Question: When did your interest in trees begin?

David Allen Sibley: It has always been there. I don’t think there is anything unusual about that--kids like to climb trees, imagine living in a big hollow tree, etc. When I was about 8 years old, I lived near Muir Woods so seeing the Redwoods, and smelling what I now know are California Laurel trees made a big impression on me. And I think an interest in trees follows naturally from an interest in birds--looking at birds means you are looking at a lot of trees, and when you do you start to notice different things about their twigs and leaves and bark.

Q: How long have you been working on this book and what kind of research did it entail? How long did each illustration in this book take you?

DAS: Well, I've been studying trees for as long as I’ve been bird watching. Whenever I’m out in the field observing birds I’m also observing habitat. I’m noticing everything. It’s impossible for a naturalist to study just one thing, since they are all connected. But the actual work on the book began 7 years ago, soon after the bird guide was completed. I spent the first few years traveling to study and photograph different species, reading about trees, working out my painting technique for these images, and planning the book page-by-page. I'm lucky to live near Boston, where the Arnold Arboretum and Mount Auburn Cemetery offer the chance to study hundreds of species of trees right here.

It's really been the last four years that I've spent most of my time in the studio painting and writing. Just like the bird guide, each individual image would take an average of about an hour to complete, from sketch to finished painting.

Q: Were there major differences in writing this book vs. the Guide to Birds?

DAS: The obvious difference is that trees are much easier to find. When I needed to study a particular species of tree I could just walk right up to it and spend as much time investigating it as I needed. Birds are more elusive. I had to spend years in the field in order to build up enough observation time to draw them well.

Another key difference is that birds recognize each other by sight, the same way we do, and the evolution of their appearance has been guided and sharply limited by how they look to other birds. Trees can’t see, they communicate through pollen and some chemical signals, so one tree doesn’t “care” what another’s leaves or bark look like. Their appearance has evolved for purely functional reasons. The exception is some flowers and fruit, which have evolved certain visual cues to attract the animals that pollinate flowers or disperse seeds.

Q: What would you say to someone who is a beginner at tree identification? What are the key identifying features in a tree?

DAS: The first thing I suggest is to spend some time with the guide. Try to become familiar with the characteristics of certain trees. Then go through the book and mark all the species that occur in your area. This will help you become familiar with the range of species that could be present so when you see an odd leaf shape, fruit, flower, bark pattern, etc.--even if you can’t remember the name--you can remember seeing it in the guide. Since trees are so easy to approach, you can simply take a photo of the key parts of any tree, or pick up a leaf or other part that has fallen on the ground, and identify it at your leisure.

They key identifiers will always be the shape, color and size of leaves; the color and shape of twigs; the color and texture of bark; and the tree’s overall size and shape as well as habitat, any fruit or flowers, and the timing of seasonal changes. For example, in late May in the northeast, if you see a pale-barked tree with small silvery leaves just emerging (while other trees have well-developed green leaves) you can be virtually certain that is a Bigtooth Aspen. A multi-trunked, spreading tree in wetter soils, with clusters of straw-colored fruit hanging from the twigs all winter, is almost certainly a female Boxelder.

Q: How many species of trees do we have in the U.S. and Canada?

DAS: There are about 700 species of trees native to North America (depending on how you define a tree). Hundreds more species are cultivated here, and quite a few of those have escaped and are growing in the wild. In this book I left out a lot of species that are “shrubs sometimes tree-like”, and species found only in southern Florida, in favor of including commonly cultivated trees that people will see more frequently.

Q: Why did you decide to structure your guide taxonomically?

DAS: I think one of the keys to learning birds, or trees, or any other living thing, is to learn the taxonomy--the families and genera--and to understand which species are closely-related and which are not. My goal in studying trees is to gain a deeper understanding of them, and part of that is to learn the fundamental things that all maples, or all alders, etc. have in common. Presenting the species in the guide grouped with their closest relatives, helps readers begin to recognize those fundamental similarities, and then they’ll be able to look at a tree and say “that just looks like a maple” in the same way that birders recognize a wren or a thrush.

Q: You have said that in researching this book you noticed that trees demonstrate, much more so than birds, the impact that humans have had on the landscape. How so?

DAS: Bird populations in North America have certainly changed dramatically since the 1500s, but it’s still possible to see almost all the same species, and in pretty much the same places and times, as the early European explorers saw them. But the tree landscape we see today is dramatically different from what our ancestors saw 500 years ago. Here in Massachusetts, there are only small patches of old-growth forest that have never been cut. Most have been cut multiple times during the last few centuries, and what we call “mature” forest today are trees mostly under 100 years old, so they’re still pretty young with time to mature. Add to that the introduced diseases that have all but eliminated species like American Chestnut, and the addition of non-native species like Norway Maple and Northern Catalpa (it’s said that 50% of the woody plants growing now in the state of Massachusetts are non-native) and you have a very different landscape. I imagine that if Audubon or Thoreau were to come back today, they would see a lot of familiar birds, but the changes in the forests would shock them.

Q: Which tree populations are in the most danger of extinction and what can be done about it?

DAS: Several species of trees are seriously threatened by disease, in the same way that the American Chestnut was wiped out by the Chestnut Blight in the early 1900s. Whitebark Pine, Butternut, Red Bay, and some oaks in California are all experiencing severe declines. Efforts are underway to contain these diseases, to prevent new pests from entering this continent, and to find and propagate resistant trees. In many cases the disease need not be fatal, but with many of our trees stressed by acid rain and/or large scale climatic changes like drought, they are more susceptible. One of the best things we can do to ensure the survival of healthy forests is to act quickly to reduce atmospheric pollution since that has far-reaching effects among trees. More directly, we should reduce our use of paper, as old-growth forests in Canada and elsewhere are still being cut down and ground up as pulp for paper.

Q: Where is the biggest tree in North America? The oldest?

DAS: One of the fun hobbies among tree enthusiasts is searching for “champion” trees--the biggest tree of each species. American Forests maintains a national registry of “Big Trees” and many states keep their own list.

The tallest tree ever measured was a Coast Redwood in California at 377 feet tall. The largest single tree by volume was another Coast Redwood with a trunk measuring over 88,000 cubic feet of wood and estimated to weigh over 3300 tons! The oldest tree is a Bristlecone Pine in Nevada known to be nearly 5000 years old. But these records of age and volume are both challenged by the Quaking Aspen, which often grows multiple trunks from a single large root system, and can be considered a single organism. One such plant in Utah covers over 100 acres with 47,000 trunks, and contains an estimated 6000 tons of wood, making it the largest single organism known. Estimates of its age range from 80,000 years up to one million years. The average age of any individual trunk is about 130 years, new trunks are constantly being produced by the root system.

Q: Do you have a favorite tree?

DAS: That’s like asking me to pick a favorite bird! I have a lot of favorites. I like the clean bark and majestic appearance of the Sycamores, the quirkiness of the Sassafras, the clean lines of the crown of the Noble Fir, the incredible cones of Coulter Pine. I could go on and on!

Q: We are certainly seeing a fierce focus on the environment in every area of public and private life--from politics to business to the cars we drive and the light bulbs we buy. Where do trees fit into our environmental future? How important are they?

DAS: It could be argued that trees play a bigger role in our lives than any other living things. We use them for construction, paper and cardboard; for food; for heat in our homes and for cooking; for shade and ornament around our houses and offices; and for specialty items like musical instruments and sports equipment, where no other material will do. Chances are that you are surrounded by wood as you read this. In natural ecosystems trees are important on their own.

Their leaves and branches provide food and shelter to countless species of animals. Even more critical is the habitat that trees create. The air under a forest canopy can be 15 degrees cooler, and much more humid, than air over an open field. This microclimate and the leaf litter on the forest floor create an entire secondary ecosystem of other plants and animals which can only exist with a healthy forest canopy. Trees filter pollutants from the air, turn carbon dioxide into oxygen, and create rich soil. On top of all that, trees are pleasing and relaxing to be around.

(Photo © Erinn Hartman)

A Look Inside The Sibley Guide to Trees
(Click on Images to Enlarge)