sport bark thick and rough enough to splinter or even cut the unwary hand.
Great diversity can also be found in bark color. Consider the almost pure white bark of aspens or birches in contrast to the deep red of coral bark maples or Tibetan cherries. In one of nature’s many fascinating oddities, the beautiful multicolored bark of the rainbow eucalyptus tree reveals distinct shades of green, blue, orange, red, and purple!
To learn more about why bark is so important, check out our article on why you shouldn’t peel off tree bark here.
All Trees Are Characterized By Their Height
Along with the trunk, height is the other primary visual characteristic that distinguishes trees from all other plants. Plant species such as grasses, mosses, and shrubs, grow closer to the ground, often covering large amounts of area. Tree canopies, however, often tower above to obtain the sunlight needed to sustain their energy needs.
As with trunks, there is great variety in height among the many species of trees. Shorter trees, often reaching mature heights of only around 10 feet, include crabapples, magnolias, and hawthorns. Earth’s largest trees include redwoods, giant sequoias, and coast Douglas firs.
The tallest tree in the world is named Hyperion. A redwood located in California, it rises to a staggering 397.9 feet! To put this into context, a 400-foot building would include somewhere between 37 and 45 stories. Hyperion is taller than the Statue of Liberty and the Big Ben tower in London.
While there is no realistic way to identify the smallest tree in the world, two species stand out. The crape myrtle, native to China and Korea, can be as short as 3 feet when fully grown. The ‘Viridis’ Japanese maple rarely grows beyond 6 feet, spreading out from its trunk into a more bush-like shape.
Another way to measure a tree is by the breadth of its canopy. Thimmamma Marrimanu, a banyan tree in one of the driest regions of India, features the largest single tree canopy on earth. Its circumference spreads a nearly incomprehensible 2,775 feet covering over 5 entire acres of land!
All Trees Are Known For Their Longevity
Among all plants, trees have the unique ability to live for truly mind-boggling periods.
Consider Methuselah, an ancient bristlecone pine in the White Mountains of California, largely regarded as the oldest single specimen on earth. As of 2022, Methuselah checks in at the ripe old age of 4,854! Not only does this transcend generations after generations of mankind, but entire civilizations have also come and gone while it continues to stand marking time.
The Sarv-e Abarqu, also known as the “Zoroastrian Sarv,” is a cypress tree in Iran. Dendrologists believe it is at least 4,000 years old and likely the oldest living organism in Asia.
In a true testament to trees’ incredible ability to survive in the harshest of conditions, consider the Chestnut Tree of One Hundred Horses. Located on the Italian island of Sicily, the oldest chestnut tree in the world lives just 5 miles from the crater of Mt. Etna, one of the most active volcanoes on the planet.
The tree’s name originated from a legend about a company of 100 knights who were caught in a torrential thunderstorm. According to the legend, all of them were able to remain safe under the tree’s massive canopy.
While individual specimens come and go, tree colonies are often many times older. A tree colony is defined as a group of genetically identical trees (clones) connected by a single root system. Pando, or “trembling giant,” is a clonal colony in south-central Utah estimated to be 80,000 years old!
Along with indeterminate growth, these extraordinary ages are achieved as a result of different tree species’ ability to adapt and thrive in many different conditions and environments.
All Trees Have A Complex Root System
Most plants have roots, specialized tissues primarily located beneath the soil that absorb and transport water and nutrients to sustain the life and growth of the plant. Even those that don’t, such as mosses and liverworts, have structures called rhizomes that serve a similar general function. Nowhere in the plant kingdom are root systems as intricate and advanced as those of trees.
The size and scale of a tree’s root system are remarkable. The largest, tallest trees often boast individual roots that surpass 15 feet in diameter. These largely underground roots alone far exceed the visible portions of even the largest species of shrubs and bushes.
Tree roots extend out radially and horizontally from the base in all directions. Because their primary function is to transport water and nutrients, roots rarely extend below the top several feet of soil. This is true whether the tree is 20 years old or 2,000. Roots sustain the tree not because of their depth so much as their length.
Each time it rains, more water reaches the ground outside the area covered by a tree’s canopy than that directly underneath. This line of demarcation is known as the drip line. This “line” is a circle or oval as it marks the distinction between these two areas as if you were looking straight down from a bird’s-eye view and tracing the canopy’s outline.
Tree roots can extend 4-7 times beyond the drip line. For example, consider a tree with a drip line of ten feet from the base of the trunk in all directions. This would be equivalent to stretching a ten-foot length of rope from the trunk and walking around the circumference of the tree while keeping it taut. To accurately capture the true expanse of the entire root network, you would need to do the same thing with a 40-70 foot length of rope!
Even using a conservative multiple of 5, a tree with a modest 30-foot canopy planted right in the middle of a football field would have a root system extending to both endzones!
Some trees, like birch trees, have far spreading roots that need to be taken cautiously when planted near homes. But many trees generally fit into the characteristics above.
All Trees Live In Communities
Not only do trees often grow in colonies, but they also have a highly sophisticated means of communicating with each other and with other tree species.
In The Hidden Life of Trees: What They Feel, How They Communicate, German forester Peter Wohlleben outlines the science behind a growing belief that trees behave as a sort of superorganism.
Through their collective root networks, trees can send chemical, hormonal, and even electrical messages to each other to enhance growth and survival. As one specific example, when one or more members are cut or otherwise injured, the healthy members of the colony will divert additional resources and healing compounds to the wounded.
Trees can also warn others of danger by emitting pheromones and other scent signals through the air!
In one stunning example, consider the wide-crowned umbrella thorn acacia, a favorite food of giraffes. When acacia leaves begin to be chewed by a giraffe, they release ethylene gas as a distress signal. Neighboring acacias detect the gas and begin pumping more tannins into their leaves. It just so happens that when consumed in large quantities, these tannins can sicken or even kill giraffes.
Trees can even communicate with and form mutually beneficial alliances with other species. For example, Douglas firs and birches are known to often share resources and communicate with each other regarding dangers that could affect them both.
In forest settings, larger more mature specimens of a given tree species will pump sugar, water, carbon, and other vital resources into the individual roots of young saplings that emerge in fully shaded areas. These so-called “Mother Trees” increase seedling survival rates by as much as 400%. If too many are cut down, entire tree networks can collapse.
While all plants interact with each other to some degree, trees exhibit an unparalleled ability to communicate with each other in highly advanced ways that scientists are just now scraping the surface of.
Where Do Trees Come From?
Depending on who you ask, you’re likely to get several different answers. Some might quickly identify water. Others might just as quickly point to nutrients in the soil. Seeds are another frequently cited response.
As is so often the case with scientific questions, the answer isn’t as simple as you might think. Let’s dig deeper to understand where the matter that forms the actual substance of trees really comes from. The true answer will likely surprise you.
All living things, plants or otherwise, need water to survive. The same goes for food (minerals and nutrients). Ditto again for reproduction. Curiously though, none of these factors account for the actual existence of what we see, touch, and use as the wood that makes a tree unique from other living things.
Believe it or not, the true secret is air! Unconvinced? The late great Richard Feynman, theoretical physicist, Nobel Prize winner, educator, and arguably one of the smartest human beings who ever lived, told the BBC in 1983 that trees are made from thin air. Not theoretically, not hypothetically, but literally.
As he and other scientists describe, trees, along with all other living things, are made of carbon. The carbon that comprises trees comes directly from the sun’s ability to separate carbon dioxide in the air into its atoms of carbon and oxygen. Each molecule of carbon dioxide consists of one atom of carbon and two atoms of oxygen.
When sunlight splits carbon dioxide, the two atoms of oxygen are cast off into the air as by-products. The carbon atom, however, joins with water and several other base resources to form the substance that permeates each cell of the tree. As water also originates from the air, regardless of where it is currently located at any given moment on earth, one can begin to appreciate the concept that trees are made from air.
Grasping and understanding this stunning revelation, however, is another matter altogether. Mr. Feynman offers an example we can all at least relate to, to assist us in that effort. Consider a burning log of wood in a fire. The wood contains physical matter, predominantly carbon that was formed when the sun separated carbon dioxide into carbon and oxygen.
When heated to a sufficient degree, the log begins to burn, emitting heat and light in the process. The more wood, the more heat, and light. Feynman explains that the heat and light we feel and see is the sun’s energy that was stored in the wood when it was formed.
As the log burns down, carbon is released back into the air where it immediately joins with oxygen to reform carbon dioxide. When the entire amount of energy held in the log is exhausted, it ceases to exist in material form, having released all of its chemical energy back into the air. Talk about enlightening!
That’s A Wrap!
Whatever your relationship with trees happens to be, understanding the eight traits outlined above can help deepen your appreciation of these truly incredible specimens.
To recap, most trees all share the same features of:
- Indeterminate growth
- Annual rings
- Root system
- Community life
The next time you escape to the woods, whether via book, movie, magazine, or in the flesh, may you enjoy a rediscovered or newfound sense of joy in sharing our earth with such wonderful companions!
Franklin, Jennifer, and Mercker, David. (2009). “Tree Growth Characteristics.” University of Tennessee Institute of Agriculture.
Coder, Kim D. (2018). What Is A Tree? Warnell School of Forestry & Natural Resources, University of Georgia, Thompson Mills Forest & State Arboretum Outreach Product. ARBORETUM-18-06. Pp.9.
Piovesan, G. and Biondi, F. (2021), On tree longevity. New Phytol, 231: 1318-1337.