Tree Identification and Selection: How to Study Efficiently

Identification and Selection makes up 8% of the ISA Certified Arborist exam, about 16 of the 200 questions, and you don't need to identify hundreds of species to pass it. Genus-level recognition, combined with the 'right tree, right place' selection principle, covers most of what gets asked.

Domain Review: This topic is one of the 10 domains covered on the ISA Certified Arborist exam.

This domain scares people who never went through forestry school more than its 8% weight really justifies. Picture a wall of unlabeled leaf specimens and you can see why. But the exam isn't built that way. It rarely shows a single leaf and asks for the species. More often it asks how you'd narrow down an unfamiliar tree, or what selection factor caused a maintenance problem years down the road, which rewards a process you can learn in a weekend over species trivia you'd need years of fieldwork to memorize.

Taxonomy: How Scientific Names Work

Every scientific name has two parts. The genus comes first and is capitalized: Quercus for oak, Acer for maple, Pinus for pine. Think of it as the tree's last name, the broader group it belongs to. The specific epithet comes second, lowercase: rubra meaning red, alba meaning white. Put them together and you get the species: Quercus rubra, red oak. Both words are italicized, always. A cultivar sits one level below species, a variety propagated by humans for a specific trait, and it's written in single quotes and roman, non-italic, type after the species name, like Acer rubrum 'October Glory.'

Above genus sits a distinction the exam draws on more than people expect: gymnosperm versus angiosperm. Gymnosperms are the cone-bearing trees, mostly conifers with needle or scale-like leaves, pines and spruces and firs and junipers among them, and their seeds sit exposed on the cone scale rather than enclosed in fruit. Angiosperms are the flowering plants, which covers essentially every broadleaf tree you'd plant on a street or in a yard, with seeds that develop inside a fruit or a seed coat derived from a flower. It's a coarser filter than genus, but often the fastest one: needle or scale foliage narrows you to gymnosperms immediately, and anything with broad, flat leaves is almost certainly an angiosperm.

You won't be asked to write out a full taxonomic hierarchy on the exam. You will be asked to read a scientific name correctly, know which part is capitalized, and recognize that two trees sharing a genus, two different oaks, say, are more closely related to each other than either is to a tree in a different genus entirely.

Rank	Example	Capitalized?	Italicized?
Genus	Acer	Yes	Yes
Specific epithet	rubrum	No	Yes
Species (genus + epithet)	Acer rubrum	Genus only	Yes
Cultivar	'October Glory'	Yes, in single quotes	No

Morphology: Reading a Tree Before You Know Its Name

Say you're standing under a street tree you can't place. Before you look at leaf shape or bark texture, check the branching arrangement: do the twigs and buds come off the stem in pairs, directly across from each other, or do they alternate, staggered first on one side and then the other? This is the fastest filter in the whole domain, because most trees are alternate. Only a handful of common genera are opposite, and they're worth memorizing as a group with the mnemonic MAD Horse: Maple, Ash, Dogwood, and Horsechestnut. Spot opposite branching, and you've just eliminated the majority of species you might otherwise be guessing between.

From there, leaf type narrows it further, and MAD Horse is a good group to practice on because it isn't uniform. Maple and dogwood both carry simple leaves, one blade per stalk. Ash breaks the pattern with a pinnately compound leaf, several leaflets arranged in rows down either side of a central stalk, and horsechestnut breaks it a different way, with a palmately compound leaf, five to seven leaflets radiating out from a single point like fingers from a palm. So within one opposite-branched group, you've already got a three-way split just from leaf type: simple (maple, dogwood), pinnately compound (ash), palmately compound (horsechestnut). That's the layered reasoning the exam rewards, one filter stacked on another, rather than pattern-matching a whole leaf against memory.

Margin is the next filter once you're down to a short list: is the edge of the leaf serrated (toothed), lobed (rounded or pointed indentations, like a classic oak or maple outline), or entire (smooth, no teeth or lobes at all). When there's no leaf to look at, winter fieldwork or a young tree with sparse foliage, bark texture, bud arrangement, and overall form become the backup filters. The logic doesn't change. You're still stacking observable traits, branching first, then whatever's available next, until a huge field narrows down to a genus you can name with confidence.

Selection: Right Tree, Right Place

Identification only gets you halfway. The other half of this domain is selection: matching a species to the site before it goes in the ground, because a wrong call made at planting shows up as a maintenance problem five, ten, twenty years later, and by then it's a removal instead of a five-minute fix. For the planting mechanics themselves, see installation and establishment; this domain is about the decision before the shovel goes in the ground.

Run through the same checklist the exam does. Hardiness zone first: can the species survive the site's winter minimum at all, before anything else matters. Soil volume next: does the planting space have room for a mature root system, because a species that wants forty feet of canopy spread doesn't belong in a two-foot-wide tree pit boxed in by concrete on both sides, no matter how well it would otherwise suit the site. pH tolerance is the one that connects straight back to soil chemistry: a species with poor tolerance for alkaline soil, pin oak is the standard example, is a bad match for a site with high-pH fill soil, and you'll get the chlorosis problem covered in that domain no matter how well you handled everything else. And pest and disease pressure has to factor in before you plant, not after: putting an ash in the ground where emerald ash borer is established, or an American elm without a Dutch elm disease management plan, is signing up for a problem that's already well understood before you dig the hole, one you'll see again from the treatment side in diagnosis and treatment.

Here's how that plays out on an actual site. Say you're picking a street tree for a narrow downtown planting strip with overhead utility lines, exactly the kind of site-constrained decision urban forestry deals with block by block. Soil volume rules out anything that wants a wide root system or a large mature canopy right away, narrowing the field before you've thought about species at all. The overhead lines rule out anything that gets tall enough to need aggressive utility pruning, which just creates a maintenance headache and a worse-looking tree for the next twenty years. Then you'd check local pH and drainage against whatever species survive those first two cuts, and check regional pest pressure last, since that changes faster than the other factors and is worth confirming close to planting time rather than trusting an old list. Notice that identification skill barely enters into this process. Selection is a site-analysis problem wearing a tree-species costume, and the exam questions in this domain test the analysis, not a plant encyclopedia in your head.

Study Tip: Drill the Filter, Not a Species List

Don't memorize a list of random trees. Learn MAD Horse cold, since opposite branching alone eliminates most of the field, and practice stacking leaf type and margin on top of it the way the walkthrough above does. Then spend the rest of your prep time on the selection checklist: hardiness zone, soil volume, pH tolerance, pest pressure, in that order. It's a shorter list to master than it looks, and it's worth more exam points than memorizing bark texture for fifty species you'll forget by test day.

Frequently Asked Questions

How many tree species do I need to know for the ISA exam?

Far fewer than most candidates expect. Identification and Selection is 8% of the exam, about 16 questions, and most of them reward genus-level recognition and site-selection reasoning over identifying an obscure species from a single photo.

Do I need to know scientific names for the ISA exam?

Yes, at the genus level. Scientific names show up in questions across several domains, not just this one, and recognizing a genus like Quercus, Acer, or Pinus on sight covers most of what's asked without requiring you to memorize every specific epithet.

How is a scientific name structured?

Genus first, capitalized, followed by the specific epithet, lowercase, and both italicized, for example Acer rubrum. The genus tells you the broader group the tree belongs to; the epithet identifies the particular species within that genus. A cultivar name, if there is one, follows in single quotes and isn't italicized.

What's the difference between a gymnosperm and an angiosperm?

Gymnosperms are the cone-bearing trees, mostly conifers with needle or scale foliage, and their seeds sit exposed on the cone rather than enclosed in fruit. Angiosperms are flowering plants, which covers essentially every broadleaf tree, with seeds that develop inside a fruit or seed coat. It's a broader split than genus, but foliage type, needles or scales versus broad flat leaves, sorts a tree into one category or the other almost instantly.

What is the MAD Horse mnemonic?

It's a memory aid for the small group of common trees with opposite branching: Maple, Ash, Dogwood, and Horsechestnut. Since most trees have alternate branching, spotting opposite branching on an unfamiliar tree immediately narrows your options to this short list, which is what makes branching arrangement the fastest first filter in tree identification.

What's the difference between a simple and a compound leaf?

A simple leaf is a single, undivided blade attached to the stem by one stalk. A compound leaf has multiple leaflets attached to a single stalk, arranged either pinnately, in rows down either side like ash, or palmately, radiating from one point like horsechestnut. Compound leaves are the exception rather than the rule, which is exactly why they're useful for narrowing down an identification.

What does 'right tree, right place' mean?

Matching a species to its planting site, hardiness zone, mature size, soil volume, pH, drainage, and overhead or underground conflicts, before it goes in the ground. Most long-term urban tree failures trace back to a selection mistake made at planting rather than to poor maintenance afterward, which is why this principle carries real weight on the exam.

Why does pest resistance matter when selecting a tree species?

Because pest and disease pressure is often predictable before you plant. Putting an ash in the ground where emerald ash borer is established, or an American elm in place without a Dutch elm disease management plan, creates a foreseeable problem rather than an unlucky one. Selection questions on the exam often test whether you'd catch that risk before planting, not just whether you can treat it afterward.

What features should I use to identify trees?

Start with branching arrangement, opposite versus alternate, since it's the fastest filter and narrows the field immediately. Then check leaf type, simple or compound, followed by leaf margin, serrated, lobed, or entire. When leaves aren't available, bark, buds, and overall form serve as backup features, but the process stays the same: stack one observable trait on top of the next until a wide field narrows down to a genus.