A Guide to the Various Oaks Growing in Missouri
Introduction
Oaks are unique in many ways, but most strikingly is how well oaks represent the classic stereotype of a tree. Most everyone has had an experience with oaks, be that collecting the acorns that litter the ground or climbing on their large, low-hanging branches as children; or raking their leaves off the street come fall time. Oaks tend to be large, with the largest oaks growing up to 100 feet high and can a trunk diameter of up to six feet. Even more impressive, though, is the width of the oaks’ canopy, which can be up to twice the size of the tree’s height. Oaks are deciduous trees, meaning their leaves drop come fall, leaving a looming skeletal figure in place of the once lively canopy. Additionally, oaks account for the highest lumber production of any hardwood tree, being beaten out only by three other species of conifers (softwoods)[1].
Pictured below is a photo displaying the sheer width of an oak’s canopy.
Oak Tree And Sun
History of Human Use of Oaks
Oaks, classified in the genus Quercus, have an approximated 600 species across the world, 56 of which are native to North America, and 21 of which are native in Missouri [2]. Oaks have been supplying us with hearty wood for centuries; however, lesser known is that acorns and the bark of oaks have been used to treat inflammation and diarrhea. Additionally, early Europeans and Americans would make flour for baking by grinding down acorns [3]. However, extensive use of oaks did not start until the 1500s, when the Spanish discovered oak’s superiority in ship building. Many of the species used to cross the Atlantic Ocean to the new world were made out of oak. Demand for oaks reached an all time high during the shipbuilding golden ages of the late 1700s to mid 1800s in both the US and Europe. In fact, some oak species were so popular, that this demand decimated their populations globally [4]. Nowadays, the oak wood is still widely used, particularly in hardwood floors, but more interestingly: in bourbon/whiskey barrels. White Oaks are used extensively in the whiskey making process, and the interaction between the wood and the alcohol is part of what gives each type of whiskey its unique flavor. In fact, a common saying is: whiskey without an oak barrel is just moonshine!
Taxonomy
Kingdom: Plantae
Phylum: Anthophyta
Class: Magnoliopsida
Order: Fagales
Family: Fagacaea
Genus: Quercus
Biology
Oaks belong to the genus Quercus and there are two distinct groups within the genus: White Oaks and Red (or black) Oaks. While obviously different, both groups have some key similarities. Both types of trees reproduce the same way. Oaks are hermaphroditic, meaning they have both male and female flowers on the same tree. Pollination of these flowers occurs by wind; female flowers will only open once the male flowers on the same tree have dispersed all their pollen [5]. This action is to promote cross breeding (and prevent self pollination), and is partially why oaks hybridize so easily. The fruit of the tree, acorns, can take one to two years to reach maturity depending on the type of tree. Acorns are a staple in the diet of many wild animals, and therefore will only occasionally have the opportunity to grow into a new tree. Additionally, oaks hybridize with ease, meaning that one species of oak can cross pollinate with another, and grow a hybrid mix of the two trees. Unlike most hybrids, oak hybrids are “viable,” and can reproduce on their own. Because of this, identification of oak trees is much less straightforward than for other genuses as extensive variation can happen from tree to tree of the same species.
Similarities aside, there are a couple of distinguishing features between these two types of oaks. These differences are apparent in the leaves, acorns, bark, and even the wood of the tree.
- White Oaks have lobed leaves that are not serrated, while Red (or black) Oaks have bristle-tipped leaves. This is the most distinct feature between the two families of trees.
- The bark of White Oaks tends to be lighter in color while the bark of Red Oaks varies from a dark grey to brown.
- For a significantly harder to see distinction: the acorns of White Oaks reach maturity after one year, while those of Red Oaks reach maturity after two years. Additionally, the acorns of Red Oaks have hairs on the insides of their cups, while White Oak acorns do not.
- Lastly, the timber of the Oak varies greatly between Red and White Oaks. The Red Oak wood is quite porous and has a reddish tinge while White Oak is leakproof and has a lighter look to it.
Identification
Leaves
When identifying oaks, it is important to determine which family the oak belongs in first. This step is generally the easiest, as a quick glance at the leaves can determine the family. Below are two pictures of the most widely-recognizable oaks from each family. A Pin Oak (Quercus palustris) and a White Oak (Quercus alba).
A Pin Oak leaf has deep nodes and comes to bristled points at the end of each lobe, as pictured below.
Below is the other type of oak, a White Oak. This photo was taken near the end of fall, as the leaves of this oak have changed colors from the green of summer indicating they are ready to drop. The smooth, rounded lobes of the leaves provide stark contrast to the sharp-tipped leaves of the Pin Oak. Although not important when distinguishing Red Oaks from White, the “middle” lobes of the White Oak are its largest and help distinguish it from other oaks in the White Oak family.
Bark
Come end of fall and winter, oaks will drop their leaves. While the lack of leaves is definitely inconvenient when it comes to oak identification, it is by no means makes it impossible. By looking at the bark, it is easy to identify which family the tree belongs to, and often the specific species of tree.
White Oak Bark
The bark of White Oaks is typically quite rigid and furrowed. Which means, in other terms, it is not very smooth. The bark, as aforementioned, tends to be a lighter grey.
Pictured above is the bark of a Swamp White Oak (Quercus bicolor). The ridges and furrows of this particular oak are extremely pronounced which leads to easy identification.
Red Oak Bark
The bark of Red Oaks is much smoother than their White Oak counterpart. The bark is also a much darker color than the bark of White Oaks. In fact, that is where the alternate name for Red Oak comes from: Black Oak. Additionally, a small, but noticeable (if you look closely) feature of Red Oak bark is the red tinge in between the ridges in the bark.
Above is a picture of one of the most common Oaks’ bark: the Pin Oak (Quercus elipsoidallis). Comparing the bark of this tree to the bark of the Swamp White Oak, the differences are quite obvious. The Pin Oak’s bark is much smoother and the tinges of red are quite visible within the ridges in the tree [6].
Entire (unlobed) Red Oaks
Weirdest of all, are the oaks whose leaves are entire, meaning their leaves have none of the lobes so characteristic of the oak tree. There are three of these types of oaks found in Missouri, only one of which is found in more than 10 counties. The three oaks (starting with most common) are: Shingle Oak (Quercus imbricaria) , Willow Oak (Quercus phello), and Water Oak (Quercus nigra). These oaks, while Red Oaks through and through, deserve their own identification section, as their leaves are tell-tale giveaways when it comes to identifying which oak is which. The leaves of these oaks are wildly different than those of the other oaks. Due to their uniqueness, “entire” oaks should be easily identified against other oaks.
Reproduction
While there are many aesthetic differences when it comes to oaks, what remains the same for all species is their method of reproduction. As stated before, oaks are hermaphroditic. This means that they can reproduce asexually through self pollination (however this is rare), or reproduce sexually through pollination from another tree. Pollination occurs by wind, and the male flowers produce large amounts of pollen for a period of up to two weeks [7]. Male flowers are easily identifiable; they hang down from large tube-like bodies called catkins, waiting for the wind to carry their pollen away [8]. Female flowers are much harder to see as they are considerably smaller than male flowers; however, if you look closely enough, they occur where a bud might.
Pollen that is fortunate enough, after potentially traveling miles through the air, will come in contact with a female flower and attach itself, thus beginning the production of an acorn. Acorns are the fruit of the oak tree, and their journey to becoming a new tree is a harrowing one. Most acorns will not produce a tree, or even have a chance to germinate, as they are a popular meal in the animal community. Acorns are eaten in droves by anything from woodpeckers, to deer. Pictured below is a diagram of what the male catkins of an oak look like.
The topic of oak reproduction inevitably leads to acorns. Oak trees are the only family of trees that produce acorns. While acorns will vary from species to species, they are all generally similar when it comes to the parts of an acorn. Think of oaks like apple trees, different trees will produce different types, but in the end they are all still produce apples, or acorns. Knowing the different parts of an acorn can help an eager oak tree identifier notice certain characteristics, leading to a tree identification!
In the picture below, the basic parts of an acorn are shown. The stalk of the acorn is attached to tree, and opens up into the cup. The cup of the acorn is the “hat” like thing that sits on top of the actual nut, ensuring it does not fall too early. Inside the nut, is not only the genetic material to grow a new tree, but all the nutrients the seedling needs until it can photosynthesize by itself. The last part of the acorn, the pointy part at the end, is left over from the female flower (style). The variation in these parts can help to distinguish the type of oak the acorn belongs to.
Image Courtesy of Cronodon. For more infromation visit: https://cronodon.com/
The length and size of the actual nut as well as the amount of the nut that the cup covers are the two most common ways to distinguish an acorn.
Oaks Natural Defenses
Since oaks are trees, they are unable to react to predation and environmental changes as easily as an animal might. But that does not stop oaks from having a few tricks up their sleeves when it comes to staying alive and warding off predators. Oaks use multiple strategies to ward off predators, but here a few interesting ones:
- Tannic Acid. Oaks contain a lot of a substance called tannic acid. This acid, present in oak leaves and wood, make oak leaves incredibly undesirable to predators. The tannins cause the leaves to be quite bitter and can be toxic in large amounts. Herbivores such as horses and deer will only eat oak leaves in desperate times [9]. Additionally, most animals who feed on the tree will seldom cause the death of an oak. Although the damage may be visible and disconcerting, it almost always is not a big deal to the overall health of the tree [10]
- Selective Acorn Production (masting). Oaks are one of the few species of plants that exhibit a behavior known as masting. Masting is when oaks will produce an abundance of acorns one year, while cutting back on acorn production significantly while in between these mast years. On top of this, oaks growing together in a similar geographic area will sync up their mast years leading to vast acorn production in oak forests during mast years [11]. Scientists are not certain as to why this phenomenon occurs, however one leading theory suggests it is related to defending against foragers, hence its inclusion in this list. The theory suggests that since animals are so dependent on acorns, if the trees were to significantly lower production, the populations of these animals feeding on the acorns would diminish. After a few years of this, the populations of acorn-eating animals will be considerably lower. As soon as this is the case, the oaks ramp up acorn production giving their seeds the best opportunity to survive and develop into a new oak tree. While these theories are convincing,
- Leaf/Acorn Drop. While not a defense against predators, leaf/acorn drop is still an interesting defense mechanism the trees have. If an oak is experiencing a particularly dry year or is under a lot of stress for another reason, depending on the time of year, they may drop all their acorns and leaves. This is a telltale sign that the oak is under duress. Since maintaining leaves and producing acorns is a particularly energy demanding task, the tree may elect to abandon these parts and focus on conserving energy internally, or fighting a disease infecting it.
Exhaustive List of Oaks Native to Missouri
White Oaks
White Oak (Quercus alba)
Post Oak (Quercus stellata)
Bur Oak (Quercus macrocarpa)
Swamp White Oak (Quercus bicolor)
Chinkapin Oak (Quercus muehlenbergii)
Swamp Chestnut Oak (Quercus michauxii)
Overcup Oak (Quercus lyrata)
Dwarf Chestnut Oak (Quercus prinoides)
Red Oaks
Northern Red Oak (Quercus rubra)
Shumard Oak (Quercus shumardii)
Black Oak (Quercus velutina)
Blackjack Oak (Quercus marilandica)
Pin Oak (Quercus palustris)
Northern Pin Oak (Quercus ellipsoidalis)
Southern Red Oak (Quercus falcata)
Cherrybark Oak (Quercus falcata var. pagodifolia)
Nuttall Oak (Quercus texana)
Entire Red Oaks:
Shingle Oak (Quercus imbricaria)
Willow Oak (Quercus phellos)
Water Oak (Quercus nigra) [12]
Sources
[1]“Missouri’s Oaks.” MDC Discover Nature.
[2]Brown, Andrew. “List of All Oak Tree Species Names and Types.” TreeNames.net, TreeNames, 8 Feb. 2018.
[3]Goldstone, Kate. “The History of English Oak – A Very British Wood.” Wood Finishes Direct, 7 Mar. 2018.
[4]Coder, Kim D. Live Oak Use Historic Time-Line. Warnell School of Forestry, Dec. 2016.
[5]Wright, Jacob J. “Early Pollination of Oak Trees.” Garden Guides, 12 Mar. 2019.
[6]Rosener, Bill. “How to Tell the Difference Between Red Oak vs White Oak.” Patterns, Stencils, & DIY Projects, SunCatcherStudio, 19 Sept. 2019.
[7]Cecil, Cleveland Van. “Oak Tree Pollen & Acorns.” Garden Guides, 12 Mar. 2019.
[8]Conrad, Jim. “Oak Flowers.” Oak Flowers.
[9]Hodgson, Larry. “Tannins in Oak Leaves.” Laidback Gardene.
[10]Swiecki, Tedmund J, and Elizabeth Bernhardt. “A Delicate Balance: Impacts of Diseases and Insects on the Health of California Oaks.” Oak Diseases and Insects.
[11]Fernández-Martínez, Marcos, et al. “Masting in Oaks: Disentangling the Effect of Flowering Phenology, Airborne Pollen Load and Drought.” Acta Oecologica, vol. 43, 2012, pp. 51–59., doi:10.1016/j.actao.2012.05.006.
[12]Denison, Edgar, et al. Missouri’s Oaks and Hickories. Conservation Commission of the State of Missouri, 1994.
Images
[1]Ohio Department of Natural Resources. “Pin Oak Tree Bark.” Trees of Ohio: Pin Oak, Ohio Department of Natural Resources.
[2]Cronodon. “Acorn Diagram.” Quercus (Oak Trees), Cronodon.com.