This is Part 2 of my interview with Contributing Editor David Allen Sibley, author of The Sibley Guide to Birds, about his new book, The Sibley Guide to Trees.

Part 1 is revealing and fascinating. In it, Sibley explains how long it took to produce The Sibley Guide to Trees, where the idea of doing a tree guide came from, why he decided against doing another guide to birds, why he feels there's room for him to make a real contribution to tree identification, and, surprisingly, why he doesn't consider himself an expert on trees. Go to Part 1.

In Part 3, Sibley compares working on the tree guide with working on the Guide to Birds; he tells why he used acrylics instead of the opaque watercolors he used for the Guide to Birds; and he explains why birdwatchers are the people he expects will buy the tree guide. Go to Part 3. --C.H.

Before I got my copy of the book, I imagined that I would open it up and see beautiful paintings of whole trees in summer. And they’re not in there.

No. I imagined that that would be a big part of the book, but the more I learned and the more painted, I realized that actually identifying leafy trees in the summer is one of the more difficult things. If you’re looking at a whole tree from a distance, and if you’re too far away to make out individual leaf shape, it’s actually easier to identify a tree in the winter than in the summer. So I ended up including more winter tree silhouettes than summer tree silhouettes.

And that brings up another issue that I had a hard time dealing with in the book, and that’s the question of scale. If I wanted to illustrate whole trees in the summer, and if each one was reproduced in the book at, say, two inches high, I’d end up with a green oval for every single species. There’s very little difference between those maples, elms, walnuts, and hickories. The differences are revealed as you zoom in.

The first differences that you’ll notice are how the leaves are clumped on the tree. That starts to show up as you add a little more detail, and as you add more detail, you can start to see some of the twig patterns even through the leaves, the actual structure of the twigs, and you can start to make out the leaf shape. But for an illustration in the field guide, you have to get in pretty close to start showing those things. So that was how I ended up with very few summer tree overviews in the book. The things that I wanted to show, or the things that were useful to show, were much more detailed. If I could have made each illustration 20 feet tall, then I could have done whole trees and shown all the detail that I needed to show. Each one would have taken a year to paint.

Other guides to trees that I’ve looked at have keys -- a series of yes-no questions that lead you down, and then finally at the end of it, you get an identification. Did you consider doing anything like that?

I really wanted to avoid that. I could see using it in a general sense, or to get to a broader answer, like a family or a genus, but I really wanted the book to work the same way the bird guide does. That is, if you see something interesting, whether it’s a leaf or some odd bark or an unusual type of fruit, you can open up the book and just start flipping through the pages and try to find a picture that matches.

I think in the long run, that’s actually more helpful and more educational than working through a key, because after you’ve done that a few times -- flipped through the pages of the book, looking for things that match -- you’ll realize that if you’ve seen an odd fruit and it’s not an acorn, you can skip the whole 40 pages of oaks in the middle of the book. You start to get a sense of what the variation is in all the types of fruit. Pretty soon you’ll be out in the field somewhere and say, “There’s an odd fruit. I don’t know what that tree is, but I know I’ve seen that picture in the book, and I think I remember it being toward the end of the book,” and flip through the pages and find it and put a name to it.

By flipping through the pages and getting an overall sense of what’s out there, you subconsciously begin to understand larger patterns -- which families are similar, which families are different, what makes all the oaks similar to all the other oaks, what makes willows and poplars so similar that they’re put in the same family. You’ll develop an understanding of that simply by flipping through the pages of the book.

That sounds very much like the way a birdwatcher would use his or her field guide.

Yeah, yeah, and that’s just what I thought was lacking in the tree field guides and the keys. I think it’s partly because the trees are so easy to see -- they’re so easy to study, you just walk up and look at them under magnification -- that there hasn’t been much of an incentive for anybody to do this sort of trees from a distance. It seems like it’s sort of a silly, arbitrary, academic exercise. But I’ve always wanted a book like that, one that would allow me to identify trees from a distance and not force me to walk up and find out whether the leaves were opposite or alternate or whether there were six-bud scales or more than six-bud scales, all those kinds of questions.

Yeah, I made a note that you had chosen to de-emphasize the alternate and opposite arrangement of leaves. Why did you do that?

I don’t find it very helpful when I’m out looking at trees now. I don’t even think about that except for few cases. It’s been promoted or emphasized as one of the central identifying features of trees in several books. It’s easy to figure out, and it does divide the trees immediately into two groups, those with or without alternate leaves, but it’s not really a taxonomic characteristic.

It’s common to certain groups of trees -- maples, ashes, and dogwoods are the big three examples. But there’s one species of dogwood that doesn’t have opposite leaves. It’s called the alternate-leaf dogwood. There are other species that have, like buckthorns, nearly opposite leaves but not quite opposite. They’re called sub-opposite. And then in each of those groups, there are close relatives that don’t have opposite leaves, and scattered through the book are other odds and ends of species that do have opposite leaves and opposite branching.

It’s certainly one feature among many that’s worth looking at to quickly assign a tree to a group, but... Let me think, a good comparison for birds might be a forked tail, where a forked tail is common to swallows and terns, and then scattered through the book, there might be a dozen other species that have obviously forked tails, but it’s not really all that informative. It’s not like the primitive species have opposite leaves and the more modern species have alternate.

It reminds me of something you wrote in Sibley’s Birding Basics: “Sorting birds into groups requires you to rank differences, deciding which ones are important and which are not. The immediate problem is deciding which of the observed differences are important.” That’s the work of the field guide, isn’t it, or is that what you saw as the work of this field guide?

Yeah, that’s a huge part of this, and I learned it slowly as I studied trees that some things are more important than others. Leaf shape is really important, but it’s also extremely variable, so you can put some emphasis on leaf shape, but if the leaf shape really seems out there, you have to allow that 10-percent chance that it’s leading you astray.

And that, too, is very much like birds, isn’t it?

Yes. The field guide is a huge part of teaching people what’s important and what’s not, but you also have to learn that gradually through your own field experience, what to look at first and what’s the most important feature that you’ve seen. That’s a never-ending question. We see it all the time with birds. Even at the highest levels, there’s a couple discussions going on right now on ID Frontiers, Frontiers of Bird Identification, where some real oddball birds are being discussed. Each camp has its proponents. They’re just selecting the features that they think are most important, and it’s leading each camp to a different identification.

What sort of reaction have you gotten from your close birding friends about the fact that you’ve left the world of birds and are now in the world of trees?

[Chuckles] I’d say they’re polite about it. [Laughs] Actually, the overwhelming reaction that I’ve gotten from birders over the last few years when I’ve mentioned this project is something along the lines of “Aw, that’s fantastic! I own a few tree field guides, and I’m not happy with any of them. I really want a good tree field guide.” So I’ve been really reassured over the last few years that I was on the right track, that my desire for a tree field guide like this would be shared by a lot of people. I think my birding friends think it’s interesting, and they can understand the challenge or the need for it and why I would find the challenge interesting.

I think you’re right. I know lots of people that have attempted to make sense of tree guides and have been put off by the terminology. It’s a frustrating thing, because I think there is a huge interest in trees and the world that the birds live in, but how do you get at it?

I started out with the goal of keeping the technical terms to a minimum, and I think in the end, we didn’t even include a glossary in the book. You know there are 20 or 30 different technical terms for slightly different variations of hairiness or fuzziness. I just did away with all of that, and I used a couple of different terms like “hairy” or “fuzzy” or “woolly,” but I didn’t make much of an effort to distinguish variations of hairiness. Technical terminology is really important for botanists in the way ornithologists have a whole language describing feathers and details of the bird, but from an identification point of view, I found that almost none of it was really necessary.

This is Part 2 of a three-part interview. Go back to Part 1. Go to Part 3.

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