Radio & Podcasts

Why does a tree drop its leaves?

During the long days of spring and summer, trees convert the sunlight that hits their leaves into energy — a process that requires water. But cold temperatures freeze water in the soil, so it's not available for uptake. And the short winter days means that there simply isn't enough sunlight energy to maintain those leaves. But instead of just tossing them away like used-up candy wrappers, trees break down the leaves’ chlorophyll, transport the nutrients to their roots, and then recycling them in the spring.

The process of moving out the chlorophyll reveals the yellow and orange of other leaf pigments. They were there all along, just masked by the chlorophyll.

Timing is critical. Trees have to maximize leaf-time to capture energy, but avoid risks of freezing before completing that nutrient recycling process.

So how do trees tell seasonal time without a calendar? You might think that the air temperature is the trigger. But the answer is photoperiod — the balance of daytime and nighttime, a far more reliable indicator of seasonal change.

What I think is cool is that trees don’t assess the length of the day — rather, they measure the duration of the night!

A pigment called phytochrome is the key. It has two interconvertible forms. One is activated by red light – which leaves get during the day. The other is activated by far-red light, which dominates at night. The proportion of these two forms is the trigger that signals it’s time for the leaves to fall.

So, when you check out our colorful mountains, remember that it's not the temperature that triggers the show — it’s the length of the night.

The other day, as I lugged home 300 pounds of apples from my neighbor’s yard, I thought about the purpose of … fruit.

Flowering plants — like apple trees — have evolved so that their seeds will land in the best place to flourish, the very definition of biological fitness. If tree seeds remain in their own shade, the seedlings would compete for resources with their parent, a bad outcome for both generations.

So, how can a tree that’s rooted in the ground send its seeds out into the world? Marketers of any product — be that a box of cereal or an apple — know that it’s all about packaging.

Take a walk along the cereal aisle of a grocery store. Corporations design cereal boxes so kids will clamor for the package with the brightest colors and the best prizes. Similarly, the bigger and brighter the apple, the more inclined a potential disperser is to choose it.

Archeological records reveal that the ancestors of apples were very small, more like the modern crabapple. Their seeds were spread by birds.

Other fossil evidence shows that as apples evolved, they were dispersed by the now-extinct megafauna that roamed the northern hemisphere 12,000 years ago. Those woolly mammoths and bear-sized beavers were well-suited to gathering and distributing larger apple fruits.

But fruit dispersal is useless if seeds are chewed up or destroyed.

So, many trees have evolved seeds that contain distasteful or poisonous compounds. Each apple seed contains amygdalin, a chemical that, when chewed, changes into cyanide, one of the deadliest of human poisons. But don't worry. You'd have to consume the seeds of 40 apple cores before suffering ill effects.

Over the centuries, horticulturists have deliberately selected trees that favor certain traits, creating the 7,000 apple varieties grown today.

So, next time you choose a big juicy apple and toss the core out the car window – with unchewed seeds — you’re playing right into evolution’s hands.

People have used cork for millennia. It's light, buoyant, and elastic, thanks to the 40 million air cells that occupy each cubic inch. Today, we use cork for fishing floats, floor tiles, and of course, bottle stoppers.

The tree that produces it is called cork oak. It has evolved a spongy bark to protect against fire and disease. Unlike other trees, its bark separates along the dead cork cambium so that the inner bark can regrow for many harvests.

Most of the 13 billion corks produced each year come from Spain and Portugal. Each cork oak lives for 250 years, and makes enough cork for 60,000 bottles of wine.

Skilled workers harvest the bark, slicing and peeling it into sheets the size of a yoga mat. The bark weathers for a year, and is then soaked, flattened and sorted. Wine corks are punched by hand, printed with logos and then sent off to candle-lit dinners in our homes and restaurants. The process provides jobs for some 30,000 people.

Cork forests also offer critical habitat for two endangered animals — the Iberian Lynx and the Iberian Eagle.

But recently, the wine industry has started using synthetic corks made from petroleum to reduce costs and the risk of contaminants. Cork oak forests are being replaced by pine plantations to make wood pulp, with negative impacts on centuries-old human and animal communities.

So, the next time you order a bottle of wine, let your server know you'd like a bottle with a real cork — from a real tree. As with so many of our actions, this seemingly simple decision can affect people and trees half a world away.

I have a small white scar on my elbow. It’s from a dog bite I got on my newspaper route in 7th grade. I’ve long imagined throwing a “scar party,” in which my guests take turns describing the marks left on their bodies by events that might have been otherwise forgotten.

So, what about trees — what of their history can we read from their bodies?

On a recent walk in Liberty Park, I noticed an odd branch on a small maple tree that started growing horizontally but then took a sharp vertical turn. A raised circular collar of scar tissue revealed that the branch above it had broken off. In response, the lower branch grew upward instead of outward, a record of an earlier injury.

Mountain trees strategically grow different types of wood that resolve the stresses imposed by wind and heavy snow loads. A spruce tree grows "tension wood" on its trunk's uphill side and wider-ringed "compression wood" on its lower side, which creates a big bellied trunk to let the tree straighten up and stand tall.

One tree that silently conveys its history is the 70-year-old Cedar of Lebanon that stands at the east entrance to Salt Lake City's Temple Square. Years ago, the tree survived a potentially fatal blow when a chunk of ice fell from the temple's roof, completely shearing off the tree’s top. Gardeners guided a lateral branch to grow vertically, leaving a distinct wiggle in its trunk.

But any time you walk in our city or forests, take a moment to appreciate the body language of trees. Without words, the arc of a branch or the slant of a trunk reveals that trees can survive damage and carry on living — their scars giving voice to their stories.

For decades, the makers of baseball bats used wood from ash trees to provide just the right feel for hitting homers. The density and loose grain of ash wood gives the bat a “trampoline” effect, producing a boost that can drive the ball further and faster.

But recently, ash bats have been sidelined due to a tiny beetle, the Emerald Ash Borer, which was accidentally introduced to the United States from Asia in wood packing material. The larvae feed on ash tree’s inner bark, blocking the transport of water and nutrients. So far, these insects have killed over seven billion ash trees.

The beetle has spread rapidly throughout the country because here, they have no natural enemies. Researchers are exploring whether predators in the beetles’ native habitats could be brought over to control the insect spread across the country.

Ash bats also took a hit in 2001, when Barry Bonds hit 73 home runs using bats made from sugar maple trees. Suddenly, everyone wanted a maple bat. What’s the difference between these two woods? The greater density of maple wood means players can lean into hitting the ball with as much power as they can give it.

And bats are not the only way trees show up in the batter’s box. Just before they step up to home plate, most players rub sticky dabs of pine tar – derived from Scot’s pine trees — on their bat handles. It improves their grip, giving the ball more pop on contact.

Trees have been part of sports history for years, from basketball floors to golf clubs to pool cues. But baseball is one of the few sports that still relies on wood’s natural properties to play — and win — the game. Maybe we should be singing, “Root root root for the home tree.”

This morning, our lawn looked like the aftermath of some sort of tree party. But instead of scattered confetti, big pieces of bark were strewn all over, deposited by the giant sycamore tree that grows in our neighbor's backyard.

These trees thrive in city settings because of their rapid growth and tolerance of pollution.

Sycamores are easy to spot thanks to their distinctive thin bark that looks like army camouflage. As the irregular blotches of tan and green pieces of bark flake off, or exfoliate, they reveal a creamy inner trunk, smooth to the touch.

On the surface, this pattern seems simple to explain: As the sycamore trunk gets bigger in diameter, the older bark has to push outward, cracking off its rigid plates.

But of the over 70,000 species of trees on our planet, only a handful of them shed their bark. Why? Botanists don’t have a definitive answer yet, but I’ll share some current theories.

For starters, bark is a dead layer of tissue that protects trees from losing moisture, from fire and from animal attacks. Since thin-barked trees lack the protection that thick bark provides, one theory is that shucking off old bark helps jettison insects and disease-causing microbes, like snakes when they shed their skins. And sycamores’ pale trunks make caterpillars more visible for birds to find and pick off, an additional mode of protection.

Most photosynthesis — how trees capture energy — takes place in green leaves. But another theory claims that think-bark trees like sycamores may capture energy through their trunk surfaces. It’s not an entirely out there idea — the process is often found in desert plants, where rates of stem photosynthesis can reach up to 60% of the leaf photosynthetic rate.

But this research is in its infancy, so stay tuned for updates!

And although hosting a sycamore may not be your best choice if you want a perfectly manicured lawn, these trees are great for providing compelling questions about nature, which I think makes that messy, post-party look of your lawn entirely worth it.

I’m the first to admit it. Getting older has its drawbacks. I can’t run as fast nor dance as wildly as I did when I was 20. But there are some bonuses to getting older. My favorite? I can take a reflective walk — instead of a bone-crunching mountain bike ride — when I visit our foothills.

Aging has benefits in the world of trees, too. In the past, timber companies viewed dead trees as economically worthless. But ecologists have learned that snags — big, old, standing trees — become "biodiversity hotspots," providing unique habitats that sustain wildlife and expand the carbon storage capacity of our forests.

In North America, more than 100 species of animals need snags for their food, nests and shelter. 85 species of birds — called cavity-nesters —hole up in snags. Their presence helps control insect pests. A single snag-dwelling swallow can eat up to 1,000 mosquitoes a day.

If you were a woodpecker, which you choose to make your home: a young tree or an old tree? Excavating a bird-sized hole in a tree softened by heart-rot fungi is much easier than drilling into the solid trunk of a young tree. Heart-rot fungi doesn’t kill the tree, but just softens the resistant center. But it can take those fungi decades, even centuries, before that softening begins.

However, sustaining those elements is now a challenge, since rotations of forest harvesting are getting shorter, and the number of snags is getting smaller.

In response, foresters have developed the practice of “veteranisation,” accelerating the aging process of younger trees to create “tree veterans” in our landscapes. Although intentionally damaging younger trees sounds odd, these practices speed the development of habitats offered by older trees.

Techniques of veteranisation include bark stripping and breaking off branches. The weirdest — but most efficient — practice is to introduce heart-rot fungi into healthy trees. Foresters actually climb up snags, drill holes in the trunk and insert plugs of the fungi into the holes, hurrying the process of decay, and making cavity-excavations easier for home-seeking wildlife.

It’s kind of comforting for a snag-to-be like me to know that as we age, we can still enrich our own habitats.

It’s the smallest instrument in the orchestra pit and it makes no sound. But the conductor’s baton is what unifies and shapes the composer’s score for listeners.

Batons are almost always made from trees. These lightweight wooden shafts taper down to a teardrop grip called a "bulb” that gives the conductor something substantial to hold onto. Most batons are painted white so that the musicians see it in the dim light of the pit.

Early on, batons were made from exotic hardwoods, such as ebony and Brazilian rosewood, trees that grow in rainforests of South America and Africa. But these species are now threatened or endangered, so conductors use other woods, like oak and walnut, due to ethical concerns about forest destruction and the rights of the indigenous people who live in those forests.

Leonard Bernstein, the legendary conductor of the New York Philharmonic, began his conducting career with batons made of maple. But they tended towards brittleness, so he switched to birch wood, crafted by the orchestra’s tympanist. It’s reported that Bernstein was buried in Brooklyn’s Greenwood Cemetery, along with the score of Mahler's Fifth Symphony and his beloved baton.

I spoke with Jared Oaks, the Music Director and conductor of the Ballet West Orchestra, who is currently busy conducting live music for Ballet West’s "Nutcracker" performances. He said that for him, the baton is not just about keeping the beat, it’s also about expression. When he gestures with his baton, he thinks of it as an invitation to the musicians, asking and guiding them to join him in creating the music.

Oaks uses a baton with a shaft of fiberglass, but the bulb he cradles in his palm is made of compressed wood of different species. He plans to make his own baton out of driftwood that he collected years ago.

Every orchestra is full of instruments made of wood, but the piece most of us overlook is that slim little shaft made of trees that brings all of the instruments together to create the sound of music.

With the holidays come evergreen wreaths on people’s doors and windows — which got host Nalini Nadkarni asking: Where does all of this holiday greenery come from?

Most of our wreath greens are harvested from Pacific Northwest forests, whose climate is ideal for growing fir, cedar and holly.

Wreath materials are a "non-timber forest product,” meaning that they are harvested without cutting down trees — like nuts, berries, mushrooms and medicinal plants. Nationwide, these products generate about $5 billion a year, with roughly half coming from the evergreen boughs and pine cones collected for the winter holidays.

Although wreaths are small in size, the business of harvesting, shaping and shipping them is huge. Each year, harvesters gather more than 10,000 tons — that's 20 million pounds! — of evergreen boughs, and nearly 14,000 bushels of pine cones to create the 5 million wreaths and garlands that are sent around the world.

Most harvesters obtain state or federal permits to gather greens from public land, or from land owned by private timber companies. And many greenery companies proudly emphasize their sustainable practices of trimming boughs, not cutting whole trees or using old growth forests.

As with agricultural crops, wild greenery is subject to the weather. Evergreen trees require cold temperatures to trigger dormancy as harvesting limbs while trees are still actively growing can stress the parent tree, causing them to shed their foliage more quickly. In 2015, trees in the Northwest experienced a "Godzilla El Niño" year, with record high winter temperatures. That delayed the harvest, and created large economic losses for the whole industry.

Some companies are now experimenting with “bough orchards.” In Denmark, for example, trees are managed for greenery production by leaving a skirt of branches on the lower part. As the tree grows taller, limbs are harvested from the skirt, a practice that can be repeated for 30 years.

So, when you deck your halls with boughs of holly — or other tree species — give a holiday cheer to the forests of the Pacific Northwest for the beauty they provide.

Given the biological purpose of mistletoe, Nalini Nadkarni finds it pretty strange that this parasite is also a symbol of love.

The Celtic Druids observed that mistletoe could flourish in sacred oak trees during the harsh winters of the British Isles. It became their symbol of vitality and fertility. The custom of kissing under mistletoe arose in England in the mid-1800s, and today, we chime in on popular songs about mistletoe like the tune, “I Saw Mommy Kissing Santa Claus.”

Knowing the biology of mistletoe reveals some contradictions about it as a symbol for love. After all, mistletoes are parasites. They infect trees to get resources from their hosts by penetrating the bark with a root-like tissue called a haustorium, which connects the mistletoe to the tree’s transport systems, diverting water, nutrients and sugars to itself.

Mistletoe can reduce the growth of some economically important trees, like Lodgepole pine and Douglas fir. But from an evolutionary standpoint, all parasites — from tapeworms to malarial vectors — are moderate in their takings, since diverting too much from the host would mean literally eating themselves out of house and home.

Nearly 250 species of birds disperse mistletoe. The seeds are surrounded by a sticky goo called viscin, which cements the seeds where they fall. Some birds wipe their bills onto branches; other birds excrete seeds in their droppings, manifesting the Old English origin of the word mistletoe: “mistel” meaning dung and “tan” meaning twig. Hence, dung-on-a-stick.

Many other animals — including porcupines and lizards — eat mistletoe berries and leaves as a critical part of their diet. In one experiment in Australia, mistletoe was removed entirely from study sites. Those areas ended up with 25% fewer bird species than neighboring sites where mistletoe was left intact.

So, as with many interactions with trees, the meaning of mistletoe is wonderfully complex. Mistletoes can reduce growth of their hosts, but they also provide a bounty of resources for the organisms surrounding them.

That’s something to reflect on when you’re invited beneath that dung-on-a-stick for a holiday kiss.

As an ecologist, I have shelves full of scientific and literary books on trees. One of my favorites is an essay by the German writer Hermann Hesse, who received the Nobel Prize for Literature in 1946. As we head into the New Year, I’m sharing with you this selection from Hesse’s piece titled, “On Trees.” I hope you find it as powerful as I do.

For me, trees have always been the most penetrating preachers. I revere them when they live in tribes and families, in forests and groves. And even more, I revere them when they stand alone. They are like lonely persons. Not like hermits who have stolen away out of some weakness, but like great, solitary men, like Beethoven and Nietzsche. In their highest boughs the world rustles, their roots rest in infinity ... Nothing is holier, nothing is more exemplary than a beautiful, strong tree. When a tree is cut down and reveals its naked death-wound to the sun, one can read its whole history in the luminous, inscribed disk of its trunk: in the rings of its years, its scars, all the struggle, all the suffering, all the sickness, all the happiness and prosperity stand truly written, the narrow years and the luxurious years, the attacks withstood, the storms endured.

And every farm boy knows that the hardest and noblest wood has the narrowest rings, that high on the mountains and in continuing danger the most indestructible, the strongest, the ideal trees grow.

Trees are sanctuaries. Whoever knows how to speak to them, whoever knows how to listen to them, can learn the truth ... So the tree rustles in the evening, when we stand uneasy before our own childish thoughts: Trees have longer thoughts, long-breathing and restful, just as they have longer lives than ours. But when we have learned how to listen to trees, then the brevity and the quickness and the childlike hastiness of our thoughts achieve an incomparable joy. Whoever has learned how to listen to trees no longer wants to be a tree. He wants to be nothing except what he is.

That is home. That is happiness.

On a recent camping trip in Nevada, I visited a display of petrified wood. A tall chain link fence surrounded the logs — as if they were in prison — to protect them from people who might grab pieces as souvenirs.

What makes these logs so special? Petrified wood is a very particular type of fossilized wood. Those logs are over 200 million years old, but amazingly, still retain the same shape and structure as when they were alive. Biologically, the original trees are related to our current-day ginkgos and Norfolk Pine trees.

The process of trees turning to stone started pretty simply: water washed logs into ancient river systems, burying them so deeply that it cut oxygen off. Then, bacteria and fungi — which need oxygen — couldn't decompose them. Instead, water, containing all kinds of dissolved minerals, flowed through the porous wood, replacing every single cell of organic material with crystals.

Petrified forests can be found all around the world. In Utah, we can see petrified wood in Grand Staircase–Escalante National Monument.

Another reason petrified wood must be protected is because of its beauty. Each piece is a giant sparkly quartz crystal, with a rainbow of colors produced by imbedded minerals: cobalt creates greens, iron oxides create reds and manganese makes pinks and oranges.

So trees can turn into rocks. Can rocks turn into trees? Yes! Trees get their nutrients, water and support from soil, which is originally derived from rock.

The dynamic process of soil formation happens through the slow but unstoppable forces of physical, chemical and biotic weathering. When moisture seeps into cracks and then freezes and expands, it literally busts the rocks apart. And when water combines with carbon dioxide, either from the air or in the soil, that reaction creates carbonic acid, which can chemically dissolve rocks.

So, trees can turn to rocks and rocks can turn into trees, if we give them enough time.

But rock time and tree time pass in different scales. As poet Bill Yake wrote, "To the rocks, the trees are just passing through."

I recently took the train from Salt Lake City to California, my first such experience in decades. Each mile, my train passed over 3,000 railroad ties – nearly all of them made from trees.

Railroad ties, or cross ties, are a critical ingredient of railway tracks. They transfer the tremendous weight of cars and cargo to the track ballast, the crushed gravel bed that underlies the rails and help maintain the right gauge, or distance between the tracks.

In the 1830s, the first railroads were secured onto stone blocks. But as locomotives became heavier, wooden railroad ties were introduced. Workers hand cut rough ties from trees with cross saws and broadaxes, jobs that were replaced by sawmills in the 1940s. Today, crossties have a uniform size and thickness, 10 inches thick and 10 feet long.

Nationwide, freight trains now move nearly two billion tons of goods across an astounding 450,000,000 railroad ties. Nearly all of them are made from wood — less than 2% are made of steel or concrete.

Ties from trees persist because wood is strong, flexible and renewable, and it doesn’t conduct electricity or interfere with electronic rail monitoring. But because wood is subject to decay, cross ties must be treated with liquid wood preservatives, like creosote, a type of coal tar. Treated crossties can last for more than 30 years.

If a rail company chooses ties made from one of our deciduous trees — like oak, maple or hickory — the liquid preservative can easily permeate throughout the wood because of the open-ended cells that transport water within deciduous trees.

If the company goes for wood made from conifer trees — like Douglas-fir or hemlock — they’ll be less expensive and lighter-weight, but will be less absorptive of preservative, since their transport cells have closed ends that allow for one-way transfer of liquids.

On my recent railroad voyage, I encountered members of the Railroad Tie Association, who are as enthusiastic about railroad ties as I am about trees. That organization was founded in 1919 to promote the sound use of wood crossties. Their annual meeting takes place each October. I think I might attend! And I'll take the train there, counting those wooden ties as I go.

Even though I’m in the tropics, it’s dark, damp and still here on the ground. The sun’s intense rays are absorbed by the layers of leaves over my head. Looking upward, I feel a familiar jig of excitement. What will I find today?

It wasn’t all that long ago that scientists called the tree canopy "the last biotic frontier." Until about 30 years ago, there was no safe way to get up there — it was a world waiting for exploration.

I step into my seat harness and leg loops. Clamping my ascenders onto the rope, I’m ready to climb. As I inchworm my way up the rope, my view begins to shift. Now, I look down see how the leaves are arranged to capture every bit of light.

10 more minutes and I’m at the top. Everywhere I look, there are huge branches covered with carpets of orchids and ferns.

I’m not the first scientist to notice the diversity up here. Canopy researchers have documented over 24,000 species of flowering plants in the tree tops — far more than you’d find rooted on the forest floor.

One reason for this high diversity is that the canopy environment is very different. Up here, there's more sunlight, greater extremes of humidity and temperature and more wind, too, which let’s canopy plants send their seeds to populate other suitable homes in forests far away.

And the canopy provides a different surfaces for the plants to rest on — from skinny young twiglets to limbs as broad as picnic tables — giving them more choices about where they can flourish.

By documenting the canopy’s plants, scientists are able to have a more complete inventory of the forest community. Without this knowledge, the complex jigsaw puzzle of the rainforest would be incomplete.

Exchanging my ascenders for my rappelling gear, I start my decent. As I move through the thousand, thousand leaves, a purple-throated mountain gem flashes by, attracted to my red shirt, a wonderful bonus for this trip aloft.

We all know the saying that money doesn’t grow on trees. But given that our bills are paper, I figured that our currency is, at least, made from trees.

However, I recently discovered that not a single tree is cut down to make America's money! Is this because the U.S. Treasury wants to save forests? No, it's because wood pulp-based currency would simply not be durable enough to survive the wear-and-tear from endless encounters with human hands, wallets and vending machines.

Those bills in your pocket and purses are actually more like fabric, created from a legally-protected combination of 75% cotton and 25% linen fibers. That blend creates the unique feel of our bills — kind of rough to the touch.

So, how about people in other countries — do they carry trees in their wallets? Since 2002, Euro banknotes use bills that are pure cotton fiber. And in the land down under, Australians use currencies made of polymer, which has a waxy feel and lasts twice as long as bills made of natural fibers.

But trees have played a role in the history of money. During the 13th century, Marco Polo was astonished when he encountered money made from bark of mulberry trees at the court of Kublai Khan in China, where paper money had been in circulation since the seventh century. The idea that one small light thing could have the same value as many heavy things — with the backing of the state — was an incredible innovation. Kublai Khan’s mulberry bark currency was stamped, sealed and, Polo wrote, “issued with as much solemnity and authority as if they were of pure gold or silver.” It also included a warning to anyone tempted to make their own that all counterfeiters would be put to death.

Today, four mulberry trees grow in the courtyard of the Head Office of the Bank of England in London, a reminder of paper currency’s history.

So, even though our money neither grows on trees nor is made from trees, the next time you pay with a bill, you can think of those early bark notes and be glad the idea caught on. It’s lighter on the wallet.

One of my favorite ways to honor trees is celebrating Tu BiShvat, the Jewish holiday that commemorates the “New Year for the Trees.” This year, it begins at sunset on Feb. 5, the 15th day of the Hebrew month of Shevat.

The origins of Tu BiShvat lie with the people who were guided by the tenets decreed in the Torah, their holy book. The Book of Leviticus forbids Jews from eating fruit produced by trees in the first three years after they were planted. Fruit produced in the fourth year goes to the temple and the poor. After five years, tree owners can take the fruit for themselves. To get the accounting right, followers needed a date to mark time, like the beginning of a fiscal year, a sort of birthday for all trees.

In the 16th century, members of the mystical Jewish sect, the Kabbalah, instituted a a feast of fruits as a ceremonial meal, or Seder. Participants ate fruits associated with the Land of Israel, which were described in the Book of Deuteronomy: figs, dates, raisins, pomegranates, olives, carob and almonds.

Celebrants also drank four cups of wine, ranging from red to white, each color a symbol of a different season. They recited particular blessings, which they believed would bring human beings and the world closer to spiritual perfection.

In the 1970s, Tu BiShvat took on an ecological flavor, and became the Jewish "Earth Day," with communities implementing actions related to environmental care. Today, in many Jewish congregations, Tu Bishvat is celebrated as a time to plant trees — in Israel and elsewhere. Many contribute money to the Jewish National Fund, an organization devoted to reforesting Israel.

For me, this holiday reveals the many different values and ways of understanding trees. It started as a way to account for the economic value of trees. Then, it manifested symbolic links to Jewish spirituality. And now, it’s an opportunity to both celebrate the many benefits that we receive from trees, and an inspiration to return those gifts.

The diversity of tree species in tropical forests is mind-boggling. Costa Rica alone hosts nearly 2,000 types of trees!

A perennial challenge for tropical biologists like me is how to identify the tree species we study.

Traditional plant taxonomy — the science of categorizing and naming living things — is based on a classification system that uses the shape and color of their flowers, fruits and leaves to give each species a unique, two-word Latin name.

But, often, trees don’t have accessible flowers, fruits or leaves. One classification alternative is the approach of tree architecture.

This concept was developed in the 1970s by a trio of botanists who recognized that tropical trees, like buildings, have distinctive forms.

These botanists grew a variety of trees from seeds, and then categorized the trees from aspects of growth. Do limbs grow from the base or throughout the trunk? Is growth continuous or seasonal? And so on. Then, they combined these tree forms to just 23 “architectural models,” and labelled each one after famous botanists, naming the first three models after themselves.

The system was wildly attractive, because it simplified the huge diversity of tropical forests. But its application is limited since trees conform to their model for only a short time. As they grow, tree shapes change because of external factors like shade and wind. So today, botanists don’t rely on tree architecture as a way to classify trees. But it has advanced our theoretical understanding of trees, especially how trees repair themselves after damage.

On your next walk, look at the structure of the trees you pass. You probably won’t classify them by their form, but it’s another way to see and appreciate trees.

We know that when it comes to people, unassuming doesn’t mean uninteresting. The same holds true for trees.

The Gambel oak has never broken any records for height or longevity, but a little research reveals its hidden complexity.

Named after William Gambel, a 19th century plant collector, Gambel oaks live in our Wasatch foothills and all over the West. Most are shorter than 10 feet tall, with rough bark, crooked branches, and an abundance of acorns that turn from green to golden brown in the fall.

These acorns feed squirrels, magpies, wild turkeys, porcupines and black bears. The Colorado Hairstreak Butterfly, the state insect of Colorado, is entirely dependent on them for food and egg-laying sites.

And for millennia, Gambel oaks have been a critical food source for indigenous people of the Southwest. Once gathered and processed, the acorns are ground into flour to make a host of sustaining and sustainable foods.

Gambel oaks also feed our scientific curiosity. Because the trees grow in clumps, scientists have wondered if the individual trunks in each cluster might actually be a single genetic individual, like the clones of our aspen trees. Until recently, that idea was only speculation.

In 2014, Jake Chalmers, then a graduate student at the University of Utah, investigated this hypothesis. He took leaf samples from individual oak trunks growing in different clumps, and then identified their genetic makeup. Each clump had a unique separate genetic signature.

That means that a landscape that looks like its housing a whole bunch of individual trees is actually home to a much smaller number of genetic organisms. It’s sort of like discovering a friend is a triplet and that there are two other individuals who share her exact genetic make-up.

So, on your next hike in our foothills, pause and appreciate the humble Gambel Oak. Their modest appearance hides their critical values — just like those wonderful unassuming people in our lives.

It’s hard to know exactly when humans started using bark cloth, but it’s likely that it predates weaving. Historians have found examples of bark cloth from across the Pacific, Indonesia, Africa and Asia.

Some of the earliest evidence of bark cloth come from Uganda, where skilled workers still create the textile through processes they developed centuries ago. First making a slit at the base of the Mutuba tree, workers cut through the top layer of bark. From there, they separate the inner bark from the tree in one large piece, and wrap the tree trunk in Mutuba leaves to protect it as it heals.

Then, they scrap and boil the inner bark to make it flexible. After beating it down to a single layer, they wring it out and stretch it taut. After several days of drying in the sun, the fabric — now strong and soft — is ready.

Although most tree species would die from this process, Mutuba trees can live through annual harvests for 40 years, with a single tree producing 8,000 square yards of cloth in its lifetime — enough to cover two football fields!

Historians have found evidence of bark cloth dating back 700 years, when only royalty were allowed to wear it. But its uses have diversified — it’s been used as currency, in religious ceremonies and as a symbol of protest against colonialism

In 2005, UNESCO named Ugandan bark cloth a “masterpiece of intangible cultural heritage.” And today, you can find bark cloth on the fashion runway! Ugandan born designer Jose’ Hendo creates bark cloth for her London-based collections, fostering culture of environmentally friendly and sustainable fashion.

The clothing we wear is a necessity, a fashion statement and, sometimes, a symbol about what we value. I can't imagine wearing anything better than an elegant garment made entirely of trees.

Our country has over 150 million telephone poles — that's half a pole for every person!

Although we don’t much notice them under normal conditions, windstorms wake us up to their importance. After Hurricane Katrina, New Orleans needed 72,000 pole replacements from around the country.

That kind of spike in demand makes pole-growing a complex business. Pole tree plantations, which grow southern yellow pines and Douglas fir, must be thinned and pruned to create trees that are at least 60 feet tall and free of knots.

As smart grids and underground power lines become more prevalent, I wonder: How much longer will we be using traditional wooden poles? Well, don’t expect to see them disappear any time soon. Wooden poles are more affordable, easier to transport and need less energy to manufacture than steel or concrete poles. They’re also non-conductive, which makes them safer for utility workers. And wooden poles store carbon, a small but real contribution to mitigate climate change.

And so many birds use these structures! Swallows, crows, ospreys and mourning doves all roost on poles. In the wide-open spaces of the West, many raptors hang out on the wires for an uninterrupted view of the prey that scuttles below.

But perching on these wires does carry risks. Power lines electrocute tens to hundreds of thousands of birds annually. Many power companies have taken measures to mitigate these dangers, by insulating existing wires and adding fiberglass perch guards on transformer poles.

Electrical poles are so ubiquitous in our landscape that we hardly even see them. But each one started its life as a seeding, providing us with a more connected world.

I’ve often been called a tree-hugger. But I know that getting anywhere near the poisonous Manchineel tree is a very bad idea.

This tall, handsome tree grows in sandy soils along the coastlines of the Caribbean and South America, but it literally oozes toxic chemicals from all of its parts.

And I do mean all: it’s fruit, "manzanillas de la muerte" or "little apples of death," have a deceptively sweet flavor. But after one bite, you'll notice a weird peppery taste, which then turns into a burning, painful tightness in your throat. Contact with the sap can cause your skin to blister. And smoke from burning manchineel wood can cause temporary blindness.

The poisonous nature of this tree seems counter to the principles of evolution. Most trees have evolved fruits to attract animals to spread their seeds to places that are safe for germination. But this tree’s toxicity means that typical dispersers — birds and mammals —completely avoid them. Instead, these seashore-dwelling trees drop their fruits into the water, and the tides and currents of the ocean disperse the seeds to successfully colonize new places.

What about trees that are toxic to other plants? Some trees, like black walnuts, exhibit allelopathy, a sort of chemical warfare among plants when one plant releases chemicals that suppress the germination or growth of other plants. These compounds move into the soil from decomposing leaves or roots, which stunts or kills the neighboring plants, leaving more nutrients, water and sunlight available for the plant aggressor.

But some poisonous trees have positive properties. People use the wood of the machineel tree to make furniture, felling the trees by burning instead of using axes. And these trees act as natural windbreaks countering the forces of beach erosion, which is critical in the face of rising sea levels. Indigenous groups have also used manchineel as medicine, using its fruits as a diuretic.

So, although there are a few trees that cause harm, even the most toxic of them have uses that redeem them.

No chocolate fan has to wonder why the scientific name of the cacao tree translates from the Greek to "the food of the gods."

The tree evolved in the South American Andes. Nearly 4,000 years ago, the earliest chocolatiers were the Olmecs in southern Mexico. Later, the Mayans and Aztecs consumed chocolate in ceremonies and celebrations, and used cacao beans as currency.

In the 1500s, chocolate mania spread throughout Europe, where people added cane sugar, cinnamon and other flavorings. Today, cacao plantations cover over 25 million acres all over the tropics.

The complex process of changing cacao beans to chocolate bars starts by harvesting the fruit. Each pod holds about 40 bean-shaped seeds, surrounded by a white pulp. The seeds are fermented, dried, roasted, cracked and shelled. The resulting nibs are ground into cocoa butter and cocoa powder, and are finally combined with sweeteners to create the chocolate we love.

But what most interests me about the cacao tree is its unusual reproductive biology. Given the great glory of chocolate, the cacao tree is surprisingly small, crookedy and kind of unassuming, never growing much taller than 20 feet high.

And, unlike 99% of the other trees in the world, which sprout flowers and fruits from new shoots, cacao pods grow directly from the trunks and main branches, a growth pattern called cauliflory, from the Latin "stem flower."

Figs, papayas, jackfruit and redbuds are also cauliflorous. Why do some trees do this? Botanists speculate that the pattern evolved because the animals that disperse the cacao tree’s seeds, like 30-pound howler monkeys, are too heavy to reach fruit at the end of the tree’s slender branches. Cauliflory gives them solid support as they can move seeds to safe sites.

Today, as I look out my window at our snowy Utah landscape, I’m glad for whatever evolutionary pathway created this curious growth form — and provided me with a tasty cup of hot chocolate, created by the cacao tree.

People tend to venerate extremes — the tallest waterfall, the oldest living person, the longest snake in captivity.

And so it is for trees. In 1940, the American Forests Organization established “The National Register of Big Trees” to identify and honor the "champion tree" for each species.

Each champion is determined by adding three measurements: the circumference of the trunk, the height of the tree and the spread of the crown. The tree with the most points is the champion for that species. And, because trees are continually growing and dying, updating the list is a never-ending task.

Finding the precise height of a tree is done by combining information from two devices. A clinometer measures the angle from the viewer on the ground to the top of the tree. And a laser rangefinder records the exact distance from an eye-level point to the tree’s top twig. With application of a little trigonometry, the judges can calculate a tree’s height right down to a millimeter.

In 2021, there were 561 trees in the national registry. The biggest champion is a giant sequoia in California, while the smallest is the southern Bayberry in North Carolina.

Utah’s state tree is the trembling aspen, but sadly, the champion of that species is nowhere near our state. It’s in Chippewa County, Michigan.

In Utah, you will find the champion trees of Rocky Mountain Juniper, Blue Spruce, Joshua tree and Limber Pine. They’re our winners!

But this national register of champion trees doesn’t tell the whole story. I think that a tree’s value is not about size or symmetry, or whether it’s the tallest or smallest of its species. A tree is great simply because it’s a tree.

Spring is here, the time when the sap in trees is rising. So, let’s think about the making of maple syrup!

The sugar in maple tree sap is created through the process of photosynthesis, which converts sunlight energy into sugar. When temperatures drop in the fall, trees convert that sugar into starch, which they store in their trunks and roots.

As winter’s grip loosens, the tree converts that starch back into sugar, which passes into the xylem, the part of the tree's transport system that carries water and dissolved materials upward. Boring a hole into a tree severs the sap-carrying wood fibers, so the sap drips out of the tree and into a waiting bucket — becoming part of the United States’ $150 million-dollar maple syrup industry.

One of my students, Max, grew up in Wisconsin, where he and his siblings would stir big vats of sap, guess the yield of each tree and griddle up stacks of pancakes for the first run of the season.

Sap collection methods have evolved from the early hand-collecting from individual trees and using horse-drawn carts into more mechanized operations. Max’s family now uses plastic tubing and gas-powered pumps to deliver sap directly to the heating vats that transform the watery sap to the gooey goodness we pour on our pancakes.

But this delightful symbiosis between people and trees is becoming endangered, due to climate change. As summer temperatures warm, a tree’s rate of respiration increases faster than its rate of photosynthesis, which decreases the sugar content for the following year’s harvest. And decreasing snowpacks make the soil more likely to freeze, leading to increased death of tree roots, which can reduce tree growth by as much as 40%.

Sugar producers are now working on climate adaptation actions such as shifting the tapping season and thinning the density of sugarbush trees to ensure that pancake-eaters will be able to enjoy those dollops of converted sunlight far into the future.

Take a look at the packet of disposable chopsticks you get with your next order of sushi — you know, the snap-apart kind, tucked in a paper sleeve. These throwaway chopsticks are clean and convenient, but they contribute to a bento box of environmental problems.

Chopsticks originated in China nearly 5,000 years ago. In Japan, jade and ivory chopsticks were originally used only for religious ceremonies. Chopsticks made of chestnut wood and persimmon wood were said to bring wealth and long life.

But in the late 1800s, as chopsticks became more common, the Japanese started using disposable utensils made of bamboo and wood, called “wari-bashi.” Today, nearly 80 billion pairs of disposable chopsticks are produced each year (about 10 pairs for every person in the world), costing the planet an estimated 10 to 20 million trees a year.

Some disposable chopsticks are made from quick-growing bamboo, but nearly half come from the wood of birch, poplar and spruce trees. Because domestic wood in Japan is protected by financial incentives, the source of 90% of the wood for their chopsticks come from Southeast Asia, Canada and the United States.

The wood used for disposable chopsticks is bleached and treated with chemicals like hydrogen peroxide and sulfur, making the chopsticks non-biodegradable, so that millions of them end up in landfills when they're discarded.

I’m happy that we’re now exploring innovative alternatives to this throwaway situation. Wood fibers from chopsticks are being added to biodegradable plastics, and to produce sustainable battery electrodes. And the Chinese government has imposed new taxes on them to protect the environment. But we’re still a long way from sustainably dealing with both the production and waste from these single-use utensils.

So, next time you go out for sushi, think about embracing the idea of B-Y-O-C — “bring your own chopsticks” — allowing more trees to live longer.

The baobab tree has many intriguing nicknames: the camel tree, the bottle tree, the upside-down tree and the Tree of Life.

These trees grow in dry areas of Africa, Madagascar and Australia. Their branches are skinny and few, and their trunks are disproportionately wide for their height. The record circumference of a baobab is 150 feet — the length of half a football field!

Although these trees produce only faint growth rings, we know that baobabs live amazingly long lives. Radiocarbon dating has determined that the oldest baobab is more than 2,500-years-old.

Local people call baobabs the “Tree of Life” because they have so many uses for it: eating its iron-rich leaves, drinking a coffee substitute from its seeds, and making beer and juice from the pulp from its large fruits, which have six times more vitamin C than oranges! And the ideal spot for markets in many rural villages? The shade of the baobab’s crown.

A single baobab can store over 1,000 gallons of water, which helps keep soil humid and stable. During droughts, elephants consume the juicy wood beneath the bark of the baobab.

I first read about baobabs as a child, in Saint-Exupery’s classic book, "The Little Prince.” The young prince roots out baobabs as seedlings to prevent them from taking over his tiny planet, a metaphor that instructs us to take actions against destructive forces as early as possible to avoid terrible consequences.

It’s appropriate advice for us today, considering that we’ve lost nine of Africa’s 13 oldest baobab trees to increasing drought brought on by climate change.

The baobab tree has earned each of its nicknames, but the one I love the most is the Tree of Life. My hope is that we heed “The Little Prince’s” advice and act now to allow the skinny limbs of the amazing Tree of Life to continue reaching upward.

My father is from India, so I grew up eating and loving Indian food.

And like all kids, desserts were my favorite — especially those flavored with two distinctive spices that come from the nutmeg tree, which has the beautiful scientific name, Myristica fragrans.

Nutmeg is native to the Moluccas or Spice Island of Indonesia. The trees — which can grow up to 70 feet tall and bear fruit for half a century — are now cultivated all over the tropics.

The nutmeg spice comes from the seed inside the tree’s apricot-like fruit. When the seeds dry, they shrink away from their shells, and are then either ground up and bottled or sold whole.

Nutmeg contains the two compounds, pinene and camphene, which give a nutty richness to many baked goods, curries and puddings. And even though these seeds only grow in the tropics, I love to grate a dash of nutmeg on my winter holiday eggnog.

But there can be too much of a good spice! If you consume nutmeg in large amounts — like three entire nutmeg seeds — you might experience psychoactive effects and hallucinations.

This very same tree — and the very same fruits — also gives us the spice mace, which comes from the red, lacy tissue or aril that surrounds the nutmeg seed. That material — called a "blade" of mace” — is processed into a delicate yellow-orange powder to flavor meats, pies and pickles. Mace, which has no relationship to the defensive pepper spray, tastes like a combination of citrus, cinnamon and black pepper.

So next time you cook foods that rely on the pungent flavors of nutmeg and mace, think about the one tree that gives us both — the beautifully named Myristica fragrans.

The roots of Arbor Day began over 150 years ago in like an unlikely spot: Nebraska City, Nebraska, heart of the Midwest, where the habitat is treeless prairies.

But it was that very lack of trees that prompted this holiday. The Nebraska Territory pioneers deeply missed the beauty of the eastern hardwood forests they had left behind, and they needed trees for windbreaks, fuel, timber and shade.

It was J. Sterling Morton, a Nebraska newspaper editor, who urged his fellow citizens to plant trees. In 1872, Nebraska's governor declared Arbor Day an official state holiday, and in 1970, it became a national celebration.

Shortly afterwards, the Arbor Day tradition of planting trees in schoolyards became popular. I remember watching my elementary school principal dig a deep hole to plant a pint-sized maple sapling in the school courtyard. Each time I visit my home town in Maryland, I stop to say hello to that tree, now a handsome specimen, over 40 feet tall.

So how will you celebrate Arbor Day? If you live in the Salt Lake City area, you could enjoy a free stroll through Red Butte Garden, Utah's official arboretum, or go to Cottonwood Park, where the Salt Lake City Urban Forestry Division — the tree heroes of our urban landscapes — provide guidance on planting the right tree in the right place at the right time.

Most holidays — like the Fourth of July — commemorate something that happened in the past. But Arbor Day is about the future. It celebrates those incredible plants that provide clean air and water, healthy communities, and beauty – now, and for years to come.

Although juniper trees are the most widely distributed tree species in the world, they’ve never won first prize for being the biggest or tallest tree.

But even if they don’t set any world records for size, they are among the oldest — and right here in Utah, you can find the Jardine Juniper, which has lived high in the Bear River Mountains for over 1,500 years.

With its contorted limbs stretching upward, only small bunches of foliage adorn its uppermost branches. But these little clumps of energy-giving leaves have kept this tree alive longer than most civilizations on our planet.

When you hike the 10-mile trail to stand in the presence of the Jardine Juniper, it’s amazing to think that when this tree was just a seedling, neither the Aztec nor Inca empires existed and Christianity was still a young religion. The tree grew and flourished through the Renaissance, the signing of the Declaration of Independence and the Industrial Age, all the while deepening its roots to sustain itself through centuries of drought and forest fires.

The Jardine Juniper is not the only ancient Juniper in our state. In Utah’s West Desert, where rainfall is less than eight inches a year, there are trees that are nearly 2,000 years old — and less than eight feet tall.

With so many of these beautiful trees around, take note next time you come upon a juniper, and maybe give it a nod — a touch of respect for these elegant elders.

Clarification: A previous version of this article stated that when the Jardine Juniper was still a seedling, Native American tribes lived all over North America. As Native American tribes still live across North America, that line has been revised.

What’s the first thing you do when you get into your car after it’s been sitting in the hot sun all day? Open the windows! Well, trees also need windows in their trunks and branches to let air circulate.

If you’re thinking: "Wait! I learned in botany class that the exchange of gases occurs in the leaves of trees, not the bark," you are correct! Leaves have tiny holes in their surfaces — called stomata, or “little mouths” — that open and close to let tree leaves draw in carbon dioxide and release oxygen for photosynthesis.

But trees also respire, just as we do, pulling in oxygen and releasing carbon dioxide. And respiration occurs in the cells of all living tree tissues, not just the leaves.

So how can this exchange take place in the parts of trees that aren’t leaves, like trunks and branches, tissues that don’t have stomata? It’s a puzzle, because tree bark is the first line of protection for a tree. It acts as an impermeable layer that prevents insects and diseases from getting to a tree’s interior structures.

Think of the trunk of a cherry tree, with its narrow lines etched into the bark. Those lines, called lenticels or “little windows,” are actually portals in the bark that let the tree breathe. They look like lens-shaped slits in the bark, allowing gases to pass between living cells on the inside and the air outside.

Botanists also use the shape of lenticels to identify trees. Some trees, like birch and cherry, have really prominent lenticels, but most are invisible to the human eye. But, even if you can’t see them, they’re there! Helping trees survive through even the hottest days they encounter.

On a road trip last week, I counted the number of different types of cars that whizzed by, responding to that human urge to tally the diversity around us.

I came up with 42 models, a tiny fraction of the many cars manufactured over the years.

As a tree lover, of course I then wondered: How many different types of trees are there? A recent scientific paper — co-authored by 143 authors from 62 countries — gave me the answer.

Based on an exhaustive inventory of 5 million trees in 100,000 forest plots, their estimate was …. over 73,000 tree species. And that’s just the number of species living today. It doesn't include those that have gone extinct since trees evolved, 380 million years ago.

Not surprisingly, tree diversity is highest in the tropics, where life has evolved without the glacial interruptions that polar and temperate regions have experienced over geologic time. And South America is the champion continent for tree diversity, hosting a whopping 43% of the world’s estimated tree species.

Based on their statistical analyses, the authors also suggested that there are another 9,000 species yet to be discovered, many of which they predict will be extremely rare, and thus vulnerable to human changes in land use and climate.

Why did these 143 scientists invest years in tallying tree species? Well, just as automobile dealers need accurate inventories of their cars to avoid running out of the models their customers want, so does this tree species inventory serve as a step to guide protection of trees and the resources they provide for animals, plants, water, air, soil — and people.

When we describe trees, we generally focus on their solid place within a landscape. For an exact description, we measure the size and shapes of their trunks, their foliage and their flowers.

But one forest researcher, Dr. Roman Dial, studies the negative space of the forest by literally measuring the forest parts that are not occupied by solids.

Dr. Dial first shoots a horizontal line over two tree canopies with a crossbow, and ties vertical ropes at intervals along that line. Then, hanging from a harness on each of those ropes, he uses a laser range-finder to measure the distance between his eye and the nearest solid object, like a branch, a leaf or a trunk, at eight compass directions around him.

He then calculates the volume and shape of the airspace around him, and creates images that look like giant asymmetrical vases placed among the solid trees. Those images help us understand how a bird or grain of pollen or even a molecule of air would “see” and navigate its way through the forest interstices.

His work has also shed light on the evolution of forest-dwelling animals. Scientists have wondered why rainforests in South America support many species of canopy-dwelling mammals, like opossums and sloths, that can hang from branches with prehensile tails or claws. But very few mammals in those forests have evolved to glide from tree to tree.

In contrast, rainforests in Asia have almost no hanging animals. But those habitats support many species of gliding mammals, along with snakes, lizards and frogs. Being able to glide from one treetop to another lets an animal move more quickly in search of food or in flight. Dr. Dial's images document that there are far larger volumes of airspace between trees in Asian forests than in South American forests.

Measuring the air space between trees provides insights about many subtle aspects of forests, helping us think about how pollen grains, birds — and gliding lizards — navigate the complex three-dimensional spaces of forests.

Envision a bolt from the sky striking a single tall tree in an open field.

That electric charge travels through the layer of moist sap just beneath the bark, heating and expanding as it goes, then blasting off the bark, and killing the tree.

That’s what lightning can do to just one tree. But what about a whole forest?

Over the past two decades, a team of researchers has monitored the number of lightning strikes at their research site in Panama. They then estimated that tropical forests around the globe experience over 60 million strikes each year, which can kill up to 40% of the largest trees in those forests.

Obviously, the tallest trees are the most directly vulnerable. But these researchers discovered another mortality factor — the presence of woody vines, or “lianas.” These plants germinate in the forest floor soil, and then grow into the tree canopy, using understory stems for support. Researchers found that the greater the density of lianas, the larger the number of small trees lightning damaged or killed.

With their long stems filled with water, lianas are act as super conductors for electricity. They physically connect canopy trees to the forest understory, like jumper cables between cars, delivering deadly electrical current from tall trees to the small trees that would otherwise be unaffected by a lightning strike. The death of those smaller trees and saplings can ultimately change the future composition of the whole forest.

As their research deepens, these scientists better understand the complex interconnections among trees, knowledge that comes not as a bolt of lightning, but through careful observations over time.

Many cultures and religions celebrate the Tree of Life. But trees also have deep associations with death. People all over the world have laid their dead to rest inside of caskets and coffins made of wood, a tradition that traces back to the burial boxes created in ancient China and Egypt.

In the United States, local furniture makers moonlighted as undertakers, making each coffin individually. During the Civil War, thousands of coffins were needed to transport the thousands of dead soldiers, which opened the era of mass-produced caskets. Since then, their manufacture has evolved into a $1.2 billion industry.

Caskets and coffins are made from of a variety of tree species, each type reflecting the resources of the departed. On the high-cost end, walnut, cherry and mahogany are used. Less expensive are those of poplar and pine, symbols of frugality and simplicity.

In the last two decades — partly due to rising funeral costs — people have increasingly sought funeral arrangements that don’t include caskets, such as cremation.

But there’s another reason for this decline. Because conventional burials involve toxic embalming fluids, the option of green burial is increasingly appealing to those who want to reduce their impact on the Earth, even after they die. Those burial containers are made of quickly decomposing bamboo and willow, and a tree is often planted in the hand-dug burial space, a symbol that life can spring from death.

Over 200 green burial cemeteries exist in the U.S., including a certified green burial garden in the natural landscape of the foothills of Bountiful, Utah.

Trees participate in the natural cycles of life and death, in the past and present, and in real and symbolic ways.