Oaks are notorious for being tricky to identify to species. Some Oaks can be confounding even to someone well versed in their identification and not all trees can be identified to species from photos or even in the field, especially early Spring growth and saplings. My hope with this short document is to provide a method for people who are learning about Oaks and want a set of tools to help get better identifications here on iNat as well as to learn more about how to identity oaks for themselves.
This is not a comprehensive guide to identifying Oaks. I have a goal of one day writing that book, but this is not it!
How To Document An Oak With Photographs
Before we get into the details, there are few things to discuss. First is terminology. Botany is jam-packed full of lingo. I am trying to avoid as much of it as I can, but there are certain words that can not be avoided. Here they are with some quick definitions:
- bud - the place of new growth on a plant; in Oaks there are always terminal buds which occur at the end of a twig and are in a clustered group as well as lateral (axial) buds that occur along the twig, on branches, and even on the trunk
- glabrate - becoming glabrous
- glabrous - smooth, hairless
- pubescence - hairiness, short soft hairs
- pubescent - covered with short soft hairs; bearing any kind of hairs
- twig - the smallest (and most recent) growth on a tree; generally where new growth occurs
- vestiture - literally, clothing; how the leaf, the stem, etc are "dressed"
Not everyone is going to have camera equipment that can capture the details (sometimes small or hard to photograph) discussed below. This is OK. Even if your camera can capture all of the details there is one thing that you can do that will not only improve the chances of getting an ID but also help you learn, take notes! A lot of features such as the feel of a leaf can not be captured in any photograph.
If you do these things it is highly likely you will be able to get an identification. If you want to make sure that your Oak is impossible to ID, then read the later section about what not to do.
Where is the Tree Located?
Habitat info can be a huge help in figuring out what kind of Oak (or any other plant) you are looking at. Before you even start thinking about photos do a quick assessment of the habitat. Is wet or dry? What is the soil like (sandy, clay, alkaline/limestone, etc.)? On top of a hill or mountain? Or in a floodplain or river bank? Or on a prairie, barren, or glade? Getting into all of the details of the various habitats is beyond the scope of this short article, but at least think about it and add it any habitat notes to your observation. It can be tremendously useful.
Growth Form and Bark of Mature Trees
If the tree is mature the overall growth form can be a critical clue. Step back and get a photo showing as much of the tree as you can. This can be difficult/impossible for large trees, just try and get as much as you can. If the tree is mature go ahead a grab and photo of the bark at about chest height.
Leaves
Approach the tree and take a photo that shows a selection of leaves. Oak leaves are highly variable, even on a single tree, and quite often a photo of a single leaf is not enough evidence.
Find a leaf, preferably alive or recently fallen, and get a photograph of the underside of the leaf. If you camera will allow it, try and get any hairs that may be present in focus. If your camera can not close focus to show any hairs, then take notes of what you see. Understand that without this information some Oaks can not be identified to species. Even in the Winter the leaf texture can be useful, just make sure that the leaf you are looking at came from the same tree that you are looking at. Leaves blow around and can be displaced well away from the tree that they came from.
Buds & Twigs
If you can reach a live limb, find and photograph a terminal bud cluster. In Winter this is pretty much mandatory. In early Spring, right before bud burst the shape changes and they can be very misleading. In later Spring and early Summer as the buds develop, they are not as useful. But from later Summer, through Fall and Winter (when the buds are mature) they alone can make the ID. As you document buds, you can try and document the twig vestiture and/or branch texture as well.
Acorns
Look for acorns. Depending on the season and whether the tree is mature or not you may not find any. If you do, get a photo of an acorn and cap. The cap is actually more important for ID than the acorn itself. If the cap is not too old and worn the tree can often be IDed from this alone. Both the outside and the inside of the cap can be useful evidence for supporting an ID. It is fairly easy to photograph the outside of the cap showing the shape of the scales and the overall shape of the cap. The inside it tricker to photograph and unless you have a camera that can close focus it will be difficult to get a useful photo. Like with the leaf vestiture, take notes if your camera can not resolve the details.
Galls
Oaks have a very diverse and interesting gall fauna associated with them. Most of these galls are formed by tiny wasps. If you see any abnormal looking growths on the tree take photos. These can be uploaded as separate observations and sometimes the gall alone can ID the tree. If you get interested in galls (and why wouldn't you!) you can learn more at Gallformers. Host identification is critical for gall identification so your Oak identification skills will serve you well if you go on a gall journey.
Specific Scenarios
I have the most experience with the Oaks of Eastern North America so this section is biased towards those, but the general process outlined earlier works for any Oak (or tree for that matter).
Oak Sections
In Eastern North America there are 4 Sections of Oaks that you run across:
These are the Red Oaks. In most cases it is easy to tell that you are looking at a Red Oak as the leaves will have distinct bristle-tips, the acorn cap scales will be flat, and the interior surface of the acorn cap will have at least some pubescence.
The Harsh Reality of Eastern Lobatae Oaks
A great many, maybe even a majority, of the Lobatae Oak observations on iNat are not identifiable to species. This in spite of the fact that there are a huge number of them that are IDed to species, often as Research Grade. The primary reason for this is that many Eastern Lobatae Oaks can not be IDed from the leaves, which is most often the evidence present in an iNat Oak observation.
There is a particular group that creates the most challenge. It consists of the following species:
With sufficient evidence, all can be IDed. Some are made easier due to limited ranges, but even those overlap with at least one of the others on the list. Leaf shapes with these can be very misleading, especially with saplings. In general small trees of any of these, other than sometimes Q. velutina, can not be IDed to species.
The other group that is tricky are the Lobatae oaks with (mostly) un-lobed leaves:
These are made somewhat easier due to range limitations. Q imbricaria is a tree mostly of the Midwest US with some populations present in the East. Q. laurifolia and Q. hemisphaerica are strictly southern oaks (NC to TX) with Q. phellos occurring across a wide range from NY to TX. Q. phellos is used heavily in cultivation and can show up outside of its natural range. Another gotcha is that to the untrained eye the Virentes Section Oaks (see later section) can look a lot like these.
Further complicating matters, there is a lot of hybridization amongst Lobatae oaks.
These are the White Oaks. Generally if it is not a Lobatae Oak and you are not in an area dominated by Live Oaks (see next section) then it is most likely a Quercus Oak. The leaves will not have bristle tips, the acorn cap scales will be warty, and the inner surface of the acorn cap will be glabrous (no hairs).
The "Chestnut Leaved" Oaks
There are 4 species of "chestnut leaved" oaks that occur in North America (all in the East). There are 2 others that can at times appear "chestnut leaved" but upon closer examination are not.
These four share many similarities in leaf shape and dentation (marginal teeth). So much so, that the Flora of North America has this to say about them:
The four species of the chestnut oak group in eastern North America (Quercus montana, Q. michauxii, Q. muhlenbergii, and Q. prinoides) are somewhat difficult to distinguish unless careful attention is paid to features of leaf vestiture and fruit and cup morphology. Attempts to identify these species mostly or solely on basis of leaf shape and dentition (as in many other oak species complexes) have resulted in a plethora of misidentified material in herbaria and erroneous reports in the literature.
The other two that can sometimes be confused as being "chestnut leaved" are:
These are the Eastern Live Oaks. The two biggest clues that you are looking at a Virentes Oak are: the leaves are not deciduous (all falling off in the Fall) and you are in a Southeastern US State (SE VA to TX). There are four species in the Virentes Section that occur in the US (none in Canada) and three of them can be difficult to separate without sufficient evidence.
These are cultivated and sometimes invasive European Oaks.
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Quercus acutissima (Sawtooth Oak): across the Eastern US; most often, and frequently, encountered in cultivation but there are areas where they have escaped cultivation, even well away from civilization (at one time they were planted on Federal lands as mast for wildlife since they produce copious acorns).
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Quercus cerris (Turkey Oak): Northeastern US, mainly NY and MA; sometimes cultivated, with established escaped populations in NY and MA
Things To Do If You Really DO NOT Want a Species Level ID
- Photograph just the trunk
- Photograph the top side of a single leaf that is not even on the tree
- Photograph the entire tree from a great distance
- Photograph the tree from a car while hurtling down the highway
- Use blurry photos
- Take a single photo of the most abnormal leaf on the whole tree
About Those Photos of Q. Montana Trunks...
| Is this Q. montana (Chestnut Oak)? A lot of people might quickly agree that it is? |
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| What about this one? |
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| Or this one, which was taken in the Blue Ridge mountains in Virginia which are loaded with Q. montana? |
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None of these are Q. montana. The first is Sassafras albidum (Sassafras), the second Oxydendrum arboreum (Sourwood), and the third Q. rubra var rubra (Northern Red Oak). These are all examples where insufficient evidence of a common species (and common observation type) can lead to erroneous identifications.
