The Forest in Fall: Tree Identification and Leaf Color

Published on the Black Rock Forest website on September 26, 2018

Betsy Ladyzhets, a senior at Barnard College and undergraduate researcher at Black Rock Forest, walks through the basics of differentiating between common tree species in the forest.  This article is adapted from a post on her research blog of the same title, posted on June 12, 2018.

 Take a walk through Black Rock Forest, and notice the life all around you.  Listen for bird song in the canopies or watch for little orange salamanders on the trails.  Count clambering squirrels racing up the trees or keep your eyes peeled for deer in the underbrush.  Or, if would prefer a study subject that isn’t actively trying to run away, you can look at the trees.

But at which trees are you looking?  This is a question more easily asked than answered.  Of the 450-odd tree species commonly found in North America’s forests, 65 have been documented at Black Rock Forest, and researchers are finding more every year as species from southern areas move north with the warming climate.  Still, identifying your friends along the trails is possible with a field guide (I recommend A Peterson Field Guide to Eastern Trees), attention to detail, and a persevering spirit.

I am far from an expert in tree identification myself, but I have had a lot of practice, as my research this past summer required me to collect leaves from over twenty different species.  And I have had a lot of practice directing others to help me find the right trees, whether those others were forest staff members helping me with sample collection or friends forced to listen to my babbling about leaf shape.

Here are the primary markers I use for determining the identity of a tree or shrub.

Location:

Before you begin examining the details of a tree, it is useful to consider where the tree lives.  A tree, like any living thing, occupies a specific niche.  “Niche” is a term used by ecologists to describe a set of environmental conditions specific to one particular species, which give it a unique place in its larger ecosystem.  This set of conditions includes what a species eats, when it reproduces, and where it lives.  That last category is useful in identifying trees, as you can use information about which species live in which areas of the forest to figure out at what you might be looking.

For example: bear oak, or Quercus ilicifolia, a shrub species in the oak family, is only found in high elevation areas, such as the peak of Black Rock Mountain.  Thus, even if you see a plant with similar leaves, if it’s not growing on a high point in rocky soil, you should not expect it to be bear oak – it’s more likely a different type of shrub.  Meanwhile, eastern cottonwood, or Populus deltoides, is an invasive species that currently only grows just outside Black Rock Forest, along the highway where the native forest has been cut away.  Thus, even if you see a plant with similar bell-shaped, spiky-edged leaves, if it’s not growing on the side of a large road, you should not expect it to be eastern cottonwood.  Descriptions of these locational habits can be found in most field guides and encyclopedia sites, along with descriptions of physical characteristics.

Leaf shape:

Okay.  So, you’re in a particular spot in the forest, and you have an idea of which trees live there.  How do you begin to differentiate one tree from the next?  Here, leaf shape is my go-to identifier.  The leaves of many species at Black Rock Forest have characteristic forms: they’re rounded or pointed, simple or compound, smooth-edged or jagged.  This identifier is most useful for established species that you would come across often, such as those in the oak and maple families.  Trees in both of these families have leaf shapes that appear similar from a distance, but are distinctive up close.

Red maple (Acer rubrum) and sugar maple (Acer saccharum) provide a common example.  Although these two closely related trees have leaves similarly segmented into large, petal-like points, red maple has rounder, more jagged leaves while sugar maple has smoother, sharper leaves.  Forest historian John Brady swears that silver maple (Acer saccharinum), whose leaves have the sharp points of silver maple and the jagged edges of red maple, can also be found in the forest, but I have yet to successfully isolate it.

Red oak (Quercus rubrum), chestnut oak (Quercus montana), and black oak (Quercus velutina) also have similar leaf shapes.  But red oak has more divided leaves with individual points, while chestnut oak has leaves with rounded edges and black oak has fewer points and deeper divisions.  Black oak additionally stands out because of its darker colored bark – more on that identifier later.

Leaf color:

As September marches on towards October and the forest begins to cool off, you will see a difference in leaf color as well as leaf shape.  Deciduous trees, or trees with broad leaves rather than needles, take a break from producing their own energy during the winter, relying instead on stored-up sugars from the spring and summer.  They prepare for this hibernation in the fall by breaking down chlorophyll, a compound found in leaves that absorbs energy from sunlight in the first step of the food production process.  Chlorophyll is a strong green pigment which dominates leaf color in the warmer seasons.  But when this compound breaks down, room is made for the other pigments underneath – this causes the trees to change from bright, grassy green to the rich tapestry of reds, golds, and browns that you can see from any high point in the forest.

For some tree families, it’s easy to remember which species turn which colors in the fall, because the colors are present in the species’ names.  Red maple leaves turn a distinctive bright red, as do red oak leaves.  White oak leaves settle into a rich, burnished brown.  For other species, the color shift is less nominally inherent, but still unique: leaves of trees in the birch family, for example, change to shiny gold.

If you examine leaf color combined with leaf shape, distinguishing common Black Rock Forest species is as easy as following a blue trail.  The trees in this forest begin their transformation to fall fashion in late September; the peak of the season is late October, and leaves are falling to the ground by early November.  The best weekends to witness these colors and pick out individual species among the foliage would be October 20 to 21 and 27 to 28, although you’ll see a beautiful array any time between now and November.

Leaf texture:

When leaves of different species have similar shapes and colors, another useful means of identification is their texture.  This can be a difficult quality to evaluate, as it’s not sufficient to just look at leaves from a distance – you have to actually go and grab a few to determine if they are leathery, smooth, or soft.  For some species, though, getting up close and personal is necessary because their leaf textures are so distinctive.

Black gum (Nyssa sylvativa), for example, has a similar leaf shape to American ash (Fraxinus Americana): both species have rounded leaves with one point at the end, and no jagged edges.  But black gum leaves have a distinctive, leathery texture, and thus can be isolated from ash leaves.  Slippery elm (Ulmus rubra) also has a unique, rough texture: rubbing the leaves of this species between your fingers feels like rubbing sandpaper.

Even the texture isn’t always enough to confidently identify slippery elm, though, as I discovered during my research last summer.  Kate Terlizzi, Black Rock Forest’s research manager, informed me that hop hornbeam (Ostrya virginiana) and black birch (the Betula genus), both native BRF tree species, have leaves that are practically identical to those of slippery elm – and hop hornbeam leaves even have a very similar texture!  Even though the three trees aren’t even in the same family!  It’s enough to make any aspiring naturalist want to plop down in the science center and never venture out into the forest again.  But don’t despair – there is one more identification tactic yet.

Bark:

Bark is a useful identifier, but a tricky one, as field guides and other resources usually describe the bark of older, mature trees while someone actually hiking through the forest would see younger saplings.  Still, by honing in on older sections of a tree (usually the trunk near its base), you can sometimes solve debates about its identity once and for all.

The case of slippery elm v. hop hornbeam, unsolvable by leaves alone, falls to bark: slippery elm has rough, intensely textured bark, while hop hornbeam has more regular, peeling bark.  (Black birch, meanwhile, has smooth bark with horizontal stripes.)  I was excited when I finally mastered this three-pronged identification last summer… and less excited when I realized that I had misidentified many trees in the past, rendering many hours of work useless.

Another tree that can be easily identified by bark is American beech, or Fagus grandifolia, a member of the oak family.  American beech has smooth, gray bark that can appear almost shiny, making it easy to spot in a forest of darker, rougher oaks and maples.

Practice, practice, practice!

Tree identification is incredibly challenging.  It depends not only on the general characteristics of a species, but also on the individual life history of a particular tree.  Did this tree grow in a sunny area or a shady area?  Near a road or in a deep forest?  On top of a mountain or in a valley?  The answers to all of these questions correlate to changes in a tree’s shape, size, and color.  Even experts who have worked at Black Rock Forest for years, such as Dr. Schuster and John Brady, can occasionally label a tree wrong.  This is why, if you really want to be certain that you know which tree you’re looking at, it is best to cultivate multiple opinions.  You can also utilize online resources: iNaturalist, for example, is an application collecting observations from citizen scientists and experts alike to build a database of plant and animal sightings.

The best way to hone your identification skills is practice – and no moment of practice is too small.  The next time you’re on a hike, strolling through a new neighborhood, or even walking through your yard, take a moment to notice details about the trees around you.  Are the leaves smooth or jagged?  Is the bark rough or smooth?  Does the trunk have ridges from fungi or do the branches have pockmarks from pests?  What animals might rely upon this tree for food and shelter?  How does it fit into the landscape around it?

To put it simply: what story is this tree telling?  Even if you cannot come to a confident conclusion about a tree’s identity on your own, you can still make discoveries about its place in the forest, and its connection to you, the human traipsing through its home.