WHAT I VISUALIZE when I hear the word “tree” changed after a conversation with forest ecologist Christopher Earle, a co-author of two new titles in the Princeton Field Guides series: “Trees of Eastern North America” and the corresponding Western volume. Are you open to an expanded view, too (and to perhaps win the right book for the trees of your area)?

Chris Earle, from Olympia Washington, has a doctorate from the University of Washington, and describes himself as a “complete biophile,” someone who loves living organisms, with the trees leading the list, it seems. Since 1997, he has been the passionate creator of conifers.org–a massive Gymnosperm database described as, “the web’s premier source of information on conifers and their allies.”

Basic information about the biology of every species of conifer in the world is arranged by species, with essays by Chris rounding out the exhaustive coverage.

Ever wonder what the most common tree on the planet is, or how the giant sequoias are so successful, and impressive—what’s their secret, genetically speaking? Or why Arborvitae is called the tree of life? Or what advantage conifers have over flowering trees, that allows them to cover nearly as much territory on earth—despite the fact that there is a tiny number of conifer species by comparison?

On the latest radio podcast (listen in now, or read along, or both), we talked about all that, and more. Most green links naming particular tree species throughout the interview will take you to much deeper profiles on Chris’s conifer website. Plus: Enter to win the books, “Trees of Eastern North America” and “Trees of Western North America” (affiliate links) at the bottom of the page.

a q&a on trees, with christopher earle

Q. I suppose we should start with the word tree—something everyone thinks they have a clear impression of, perhaps, but yours goes a bit deeper. What is a tree?

A. For one thing: One of the challenges we had creating the books was deciding what’s the difference between a tree and a shrub. There is no conventional definition; no accepted scientific definition. Most people regard a tree as something that’s taller than themselves; some people put a number like 20 feet on it.

But what you think is a tree partly depends on where you live. If you live out in the Western desert, something that’s 10 or 15 feet tall may qualify as a tree, and the same thing here in Olympia, Washington, may be a shrub.

We had a to draw a somewhat arbitrary line, but basically everything in the book is taller than a human, and most of it has a single trunk.

In most cases, you can think of a tree as a colonial organism. People tend to anthropomorphize about them, but you can’t cut off your finger and stick it in the ground and grow a new person. You can do something like that with a tree. Many trees are very easy to layer, or put a branch in water and it will generate roots and it can regrow. That’s because every part of a tree is essentially autonomous; they all work together, they know whether you should be a cell of the wood or a cell of the bark or the leaf, because of communication using growth chemicals. All the cells share the same genetic material, and their development is controlled by growth chemicals between different parts of the tree.

Essentially, a tree can be thought of as a giant colony. In one way, it’s like a coral colony, in that it’s growing on a dead substrate.  Most of the wood in a tree—and certainly the heartwood—is dead. It provides a structural support that allows the tree to rise up into the sky and be taller than the neighboring plants. That gives it access to sunlight, which in most forests is the primary thing that trees compete with each other for.

Q. So it’s bigger than a person; it’s a colonial organism; it has woody tissue that allows it to stand tall. When we spoke recently, you said something that stuck in my head: “A tree has an economy,” was your statement. What is a tree’s economy, then?

A. That’s a related idea: It’s a colony, and it’s sort of like a business, or a small country. It has to have access to raw materials, and if you’re a tree, the raw materials are nutrients found in the soil; water, which also comes from the soil; and carbon dioxide, which comes directly out of the atmosphere.

Then you need power to run the factory, and the power in a tree comes from sunlight. The sunlight powers the conversion of carbon dioxide and nutrients into all of the chemicals the tree needs to function: wood, living materials, chemicals that repel herbivores, and things like that.

Finally, of course, every economy has to have a place to spend this good stuff. A tree spends it on producing leaves, which allow it to continue its growth; and producing wood, which allows it to grow tall as we discussed, and also the specialized cells in the wood that move material between the roots and the leaves. And it has to produce roots. A very large fraction of a tree’s energy goes into its roots. That’s actually the other main area of competition for trees–sending those roots out into the soil and getting access to nutrients before their neighbors can.

Then a tree has to also spend some of this energy it has acquired producing reproductive structures: flowers, fruits, seeds. And surprisingly a large percent of its energy goes into producing defensive chemicals, which for the most part are complicated chemicals meant to deter herbivores—most of which are insects.

Q. You talk about the tree-versus-shrub decision for what got into the books or not, but what else helped create the list of included species? We should be clear with people: This is not a set of horticultural books—not all the ornamental trees that you might be able to buy.

A. Correct. This is not all the trees. There are actually about 25,000 or 50,000 trees in the world, depending what you call a tree and what you call a shrub.

Q. As in 25,000 to 50,000 distinct species?

A. Yes, different species. And that set of books would fill an entire bookshelf.

Q. And these Princeton Field Guides are pretty hefty books, but not that big.

A. There are about 800 or 900 in the new books, and those are all the native or naturalized species that live in North America—the continental U.S. and Canada.

By native, that means presumably they were here waiting when Columbus showed up. And naturalized means they’re been brought in later, but have taken to the country, and don’t need human help to reproduce here.

Q. Examples?

A. A good example of the naturalized species is the Scots pine (Pinus sylvestris). You see them everywhere. They are a regular constituent of many Eastern forests, but they weren’t here before 1500 or 1600 or so.

Q. Speaking of conifers: You have a particular expertise in them, I know, but conifers and their allies take up a small chunk of these new field guides, compared to the pages devoted to Angiosperms, or flowering trees.  Is that representative of the balance on the planet–conifers to flowering trees–or just the way the books happened to shape up?

A. The flowering plants (the Angiosperms) and the conifers (which include the Gymnosperms) have very different ways of living on the planet.

There are about 350,000 different species of flowering plants that have been named so far, and no doubt a good number waiting to be discovered in places like tropical rain forests. A good-sized number of that total, about 50,000, are trees.

With conifers, on the other hand: There are only 650 species on the planet.

Q. Wow.

A. With those I’d say better than 90 percent are trees. But the conifers have been here a lot longer—about 300 million years in fact, about three times as long. And they have adapted to a very wide range of environments.

They are particularly good at living in the mountains, and in cold climates. As a result, the conifers cover nearly as much of the landscape as the flowering trees do.

Q. So there are many fewer species—but that doesn’t limit their territory.

A. Yes, like the Scots pine we mentioned. That’s the single most widely distributed tree species in the world. It’s now found through most of Northeastern North America, and covers all of Europe and Asia, from England to Siberia.

Q. I wouldn’t have guess it would be that tree.

A. Other examples: black spruce, white spruce, Siberian larch–each of these trees has a range that covers 4,000 or 5,000 miles, whereas a lot of flowering plant species have extremely limited ranges. In fact, there are a lot of flowering trees that only have been collected ever once or twice. They live in obscure parts of the tropics and very little is known about them.

Q. Speaking of tree economies: It sounds as if the conifers are good at setting up a smart economy. [Laughter.]

A. They certainly are very competitively effective, and one of the reasons is that most of them are evergreen trees. The great thing about being evergreen is that you can do photosynthesis all year round.

Q. Even in times when it’s quite cold, as long as there is light?

A. There are some exceptions—places where it gets really cold in the winter. You may think you live in a place that gets cold in the winter, but if the ground doesn’t freeze, then the tree is probably able to function.

Also, of course, there are a lot of deserts that conifers don’t live in, and trees don’t live in, due to limitation of water. But being evergreen definitely gives conifers an advantage in environments that are fairly common in the planet. So that’s one reason they are so widespread.

Q. And their great headstart—of hundreds of millions of years? Does that factor in?

A. It doesn’t really that much, because nature really has no sense of nostalgia. A lot of people like to talk about how some species are “living fossils,” because they have been around for hundreds of millions of years without changing. But the reason is that they solved a really hard problem really early in history, and they’re continuing to be effective competitors against species that evolved much later.

Q. Conifers have a powerful place in our consciousness, don’t they?

A. They have considerable value of various kinds: One is the spiritual value—anybody who has ever walked through a redwood forest knows what that’s about, and some of the great old forests of oaks back East are spiritually potent as well.

The fact that conifers are evergreen means they are symbolic of eternal life. If you wandered through an 18th or 19th century graveyard you’ll see a lot of yew trees planted there, and a lot of conifers on tombstones and sculptures. That’s because the evergreen nature of them is symbolic of life after death, of the eternal life of the spirit.

Conifers are also important on a much more practical level: They are a source of wood for houses, furniture and so forth. Most listeners are probably in houses that are built of some kind of conifer wood. Some cultures also use them to make clothing, and rope. The native peoples of the Pacific Northwest use the Western redcedar for practically everything: houses, clothing, boats, storage implements, medicine, shredded the bark and used it for diapers…

Conifers are the source of many medicines—such as the yew tree being the original source of taxol, still the most effective treatment for ovarian cancer (though we have learned ways to synthesize it in the laboratory).

Conifer resins were traditionally used for many medicines, though that has fallen by the wayside with more sophisticated technology of modern medicine.

Many conifers produce nuts that are an important food source, such as pesto from pine nuts. They’ve been a staple food for people in the Pacific Northwest, the Himalaya, for the aboriginal people of Australia, who ate a lot of pine nuts as well.

So many different uses pervading our culture.

[A report from the Food and Agriculture Organization of the United Nations on conifers in our culture.]

Q. You told me the story of the Arborvitae, or “tree of life.”

A. On his second expedition to Canada, when Jacques Cartier arrived in what we now call Quebec, he and most of his crew were sick with scurvy, and many had died. They picked up a young native boy who gave them the branches of Thuja occidentalis, and told them to eat the foliage [photo above], and they were cured. They were so impressed that they brought specimens of the tree back to France. It was the first North American tree to be introduced to Europe.

Q. So getting back to a tree’s economy and biology: A small segment of conifers are deciduous–the Eastern larch, for instance (Larix laricina). Can we talk about how their strategy works compared to the more common evergreen types?

A. Most of the larches, for example, live in very cold climates where they do experience frozen ground for many months of the year. In that environment, they have no option but to close down.

That eliminates their main competitive advantage over the flowering plants, so they have adapted by producing a leaf that requires very little energy to produce, and that works great in the summer months, though late in the summer they tend to start getting eaten up by the bugs. But then they have to shut down anyway. Before they do, the tree absorbs much of the Nitrogen out of the leaves, and brings it back into the tree—which is why the leaves turn that lovely yellow color in the fall. This is the same strategy that’s used by most of the Eastern deciduous trees, and why they show those beautiful colors.

Q. Among the conifers, the number that are deciduous—is it a very small fraction?

A. It’s pretty small, and it’s scattered through various types of conifers and they live in some strange places. Like the bald cypress (Taxodium distichum) [top-of-page photo] which lives in the Southeastern United States [often in wet areas].. There are some relatives of the cypress, and some relatives of the larch—and those are the main deciduous conifers.

Q. Are there some trees that you want to highlight, just because of personal affection for them, the way I love the Japanese umbrella pine (Sciadopitys verticillata) simply because it was the first plant I planted when I started my garden 30 years ago?

A.  My really favorite trees I don’t love for scientific reasons, either. My favorite is probably the Great Basin bristlecone pine (Pinus longaeva) [photo above] the oldest known tree in the world. There is at least one specimen that’s over 5,000 years old. The real reason I like it is that I haven’t been any place this tree grew that wasn’t gorgeous.

Q. So it know how to pick a good neighborhood. [Laughter.]

A. It really does. And they grow with extreme gnarliness, and great character. They’re great subjects for photography; very memorable.

They’re close relatives of the foxtail pine (Pinus balfouriana), which lives in the high Sierra of Southern California and also a little bit in Northwestern California, and is another tree that falls into that same category.

Another exceptional tree is the sequoia, the coast redwood. It’s incomparable—the tallest tree in the world. There is some evidence that Douglas firs might have once have gotten taller, but there is also evidence that we have cut the tallest redwoods.

The largest tree in the world was a coast redwood (Sequoia sempervirens) until it was cut in about 1949, when the giant sequoia (Sequoiadendron giganteum) became the largest tree in the world.  Unlike all the other conifers, they are hexaploid, and have six copies of their genetic material in every cell—and this is sort of like carrying around a box full of spare parts with you.

It means the sequoia has lots of pre-designed adaptations to all kind of conditions it may encounter along the way. You can burn all the live foliage off the sequoia, and it will resprout–even if the tree is 300 feet tall. The tree typically lives much longer than any of its branches; an old sequoia may be on its eighth or tenth set of branches. Even if you burn them down completely, they’ll resprout from their roots. [Read an illustrated presentation by Chris on conifer longevity.]

enter to win the new field guides

I‘LL BUY THE RIGHT volume of the new Princeton University Press Princeton Field Guides to Eastern trees or Western ones, for each of two lucky readers. All you have to do to enter is answer this question in the comments box at the very bottom of the page, below the last comment:

If my favorite tree is Japanese umbrella pine for sentimental reasons, and Chris Earle loves the Great Basic bristlecone pine, what about you? Any favorite, for whatever reason at all?

Also tells us where you are–East or West?

No answer, or feeling shy? That’s fine; just type in something like, “Count me in,” and I will, but a reply is better. I’ll pick two random winners after entries close at midnight Monday, October 6. Good luck to all. US and Canada only.

more from christopher earle

• visit conifers.org
• order “Trees of Western North America“
• order “Trees of Eastern North America” (affiliate links)

prefer the podcast?

CHRISTOPHER EARLE was my guest on the latest radio podcast. You can listen anywhere, anytime: Locally, in my Hudson Valley (NY)-Berkshires (MA)-Litchfield Hills (CT) region, “A Way to Garden” airs on Robin Hood Radio on Monday at 8:30 AM Eastern, with a rerun at 8:30 Saturdays. Get it free on iTunes, the Stitcher app, or streaming from RobinHoodRadio.com or via its RSS feed. The September 29, 2014 show can be streamed here now. Robin Hood is the smallest NPR station in the nation; our garden show marked the start of its fifth year in March, and is syndicated via PRX.

(Photos courtesy of Chris Earle and conifers.org; “big tree” of Taxodium distichum; at Cat Island, Louisiana, ©Vladimir Dinets, 2012.)